Vector magnetometer system and methods

Abstract

A method of magnetic survey including: acquiring calibration measurements with bird including a vector magnetometer, a plurality of inertial sensors, and a GNSS transceiver, in a known magnetic field; determining calibration parameters using the calibration measurements; acquiring vector magnetic measurements with the vector magnetometer, orientation measurements with the inertial sensors, and position measurements with the GNSS transceiver, while towing the bird with an aerial vehicle along a flight path; and correcting the vector magnetic measurements to provide corrected magnetic vector measurements by: calibrating the vector magnetic measurements using the calibration parameters; aligning the vector magnetic measurements to a position measurement coordinate space using the orientation measurements; and providing the corrected vector magnetic measurements with the position measurements.

Description

[0001] VECTOR MAGNETOMETER SYSTEM AND METHODS

[0002] TECHNOLOGICAL FIELD

[0003] The present disclosure, in some embodiments thereof, relates to the measurement of vectorial magnetic fields and, more particularly, but not exclusively, to airborne measurement of vectorial magnetic fields.

[0004] BACKGROUND ART

[0005] Background art, where each art is incorporated in its entirety by reference, includes the below list. In the following document these arts are referred to by name of the first listed author, followed by year of publication of the document e.g., (Name, year).

[0006] Aiken et al International Geomagnetic Reference Field: the thirteenth generation Earth, Planets and Space 73, Article number: 49 (2021).

[0007] Ben-Avraham, Z., Rosenthal, M., Tibor, G., Navon, H., Wust-Bloch, H., Hofstetter, R., &Rybakov, M. (2014). Structure and tectonic development of the Kinneret Basin. Lake Kinneret: Ecology and Management, 6, 19-38.

[0008] Guspi, F. (1987). Frequency -domain reduction of potential field measurements to a horizontal plane. Geoexploration, 24(2), 87-98.

[0009] Parker, R. L., & O’Brien, M. S. (1997). Spectral analysis of vector magnetic field profiles. Journal of Geophysical Research, 1O2(B 11), 24815-24824.

[0010] Acknowledgement of the above references herein is not to be inferred as meaning that these are in any way relevant to the patentability of the presently disclosed subject matter.

[0011] GENERAL DESCRIPTION

[0012] Following is a non-exclusive list of some exemplary embodiments of the disclosure. The present disclosure also includes embodiments which include fewer than all the features in an example and embodiments using features from multiple examples, even if not listed below.

[0013] Example 1. A method of magnetic survey comprising: acquiring calibration measurements with a bird comprising a vector magnetometer, a plurality of inertial sensors, and a GNSS transceiver, in a known magnetic field; determining calibration parameters using said calibration measurements; acquiring vector magnetic measurements with said vector magnetometer, orientation measurements with said inertial sensors, and position measurements with said GNSS transceiver, while towing said bird with an aerial vehicle along a flight path; and correcting said vector magnetic measurements to provide corrected magnetic vector measurements by: calibrating said vector magnetic measurements using said calibration parameters; aligning said vector magnetic measurements to a position measurement coordinate space using said orientation measurements; and providing said corrected vector magnetic measurements with said position measurements.

[0014] Example 2. The method according to Example 1, wherein said acquiring calibration measurements comprises acquiring vector magnetic measurements with said vector magnetometer, and orientation measurements with said inertial sensors in a known magnetic field.

[0015] Example s. The method according to Example 2, wherein said determining calibration parameters comprises determining one or more of scale factors, offsets, and orthogonality correction calibration parameters using said calibration measurements and said known magnetic field.

[0016] Example 4. The method according to any one of Examples 2-3, wherein said determining calibration parameters comprises determining alignment parameters for alignment of orientation measurements with the vector magnetic field measurements.

[0017] Example 5. The method according to any one of Examples 1-4, wherein said bird comprises a scaler magnetometer; and wherein acquiring magnetic measurements comprises acquiring scaler magnetic measurements with said scaler magnetometer.

[0018] Example 6. The method according to Example 5, comprising verifying said vector magnetic measurements using said scaler magnetic measurements. Example 7. The method according to any one of Examples 1-6, wherein said providing is during said towing.

[0019] Example 8. The method according to any one of Examples 1-7, wherein said providing is to a user via a user interface.

[0020] Example 9. The method according to any one of Examples 1-8, comprising identifying one or more crossover point in said flight path.

[0021] Example 10. The method according to Example 9, comprising determining one or more crossover calibration parameters using measurement data for said one or more crossover point.

[0022] Example 11. The method according to Example 10, comprising one or more of verifying and adjusting said calibration parameters using said crossover calibration parameters.

[0023] Example 12. The method according to any one of Examples 9-11, wherein said identifying comprises identifying, in said position measurements, one or more crossover points having a same location having a same latitude and longitude.

[0024] Example 13. The method according to any one of Examples 10-12, wherein said determining comprises determining differences in crossover point corrected magnetic measurements; calculating the crossover calibration parameters which minimize said differences in crossover point data.

[0025] Example 14. The method according to any one of Examples 1-13, wherein said bird includes a towable structure.

[0026] Example 15. The method according to any one of Examples 1-14, comprising: evaluating said corrected vector magnetic measurements provided with said position measurements; and determining one or more trajectory recommendation based on said evaluating.

[0027] Example 16. The method according to Example 15, wherein said evaluating comprises determining and evaluating a density of said corrected vector magnetic measurements using said position measurements.

[0028] Example 17. The method according to Example 16, wherein said evaluating comprises comparing said density with a desired density.

[0029] Example 18. A vector magnetic measurement system comprising: a towable bird structure comprising: a vector magnetometer; a plurality of inertial sensors; a GNSS transceiver; and one or more antennas configured to receive and transmit signals to and from said GNSS transceiver; a connector configured to connect said towable bird to a towing aircraft; and circuitry configured to: determine calibration measurements using magnetic field measurements acquired with said vector magnetometer in a known magnetic field; receive vector magnetic measurements from said vector magnetometer, orientation measurements from said inertial sensors, and position measurements from said GNSS transceiver, while said towable bird is towed by an aerial vehicle along a flight path; and correct said vector magnetic measurements to provide corrected vector magnetic measurements by: calibrating said vector magnetic measurements using said calibration parameters; aligning said vector magnetic measurements to a position measurement coordinate space using said orientation measurements; and provide said corrected vector magnetic measurements with said position measurements.

[0030] Example 19. The system according to Example 18, wherein said bird comprises a scaler magnetometer.

[0031] Example 20. The system according to any one of Examples 18-19, comprising an aircraft module configured to be hosted by said aircraft.

[0032] Example 21. The system according to Example 20, wherein said aircraft module comprises a power supply, wherein said connector comprises one or more electrical connector between said power supply and circuitry of said towable bird.

[0033] Example 22. The system according to Example 21, wherein said aircraft module comprises processing and memory circuitry which hosts at least a portion of said circuitry.

[0034] Example 23. The system according to Example 22, wherein said connector comprises one or more data connector configured to transmit data between said aircraft module processing and memory circuitry and circuitry of said bird. Example 24. The system according to any one of Examples 20-23, wherein said aircraft module comprises a user interface, wherein said circuitry is configured to provide said corrected vector magnetic measurements with said position measurements to said user interface which is configured to display said corrected vector magnetic measurements with said position measurements.

[0035] Example 25. The system according to any one of Examples 18-24, wherein said bird comprises processing and memory circuitry which hosts at least a portion of said circuitry.

[0036] Example 26. The system according to any one of Examples 18-25, wherein said bird comprises: a bird housing comprising a hollow chamber; a rigid element rigidly mounted to walls of said hollow chamber, onto which rigid element are rigidly attached said vector magnetometer, said plurality of inertial sensors, and said one or more antennas.

[0037] Example 27. A method of magnetic survey comprising: acquiring vector magnetic measurements, orientation measurements, and position measurements with a bird comprising a vector magnetometer configured to acquire the vector magnetic measurements, a plurality of inertial sensors configured to acquire the orientation measurements, and a GNSS transceiver configured to acquire the position measurements, while towing said bird with an aerial vehicle along a flight path; processing said vector magnetic measurements using said orientation measurements and said position measurements to provide orientation-independent and altitude-normalized corrected magnetic measurements; determining calibration parameters using said corrected magnetic measurements by: identifying, in said position measurements, one or more crossover points having a same latitude and longitude; determining differences in crossover point corrected magnetic measurements; calculating the calibration parameters which minimize said differences in crossover point data.

[0038] Example 28. The method according to Example 27, wherein said bird includes a towable structure.

[0039] Example 29. The method according to any one of Examples 27-28, comprising calibrating said corrected vector magnetic measurements using said calibration measurements.

[0040] Example 30. The method according to any one of Examples 27-29, wherein said processing comprises aligning said vector magnetic measurements to a position measurement coordinate space using said orientation measurements.

[0041] Example 31. The method according to any one of Examples 27-30, comprising providing said corrected vector magnetic measurements with said position measurements.

[0042] Example 32. The method according to Examples 27-31, wherein said determining calibration parameters comprises determining one or more of scale factors, offsets, and orthogonality correction calibration parameters.

[0043] Example 33. The method according to any one of Examples 27-32, wherein said bird comprises a scaler magnetometer; and wherein acquiring magnetic measurements comprises acquiring scaler magnetic measurements with said scaler magnetometer.

[0044] Example 34. The method according to Example 33, comprising verifying said vector magnetic measurements using said scaler magnetic measurements.

