Auxiliary data for satellite control

Abstract

The present invention relates to a method of generating assistance data for controlling a satellite moving in an Earth orbit, the method comprising the steps of receiving (101) tracking data for the satellite, applying an orbit determination algorithm including the steps of estimating (102) an orbit of the satellite based on the tracking data and predicting (103) future ephemeris data of the satellite based on the estimated orbit, generating (105) assistance data including the predicted future ephemeris data, and transmitting (106) the assistance data to the satellite for use in attitude and orbit control of the satellite.

Description

[Background technology]

[0001] The present invention relates to a method and a ground segment for generating assistance data for controlling a satellite moving in an earth orbit, and to a satellite moving in an earth orbit using the assistance data.

[0002] On-board knowledge of the satellites' positions is of crucial importance for all space missions. Earth observation missions require very accurate knowledge of the satellites' positions, especially when using small satellites. Positioning errors of more than 500 m may already be unacceptable for certain payloads, for example Synthetic Aperture Radar (SAR) imagery. Satellites with an on-board Global Positioning System (GPS) receiver can accurately determine their orbital state without the assistance of a ground segment. An on-board GPS receiver may therefore provide an accurate navigation solution. However, this system relies on regular GPS measurements, which are not always available.

[0003] GPS outages can occur for a variety of reasons, including poor availability and quality of GPS signals in certain locations or satellite orientations. Other reasons could be incorrect installation of the GPS equipment or antennas onboard the satellites. Additionally, GPS signals can be degraded or blocked, resulting in inaccurate data or a complete loss of GPS signal.

[0004] Without regular updates from the GPS receiver, the internal orbit propagator in the satellite's Attitude Determination and Control System (ADCS) could start to deviate from the true orbit. For example, if a satellite loses GPS signal lock for just one hour, this could result in an error of up to 6km in the satellite's trajectory. Therefore, a more accurate idea of ​​the satellite's current location is needed in the event of a GPS outage.

[0005] To control satellites moving in Earth orbit, it is desirable to have precise information on-board about the satellites' positions. This applies, for example, to situations where measurements from the on-board GPS receiver may not be available or may not be accurate. It is therefore necessary to provide precise information on the satellites' positions on-board to ensure the necessary ADCS functionality even during GPS outages.

[0006] The embodiments described below are not limited to implementations that address some or all of the shortcomings of known approaches described above. Summary of the Invention

[0007] This Summary is provided to introduce a selection of concepts in a simplified form that are more fully described in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter.

[0008] In a first aspect, the present invention provides a method of generating assistance data for controlling a satellite moving in an Earth orbit, the method comprising the steps of receiving tracking data for the satellite, applying an orbit determination algorithm including the steps of estimating an orbit of the satellite based on the tracking data and predicting future ephemeris data for the satellite based on the estimated orbit, generating assistance data including the predicted future ephemeris data, and transmitting the assistance data to the satellite for use in attitude and orbit control of the satellite.

[0009] In some embodiments, the tracking data includes Global Navigation Satellite System "GNSS" sensor data, Satellite Laser Ranging "SLR" measurements, or satellite radar measurements.

[0010] In some embodiments, the tracking data indicates a series of times and the corresponding positions and velocities of the satellites.

[0011] In some embodiments, the method further comprises determining a current position of the satellite based on the aiding data when data from a GNSS or GPS sensor on board the satellite is unavailable or unreliable.

[0012] In some embodiments, at least the steps of applying the orbit determination algorithm and generating the assistance data are performed on the ground.

[0013] In some embodiments, estimating the orbit of the satellite is further based on a previously estimated orbit.

[0014] In some embodiments, the tracking data corresponds to a predetermined time interval, optionally corresponding to the time between two ground station passes of the satellite.

[0015] In some embodiments, the tracking data includes one or more gaps corresponding to one or more tracking cessations during a given time interval.

[0016] In some embodiments, the method is repeated for each pass of the satellite, and optionally tracking data is received for each pass of the satellite and generated assistance data is transmitted for each successive pass of the satellite.

[0017] In some embodiments, the method is repeated for a second satellite, and optionally for multiple satellites in parallel.

[0018] In some embodiments, the auxiliary data includes predicted future ephemeris at time intervals of up to 1 minute, optionally 2 minutes, and further optionally 5 minutes, over a period of at least 6 hours, optionally at least 12 hours, and further optionally at least 24 hours.

[0019] In some embodiments, the aiding data includes predicted future ephemeris covering at least one orbital period of the satellite.

[0020] In some embodiments, the method further includes verifying the accuracy of the results of the orbit determination algorithm prior to generating and transmitting the assistance data.

[0021] In some embodiments, estimating the orbit includes filtering the tracking data to provide filtered ephemeris for the satellites.

[0022] In some embodiments, a Kalman filter is used to process the tracking data sequentially.

