Aeronautical radio direction finding pod and pod deployment method

Abstract

Aeronautical radio direction finding pod and method of implementing the pod. Aeronautical radio direction finding pod 1, comprising a frame 11, a support member 12 of the frame for attaching to an aircraft 50 along a principal axis of force, an interferometer for receiving electromagnetic waves of a band selected from one or more of the radar bands P, L, S, C, X, Ku, K and Ka, the interferometer comprising at least two antennas, the antennas being supported by the frame 11, and having a radiation pattern with an aperture of -3 dB of at least 30° centered on a line of sight and beyond 80° from the line of sight a level at least 15 dB lower than that of the maximum radiation.

Description

Title of the invention: Aeronautical radio direction finding pod and method for implementing the pod

[0001] The invention relates to the field of airborne detection of radio-electric emitting sources, in particular radar.

[0002] The precise location of a radio wave transmitter is a need expressed in multiple circumstances, such as spectrum monitoring in an environment where spectral resources are needed by an increasing number of users and applications.

[0003] Mounting a radio direction finding device on an aircraft offers significant advantages over ground-based installation: greater range, reduction of multipath effects due to terrain or overground, i.e. vegetation and buildings, rapid movement allowing estimation of the distance to a stationary transmitter by a single device, where static triangulation would require two synchronized ground installations.

[0004] Phase goniometry or interferometry requires at least two antennas, the phase difference of the received signals being related to the angle between the direction of arrival of the wave and the line defined by the phase center of the two antennas.

[0005] Aircraft specializing in the collection of electromagnetic intelligence are thus equipped with multiple antenna arrays to cover the frequency bands and polarizations of interest, and whose layout is optimized to cover a wide angular sector, while being relatively insensitive to interactions of incident waves with the aircraft's structure, which would disrupt the measurement of phase differences. These specialized aircraft feature various protrusions, such as radomes protecting these antenna arrays. Such specialized aircraft are expensive to acquire and operate, and are available in limited numbers.

[0006] Such an optimized implantation is incompatible with a device that is easy to assemble and disassemble, portable on different types of aircraft depending on availability and type of mission.

[0007] The Applicant has identified a need for airborne detection equipment for radio emission sources, carried by an aircraft in a removable manner.

[0008] The invention improves the situation.

[0009] According to one aspect of the invention, an aeronautical radio direction finding pod comprises a frame, a support element for the frame for attaching it to an aircraft along a principal axis of force, an interferometer for receiving electromagnetic waves of a band selected from one or more of the radar bands P, L, S, C, X, Ku, K and Ka, the interferometer comprising at least two antennas, the antennas being supported by the building and presenting a -3 dB aperture radiation pattern of at least 30° centered on a line of sight and beyond 80° from the line of sight a level at least 15 dB lower than that of the maximum radiation.

[0010] The pod can be carried by a light aircraft, a drone, a heavy aircraft, or a helicopter. The pod can be removed at the end of the mission, making pod maintenance easier and independent of aircraft maintenance. The aircraft follows a simple flight plan, and the pod benefits from excellent deployment conditions. The aircraft is banked so that the useful observation area is within the main radiation lobe of the antennas and outside the areas of interaction with the carrier structure. An analysis of the evolution of the angular bearings during the turn associated with the carrier's bank angle is performed in correlation with the evolution of its heading.

[0011] In a particularly economical embodiment, the antennas are arranged in pairs for a given frequency band.

[0012] In another embodiment, the antennas are arranged in a triplet for a given frequency band. Ambiguities are reduced.

[0013] In one embodiment, the device comprises an interferometer for receiving electromagnetic waves in the S and C bands and an interferometer for receiving electromagnetic waves in the X and Ku bands. The cost is reduced.

[0014] In one embodiment, the device includes an S-band electromagnetic wave receiving interferometer, a C-band electromagnetic wave receiving interferometer, an X-band electromagnetic wave receiving interferometer and, where applicable, a Ku-band electromagnetic wave receiving interferometer. Direction finding processing is facilitated.

[0015] In one embodiment, the device comprises an enclosure transparent to said electromagnetic waves, the enclosure surrounding the frame and the antennas. The antennas are arranged inside the enclosure for their protection and to avoid degrading the drag.