[0045] Example 35. The method according to any one of Examples 27-34, wherein said providing is during said towing.

[0046] Example 36. The method according to any one of Examples 27-35, wherein said providing is to a user via a user interface.

[0047] Example 37. A vector magnetic measurement system comprising: a towable bird comprising: a vector magnetometer; a plurality of inertial sensors; a GNSS transceiver; and one or more antennas configured to receive and transmit signals to and from said GNSS transceiver; a connector configured to connect said towable bird to a towing aircraft; and circuitry configured to: receive vector magnetic measurements from the vector magnetometer, orientation measurements from the plurality of inertial sensors, and position measurements from the GNSS transceiver; process said vector magnetic measurements using said orientation measurements and said position measurements to provide orientation-independent and altitude-normalized corrected magnetic measurements; determine calibration parameters using said corrected magnetic measurements by: identifying, in said position measurements, one or more crossover points having a same location; determining differences in crossover point corrected magnetic measurements; calculating the calibration parameters which minimize said differences in crossover point data.

[0048] Unless otherwise defined, all technical and / or scientific terms used within this document have meaning as commonly understood by one of ordinary skill in the art / s to which the present disclosure pertains. Methods and / or materials similar or equivalent to those described herein can be used in the practice and / or testing of embodiments of the present disclosure, and exemplary methods and / or materials are described below. Regarding exemplary embodiments described below, the materials, methods, and examples are illustrative and are not intended to be necessarily limiting.

[0049] Some embodiments of the present disclosure are embodied as a system, method, or computer program product. For example, some embodiments of the present disclosure may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module” and / or “system.”

[0050] Implementation of the method and / or system of some embodiments of the present disclosure can involve performing and / or completing selected tasks manually, automatically, or a combination thereof. According to actual instrumentation and / or equipment of some embodiments of the method and / or system of the present disclosure, several selected tasks could be implemented by hardware, by software or by firmware and / or by a combination thereof, e.g., using an operating system.

[0051] For example, hardware for performing selected tasks according to some embodiments of the present disclosure could be implemented as a chip or a circuit. As software, selected tasks according to some embodiments of the present disclosure could be implemented as a plurality of software instructions being executed by a computational device e.g., using any suitable operating system.

[0052] In some embodiments, one or more tasks according to some exemplary embodiments of method and / or system as described herein are performed by a data processor, such as a computing platform for executing a plurality of instructions. Optionally, the data processor includes a volatile memory for storing instructions and / or data and / or a non-volatile storage e.g., for storing instructions and / or data. Optionally, a network connection is provided as well. User interface / s e.g., display / s and / or user input device / s are optionally provided.

[0053] Some embodiments of the present disclosure may be described below with reference to flowchart illustrations and / or block diagrams. For example illustrating exemplary methods and / or apparatus (systems) and / or and computer program products according to embodiments of the present disclosure. It will be understood that each step of the flowchart illustrations and / or block of the block diagrams, and / or combinations of steps in the flowchart illustrations and / or blocks in the block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general-purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions / acts specified in the flowchart steps and / or block diagram block or blocks.

[0054] These computer program instructions may also be stored in a computer readable medium that can direct a computer (e.g., in a memory, local and / or hosted at the cloud), other programmable data processing apparatus, or other devices to function in a particular manner, such that the instructions stored in the computer readable medium can be used to produce an article of manufacture including instructions which implement the function / act specified in the flowchart and / or block diagram block or blocks.

[0055] The computer program instructions may also be run by one or more computational device to cause a series of operational steps to be performed e.g., on the computational device, other programmable apparatus and / or other devices to produce a computer implemented process such that the instructions which execute provide processes for implementing the functions / acts specified in the flowchart and / or block diagram block or blocks.

[0056] Some of the methods described herein are generally designed only for use by a computer, and may not be feasible and / or practical for performing purely manually, by a human expert. A human expert who wanted to manually perform similar tasks, might be expected to use different methods, e.g., making use of expert knowledge and / or the pattern recognition capabilities of the human brain, potentially more efficient than manually going through the steps of the methods described herein.

[0057] BRIEF DESCRIPTION OF THE DRAWINGS

[0058] In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: FIG. 1A is a simplified schematic of a magnetic field measurement system, according to some embodiments of the disclosure;

[0059] FIG. IB is a simplified schematic of a bird for airborne magnetic field measurements, according to some embodiments of the disclosure;

[0060] FIG. 2 is a flowchart of a method of determining calibration parameters, according to some embodiments of the disclosure;

[0061] FIG. 3 is a flowchart of a method of vectoral magnetic field measurement, according to some embodiments of the disclosure;

[0062] FIG. 4 is an illustration of a measurement flight track, according to some embodiments of the disclosure;

[0063] FIG. 5 is a polar plot of magnetic field measurements and calibrated magnetic field measurements, according to some embodiments of the disclosure;

[0064] FIG. 6 is plots of triaxial magnetic field measurements, according to some embodiments of the disclosure;

[0065] FIG. 7 illustrates plots of magnetic field, bird angle, and altitude, with distance, for a measurement profile, according to some embodiments of the disclosure;

[0066] FIG. 8 illustrates power spectral density with wavenumber and wavelength, according to some embodiments of the disclosure; and

[0067] FIGs. 9A-D illustrate mapping of errors in crossover data for magnetic fields, according to some embodiments of the disclosure.

[0068] DETAILED DESCRIPTION OF EMBODIMENTS

[0069] The present disclosure, in some embodiments thereof, relates to measurement of vectorial magnetic fields and, more particularly, but not exclusively, to airborne measurement of vectorial magnetic fields.

[0070] Overview

[0071] A broad aspect of some embodiments of the disclosure relates to an aero-towed vector magnetometer system where a structure (herein termed a “bird”) housing a vector magnetometer is attached to (e.g., suspended from) an aerial vehicle which is then flown over a survey region. In some embodiments, the bird is suspended at a distance from the aerial vehicle, a potential benefit of which is, in measurements of the vector magnetometer, the reduction in magnetic noise associated with the aerial vehicle itself. Exemplary suitable vehicles include manned and / or unmanned rotary (e.g., helicopter, drone) and fixed-wing aircraft (e.g., jet, glider).

[0072] The bird structure or housing may be aerodynamically designed to maintain stability and control during flight, for example, minimizing drag and oscillations while being towed at different speeds and altitudes. The bird structure or housing may incorporate aerodynamic surfaces such as one or more of fin / s, wing / s, and stabilizer / s to optimize its flight characteristics and / or ensure a controlled trajectory relative to the towing vehicle.

[0073] In some embodiments, while vector magnetic measurements are acquired, measurements of location and orientation of the bird are acquired using devices hosted by the bird itself. Where, in some embodiments, the location and orientation measurements enable mapping of vectoral magnetic fields using the magnetic measurements. A potential advantage of hosting of both the magnetic sensors and one or more of the location and orientation sensing circuitry by the bird is increased accuracy of mapping of the magnetic measurements using the orientation and / or location measurements.

[0074] Where location measurements may provide geographical location for the vector measurements (e.g., for mapping), and orientation measurements of the bird may be used to orient the vector magnetic measurements with respect to the location. Orientation measurements of the bird (e.g., itself) potentially enable accurate vector magnetic measurements. Potentially increasing accuracy of vector magnetic measurement, for example, when bird movement is not the same as that of the aircraft and / or changes and / or is unpredictable. Such measurement of orientation of may be advantageous and / or mitigate accuracy issue / s introduced by suspension of the bird from the aircraft (e.g., at distances with associated reduction in magnetic noise) where the bird may be prey to changes in orientation e.g., associated with wind, and / or where the bird has differences in trajectory with respect to the aircraft.

[0075] An aspect of some embodiments of the disclosure relates to real-time acquisition of vectorial magnetic measurements. Where, in some embodiments, processing of magnetic, position, and orientation measurements is performed during real time (e.g., while the bird is being towed).

[0076] In some embodiments, real-time acquisition of data enables adaptive control of the towing aircraft's flight path based on features derived from the acquired data. This adaptive control may be implemented either automatically by an onboard processor (e.g., if the aircraft is an autonomous aircraft) or semi-automatically by providing recommendations (e.g., flight path recommendations) to a human operator.

[0077] The collected geomagnetic data may include various features relevant to the quality and utility of future measurements. Such features may include, but are not limited to, the density of measurements within a given region, the accuracy of the measurements (e.g., as determined by comparing data at crossover points) and the detection of anomalies and / or patterns indicative of areas of interest. Based on these features, the system may dynamically adjust the aircraft's trajectory, optionally including altitude.

[0078] In some embodiments, the processor is configured to analyze the acquired data in real time to identify regions where additional measurements are required to enhance data density and / or improve accuracy. For instance, if data acquired at crossover points indicates inconsistencies, the system may instruct the aircraft to revisit those locations to obtain additional measurements. If a particular geomagnetic anomaly is detected, the system may guide the aircraft (automatically or by instructions communicated to the pilot) to perform further localized scanning of the anomaly to gather more detailed information.

[0079] The adaptive control of the aircraft may be executed in an autonomous mode, wherein the processor directly controls the flight path adjustments based on pre-defined criteria and real-time data analysis. Alternatively, the system may operate in a semiautomatic mode, wherein the processor generates recommendations and presents them to the user, who can then manually adjust the flight path based on the provided insights.