[0023] In some embodiments, predicting future ephemeris data for the satellite includes propagating forward in time from the filtered ephemeris.

[0024] In some embodiments, a dynamic model is used to predict future ephemeris data for a satellite.

[0025] In some embodiments, the method is a computer-implemented method.

[0026] In a second aspect, the present disclosure provides a ground segment for generating assistance data for controlling a satellite orbiting the Earth, the ground station system being configured to perform the method according to the first aspect.

[0027] In a third aspect, the present disclosure provides a ground segment for generating assistance data for controlling a satellite moving in an orbit around the Earth, the ground segment including: a receiving module configured to receive tracking data for the satellite; an orbit determination toolkit "ODTK" configured to apply an orbit determination algorithm including estimating an orbit of the satellite based on the tracking data and predicting future ephemeris data of the satellite based on the estimated orbit; an assistance data module configured to generate assistance data including the predicted future ephemeris data; and a transmitting module configured to transmit the assistance data to the satellite for use in an attitude determination and control system "ADCS" unit of the satellite.

[0028] In some embodiments, the components of the ground segment are distributed across multiple locations, optionally different ground locations.

[0029] In a fourth aspect, the present disclosure provides for use of a ground segment according to the second or third aspect to generate assistance data for controlling one or more satellites moving in orbit around the Earth, each of the one or more satellites being configured to determine its position based on the assistance data.

[0030] In a fifth aspect, the present disclosure provides a satellite moving in an Earth orbit, the satellite including a receiving module configured to receive assistance data generated for the satellite in accordance with a method according to the first aspect, and an Attitude Determination and Control System "ADCS" unit for controlling the satellite in orbit, the ADCS unit configured to determine a current position of the satellite based on the assistance data.

[0031] In some embodiments, the satellite includes an on-board computer configured to select latest predicted ephemeris data from the aiding data based on a current on-board time of the satellite, and an orbit propagator forming part of the ADCS unit and configured to numerically propagate the selected predicted ephemeris data based on the current on-board time to determine a current position of the satellite.

[0032] In some embodiments, the satellite further includes a tracking module for tracking the satellite, optionally the tracking module being a GPS sensor module, and the ADCS unit is further configured to determine a current position of the satellite based on sensor data of the tracking module, and if the tracking module is not functioning, the ADCS unit is configured to switch from the sensor data to the aiding data.

[0033] In some embodiments, the satellite is a small Earth observation satellite and / or a radar satellite.

[0034] Some embodiments of the present disclosure provide a system including one or more computing systems, each including at least one processor and memory, configured to implement any of the methods or processes described herein.

[0035] Some embodiments of the present invention further provide a computer readable medium including instructions, e.g. in the form of an algorithm, that when implemented in a computing system forming part of a satellite operating system, causes the system to perform any of the methods or processes described herein.

[0036] Features of different aspects and embodiments of the invention may be combined as appropriate and in any aspect of the invention, as will be apparent to those skilled in the art. Embodiments of the present invention will now be described, by way of example only, with reference to the following drawings, in which: [Brief description of the drawings]

[0037] [Figure 1] FIG. 2 is a block diagram illustrating a process for generating assistance data for controlling a satellite orbiting the Earth. [Diagram 2] 1 is a schematic diagram of a satellite and a ground segment, the ground segment configured to generate assistance data, and the satellite configured to use the assistance data to obtain precise knowledge of a position on the satellite; [Diagram 3] This visualizes the on-board use of aiding data in a scenario where the satellite's GPS receiver is temporarily inactive. [Figure 4] FIG. 1 is a perspective view of a satellite including an ADCS unit for controlling the satellite as it orbits the Earth. [Diagram 5] FIG. 5 is a partial perspective view of the satellite shown in FIG. 4, showing components for controlling the satellite's attitude. [Figure 6] The figure shows test results comparing the position determined by the on-board propagator based on aided data with the actual orbit data based on GPS measurements. [Figure 7] 1 is a graph comparing predicted future ephemeris data with actual orbits. [Figure 8] 1 is a graph showing measurement residuals of 24-hour GPS data. [Figure 9] 9 is a graph showing position consistency statistics according to FIG. 8; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] Common reference numbers are used throughout the figures to denote similar features.

[0039] Embodiments of the invention are described below by way of example only. These examples represent the best ways currently known to applicant for practicing the invention, but are not intended to be the only ways that can be accomplished. The description sets forth the functions of the examples and a sequence of steps for constructing and operating the examples, although the same or equivalent functions and sequences may be accomplished by different examples.