[0016] In one embodiment, the antennas exhibit a -3 dB aperture radiation pattern of at least 60° centered on a line of sight, and beyond 80° from the line of sight, a level at least 10 dB lower than that of the maximum radiation. The antennas provide a high-quality signal. The phase center of the antennas remains stable within the angular sector of observation in which transmitters are detected.

[0017] In one embodiment, the antennas of an interferometer of a first frequency band are nested within the antennas of an interferometer of a second frequency band, the second frequency band being lower than the first frequency band. The pod is compact.

[0018] In one embodiment, the device comprises a signal receiver connected to said antennas of at least one interferometer, the receiver comprising a device for separating the received signal into radar pulses, the pod further comprising a device for selecting radar pulses from the receiver and for calculating a phase difference between the antennas of at least one interferometer, an angular bearing calculator based on the evolution of the phase difference from the phase difference calculator and gyroscopic information. The angular bearing is precise.

[0019] In one embodiment, the device includes a gyroscopic unit configured to provide a georeferenced angular bearing.

[0020] In one embodiment, the device includes an inertial measurement unit configured to provide geolocation of a source of a received emission. The geolocation is precise.

[0021] In one embodiment, a method for deploying the above-mentioned pod during the flight of an aircraft carrying said pod, comprising banking the aircraft to orient the antennas' line of sight towards at least one area of ​​interest, and eliminating angular ambiguities in the interferometry by comparing the variation in angular bearings during the turn associated with the aircraft's bank angle to the variation in gyroscopic information. Generally, waves from distant transmitters are contained in a near-horizontal plane, and their interaction with the aircraft's wing, if the aircraft is flying at a shallow bank angle, degrades the expected direction-finding accuracy.Thus, in one embodiment, the aircraft, in order to perform a goniometric measurement, enters a steep banked turn, which, with antennas having small side lobes in the plane of the wing, significantly reduces the interaction of the received waves with the wing and restores goniometric accuracy.

[0022] In one embodiment, the pod is passive, particularly in the frequency bands of said antennas.

[0023] In one embodiment, the pod is devoid of an emitting element in the P, L, S, C, X, Ku, K and Ka radar bands.

[0024] In one embodiment, the pod is provided with a remote data link for transmitting telemetry.

[0025] In one embodiment, the pod is removable.

[0026] In one embodiment, the antennas of an interferometer are preferentially mounted on a pod with standardized interfaces for under-wing or under-fuselage mounting, and where applicable, connectors allowing access to the network onboard electrical systems and to exchange information with avionics and / or the crew via wired connection.

[0027] In one embodiment, the pod includes a gyroscopic unit.

[0028] In one embodiment, the pod includes an inertial unit.

[0029] Other features and advantages of the invention will become apparent from an examination of the detailed description below, and the accompanying drawings, in which:

[0030] [Fig-1] schematically illustrates, in front view, an aeronautical pod of radio direction finding according to one aspect of the invention mounted under an aircraft in a steep left turn.

[0031] [Fig.2] schematically illustrates in front view an aeronautical radio direction finding pod according to one aspect of the invention mounted under an aircraft in a high banked right turn.

[0032] [Fig.3] schematically illustrates in longitudinal section an aeronautical radio direction finding pod according to one aspect of the invention.

[0033] [Fig.4] schematically illustrates in cross-section an aeronautical radio direction finding pod according to one aspect of the invention.

[0034] [Fig.5] is a block diagram of an aeronautical radio direction finding pod according to one aspect of the invention.

[0035] [Fig.6] is a block diagram of an aeronautical radio direction finding pod according to another aspect of the invention.

[0036] [Fig.7] represents curves illustrating the evolution of five angular bearings likely to correspond to the same phase differences of a signal coming from a long-distance transmitter.

[0037] The attached drawings may not only serve to complete the invention, but also contribute to its definition, if necessary.

[0038] The pod has a longitudinal axis that may correspond to the aircraft's longitudinal axis or roll axis. The pod has a transverse axis that may correspond to the aircraft's transverse axis or pitch axis, with the aircraft's wings being oriented along the transverse axis or at an angle to it. The pod has an elevator axis that may correspond to the aircraft's elevator axis or yaw axis. The elevator axis may be vertical when the aircraft is flying at zero bank angle.