[0080] By leveraging real-time data acquisition and analysis, the system potentially enables efficient and targeted data collection, potentially reducing redundant measurements while ensuring comprehensive coverage of areas of interest. Additionally, the ability to adapt the flight path dynamically improves the overall effectiveness of the geomagnetic survey process, leading to higher-quality data for subsequent analysis and interpretation.

[0081] Optionally, in some embodiments, processed vectoral magnetic fields are displayed to a user e.g., via a user interface. Where the user is airborne on the towing vehicle (or in communication with an airborne person), measurement results may be used to guide measurements e.g., for additional (e.g., denser) measurement of a region of interest identified in acquire measurements. An aspect of some embodiments of the disclosure relates to calibration of magnetic field measurements using calibration measurements (e.g., including vector magnetic measurements and orientation measurements) acquired using the bird where the bird is located in a known magnetic field and / or where the bird is not being towed (e.g., grounded) and / or where the bird is not attached to an aircraft. In some embodiments, the calibration parameters are then used to process raw measurement magnetic field data e.g., for display to a user e.g., for use in determining feature / s of additional measurements to be acquired. Potentially enabling use of acquired measurements (e.g., to guide later measurements) during a measurement acquisition session (e.g., during a single flight).

[0082] In some embodiments, calibration measurements are performed at a base station where the known magnetic field is provided by one or more of base station measurement data and / or modeling of the earth’s magnetic field and / or database information regarding the earth’s magnetic field. Alternatively, in some embodiments, magnetic isolation and / or magnetic field generation device / s are used (e.g., in a lab set up) to expose the bird to known magnetic field / s. In some embodiments, calibration measurements are acquired when the bird is positioned in a plurality of orientations.

[0083] In some embodiments, the calibration parameters determined prior to flight are used without adjustment during flight. In some embodiments, the calibration parameters are determined as part of factory calibration e.g., provided in a memory (local and / or external) of the system which performs processing of data acquired.

[0084] In some embodiments, calibration parameters compensate for one or more of sensor characteristics, magnetic noise, and misalignment.

[0085] Where, in some embodiments, calibration parameter / s compensate for misalignment between axes of orientation measurements and measurement axes of the triaxial magnetometer.

[0086] Where, in some embodiments, calibration parameter / s compensate for sensor characteristics including non-ideality of the sensor / s and / or differences between sensors, e.g., where gain and / or offsets of sensors are individually magnetic field calibrated (e.g., each of three magnetic field sensors of the triaxial magnetometer are calibrated).

[0087] In some embodiments, calibration parameters compensate for magnetic noise, for example, associated with the bird itself, e.g., magnetic noise associated with of one or more of, hard iron, soft iron, and eddy current magnetic fields. Where, without wanting to be bound by theory, it is theorized that hard iron effects include residual magnetic fields associated with magnetization (e.g., permanent magnetization) of element / s of the bird. It is further theorized that soft iron effects include magnetic fields induced in element / s of the bird by the activity of electrical components (e.g., electrical processing and / or memory circuitry / s, electrical connections (power and / or data), measurement device / s, power supply / s) of the bird during operation of the bird. It is further theorized that eddy current magnetic fields are those magnetic fields generated by eddy currents flowing in elements of the bird where the operation of the bird generates (e.g., electromagnetically) the eddy currents.

[0088] In an exemplary embodiment, measurements acquired during rapid changes in orientation of the bird (e.g., associated with sharp flight path turns of the aerial towing vehicle) are excluded e.g., during data processing. A potential benefit being, it is theorized, that a majority of eddy current noise is associated with rapid changes in angle of the bird. Which may be advantageous in those embodiments, for which eddy current noise is not calibrated for.

[0089] In some embodiments, magnetic anomalies to the earth’s magnetic field are extracted from magnetic measurements, the extracted anomalous magnetic field characteristics may be indicative of geological features such as tectonic features (e.g., fault line / s), mineral deposits, environmental features, and types of human activity. Vectorial magnetic measurement of anomalous magnetic fields may provide additional information as to the magnetic feature / s and / or activity e.g., in comparison to scalar total magnetic field measurements. In some embodiments, the magnetometer system is used for vectorial geomagnetic field mapping of anomalous magnetic fields. Potentially, vector measurements provide magnetic field information in situations where the total magnetic anomalous field is small e.g., at equatorial regions and / or where source / s of the anomalous magnetic field are covered e.g., with one or more of ice, sediments, and vegetation.

[0090] In some embodiments, speeds of aerial towing enable extraction of anomalous magnetic fields from diurnal variations in the earth’s magnetic field (e.g., associated with solar activity and / or rotation of the earth). Where, in some embodiments, where speeds are such that the earth’s magnetic field may be assumed to be constant during data acquisition times e.g., as data acquisition (e.g., as associated with speed of travel) is assumed to be sufficiently fast with respect to the rate of diurnal variations. The fast movement of the aerial towing vehicle potentially facilitates differentiation between the magnetic signals associated with geological anomalies (e.g., having smaller spatial scales) and the signals related to daily variations in external magnetic fields (e.g., having larger spatial scales).

[0091] Potential advantages of aerial towing include the ability to measure magnetic fields in three axes and / or low per-kilometer survey costs e.g., in contrast to marine towed systems which may be more expensive per-kilometer and / or possibly provide (e.g., only) horizontal and vertical components of the anomalous magnetic field.

[0092] Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not necessarily limited in its application to the details of construction and the arrangement of the components and / or methods set forth in the following description and / or illustrated in the drawings and / or the Examples. The invention is capable of other embodiments or of being practiced or carried out in various ways.

[0093] Exemplary system

[0094] FIG. 1A is a simplified schematic of a magnetic field measurement system 100, according to some embodiments of the disclosure.

[0095] In some embodiments, system 100 includes a towable structure 102 also herein termed “bird” which houses one or more magnetic measurement devices 116, 118. In some embodiments, bird 102 is mechanically attached to an aerial vehicle 104 (also herein termed “aircraft”. Optionally, in some embodiments, system 100 includes an aircraft module 164 hosted by aircraft 104.

[0096] In some embodiments, bird 102 includes a multi-axial (also herein termed “vector”) magnetometer 116 configured to provide magnetic field strength measurements in more than one directional axis. In an exemplary embodiment, magnetometer 116 is a triaxial magnetometer. Where vector magnetometer 116 may include a plurality of magnetometers arranged to provide measurement of magnetic field strength in different directions. Although illustrated as a single device located within bird 102, vector magnetometer 116, in some embodiments, is implemented using separate devices which are optionally located in different portions of bird 102. In some embodiments, magnetometer 116 is implemented using one or more magnetometer types e.g., one or more of rotating coil, hall effect, magneto-resistive, fluxgate, and optically pumped vector magnetometers. In an exemplary embodiment, vector magnetometer 116 includes a ring core fluxgate magnetometer. In an exemplary implementation (e.g., for which exemplary measurement results are presented hereinbelow), a digital triaxial ring core fluxgate magnetometer by Magson GmbH, having a noise level of <0.015nT / Hz was used.

[0097] Optionally, in some embodiments, bird 102 includes one or more scalar, also termed “total field” magnetometer 118. In some embodiments, scalar magnetometer / s 118 include one or more type of scalar magnetometer e.g., optically pumped, proton precession, and Overhauser effect magnetometers. In an exemplary embodiment, scalar magnetometer 118 includes an optically pumped magnetometer e.g., a rubidium optically pumped magnetometer. In the exemplary implementation, scalar magnetometer 118 included a QuSpin® rubidium optically pumped Total-Field Magnetometer (QTFM).

[0098] In some embodiments, bird 102 is configured so that vector magnetic field measurements have a sufficiently high signal to noise ratio (SNR) with respect to internal magnetic noise generated by the bird itself e.g., noise associated with electrical components of the bird. In some embodiments, magnetic field measurements are acquired at different distances from bird electrical components. Where a difference in measurements may be used to provide an approximation where one or both of magnetometers 116, 118 are located at positions exposed to under a threshold level of magnetic noise. In the exemplary implementation, the threshold was about 0.5 nT.

[0099] In some embodiments, a size of bird 102 and / or electrical component / s and / or magnetic shielding of electrical components of bird 102 are configured to provide hosting location / s on and / or within bird 102 for magnetometers 116, 118 with sufficiently low magnetic noise.

[0100] In some embodiments, for example, where measurement is directed towards characterization of geological magnetic feature / s, signals may be of magnitude of 10- lOOOnT, or 50-1000nT, or 50-500nT, or lower, or higher, or intermediate ranges or magnetic fields. Where magnetic noise levels at locations of the magnetometer / s may be sufficiently low for such magnitude magnetic measurements to be acquired.

[0101] In some embodiments, for example, where measurement is directed towards identifying human artifacts (e.g., vehicle, weapon, explosive, hidden electrical equipment), signals may be of lower magnitude e.g., l-50nT, or l-10nT, or lower or higher, or intermediate ranges or magnetic fields. Where, in some embodiments, threshold magnetic noise may be about 0.1-0.5nT e.g., sufficiently low so that the lower magnitude magnetic signals may be detected from measurements. In some embodiments, bird 102 includes one or both of orientation measurement sensor / s and position sensor / s. Where orientation measurement sensor / s may include, for example, inertial sensor / s e.g., accelerometer / s and / or gyroscope / s. Where position sensor / s include antennas for receipt of signals from which position (e.g., including geographical location and / or altitude) may be determined.