[0040] The present invention provides a method and ground segment for generating assistance data for obtaining accurate on-board knowledge of the satellite's position. For this purpose, a ground-based orbit determination "OD" may be implemented, using received tracking data, such as GPS receiver measurements, to estimate the state of the satellite and then predicting the state of the satellite for a certain period of time in the future by a propagation algorithm. Assistance data is generated based on the future prediction. The assistance data is suitable for use in the satellite's ADCS unit instead of GPS measurements to obtain accurate information about the on-board position of the satellite. The present invention further provides a satellite configured to receive assistance data and equipped with an ADCS unit for determining the satellite's position based on the assistance data.

[0041] The present invention is based on the discovery that improved accuracy can be achieved by using aided data instead of unfiltered GPS measurements because the noise inherent in GPS measurements can be filtered before the aided data is generated and fed to the internal orbit propagator on board the satellite. Failure to filter noise from the GPS measurements has the most significant effect of causing the orbit propagator to deviate from the true orbit.

[0042] FIG. 1 illustrates a method for generating assistance data. Beginning at operation 101, tracking data for a satellite is received. The tracking data may indicate a series of time and corresponding position and velocity points of the satellite. The tracking data may be used to determine the satellite's orbit. The tracking data may be obtained using a radar or laser ranging device at an earth station. Additionally or alternatively, the tracking data may be obtained by a Global Navigation Satellite System (GNSS) receiver on board the satellite. A well-known example of a GNSS is the Global Positioning System (GPS). In the following description, for clarity, the tracking data is described as GPS data obtained by a GPS receiver on board the satellite. The GPS receiver module is described in more detail with reference to FIG. 5.

[0043] In operation 101, tracking data may be received from the satellite itself or from a tracking station that is tracking the satellite. The tracking data may be received directly from the satellite or tracking station, or indirectly, for example, via one or more ground stations. The tracking data may be received each time a satellite passes a ground station. Typically, GPS measurements are transmitted in telemetry each time a ground station passes. In the case of GPS data, the tracking data consists of at least a position and velocity vector and a timestamp. The GPS data may correspond to GPS measurements spaced 30 seconds apart from one another. The GPS data may correspond to at least one complete orbital period.

[0044] In operation 110, an orbit determination "OD" method is applied to the tracking data. In this step, an orbit is (re)estimated based on the received tracking data (and the previously estimated orbit). OD is a method for estimating the state, such as the position and velocity, of an orbiting object, such as a satellite. OD is well known in the field of satellite operations and may be described as a filtering method that integrates observations and orbital dynamics equations to estimate the satellite's position and velocity. That is, it estimates the satellite's state variables (position and velocity) based on measurement data, such as the tracking data. The OD method may be applied by an OD process tool, commonly called a filter, which is described in more detail with reference to FIG. 2. Numerical OD methods may achieve significant improvements in accuracy at the cost of incremental propagation over time and a high computational load.

[0045] A first step 102 of the OD operation 110 is to estimate the orbit of the satellite based on at least the tracking data received in step 101. The output of step 102 is also called the filtered ephemeris. A second step 103 is to predict future ephemeris data based on the estimated orbit (or filtered ephemeris). The predicted future orbit ephemeris may correspond to the future state of the satellite at any time, such as in the near future or more than 24 hours into the future. The predicted future ephemeris data may provide a resolution of 5 seconds or include more than 24 hours of data. The predicted future ephemeris data may include data corresponding to at least two or more complete orbital periods.

[0046] One example of an OD operation 110 applies two types of filters (statistical processes) in a process that includes the following steps.

[0047] 1) BWLS (Bayesian Weighted Least Squares): This is essentially a curve fitting process. It does not use a dynamic model and is used to make a better initial guess to proceed to the next filtering step. Here, all the tracking data is processed at once.

[0048] 2) The next step is Order Statistical Filtering (OSF). This is a Kalman filter, where the data is processed in order. Here, a dynamic model is used to give the most accurate updates. The optimal state minimizes the uncertainty in the state estimation. The model uncertainty is determined from the size and variability of this uncertainty (process noise). Here, we are moving from old data to new data.

[0049] 3) 2nd OSF: Also called the Smoother. Same as (2) but in reverse time. Kalman filters generally try to minimize state uncertainty knowing the convergence (uncertainty level) of the previous state and use a dynamic model to make an optimal guess.

[0050] In operation 104, the accuracy of the results of the OD method in operation 110 is verified. To ensure that the prediction is accurate, the uncertainty must be low. Only then does sufficient knowledge exist about the orbit and the prediction is accurate enough to be used as a basis for controlling the satellite in orbit. The verification step 104 is optional and may be applied before generating and transmitting the assistance data in steps 105, 106. For example, the verification step 104 may be performed only if the tracking data received in step 101 contains one or more gaps. If the accuracy of the verification is low, the OD method of operation 110 may be repeated or paused until more tracking data for the satellite is received. If the prediction is verified to be accurate, assistance data for the satellite is generated.