[0039] The invention relates to an aeronautical radio direction finding pod 1. A pod 1 is a device mechanically and removably attached to an aircraft 50. The aircraft 50 is capable of flying with or without the pod 1. The pod 1 can be attached to the aircraft 50, and conversely detached from the aircraft 50, on an airstrip or a suitable takeoff site. For this purpose, the aircraft 50 can be equipped with a load-harnessing device, for example of the type described in FR2968275, located under the fuselage or under A wing. Aircraft 50 can be piloted, autopiloted, or remotely piloted. Aircraft 50 can be an airplane, for example, a light propeller-driven airplane, a drone, or a helicopter.

[0040] The pod 1 comprises a frame 11, for example in the form of a beam elongated along its longitudinal axis. The pod 1 comprises a mechanical interface and an electrical interface with the aircraft 50, forming a support element 12 of the frame. The mechanical interface may include at least one lifting ring and at least one bearing surface. The electrical interface may include a connector for supplying electrical power to the pod 1 from the aircraft 50. The support element 12 of the frame may include a pneumatic interface for supplying pneumatic power to the pod 1 from the aircraft 50. The support element 12 of the frame may include an electronic interface with the aircraft 50 for data communication between the aircraft 50 and the pod 1, for example via a wired connection.

[0041] The pod 1 may include an enclosure 13 transparent to electromagnetic waves. The enclosure 13 surrounds the frame 11 and the other components of the pod 1 to protect them and reduce aerodynamic drag. The enclosure 13 is supported by the frame 11.

[0042] The pod 1 comprises at least one interferometer 2, 3, 4, 5 or group of receiving electromagnetic waves of a band selected from one or more of the P, L, S, C, X, Ku, K, and Ka radar bands. The antenna group comprises at least two antennas. The antennas are supported by the frame 11. The antennas are arranged in the enclosure 13. The enclosure 13 surrounds the frame 11 and the antennas, as well as, where applicable, other components. The antennas may be distributed along the longitudinal axis.

[0043] According to some embodiments, the pod 1 comprises one, two, or more antenna groups. Each antenna group is dedicated to one, two, or three of the frequency bands. By way of example, the pod 1 comprises:

[0044] - an S-band antenna group.

[0045] - a group of C-band antennas.

[0046] - an X-band antenna group.

[0047] - a Ku band antenna array.

[0048] - a group of Ka-band antennas.

[0049] - an S-band antenna group and a C-band antenna group.

[0050] - an X-band antenna group and a Ku-band antenna group.

[0051] - a group of P-band antennas and a group of L-band antennas.

[0052] - an S-band antenna group, a C-band antenna group and a group of X-band antennas.

[0053] - an S-band antenna group, a C-band antenna group, a group of X-band antennas and a group of Ku-band antennas as illustrated [Fig.3] which represents the antenna positions constituting four two-antenna interferometers 2, 3, 4, 5. Interferometers 2, 3, 4, 5 are nested, maintaining approximately the same length expressed in wavelengths at the center frequency of each of the bands. In other words, the distance between two antennas of each interferometer 2, 3, 4, 5 is equal to the product of the wavelength of the center frequency of the band of said interferometer 2, 3, 4, 5 and a constant coefficient for all interferometers 2, 3, 4, 5 in pod 1. More precisely, the antennas of the Ku-band interferometer are arranged between the antennas of the X-band interferometer. The antennas of the X-band interferometer are arranged between the antennas of the C-band interferometer. The antennas of the C-band interferometer are arranged between the antennas of the S-band interferometer.The antennas of the long-wavelength interferometer are arranged around the antennas of the shorter-wavelength interferometer. The goniometric measurement obtained by the antennas of an interferometer 2, 3, 4, 5 is more precise the more accurate the measurement of the phase difference between the signals received by the antennas and the greater the spacing between said antennas. However, increasing the spacing between said antennas creates angular ambiguities, as several directions of wave incidence can produce the same phase difference. Angular ambiguities can be eliminated by comparing the evolution of the possible directions of wave incidence during a turn, particularly for a stationary or quasi-stationary source.

[0054] Each antenna has a -3 dB beamwidth of at least 30° centered on a line of sight. The line of sight is usually the horizontal stabilizer axis. The antennas in an antenna array are identical and share a common line of sight. The beamwidth may advantageously have a -3 dB beamwidth greater than 60°. In the example shown in Figures 1 and 2, the beamwidth is 60°. An aircraft bank angle of 60° in roll allows coverage of an area between the horizon at 0° and -60° below pod 1, i.e., between +30° and +90° relative to the vertical. Beyond 80° of the line of sight, the beamwidth of each antenna is at least 10 dB lower than the maximum beamwidth. This level can advantageously be more than 15 dB lower than that of the maximum radiation.