[0102] In some embodiments, bird 102 includes at least at least three inertial sensors configured (e.g., orientated with respect to the bird and / or each other) to provide triaxial orientation measurement of bird 102. In some embodiments, bird 102 includes both triaxial gyroscopes and triaxial accelerometers for example where one or more of (e.g., each of) the inertial sensors are digital sensors.

[0103] In some embodiments, the position sensor / s use global navigation satellite system (GNSS) positioning technology / ies. Although description within this document is generally with respect to GNSS positioning technologies, other positioning technologies e.g., using received externally generated signals for example, via antenna (e.g., radio wave communication with base station / s) are envisioned and encompassed.

[0104] In an exemplary embodiment, bird 102 includes more than one antennas 114, 122 for position sensing. Where, in some embodiments, the more than one antennas provide increased accuracy of position of bird 102, e.g., using GNSS positioning for each of the antennas 114, 122. In some embodiments, the plurality of positioning antennas are used to adjust and / or verify orientation of bird 102, e.g., as determined using orientation sensor measurements.

[0105] In some embodiments, e.g., to increase accuracy of positioning and orientation of bird 102, antennas 114, 122 are positioned at extremities of bird 102 where a long axis dimension of bird 102 may be selected to provide a suitable distance between antennas 114, 120. In the exemplary implementation, antennas 114, 120 are about 2m apart from each other.

[0106] In an exemplary embodiment, bird 102 includes a position measurement device 120 including both orientation sensors and connectivity to one or both location sensor antennas 114, 122. In some embodiments, position measurement device 120 outputs both a map location and orientation angles (e.g., pitch, roll, and yaw) for the bird. Where, in an exemplary embodiment position measurements using more than one antenna (e.g., two antennas) along with inertial sensor data is used to determine a position (e.g., location and altitude) of a center point of the bird and orientation angles for the bird in 3D space. In some embodiments, position measurement device 120 includes a GNSS positioning device configured to transmit and / or receive data via antennas 114, 122. In the exemplary implementation position measurement device 120 includes a VN-300 DUAL GNSS / INS device of VectorNav®.

[0107] In some embodiments, one or both of bird 102 and aircraft module 164 host processing and memory circuitry (PMC) 110, 106 respectively.

[0108] In some embodiments, the one or more PMCs 110, 106, transmit data externally and / or receive data from external sources, via cloud 112. The data being sent to and / or received from one or more external PMCs and / or user interfaces (UIs) which are not illustrated.

[0109] In some embodiments, bird 102 includes a housing 124 e.g., which may provide mechanical support and / or protection to devices and / or elements hosted by bird 102.

[0110] In some embodiments, housing 124 is sized and / or shaped and / or weighted and / or positioned within the bird to reduce movement of the bird relative to the aircraft during flight of the aircraft (and aerial towing of the bird) e.g., including one or more feature as described regarding and / or illustrated for bird 102 FIG. IB.

[0111] In some embodiments, bird 102 includes non-magnetic material, a potential advantage of which is low magnetic noise in magnetic field measurements associated with the bird itself. For example, in some embodiments, body 124 includes (e.g., is formed from) non-magnetic material e.g., fiberglass. For example, in some embodiments other bird components, e.g., connectors within body 124 and / or connectors 132 of other elements to body (e.g., screws) include non-magnetic material, for example, include (e.g., are formed of) brass.

[0112] In some embodiments, one or more components, e.g. one or more antenna / s, and / or magnetometer / s, and / or position measurement device / s 114, 116, 118, 120, 122 hosted by bird 102 are mechanically supported and / or fixedly attached to bird housing 124 via a rigid baseplate 156. A potential benefit of the rigidity of baseplate 156 is a known position and / or a static relative position of the components with respect to each other. A potential benefit of which is increased accuracy of position measurements with respect to magnetic measurements acquired by the components. In some embodiments, component / s are attached to baseplate 156 by connector / s 132. In some embodiments, baseplate comprises (e.g., is formed from) fiberglass e.g., a fiberglass composite. In some embodiments, bird 102 is suspended from aircraft 104 by one or more elongated connectors 126. In some embodiments, the one or more connectors includes 1- 3 connectors, or 1-2 connectors, a potential advantage of multiple connectors being redundancy in event of damage to a connector e.g., breakage of a connector. In an exemplary embodiment, system 100 includes a single connector 126, a potential advantage being mechanical simplicity and / or reduced magnetic noise associated with the connector.

[0113] In some embodiments, connector 126 includes a mechanical connector (e.g., rope, e.g., cable) which is sufficiently mechanically strong to support weight of bird 102 and any additional forces on the connector e.g., those forces associated with flight. For example, having a breaking load of 10-20 tons, or at least 10 tons, or about 11 tons. In some embodiments, connector 126 provides mechanical support for power and / or data cables e.g., where in some embodiments one or more power and / or data cable may be attached to (e.g., to a surface of) connector 126. Alternatively or additionally, in some embodiments, connector 126 includes one or more hollow passageway through which cable / s pass. In the exemplary implementation connector 126 hosts seven conducting (e.g., for power and / or data) wires.

[0114] In some embodiments, connector 126 includes non-magnetic material (e.g., is formed of non-magnetic material). In some embodiments, connector 126 may include synthetic fibers such as nylon, polyester, polypropylene, ultra-high-molecular-weight polyethylene (UHMWPE), aramid fibers (e.g., Kevlar®), and liquid crystal polymer (LCP), and / or may include natural fibers such as manila, cotton, hemp, and coir. In an exemplary embodiment, connector 126 includes UHMWPE fibers. In the exemplary implementation the UHMWPE figures have an overall diameter of about 2 cm and a breaking load of 11 tons.

[0115] A length of connector 126 may be selected to position bird 102 at a distance 136 from aerial vehicle. In some embodiments, distance 136 (and / or a corresponding length of connector 126) is selected to position magnetic sensor / s of magnetic measurement device / s 116, 118 away from magnetic signals associated with the aircraft.

[0116] Optionally, in some embodiments, connection between aircraft 104 and bird 102 includes one or both of data and power connections. For example, in some embodiments, connector 160 includes data connection to PMC 106 of aircraft module 164 and / or power connection to a power supply 158 of aircraft module. A potential benefit of a power supply external to bird 102 is associated space and / or weight reduction / s to the bird and / or the ability to increase power supply (e.g., battery) lifetime. In some embodiments, power supply 158 is the sole power supply to bird 102 and / or the bird itself does not include an internal power supply, a potential benefit being reduction in exposure of magnetometers of the bird to magnetic noise associated with power supply.

[0117] Alternatively or additionally to aircraft module power supply 158, in some embodiments, bird 102 includes an internal power supply (not illustrated). In the exemplary implementation, a 12 V direct current supply having with a consumption of 0.7 A is delivered from power supply 158 to bird 102 e.g., via cable / s.

[0118] In some embodiments, data passes between aircraft module PMC 106 and bird PMC 110. For example, where control signals (e.g., generated upon receipt of a user input through UI 162) may pass from PMC 106 to PMC 110. For example, where measurement data may pass (e.g., in real time) from PMC 110 to PMC 106, e.g., for display to a user (e.g., in real time, during flight) through UI 162. In some embodiments, data is transmitted between PMCs 106, 110 via connector data connections.

[0119] FIG. IB is a simplified schematic of a bird 102 for airborne magnetic field measurements, according to some embodiments of the disclosure.

[0120] In some embodiments, bird 102 is elongate having a central long axis length 152 which is longer than both dimensions perpendicular to the central long axis e.g., height 148 and a width of the bird in a direction perpendicular to a plane of the page.

[0121] In some embodiments, bird 102 has a larger and / or heavier portion, for example, head portion 142 which has larger dimension / s perpendicular to the central long axis than a tail portion 140 and a mid-section 144 of bird 102.

[0122] In some embodiments, a center of gravity of bird 102 is towards the head portion (e.g., nearer the head portion than the tail). In some embodiments, bird 102 body 124 contributes to concentration of weight at head portion 142, where, in some embodiments, the size of the head portion itself is responsible for the non-central center of mass, and where, optionally, in some embodiments, weighting element / s (not illustrated) are located within the head portion of the bird body. Potential benefit / s of weighting and / or a more front location to the center of gravity of the bird is increased stability in position of the bird during towing e.g., during towing with a single connection 126 to towing aircraft 104. The stability associated with this center of gravity may, for example, enable a wide range of towing velocities e.g., up to helicopter and / or airplane velocities. In an exemplary embodiment, the bird’s total weight is 10-50k, or 20-30kg, or lower, or higher, or intermediate weights, or ranges. In the exemplary implementation, the bird’s total weight was about 27 kg.

[0123] In some embodiments, bird 102 is attached to connector 126 by a harness 138. Where, in some embodiments, harness 138 is an element which surrounds a portion of mid-section 144. In some embodiments, harness 138 is positioned at a region of a center of gravity of bird 102. In some embodiments, harness 138 is repositionable e.g., during attachment of connector 126 to harness. A potential benefit of which being the ability to adjust the position of the harness with respect to the bird to balance suspension of bird 102.

[0124] In some embodiments, one or more portion of bird 102 includes fins 130, a potential benefit of which is increased alignment of direction of the bird long axis with direction of movement of towing vehicle 104. In an exemplary embodiment, fins 130 are located at tail region 130. Where fins 130, in some embodiments, are protrusions which are oriented generally parallel to bird central long axis. The fins potentially increasing air resistance to one or more of the bird deviating from the aircraft trajectory, turning of the bird during flight, and spinning of the bird.