[0051] In operation 105, aiding data is generated that includes predicted future ephemeris data for the satellite. The resolution of the aiding data will be lower than the resolution of the future ephemeris predicted in step 103. The resolution of the aiding data represents a compromise between the amount of data that can be successfully uploaded and stored on the satellite (e.g., due to uplink bandwidth limitations and on-board memory constraints) and the amount of data required to keep the satellite from deviating from its actual orbit. The aiding data may provide, for example, a 5 minute resolution or may include ephemeris data for at least one complete orbital period.

[0052] In operation 106, the generated assistance data is transmitted to the satellite and used for satellite attitude and orbit control. The assistance data may be transmitted to the satellite according to every ground station pass, every ground station pass, or other schedule. For example, the assistance data may be transmitted to the satellite only if time remains after other priority transmissions have been made, since the assistance data is only used if the GPS signal is lost. The satellite may also pass over one or more ground stations during its orbit. Thus, the assistance data may be transmitted to the satellite once per orbit, less than once per orbit, or more than once per orbit. On-board use of the assistance data to obtain accurate knowledge of the satellite's position is described in more detail with reference to Figures 2 and 3. The assistance data may be configured, for example, to substitute GPS measurements when the GPS is down, and is used to initialize an internal orbit propagator.

[0053] Method 100 prevents the satellite from deviating from its actual orbit when GPS is unavailable. The aiding data allows the satellite to maintain accurate attitude and orbit control even when GPS is invalid or unavailable for extended periods of time. During this period, the satellite can perform payload and orbit maintenance maneuvers that would not normally be possible. Method 100 allows the satellite to have accurate orbit information on-board at all times, with or without GPS. This allows scheduled payload activities to be performed with very good geolocation accuracy. Furthermore, prohibitively high computational burdens for constantly estimating and predicting orbit information on board the satellite are avoided.

[0054] Figure 2 shows a system consisting of a ground segment 200 and a satellite 300. Tracking and assistance data may be exchanged between the ground segment 200 and the satellite 300 for each pass of the ground station. Figure 2 shows the components required to generate the assistance data and how it is used on board the satellite to control the satellite 300. The ground segment 200 is configured to perform the method 100 described with reference to Figure 1. The same ground segment 200 may be used to generate and provide assistance data to multiple satellites, for example in a constellation, also called a fleet.

[0055] As shown in FIG. 2, the (distributed) ground segment 200 is configured with a ground station 201 at a different ground location than the ground automation 202. Alternatively, the ground station 201 may be co-located with the ground automation 202 of the ground segment 200. Additionally, the ground automation 202 of the ground segment 200 may be hosted and executed using a cloud computing provider. The ground station 201 is configured to receive tracking data from a satellite 300. In this embodiment, the tracking data originates from the satellite 300. In another example, the ground automation 202 may use radar or laser ranging device tracking data from an earth station. The tracking data is routed from the ground station 201 to the ground automation 202.

[0056] The ground automation 202 includes a receiving module (not shown as a separate component) configured to receive tracking data of the satellites 300, an orbit determination toolkit "ODTK" 210 configured to apply an orbit determination algorithm, an assistance data module 260 configured to generate assistance data, and a transmitting module (not shown as a separate component) configured to transmit the assistance data to the satellites 300. Similarly, as described for the receiving ground station 201, the assistance data from the transmitting module may be transmitted to the satellites via a ground station, for example ground station 201 at a different or the same terrestrial location as the ground automation 202.

[0057] Each time new tracking data is received from the satellite 300, the OD server 220 coordinates the execution of the OD process tool 210 to determine the orbit. For example, a commercially available software for performing the OD method is the orbit determination tool kit "ODTK". The OD server 220 is configured to control the OD process tool 210 as illustrated in FIG. 2. The OD process tool 230 may be a software module embedded in the OD server or may be standalone. The tracking data is also provided to the OD process tool 210. The OD process tool 210 is composed of a filter module 212, a storage module 213, and a prediction module 214. The filter module 212 is generally referred to as an OD filter and is configured to filter the tracking data in one or more filtering stages, which are further described below. The filter module 212 performs filtering of the tracking data and, if available, previously estimated orbit knowledge, and (re)estimates the orbit of the satellite. The estimation results (such as the previously estimated orbit knowledge) are provided to the storage module 213. For filtering, the OD process tool may maintain a dynamic model of the satellite's motion. The model is dynamic in the sense that it may be used to predict the position and velocity of the satellite at any time in the future and is updated periodically. The prediction is performed by the prediction module 214. In some examples, the model may be updated each time one or more tracking data are filtered by the filter module 212 of the OD process tool. Additionally or alternatively, the model may be updated manually by a user of the system.