[0055] The pod 1 includes a receiver 14. The receiver 14 is located in the pod 1, inside the casing 13. The receiver 14 can be attached to the frame 11. The receiver 14 receives the signals provided by the antennas of said at least one interferometer 2, 3, 4, 5. The antennas are positioned in the lower area of ​​the pod 1, in particular on the lower face of the casing 13. The receiver 14 is positioned in the upper area of ​​the pod 1. The receiver 14 is in contact with a lower surface of the frame 11.

[0056] In the embodiment illustrated in [Fig. 4], the antennas are connected by coaxial cables 15 to the receiver 14. The receiver 14 is multichannel. The receiver 14 has a number of channels at least equal to the number of antennas in an antenna array. These channels are coherent with each other, using the same reference oscillator 16 in the embodiment illustrated in [Fig. 5].

[0057] In the embodiment illustrated in [Fig. 5], the pod 1 includes a multichannel signal receiver 14. Each channel of the multichannel signal receiver 14 is connected to an antenna in an antenna array. The receiver 14 detects and analyzes the emissions received by the antennas. The receiver 14 includes a time and frequency separation device that splits the received signal into radar pulses. A computer selects the radar pulses of interest from the receiver 14. For the selected radar pulses, the computer calculates the phase differences between channels of the same antenna array and then calculates the evolution of these phase differences. The computer translates these phase differences into instantaneous angular bearings in a coordinate system. The pod 1 includes a gyroscopic unit 17 that provides gyroscopic information.The gyroscopic information on the evolution of the reference frame during the turn of aircraft 50 is used by the computer to perform a tracking of the angular bearings which eliminates ambiguous bearings.

[0058] The 14-channel multichannel receiver can be permanently connected to all the antennas, the reception band of each channel then being the radar band or bands of the corresponding antenna group.

[0059] The 14-channel multichannel receiver can also be connected via switches to the antennas of each antenna group. The 14-channel multichannel receiver can then assign a time range to each antenna group. The number of channels in the 14-channel multichannel receiver is thus reduced, but each channel then has a reception band that covers the radar bands of said antenna groups.

[0060] The multichannel receiver 14 is connected to a clock 16. The clock 16 ensures the synchronization of the channels of the multichannel receiver. The receiver 14 includes a device for separating the received signal into radar pulses. The signals amplified and filtered by the multichannel receiver 14 are advantageously digitized. Downstream processing is performed by a computer or by separate processing modules. The downstream processing is described below in the simplest configuration of a group of two antennas. The downstream processing includes separation of the received signals into radar pulse trains 18, phase difference calculation 19, angular bearing calculation 20, and georeferenced angular bearing calculation 21.

[0061] The separation of received signals into radar pulse trains 18 is also called deinterlacing. Deinterlacing is usually based on a Characterization includes level, duration, center frequency, any modulation, and the time of occurrence of each pulse. These characteristics also allow, by comparison with a reference library, the selection of radar emissions of interest. This selection can be made by an algorithm, possibly supervised by a human operator.

[0062] For the selected radar emissions, a phase difference calculation 19 is performed between pulses received at the same instant on the two antennas.

[0063] The interferometer formula well known to those skilled in the art which relates direction of origin of an emission and phase difference between antennas is reversed by the following treatment, namely the calculation of angular bearings 20. The calculation of angular bearings provides the different possible angular bearings.

[0064] The following processing is the calculation of the georeferenced angular bearing 21. The calculation of the georeferenced angular bearing 21 uses gyroscopic information from the gyroscopic unit 17, which allows for an angular tracking of the different possible bearings, in particular using Kalman filters, to identify an origin direction in a georeferenced frame.

[0065] During a flight of aircraft 50 equipped with pod 1, tilting aircraft 50 about its roll axis directs the main lobe of the antenna arrays towards at least one area of ​​interest, see [Fig. 7]. This tilting contributes to producing a turn of the aircraft. Angular ambiguities in interferometry are eliminated by the trajectory of pod 1, which makes it possible to compare the evolution of possible directions during the turn.