[0125] In some embodiments, the bird mid-section tapers cross-sectionally in one or more direction from head 142 towards tail 140.

[0126] In some embodiments, bird length 152 is l-5m, or l-3m, or about 2m, or lower, or higher, or intermediate lengths, or ranges, head length is 0.1-lm, or lower, or higher, or intermediate lengths or ranges, head width and / or height 148 are 0.1-lm or lower, or higher, or intermediate dimensions, or ranges, mid-section 144 length is 0.5-4m, or 0.5- 3, or 0.5-2, or about 1.5m, or lower, or higher, or intermediate lengths, or ranges, connector length 136 is 5- 100m, or 30-50m, or about 40m long 136, or lower, or higher, or intermediate lengths, or ranges. In the exemplary implementation, bird length 152 is about 2m and connector length 136 is about 40m.

[0127] Optionally, in some embodiments, system 100 and / or bird 102 includes one or more additional sensor. For example, one or more temperature sensor. For example, one or more altitude sensor and / or radar sensor e.g., a radar altimeter. Exemplary calibration method

[0128] FIG. 2 is a flowchart of a method of determining calibration parameters, according to some embodiments of the disclosure.

[0129] At 200, in some embodiments, a bird (e.g., bird 102 FIGs. 1A-B) is positioned in a known magnetic field. Which may be a location with stable magnetic field conditions, for example, on the ground, e.g., in a known magnetic field of a base station and / or lab setting. The magnetic field may be known as it is controlled e.g., where the bird is positioning in a lab setting having a controlled magnetic field.

[0130] Alternatively, in some embodiments, the bird may not be located in a known magnetic field and / or may be on a flight path.

[0131] At 202, in some embodiments, vector magnetometer field measurements are acquired e.g., by a vector magnetometer 116 FIG. 1A. In some embodiments, the vector magnetic measurements include three components (e.g., three spatially orthogonal components).

[0132] Optionally, in some embodiments, scalar magnetic field measurements are also acquired, e.g., by scalar magnetometer 118 FIG. 1A.

[0133] At 204, in some embodiments, (e.g., concurrently with step 202) orientation measurements are acquired for at least one bird position e.g., by position measurement device 120.

[0134] In some embodiments, the bird is then repositioned and steps 202 and 204 are repeated, to acquire magnetic and position measurements (herein termed “calibration measurements”) for a plurality of bird positions.

[0135] For example, in some embodiments, the bird is rotated through azimuthal directions with variations of pitch and roll angles corresponding to a range of expected and / or possible angular bird positions during flight (towing). Where azimuthal directions, in some embodiments, are of up to angles of 20-60°, or of up to about 40°, or lower, or higher, or intermediate ranges, or angles.

[0136] If measurements are acquired while the bird is on a flight path, repositioning may be implemented by movements of the towing vehicle e.g., the bird may be flown on a figure of eight path potentially giving different orientations to the bird.

[0137] At 206, in some embodiments, calibration parameters are determined using calibration measurements e.g., for at least one bird position. In some embodiments, calibration parameters are determined using the magnetic measurements and the known magnetic field.

[0138] Where, in some embodiments, the three magnetic measurement components are calibrated to account for sensor offsets and / or gains, and / or non-orthogonality of the three magnetic measurement signals.

[0139] In some embodiments, calibration is (alternatively or additionally) for distortion of magnetic field measurements by one or more of three different types of magnetic effects; hard iron (remanent magnetization), soft iron (induced magnetization), and eddy- current fields. Where, in some embodiments, calibration for magnetic effect / s is for those associated with the bird itself e.g., and not the aerial towing vehicle.

[0140] In some embodiments, sensor, magnetic noise, and orthogonality (SMO) calibration parameters (S, P, O respectively) are determined using the following formula:

[0141] F = S ■ P ■ B + O Equation 1

[0142] Where:

[0143] F is a vector of the measured magnetic field including the triaxial measurement components F = (Fx, Fy, FZ)T;

[0144] B = (Bx, By, BZ)Tis a known vector magnetic field at the magnetometer position, where the known magnetic field is provided and / or determined from one or both base station magnetic field data and database data e.g., as provided by international geomagnetic reference field IGRF (International Geomagnetic Reference Field) v.13 model (Aiken et al., 2021).

[0145] For example, as an alternative to performing calibration measurements at a base station, in some embodiments, calibration measurements are acquired in a lab setting where magnetic field is controlled and / or measured using external magnetometer / s to the bird to provide the known magnetic field.

[0146] S is a 3x3 diagonal matrix representing scale factors, indicative e.g., of sensor gains and / or the soft iron effect;

[0147] P is a 3x3 matrix, which transforms magnetic measurement vector data into an orthogonal coordinate system;

[0148] O = (Ox, Oy, Oz)Tis a vector representing offsets, indicative e.g., of sensor offsets and / or the hard iron effect; Therefore, in some embodiments, OPM calibrated magnetic field BM measurements are obtained using acquired measurements F, calibration parameters S, P, and O and solving:

[0149] BM= P-1■ S’-1■ (F — O) Equation 2

[0150] In some embodiments, calibration parameters include alignment calibration parameter / s, also herein termed “compensation parameters” for alignment of orientation measurements with the magnetic field measurements e.g., to compensate for difference / s in orientation angle between axes of magnetic and orientation measurements.

[0151] In some embodiments, a rotation matrix RM (e.g., a 3x3 matrix) is determined . Where rotation matrix RM translates magnetic measurements BM = (BMX, BMY, BMZ)Tfrom a magnetometer reference frame (orientation of axes of magnetic measurements) to a bird reference frame BB = (BBX, Bsy, BBZ)T(orientation of axes of orientation measurements). For example, according to:

[0152] BB= RMBMEquation 3

[0153] Where, in some embodiments, the rotation matrix is determined by minimizing a misfit (e.g., a / / -misfit) between the SMO calibrated vector magnetic measurements BM and a known magnetic field B = (Bx, By, BZ)T, where it is assumed that the axes of orientation measurements are aligned with that of the known magnetic field. For example, by minimizing / / according to Equation 4 below:

[0154] In some embodiments the calibration parameters, e.g., as described hereinabove are determined by assuming that the bird being flown in close proximity to a base station (e.g., in a calibration flight e.g. in a figure of eight calibration flight) is experiencing a known magnetic field of the base station.

[0155] In some embodiments, the calibration parameters, e.g., as described hereinabove are determined using crossover data acquired during flight of the bird e.g., as described regarding steps 316-318 FIG. 3.

[0156] Exemplary magnetic field measurement

[0157] FIG. 3 is a flowchart of a method of vectoral magnetic field measurement, according to some embodiments of the disclosure. At 300, in some embodiments, calibration parameters are received e.g., where the calibration parameters include one or more feature of those described regarding FIG. 2.

[0158] At 302, in some embodiments, vector magnetic field measurements and bird position data is acquired. Where, referring to FIG. 1A, the vector magnetic field measurements are received from vector magnetometer 118 and bird position data is received from position measurement device 120.

[0159] In some embodiments, vector magnetic field measurements include a plurality of magnetic field strength measurement signals, each signal providing a measurement of magnetic field in a different direction (measurement axis). In an exemplary embodiment, three magnetic field strength measurement signals are received, where each measurement corresponds to a different spatial direction, and where angles between the respective directions are about orthogonal or about 90 degrees.

[0160] In some embodiments, position measurement data includes at least one GNSS data set providing GNSS-derived position with time. In an exemplary embodiment, position measurement data includes two GNSS data sets, each associated with a different antenna, where the bird includes two antennas. In some embodiments, position measurement data includes inertial sensor data with time, e.g., data received from one or more accelerometer and / or one or more gyroscope.

[0161] In an exemplary embodiment, a position measurement device provides orientation data for the bird, based on inertial sensor data and, optionally, GNSS-derived position data. Where, the position measurement device (including inertial sensor / s and / or GNSS device / s) outputs a data set providing a three-axis orientation of the bird including, for example, angles for pitch, roll, and yaw.

[0162] In the exemplary implementation, position (e.g., including geographical location and altitude) and orientation angles are provided by position measurement device / s.

[0163] In some embodiments, the vector magnetic field and position measurement data is received in data sets, each set associated with an acquisition time frame. In some embodiments, the magnetic field and position measurement data is acquired with aligned timing and / or is time stamped where the data may then be time-space aligned using the time stamps.

[0164] At 304, in some embodiments, the acquired and / or received vector magnetic field measurement data and position data have different timing (e.g., data acquisition at different operational frequencies and / or where timing of acquisition is out of phase). Optionally, the two data sets are (after optional aligning e.g., using time stamps), resampled and / or interpolated to provide a data set where each vector magnetic measurement (e.g., including three values, a value for each axis) has an associated position measurement set. Where, in an exemplary embodiment, magnetic measurement axes include vertical, north-south, and east-west, and position data includes angles for pitch, roll, and yaw.

[0165] In the exemplary implementation, bird position and orientation data had a lower sampling rate (25 Hz) than measurements provided by the vector magnetometer (50 Hz). During data processing, the bird position data was interpolated to provide a data set having a frequency of 50 Hz.

[0166] At 306, optionally, in some embodiments, measurement data is smoothed and / or cleaned.

[0167] For example, in some embodiments, magnetic measurement data and / or orientation data is smoothed e.g., each axis individually. In some embodiments, magnetic and / or orientation data is de-spiked.