[0058] The desired / predicted attitude of the spacecraft is also used in the OD process to fit a dynamic model of the satellite. This attitude data may be collected at each satellite pass. The attitude data may be stored, for example, in an attitude file that is appended to with each satellite pass. By including the attitude data in the state vector data, the exact motion of the satellite can be modeled. This accurate position model, in combination with state vector measurements (such as GPS measurements) and satellite dimensions, can improve the reliability and accuracy of the dynamic model, which results in more accurate filtering of the data by the OD filter. This results in more accurate predictions made based on the dynamic model, which improves the planning of orbital maneuvers and improves the accuracy of post-processing of orbital information.

[0059] The orbit is defined by six orbital parameters: 1. The semi-major axis of the eclipse (representing the altitude), 2. The inclination, 3. The eclipse eccentricity, 4. The argument of perigee (where on the orbital arc the closest point to the Earth is located), 5. The longitude of the ascending node, 6. The angle of true anomaly. Since estimating the satellite's acceleration provides information about the orbital state and energy, the orbit can also be described by a state vector (position, velocity, time).

[0060] The OD server 220 then performs predictions based on the estimated orbits to predict future ephemeris data for the satellites. The prediction module 214 may be configured to predict future ephemeris for 24 hours or more. The prediction module 214 may further be configured to predict future ephemeris at any desired resolution, such as between 1 second and 5 seconds.

[0061] Additionally, ground automation 202 includes an OD application programming interface "API" 230. Here, tracking data for satellites, typically stored in separate files and possibly received from different ground stations, is compiled into a data set accessible via API 230 and searchable by time range. For this purpose, the data may be organized in chronological order. Via the API, an evaluation process may be triggered. For this purpose, the API may access data from filter module 212, storage module 213, and / or prediction module 214. Slackbot 240 and / or operator 250 may assist in verifying the accuracy of the results generated by OD process tool 210. Slackbot 240 may provide links to download filtered and / or predicted ephemeris and results of filter consistency tests. Ancillary data module 260 may be configured to generate auxiliary data based on predicted future ephemeris only if the results generated by the OD process tool are acceptable and / or approved. The auxiliary data module 260 may include a downconverter (not shown) to reduce the resolution of the future predicted ephemeris, if necessary.

[0062] The satellite 300 receives new assistance data from the ground segment 200, for example, after every ground station pass or according to different schedules. The new assistance data may include future predicted ephemeris covering at least one complete orbital period or a partial orbital period until the next ground station pass. For this purpose, the satellite 300 includes a receiving module (not shown). A data file containing the assistance data is stored in the memory 311 of the on-board computer 340. Each time the satellite 300 receives new assistance data, the entire assistance data file stored in the memory 311 may be replaced, saving memory resources. When the on-board computer 340 receives the information 321 that the GPS is working 321a, the assistance data stored in the memory 311 may not be used. When the on-board computer 340 receives the information 321 that the GPS is not working 321b, the processor 312 of the on-board computer 340 adjusts the internal orbit propagator to initialize the internal orbit propagator with ephemeris data predicted from the assistance data. For this purpose, the processor 312 of the on-board computer 340 selects the appropriate predicted ephemeris from the memory 311 and provides it to the satellite's ADCS subsystem 302. The predicted ephemeris is selected to be closest to the current on-board computer time. The internal orbit propagator of the ADCD 302 uses the predicted ephemeris for orbit propagation instead of using the most recent GPS measurement data. This means that if GPS goes down, the aiding data is used instead of the actual tracking data.

[0063] An example of a GPS outage that may trigger the use of aiding data could be a situation where some satellites lose or are unable to maintain lock on one or more GPS satellites for a relatively long period of time, such as an hour or more. Furthermore, this problem may be repeated many times during a day. In particular, this problem may occur when the satellites are performing complex operations, such as downlink operations. Around this activity, the number of tracked GPS satellites may suddenly decrease from a nominal value of about 15 to zero. Furthermore, if the GPS antenna is pointed towards the zenith, it may take some time for the GPS lock to be restored even after returning to the nominal flight attitude, when the GPS constellation should be well visible.