[0066] Deinterlacing of the received signals can be performed, for example, using the technique of Manon Mottier, Gilles Chardon, and Frédéric Pascal. Deinterlacing and classification of RADAR signals based on optimal transport distances. 28th Francophone Colloquium on Signal and Image Processing GRETSI 2022, Sep 2022, Nancy, France, hal-03851279

[0067] Figure 7 shows, during a 30° turn of a pod 1 comprising a two-antenna interferometer separated by four wavelengths, after compensation by the gyrometric information associated with this turn, the evolution of the five angular bearings that could correspond to the same phase differences produced by a long-distance transmitter. The true angular bearing corresponds to the one whose compensated evolution remains identically zero; the other four correspond to ambiguities or ambiguous angular bearings. In other words, an ambiguity exhibits a variation during the turn that allows it to be distinguished from the true angular bearing.

[0068] Furthermore, geolocation of a stationary or quasi-stationary transmitter can be performed. For this purpose, aircraft 50 can perform two or more Successive angular bearings for triangulation, for which a geolocation device could usefully be an inertial measurement unit. In the embodiment of [Fig. 6], the pod includes a gyroscopic unit 23 to enable the calculation of geolocation 22. If the transmitter is at a short distance, this distance can be estimated during a single turn by supplementing the analysis of the variation of the angular bearing during the turn, performed as above to eliminate ambiguities.

[0069] The results of the georeferenced angular bearing obtained from the georeferenced angular bearing calculation 21 can be stored in the pod 1, transmitted to the aircraft 50 via the electronic interface associated with the support unit 12, and / or transmitted radio frequency. Alternatively, the geolocation calculation is performed, providing coordinates of the transmitting source.

Claims

Demands

1. An aeronautical radio direction finding pod (1), having a longitudinal axis or roll axis, a transverse axis or pitch axis and a depth axis or yaw axis corresponding to a line of sight, said pod comprising a frame (11), a support member (12) for the frame for attachment to an aircraft (50) along a principal lifting axis, an electromagnetic wave receiving interferometer of a band selected from one or more of the radar bands P, L, S, C, X, Ku, K and Ka, the interferometer comprising at least two antennas, the antennas being supported by the frame (11), positioned in the lower area of ​​the pod (1),and exhibiting a -3 dB beamwidth of at least 30° centered on the line of sight and beyond 80° from the line of sight a level at least 15 dB lower than that of maximum radiation, or a -3 dB beamwidth of at least 60° centered on the line of sight and beyond 80° from the line of sight a level at least 10 dB lower than that of maximum radiation, to allow the line of sight of the antennas to be oriented towards at least one area of ​​interest.

2. Pod (1) according to claim 1, comprising an interferometer for receiving electromagnetic waves in the S and C bands and an interferometer for receiving electromagnetic waves in the X and Ku bands.

3. Pod (1) according to claim 1, comprising an S-band electromagnetic wave receiving interferometer, a C-band electromagnetic wave receiving interferometer, an X-band electromagnetic wave receiving interferometer and, optionally, a Ku-band electromagnetic wave receiving interferometer.

4. Pod (1) according to any one of the preceding claims, comprising an enclosure (13) transparent to said electromagnetic waves, the enclosure (13) surrounding the frame (11) and the antennas.

5. Pod (1) according to any one of the preceding claims, wherein the antennas of a first frequency band interferometer are nested within the antennas of a second frequency band interferometer, the second frequency band being lower than the first frequency band.

6. Pod (1) according to any one of the preceding claims, comprising a signal receiver (14) connected to said antennas of at least one interferometer, the receiver (14) comprising a device for separating the received signal into radar pulses, the pod (1) further comprising a device for selecting radar pulses from the receiver (14), and for calculating a phase difference between the antennas of at least one interferometer, an angular bearing calculator based on the evolution of the phase difference from the phase difference calculator and gyroscopic information,

7. Pod (1) according to claim 6, comprising a gyroscopic unit (17) configured to provide a georeferenced angular bearing.

8. Pod (1) according to claim 6, comprising an inertial navigation system (23) configured to provide geolocation of a source of a received emission.

9. Method of implementing the pod (1) according to any one of the preceding claims, during a flight of an aircraft (50) carrying said pod (1), comprising tilting the aircraft (50) to orient the antennas' line of sight towards at least one area of ​​interest, and eliminating angular ambiguities of interferometry by comparing the variation of angular bearings during the turn associated with the tilt of the aircraft (50) to the variation of gyroscopic information.

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