[0168] In an exemplary embodiment, magnetic measurement data is de-spiked e.g., by removing data points where there is above a threshold difference (the threshold, in some embodiments, being 0.5-5nT, or l-3nT, or about 2nT, or lower or higher or intermediate magnetic fields or ranges) between time-adjacent magnetic field measurements. In the exemplary implementation, successive measurement points having over a 2nT difference in measured magnetic field were omitted.

[0169] For example, in some embodiments, position and / or orientation measurement data is cleaned. In some embodiments, data associated with sharp turns in movement of the bird (e.g., high rate of change of yaw angle) is discarded. A potential benefit of which is removal of data where orientation data is likely to be inaccurate and / or where magnetic measurements are likely to have high eddy current noise.

[0170] In some embodiments, measurement data is corrected for potential inaccuracies associated with temperature. For example, using data provided by one or more temperature sensor hosted by the bird.

[0171] At 308, in some embodiments, calibration parameters are used to correct the measurement data e.g., according to feature / s as described regarding FIG. 2. Where, for example, the calibration parameters compensate for one or more of sensor non-idealities, magnetic noise, non-orthogonality, and misalignment. At 309, optionally, in some embodiments, scalar magnetic field measurements are used to verify and / or adjust the vector magnetic field measurements.

[0172] Where, in some embodiments, a scaler magnetic field measurement is determined from vector magnetic measurements (e.g., by summing), where the

[0173] 't t tooo determined scaler Q h al magnetic field is compared with the directly measured scalar magnetic field. In some embodiments, if the two scaler magnetic field measurements are sufficiently close (e.g., within a threshold of each other) the magnetic field measurements t t toQo are verified. a to to N x

[0174] In some embodiments, the magnetic field measurements themselves are used to verify the measurements. For example, by determining and then comparing power spectral density (PSD) for vertical anomalies with PSDs of the two horizontal measurements. In some embodiments, if the PSD of vertical and horizontal anomalies are within a threshold of each other, the magnetic field measurements are verified.

[0175] In some embodiments, if the measurements are not verified, an error message is displayed to a user and / or calibration parameter / s are adjusted and / or re-determined. In some embodiments, verification using one or more technique is performed recurrently (e.g., during a flight, e.g., periodically).

[0176] At 310, in some embodiments, the vector magnetic measurement data which is in a bird reference frame BB (also herein termed “bird coordinate space”) is rotated into geographical coordinates using orientation data for the bird, for example according to:

[0177] = RG - Equation 3 where BN,E,D are the magnetic components in geographical coordinates;

[0178] BB = (BBX, Bsy, BBZ)Tis the x, y and z components of the calibrated magnetic field which has been rotated to the bird coordinate space;

[0179] RG is a rotation matrix calculated individually for each sample based on orientation measurements e.g., pitch, roll and yaw angles.

[0180] At 312, in some embodiments, the anomalous magnetic field is extracted from the measured magnetic field. For example, by subtracting database (e.g., IGRF values Aiken 2021) values of non-anomalous magnetic field for the position. For example, by subtracting nearest base station ground magnetic field data. In some embodiments, extracting of the anomalous magnetic fields includes verifying that frequency of variation e.g., diurnal variation in the non-anomalous field is sufficiently slow with respect to the time scale of magnetic field measurements. In some embodiments, magnetic measurements are processed in profiles, where a time duration of a profile may be less than that of an entire measurement time period (e.g., flight time). The profile durations may be selected to me short with respect to timing of diurnal variations. In some embodiments, profile durations are between 0.5-10minutes, or 0.5-8minutes, or less than 5 minutes, or lower, or higher, or intermediate durations, or ranges.

[0181] For example, in the exemplary implementation monitored diurnal variations of magnetic field by a total field magnetometer (Geometries G-856 proton magnetometer) at a base station indicated that up to 10 nT variation in the strength of the ambient magnetic field occurred during the aerial survey. Which, in some embodiments, may be mapped to 10 nT temporal variations in the components of the ambient field. During data processing of the exemplary measurement, a longest profile included five minutes of flight, faster compared to the measured typical wavelength of the external field diurnal variations.

[0182] Additionally or alternatively, in some embodiments, the anomalous magnetic field is extracted using crossover point data e.g., as described regarding one or more feature of step / s 316 and 318. Where, for example, in some embodiments, calibrated magnetic measurement data at crossover points is used to determine value / s of the non-anomalous magnetic field. For example, in some embodiments, a flight path is divided into portions, each portion herein termed a “profile”. In some embodiments, crossover data for a plurality of profiles is used to determine, e.g., for each of the plurality of profiles, a constant value for the non-anomalous magnetic field which minimizes the differences in crossover magnetic measurement data for the plurality of profiles.

[0183] At 314, in some embodiments, the anomalous magnetic field values are altitude corrected e.g., to enable comparison between the measurements. Where, for example, different altitudes of the position measurements are used to adjust the magnetic fields values to those of a selected constant altitude (vertical level). In some embodiments, the altitude corrections are performed using one or more features as described in Guspf, 1987. Alternatively or additionally, in some embodiments, altitude corrections are performed using altitude measurements e.g., as provided by an onboard (e.g., hosted by the bird and / or towing vehicle) altimeter. In the exemplary implementation, altitude correction was to an altitude of 135 meters above the water level. At 315, in some embodiments, the anomalous magnetic field values are low pass filtered. The Guspi method described regarding step 314, includes low-pass filtering. However, for example in embodiments where other altitude correction method / s are employed which do not use low-pass filtering, the data what data may be low-pass filtered. Without wanting to be bound by theory it is theorized that (e.g., see discussion of FIG. 8 hereinbelow) the vector anomalous magnetic field signal (e.g., the geological- related signal) resides, and / or is stronger and / or more easily detected and / or is less noisy at lower frequencies, e.g., at wavelengths larger than a threshold wavelength.

[0184] In some embodiments, e.g., prior to display, magnetic field measurements are low-pass filtered to attenuate and / or remove frequency components having wavelength of smaller than a threshold value. In some embodiments, the threshold is selected and / or determined based on a distance between where the magnetic measurement / s are acquired (e.g., in the air at towing altitudes) and the feature / s being measured (e.g., at and / or below ground level). Where, for example, filtering may be of frequency components having a wavelength which is positively correlated to the distance.

[0185] For example, for distances between the region of interest at or below ground level and towing altitude which are in a range of 50-200m (e.g., as provided by exemplary implementation measurements), filtering of magnetic measurement data may be of frequency components having wavelength of 0.4-0.6 km, or 0.4-0.5 km, or about 0.45 km, or lower, or higher, or intermediate ranges, or distances. For larger distances, e.g., as would be provided by a towing vehicle flying at higher altitude, for example, distances of 5- 10km, filtering may be of frequency components having wavelength of 5- 10km, or, lower, or higher, or intermediate ranges, or distances.

[0186] At 316, in some embodiments, crossover points in data are identified. Where a crossover point is a repeated measurement location, in at least two directions e.g., where the bird passes through a same geographical location (longitude and latitude), but not necessarily at a same altitude. The crossover point data is for at least two measurement paths which intersect with the crossover point, though may be for more than two measurement paths. Herein where “both” paths are described, this, in some embodiments, encompasses data for more than two paths. Crossover point data may include closest measurement results to the crossover location e.g., for the at least two measurement paths. For example, where a measurement location in a first path is matched with a closest measurement location in a second flight path. Alternatively, the crossover data may include interpolated data values for one (or more) path, where the location associated with data for one or both of the paths may include interpolation between measurements e.g., at locations prior to and after the crossover location.

[0187] At 318, in some embodiments, crossover point data is compared, for example, to verify calibration parameters. In some embodiments, e.g., if the calibration parameters are not verified, crossover point data may be used to re-calculate and / or adjust the calibration parameters. In some embodiments, vector magnetic measurements at the crossover points are compared, and if differences between each axis of the 3 axis measurements are each below an associated threshold, the calibration parameters are verified.

[0188] In some embodiments, initial calibration parameters received are replaced with calibration parameters determined using crossover point data, e.g., after a sufficient number of crossover points have been passed. For example, after 1-10 crossover points have been passed, or lower or higher or intermediate numbers of crossover points.

[0189] In some embodiments, calibration parameters may not be received at step 300, but are determined using crossover point data. Where, during real time processing, data is refined as it is acquired e.g., as calibration parameters are successively refined using crossover point data as it is acquired. In this case, data may not be outputted until calibration parameters have been determined using the crossover point data. Alternatively, the non-calibrated data may be outputted and then corrected dynamically as calibration parameters are determined.

[0190] In some embodiments, crossover point data is used to determine calibration parameters. For example, parameters S, P, O as described in step 206 of FIG. 2. In some embodiments, at a crossover point, one of the two magnetic field measurements at the crossover point may be considered the “known magnetic field” B (as described in FIG. 2) and the other set may be considered as the measured magnetic field F.

[0191] In some embodiments, multiple sets of crossover point data may be used to determine (and / or adjust) the calibration parameters, where the calibration parameters may be determined (e.g., using an iterative process) as those which minimize the errors (e.g., a measure of the collective error) between the crossover point measurements, for the plurality of crossover points.