[0064] The use and availability of satellite on-board aiding data to determine the satellite's current position based on aiding data is further explained with reference to FIG. 3. In this scenario, the GPS of the satellite stops; i.e. the GPS module goes from functioning 321a to non-functioning 321b and back to functioning 321a again. The aiding data file contains ephemerides predicted into the future every Δt, for example with a resolution of 5 minutes. Thus, the aiding data file contains predicted ephemerides x, v (containing both position and velocity data) for times t1, t2, t3, etc. With a resolution of 5 minutes, there is a gap Δt of 5 minutes between successive ephemerides. When the satellite reaches time t 停止 If the tracking data is lost at time t1, i.e., the on-board GPS receiver cannot maintain lock on the GPS satellite signals, the on-board computer flight software (OBC) selects the latest predicted ephemeris (x1,v1) corresponding to time t1 and provides it to the ADCS system. The ADCS system calculates the current position and velocity based on the predicted ephemeris (x1,v1) from the OBC and numerically propagates the orbit information until it receives an update. 停止There may be a short offset period between the GPS outage at t and the ADCS system calculating the current position based on the aiding data at t start. The next update will be made to the tracking data when it becomes available again, but if the tracking data is not available again, the OBC will continue to provide the predicted ephemeris to the ADCS system. After a predefined time interval Δt' (which may be the same as the time interval Δt shown in FIG. 3), the on-board computer may again select the latest predicted ephemeris (x2,v2) corresponding to time t2. Thus, the ADCS system will calculate the current position based on the aiding data at time t2. 更新 Then, it calculates the current orbit information for the current time based on the ephemeris (x2,v2) provided by the OBC from the aiding data, and then continues to numerically propagate this information on its own until the next update is provided by the OBC. This process may be repeated as long as the GPS is unable to obtain GPS information about the satellite positions. In the scenario shown in Figure 3, the GPS calculates t GPS The lock is reacquired at , and then the ADCS system receives the GPS measurements and calculates the current position and velocity based on the latest GPS measurements. In this case, the predicted ephemeris (x3,v3) for t3 is not sent to the ADCS system.

[0065] FIG. 4 is a perspective view of a satellite 300 orbiting the Earth as an example of a platform that may use on-board aiding data. The ground segment, including all the ground elements of the spacecraft system, is indicated at 200. The ground segment 200 allows for the control of the satellite 300 as it moves in orbit around the Earth, and for the delivery of payload data and telemetry. The satellite 300 is composed of a body 310, solar panels 350, and "wings" 360. The satellite 100 further comprises an attitude determination and control system "ADCS" unit, which may be provided with one or more reaction wheels, one of which is indicated at 370. The ADCS unit is further described with reference to FIG. 5. The reaction wheels 370 apply torque forces to the satellite body 310. The ADCS unit is used to control the satellite as it moves in orbit, i.e. to guide and maintain the satellite 300 in a desired direction.

[0066] A satellite, such as satellite 300 in FIG. 4, is typically equipped with a propulsion system 390 for steering the satellite with the thrust generated. The propulsion system 390 shown in FIG. 3 is mounted on the body 310 on the opposite side of the solar panels 350. The propulsion system 390 comprises a number of thrusters 305. Four thrusters 305 are shown in the example of FIG. 4 and are configured to generate thrust to steer the satellite as needed. The thrusters 305 are typically operated to keep the satellite 300 in a particular orbit. For example, the thrusters 305 may be used to propel the satellite 300 in a particular direction relative to the surface of the Earth.

[0067] The satellite shown in FIG. 4 may be a microsatellite or small satellite, and due to its small size and high agility, the entire satellite may be manipulated to change its attitude. This type of manipulation may be performed using an ADCS unit. In one example, the satellite 300 may be a microsatellite with a mass of 100 kg. Regular satellites with a mass of about 1000 kg are generally more expensive and less agile than microsatellites. Satellites may be classified according to their mass. For example, satellites with a mass of about 1 kg to about 10 kg may be classified as cube satellites, satellites with a mass of about 50 kg to about 250 kg may be classified as microsatellites, satellites with a mass of about 500 kg may be classified as small satellites, and satellites with a mass of about 800 kg to about 1200 kg may be classified as regular satellites.

[0068] Larger satellites may offer the opportunity to implement a numerical method called an "orbital filter" onboard the satellite, but for smaller satellites, such as nanosatellites, it may not be feasible to implement an orbital filter that gives satisfactory results. For small and inexpensive satellites, design options are limited by weight and size limitations, as well as the use of commercially available off-the-shelf components.

[0069] A partial perspective view of a satellite is shown in Figure 5. The ADCS 302 is typically located within the satellite body 310 and is used to control the orientation of the satellite. The components of the ADCS 302 unit are described with reference to Figure 5.

[0070] The ADCS unit 302 consists of a set of three reaction wheels 370a, 370b, 370c located within the satellite body 310. The reaction wheels are also called momentum wheels. The reaction wheels 370a, 370b, 370c are controlled by the ADCS controller 341. The reaction wheels 370a, 370b, 370c work by using electric motors to rotate wheels within the spacecraft body 310. Due to conservation of angular momentum, simply rotating the wheels in one direction will cause the spacecraft to rotate in the opposite direction. The use of reaction wheels is a well-known method for orienting a spacecraft such as a satellite. In this example, three reaction wheels 370a, 370b, 370c are provided, one for orienting the satellite 300 on each axis. The reaction wheels 370a, 370b, 370c are shown with orthogonal axes. In another example, four or more reaction wheels may be used to better control various aspects of the satellite dynamics, such as slew rate and fine positioning control, especially for satellites with large moments of inertia.