[0192] Determining the calibration parameters using crossover point data may involve iterative optimization methods, such as least squares regression, where the calibration parameters are adjusted to minimize the sum of squared differences between the observed and predicted crossover points. Alternatively, gradient descent algorithms may be employed to iteratively refine the calibration parameters by reducing an objective function representing the collective error. In some implementations, machine learning techniques, such as neural networks or support vector machines, may be trained using the crossover point data to identify optimal calibration parameters that generalize across various conditions. Additionally, statistical methods such as maximum likelihood estimation (MLE) or Bayesian inference may be applied to estimate the calibration parameters by incorporating prior information and minimizing the uncertainty associated with the measurements. The determination of calibration parameters may further incorporate techniques such as principal component analysis (PCA) to reduce dimensionality and identify the most significant factors influencing the crossover point measurements, thereby improving the robustness and accuracy of the calibration process. These approaches may be implemented individually or in combination to achieve an optimized calibration framework that enhances measurement accuracy across varying operational conditions.

[0193] In some embodiments, crossover point data is used to determine the non- anomalous magnetic field. Where, in some embodiments, data acquired is divided into profiles, each profile including magnetic and orientation data for a time period having a short duration (e.g., with respect to diurnal variation in non-anomalous magnetic field) for example, profiles being less than 20, or 10, or 5 minutes long, or lower or higher or intermediate durations. In some embodiments, a non-anomalous magnetic field is determined for each profile. Where, in some embodiments, crossover data of different profiles is used to determine the non-anomalous magnetic field for a profile (e.g., each profile) which minimizes differences in crossover magnetic field value / s. This calculation may be performed after survey e.g., using all of the profiles together, or during acquisition, where, as measurement profiles are acquired, non-anomalous magnetic fields are redetermined using the additional crossover points data.

[0194] At 320, optionally, in some embodiments, measurement results are displayed e.g., to a user. In some embodiments, one or more of steps 302-318 are performed in real-time. Where, for example, as the towing aircraft is flown, magnetic measurements are acquired at 302, processed at step / s 302-318, and then optionally displayed. For example, to a user via a user interface for example, at an aircraft user interface e.g., UI FIG. 1A. A potential advantage of which is that a user viewing results may adjust measurement parameter / s e.g., flight plans based on the data.

[0195] Additionally or alternatively, in some embodiments, the measurement results are used. For example, where the measurement results may be used for adaptive control of the towing aircraft's flight path. This adaptive control may be implemented either automatically by an onboard processor (e.g., if the aircraft is an autonomous aircraft) or semi-automatically by providing recommendations (e.g., flight path recommendations) to a human operator. For example, by outputting recommendations to a user e.g., via a user interface e.g., within the aircraft. In some embodiments, a recommendation may include a geographical region of interest in the acquired data (e.g., in some embodiments, in response to a set of parameters entered by the user).

[0196] For example, geomagnetic features of interest may be characterized or selected prior to acquisition of measurements (e.g., fault line magnetic signature e.g., human activity magnetic signature), and when data acquired matches that of the feature / s of interest, the user may be notified, and / or a flight path change or flight path recommendation may be produced.

[0197] In some embodiments, insufficient data (density and / or accuracy) for a geographical region may trigger a recommendation to a user and / or a flight path change e.g., to return to the area in order to acquire sufficient data. Where required data density and / or accuracy may be pre-defined e.g., by the user.

[0198] Exemplary implementation

[0199] The exemplary implemented system (e.g., as described regarding FIGs. 1A-B and / or elsewhere in this document) was calibrated and used to conduct a survey above the northeastern part of the Sea of Galilee, northern Israel.

[0200] Calibration

[0201] Calibration parameters were initially determined under stable magnetic field conditions in a lab location. Magnetic field component data was compared (e.g., by summing of the magnetic field components) with base station total field measurements. During acquisition of calibration measurements, the bird was rotated in azimuthal directions with variations of pitch and roll angles ranging up to 40°, covering the angular range of the bird during typical flight conditions. Misalignment matrix RM (e.g., as described regarding step 206 FIG. 2) was determined during a short (eight minutes) ground experiment at which it was assumed that the reference magnetic field remained constant.

[0202] Exemplary measurement results and processing

[0203] FIG. 4 is an illustration of a measurement flight track, according to some embodiments of the disclosure.

[0204] FIG. 4 illustrates mapping of location of the bird, on a simplified map where a line 400 indicates a division between land and sea.

[0205] The exemplary implementation of the bird as described within this document was towed by an MD 500 helicopter at a typical flight speed of 27 m / s. Magnetic and position measurement data were acquired along 212.5 km of track lines (e.g., track lines illustrated in FIG. 4) forming an approximate grid with an average line spacing of about 350 m, providing a coverage area of 40.5 km2. A total of 255 crossover points were obtained. The bird was towed at an altitude of about 110 meters above ground and / or sea surface with ±25 meters variation during 95% of the flight. Given a sampling rate of 25 Hz of the GNSS / INS system, an average distance between successive data points was 1.08 meters.

[0206] FIG. 5 is a polar plot of magnetic field measurements and calibrated magnetic field measurements, according to some embodiments of the disclosure.

[0207] As illustrated, for example, in FIG. 5, the raw measured total fields F (annotated on the plot as “Raw data”) showed heading-dependent variations where the field tended to be stronger (~ 10244 nT) in a north- south direction compared to the east-west measured fields. The measured field had a standard deviation of 6.7 nT and a mean of 44930 nT, roughly 26 nT stronger compared to base station data e.g. indicated on the plot as “Reference field” on FIG. 5.

[0208] After applying the calibration and compensation parameters to the measured three components, as illustrated by the plot in FIG. 5 annotated as “Calibrated” measurements, standard deviation decreased to 1.67 nT and the heading dependent variations as well as the offset relative to the reference field were eliminated.

[0209] FIG. 6 is plots of triaxial magnetic field measurements, according to some embodiments of the disclosure. FIG. 6 illustrates plots for measurements SMO-calibrated 602,604,610 and then both alignment and SMO calibrated 600, 606, 608 for north-south, east-west, and vertical directions respectively. Where both the SMO-calibrated and SMO-alignment-calibrated data sets have been rotated to geographical coordinates (e.g., each measurement multiplied by the appropriate RG matrix e.g., as described regarding step 206 FIG. 2). SMO-alignment-calibrated measurements had mean deviations of north / east / vertical components reduced from 1357 / 1444 / 1442 nT to 34.4 / 98 / 34.2 nT, respectively.

[0210] FIG. 7 illustrates plots of magnetic field, bird angle, and altitude, with distance, for a measurement profile, according to some embodiments of the disclosure.

[0211] FIG. 7 illustrates for the magnetic field components (north, east, vertical), measured magnetic field components, after calibration, compensation, rotation to geographical coordinates, and extraction of the anomalous fields. In FIG. 7, an altitude corrected measurement is superimposed on the (more fluctuating) non-altitude corrected measurements.

[0212] FIG. 8 illustrates power spectral density with wavenumber and wavelength, according to some embodiments of the disclosure.

[0213] FIG. 8 plots power spectral density (PSD) of anomalous magnetic field measurement components 800 and total magnetic field 802 extracted from the survey data. Where frequency distribution of magnetic fields (PSD) is determined using the position and magnetic field measurements. FIG. 8 illustrates a general downward- sloping trend with increasing wavenumbers and wavelengths. Below 2.2 km1wavenumber (> 0.45 km wavelength), the spectra of the total magnetic field data is similar to that of the vector components, but at shorter wavelengths the total magnetic field spectra diverges from, having a lower PSD, than that of the vector components.

[0214] A test for self-consistency of the vectorial data is whether the power spectral density of the vertical anomalies equals the sum of the PSDs of the two horizontal anomalies (“power sum rule”, (Parker & O’Brien, 1997)). The power sum rule fails for wavenumbers larger than 2.2 km1(wavelength smaller than 0.45 km).

[0215] Without wanting to be bound by theory, it is theorized that agreement between the PSD of the vertical component and the sum of the horizontal components indicates self-consistency of the vectorial data (e.g., according to feature / s as described regarding “power sum rule” in (Parker & O’Brien, 1997).

[0216] FIG. 8 illustrates this agreement for wavelengths larger than 0.45 km where, it is theorized that the geological -related signal resides in a range of wavelengths larger than 0.45.

[0217] In some embodiments, e.g., according to Parker and O’Brien (1997), where measurement is of isolated vertical dipoles, the PSD of the vertical component is expected to be larger than the sum of spectra of the horizontal components. In some embodiments, it is assumed that at small wavelengths (<0.45 km), the signal is dominated by magnetic fields associated with electrical component / s of the measurement equipment and / or related to man-made (e.g., as opposed to) geological features.

[0218] FIGs. 9A-D illustrate mapping of errors in crossover data for magnetic field,

[0219] FIGs. 9A-D illustrate size of errors in crossover data for north, east, vertical, and total field (scaler field) on a mapping of the measurement flight path. Where the illustrated errors are for measurement data which has been corrected according to steps, 304, 306, 308, 310, 312, and 314 of FIG. 3.

[0220] The illustrated errors in crossover data concur with calibration parameters determined using calibration procedures (e.g., as described in and / or regarding FIG. 2). The illustrated errors, apparently, concur with an assumption that a majority of inaccuracy in magnetic field measurement is associated with inaccuracy in orientation measurements of the bird. Where error values provided by a simulation using manufacturer orientation accuracies of the exemplary implementation orientation sensors, combined with database values for the earth’s magnetic field, were found to agree with those values illustrated in FIGs. 9A-D: The noise levels of the magnetic field component data (15.6.0 / 44.5 / 14.0 nT for BN / BE / BV) are similar to the simulation values (18.1 / 45.1 / 15.7 nT for BN / BE / BV).