[0071] The ADCS unit shown in Fig. 5 further includes torque rods 305a, 305b, and 305c. The torque rods are also used for satellite attitude control. The torque rods 305a, 305b, and 305c are typically operated to maintain the satellite 300 in a particular attitude, and the operation is controlled by an ADCS controller 341, which will be described below.

[0072] The ADCS unit 302 further includes an ADCS controller 341. The ADCS controller 341 communicates with an on-board computing system 340. The on-board computing system 340 is comprised of a processor 349, a memory 348, and a telemetry unit 345. The memory 348 can be used to store auxiliary data (similar to the memory 311 in FIG. 2). The telemetry unit 345 can be configured to transmit telemetry data, including tracking data, to a ground station, such as the ground station 201 in FIG. 2, upon each pass over the ground station. The ADCS controller 341 is further configured to receive information from one or more sensors 347. The one or more sensors 347 are configured to measure various quantities during the satellite's flight, such as a sun sensor and a magnetometer for measuring local magnetic fields. The ADCS controller 341 further communicates with a GPS receiver module, including a GPS receiver 352 and a GPS antenna 353. The GPS receiver module can be a commercially available GPS receiver.

[0073] To properly control the satellite 300 moving in Earth orbit, the ADCS controller needs to know the actual position of the satellite as accurately as possible. For this purpose, the ADCS controller 341 further executes orbit propagator software, also called (internal) orbit propagator. The ADCS controller 341 including the orbit propagator can be a commercially available off-the-shelf product. If the GPS module is functional, the ADCS controller 341 is configured to calculate the current position and velocity based on the latest GPS measurements received from the GPS receiver 352. While the GPS module maintains lock, the GPS measurements are typically repeated every 30 seconds. Therefore, the internal orbit propagator is repeatedly initialized with new GPS measurements and continues to perform numerical propagation based on the GPS measurements. If the GPS module stops working for any reason, either temporarily or permanently, as in the example of FIG. 3, the on-board computing system 340 selects ephemeris data points from the aiding data stored in the memory 348 according to the on-board time of the satellite. In one example, the aiding data consists of future ephemeris data predicted every 5 minutes. When the on-board computing system 340 receives a notification from the ADCS controller 341 that the GPS is not functioning, it selects the most recent ephemeris data based on the on-board time and provides it to the ADCS controller 341. The ADCS controller 341 then calculates the current position and velocity based on the ephemeris data received from the on-board computing system 341 and numerically propagates the orbit information until it receives the next update. The next update can be either a GPS measurement if the GPS module reacquires lock, or the next predicted ephemeris data point in the aiding data. In the latter case, the internal orbit propagator is initialized with new predicted ephemeris data every 5 minutes or whenever a new ephemeris data point is available.

[0074] As an example, the certainty of the future predicted ephemeris and the results determined by the internal orbit propagator based on the uploaded aiding data was tested by simulation. For this, it is assumed that the filtered GPS receiver measurements are significantly more accurate than the predictions and can be considered as the "true" state when compared to the results. The results of the simulation are explained with reference to Figures 6-9.

[0075] Figure 6 shows a plot of the position difference between the position data determined by the on-board orbit propagator based on aiding data (on-board information about the satellite's position) and the actual orbit (filtered tracking data, such as filtered GPS measurements). With respect to Figure 6, the increments on the y-axis correspond to 20 meters and the increments on the x-axis correspond to 2 hours. To evaluate the deviation between the on-board knowledge based on aiding data and the actual orbit, the GPS lock was released for at least 12 hours. As shown in Figure 6, once the ADCS internal orbit propagator is initialized with aiding data generated according to the method described with reference to Figure 1, the on-board information of the satellite becomes very accurate for about 15 hours. Although it was found that the peak deviations increased during this period, the deviations in position still do not exceed 80 meters, which is within the acceptable range for many payload requirements. The position error of 80 meters is well below the 500 meter requirement specified for SAR imaging. Without aiding data, the deviations could be much larger, such as 6000 meters, in a similar time frame. The aiding data gives the satellites sufficient on-board knowledge of their position to continue their mission even if GPS is out of service.

[0076] Figure 7 shows a graph comparing the predicted future ephemeris data obtained by the ODTK with the actual orbit measured by the on-board GPS receiver and filtered to reduce noise by the ODTK. Lines (a) and (b) in the figure correspond to the radial and cross-track directions, respectively. From Figure 7, it can be seen that the deviations in both directions are negligible over the entire 24-hour prediction. Line (c) corresponds to the in-orbit direction. As expected, the errors in the satellite's orbit become slightly more noticeable. From Figure 7, it can be seen that the deviations in the heading direction are small (less than 100 meters) over at least 12 hours and remain within about 400 meters over the entire 24-hour prediction. Thus, the predicted future ephemeris data provides a very high accuracy for at least the first 12 hours and an acceptable accuracy for the entire 24-hour prediction.