[0221] Further checking of magnetic field measurements comparing total magnetic anomaly field (as determined by summing the components of the field) with existing measurements of sea surface total magnetic field anomalies, yielded a standard deviation of 6.2 nT as provided by Ben-Avraham (2014).

[0222] General

[0223] As used within this document, the term “about” refers to±20% The terms “comprises”, “comprising”, “includes”, “including”, “having” and their conjugates mean “including but not limited to”.

[0224] The term “consisting of’ means “including and limited to”.

[0225] As used herein, singular forms, for example, “a”, “an” and “the” include plural references unless the context clearly dictates otherwise.

[0226] Within this application, various quantifications and / or expressions may include use of ranges. Range format should not be construed as an inflexible limitation on the scope of the present disclosure. Accordingly, descriptions including ranges should be considered to have specifically disclosed all the possible subranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within the stated range and / or subrange, for example, 1, 2, 3, 4, 5, and 6. Whenever a numerical range is indicated within this document, it is meant to include any cited numeral (fractional or integral) within the indicated range.

[0227] It is appreciated that certain features which are (e.g., for clarity) described in the context of separate embodiments, may also be provided in combination in a single embodiment. Where various features of the present disclosure, which are (e.g., for brevity) described in a context of a single embodiment, may also be provided separately or in any suitable sub-combination or may be suitable for use with any other described embodiment. Features described in the context of various embodiments are not to be considered essential features of those embodiments, unless the embodiment is inoperative without those elements.

[0228] Although the present disclosure has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, this application intends to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.

[0229] All references (e.g., publications, patents, patent applications) mentioned in this specification are herein incorporated in their entirety by reference into the specification, e.g., as if each individual publication, patent, or patent application was individually indicated to be incorporated herein by reference. Citation or identification of any reference in this application should not be construed as an admission that such reference is available as prior art to the present disclosure. In addition, any priority document(s) and / or documents related to this application (e.g., co-filed) are hereby incorporated herein by reference in its / their entirety.

[0230] Where section headings are used in this document, they should not be interpreted as necessarily limiting.

Claims

CLAIMS:

1. A method of magnetic survey comprising: acquiring calibration measurements with a bird comprising a vector magnetometer, a plurality of inertial sensors, and a GNSS transceiver, in a known magnetic field; determining calibration parameters using said calibration measurements; acquiring vector magnetic measurements with said vector magnetometer, orientation measurements with said inertial sensors, and position measurements with said GNSS transceiver, while towing said bird with an aerial vehicle along a flight path; and correcting said vector magnetic measurements to provide corrected magnetic vector measurements by: calibrating said vector magnetic measurements using said calibration parameters; aligning said vector magnetic measurements to a position measurement coordinate space using said orientation measurements; and providing said corrected vector magnetic measurements with said position measurements.

2. The method according to claim 1, wherein said acquiring calibration measurements comprises acquiring vector magnetic measurements with said vector magnetometer, and orientation measurements with said inertial sensors in a known magnetic field.

3. The method according to claim 2, wherein said determining calibration parameters comprises determining one or more of scale factors, offsets, and orthogonality correction calibration parameters using said calibration measurements and said known magnetic field.

4. The method according to any one of claims 2-3, wherein said determining calibration parameters comprises determining alignment parameters for alignment of orientation measurements with the vector magnetic field measurements.

5. The method according to any one of claims 1-4, wherein said bird comprises a scaler magnetometer; and wherein acquiring magnetic measurements comprises acquiring scaler magnetic measurements with said scaler magnetometer.

6. The method according to claim 5, comprising verifying said vector magnetic measurements using said scaler magnetic measurements.

7. The method according to any one of claims 1-6, wherein said providing is during said towing.

8. The method according to any one of claims 1-7, wherein said providing is to a user via a user interface.

9. The method according to any one of claims 1-8, comprising identifying one or more crossover point in said flight path.

10. The method according to claim 9, comprising determining one or more crossover calibration parameters using measurement data for said one or more crossover point.

11. The method according to claim 10, comprising one or more of verifying and adjusting said calibration parameters using said crossover calibration parameters.

12. The method according to any one of claims 9-11, wherein said identifying comprises identifying, in said position measurements, one or more crossover points having a same location having a same latitude and longitude.

13. The method according to any one of claims 10-12, wherein said determining comprises determining differences in crossover point corrected magnetic measurements;calculating the crossover calibration parameters which minimize said differences in crossover point data.

14. The method according to any one of claims 1-13, wherein said bird includes a towable structure.

15. The method according to any one of claims 1-14, comprising: evaluating said corrected vector magnetic measurements provided with said position measurements; and determining one or more trajectory recommendation based on said evaluating.

16. The method according to claim 15, wherein said evaluating comprises determining and evaluating a density of said corrected vector magnetic measurements using said position measurements.

17. The method according to claim 16, wherein said evaluating comprises comparing said density with a desired density.

18. A vector magnetic measurement system comprising: a towable bird structure comprising: a vector magnetometer; a plurality of inertial sensors; a GNSS transceiver; and one or more antennas configured to receive and transmit signals to and from said GNSS transceiver; a connector configured to connect said towable bird to a towing aircraft; and circuitry configured to: determine calibration measurements using magnetic field measurements acquired with said vector magnetometer in a known magnetic field; receive vector magnetic measurements from said vector magnetometer, orientation measurements from said inertial sensors, and position measurements from said GNSS transceiver, while said towable bird is towed by an aerial vehicle along a flight path; andcorrect said vector magnetic measurements to provide corrected vector magnetic measurements by: calibrating said vector magnetic measurements using said calibration parameters; aligning said vector magnetic measurements to a position measurement coordinate space using said orientation measurements; and provide said corrected vector magnetic measurements with said position measurements.

19. The system according to claim 18, wherein said bird comprises a scaler magnetometer.

20. The system according to any one of claims 18-19, comprising an aircraft module configured to be hosted by said aircraft.

21. The system according to claim 20, wherein said aircraft module comprises a power supply, wherein said connector comprises one or more electrical connector between said power supply and circuitry of said towable bird.

22. The system according to claim 21, wherein said aircraft module comprises processing and memory circuitry which hosts at least a portion of said circuitry.

23. The system according to claim 22, wherein said connector comprises one or more data connector configured to transmit data between said aircraft module processing and memory circuitry and circuitry of said bird.

24. The system according to any one of claims 20-23, wherein said aircraft module comprises a user interface, wherein said circuitry is configured to provide said corrected vector magnetic measurements with said position measurements to said user interface which is configured to display said corrected vector magnetic measurements with said position measurements.

25. The system according to any one of claims 18-24, wherein said bird comprises processing and memory circuitry which hosts at least a portion of said circuitry.

26. The system according to any one of claims 18-25, wherein said bird comprises: a bird housing comprising a hollow chamber; a rigid element rigidly mounted to walls of said hollow chamber, onto which rigid element are rigidly attached said vector magnetometer, said plurality of inertial sensors, and said one or more antennas.

27. A method of magnetic survey comprising: acquiring vector magnetic measurements, orientation measurements, and position measurements with a bird comprising a vector magnetometer configured to acquire the vector magnetic measurements, a plurality of inertial sensors configured to acquire the orientation measurements, and a GNSS transceiver configured to acquire the position measurements, while towing said bird with an aerial vehicle along a flight path; processing said vector magnetic measurements using said orientation measurements and said position measurements to provide orientation-independent and altitude-normalized corrected magnetic measurements; determining calibration parameters using said corrected magnetic measurements by: identifying, in said position measurements, one or more crossover points having a same latitude and longitude; determining differences in crossover point corrected magnetic measurements; calculating the calibration parameters which minimize said differences in crossover point data.

28. The method according to claim 27, wherein said bird includes a towable structure.

29. The method according to any one of claims 27-28, comprising calibrating said corrected vector magnetic measurements using said calibration measurements.

30. The method according to any one of claims 27-29, wherein said processing comprises aligning said vector magnetic measurements to a position measurement coordinate space using said orientation measurements.

31. The method according to any one of claims 27-30, comprising providing said corrected vector magnetic measurements with said position measurements.

32. The method according to claims 27-31, wherein said determining calibration parameters comprises determining one or more of scale factors, offsets, and orthogonality correction calibration parameters.

33. The method according to any one of claims 27-32, wherein said bird comprises a scaler magnetometer; and wherein acquiring magnetic measurements comprises acquiring scaler magnetic measurements with said scaler magnetometer.

34. The method according to claim 33, comprising verifying said vector magnetic measurements using said scaler magnetic measurements.

35. The method according to any one of claims 27-34, wherein said providing is during said towing.

36. The method according to any one of claims 27-35, wherein said providing is to a user via a user interface.

37. A vector magnetic measurement system comprising: a towable bird comprising: a vector magnetometer; a plurality of inertial sensors; a GNSS transceiver; and one or more antennas configured to receive and transmit signals to and from said GNSS transceiver; a connector configured to connect said towable bird to a towing aircraft; and circuitry configured to: receive vector magnetic measurements from the vector magnetometer, orientation measurements from the plurality of inertial sensors, and position measurements from the GNSS transceiver; process said vector magnetic measurements using said orientation measurements and said position measurements to provide orientation-independent and altitude-normalized corrected magnetic measurements;determine calibration parameters using said corrected magnetic measurements by: identifying, in said position measurements, one or more crossover points having a same location; determining differences in crossover point corrected magnetic measurements; calculating the calibration parameters which minimize said differences in crossover point data.

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