[0077] The results of the example orbit determination algorithm have been evaluated by the following simulations, as described with reference to FIGS.

[0078] Figure 8 is a graph showing the measurement residual ratios of multiple GPS measurements over a 24-hour period (of which approximately 17-18 hours are shown in Figure 8). As can be seen in Figure 8, there are at least two gaps in the GPS data that are longer than an hour (the first gap is from approximately 9:30 to 11:30, and the second gap is from approximately 21:30 to 23:30). The gaps may correspond to outages in the GPS tracking of the satellite onboard. In Figure 8, data points of different shapes and styles represent the x, y, and z components of the position and velocity vectors, respectively. Measurement residual ratios that fall outside the 3-sigma limits (boundaries in Figure 8) are not considered accurate measurements and are ignored.

[0079] Figure 9 is a graph showing the position consistency statistics (test statistics) of the orbit determination results based on GPS measurements shown in Figure 8. The consistency test is used to evaluate the quality of the orbit determination. As can be seen from Figure 9, even during the time periods corresponding to GPS outages (such as between about 9:30 and 11:30 and between about 21:30 and 23:30 as shown in Figure 8), the estimation algorithm works without actual measurements. Therefore, the impact of GPS outages on the orbit estimation is not significant. In Figure 9, line (a) corresponds to the in-track direction, line (b) to the radial direction, and line (c) to the cross-track direction. Note that the deviation in the cross-track direction is negligible over the entire 18-hour estimation. The deviations in the radial and heading directions are slightly more noticeable, but are stable over the 18-hour propagation time, despite being generated from GPS measurements with gaps of more than 2 hours. Based on the test statistics shown in Figure 9, little difference is observed between the GPS and non-GPS measurement periods. Because the orbit determination is the basis for the uploaded aiding data, which includes predicted future ephemeris data, these predictions can be assumed to be accurate even when the tracking data contains gaps due to GPS outages. This demonstrates the accuracy of the orbit determination and the resulting aiding data, even when based on an imperfect record of actual GPS measurements.

[0080] For clarity, the above description has described embodiments of the invention with reference to a single user, it should be understood that in practice the system may be shared by multiple users and may even be shared by many users simultaneously.

[0081] The above embodiments are fully automatic. In some instances, a user or operator of the system may manually indicate some steps of the method to be performed.

[0082] In the embodiments described in the present invention, the system may be implemented as any form of computing and / or electronic device. Such devices may include one or more processors that are microprocessors, controllers, or other suitable types of processors that process computer-executable instructions to control the operation of the device to collect and record routing information. In some examples, such as when a system-on-chip architecture is used, the processor may include one or more fixed function blocks (also called accelerators) that implement parts of the method in hardware (rather than software or firmware). Platform software, including an operating system or other suitable platform software, may be provided to the computing-based device to enable application software to be executed on the device.

[0083] Various functions described herein may be implemented in hardware, software, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over a computer-readable medium as one or more instructions or code. A computer-readable medium may include, for example, a computer-readable storage medium. A computer-readable storage medium may include volatile or non-volatile, removable or non-removable media implemented in any manner or technology for storing information such as computer-readable instructions, data structures, program modules, or other data. A computer-readable storage medium may be any available storage medium accessible by a computer. By way of example and not limitation, such computer-readable storage media may include RAM, ROM, EEPROM, flash memory or other storage devices, CD-ROM or other optical disk storage devices, magnetic disk storage devices or other magnetic storage devices, or any other medium used to carry or store desired program code in the form of instructions or data structures and accessible by a computer. Optical disk and disk as used herein include optical disks (CDs), laser disks, optical disks, digital versatile disks (DVDs), floppy disks, and Blu-ray disks (BDs). Also, propagated signals are not included within the scope of computer-readable storage media. Computer-readable media also includes communication media, including any medium that facilitates the transmission of a computer program from one place to another. A connection may be, for example, a communication medium. For example, if the software is transmitted from a website, server, or other remote source using coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave, it is included in the definition of communication media. Combinations of the above should also be included within the scope of computer-readable media.

[0084] Additionally or alternatively, the functions described herein may be performed at least in part by one or more hardware logic components. For example, hardware logic components that may be used may include field programmable gate arrays (FPGAs), application specific integrated circuits (ASICs), application specific standard products (ASSPs), systems on a chip (SOCs), complex programmable logic devices (CPLDs), etc.

[0085] Although illustrated as a single system, it should be understood that the computing device may be a distributed system, so that, for example, multiple devices may communicate over a network connection and jointly perform tasks described as being performed by the computing device.

[0086] Although illustrated as a local device, it should be appreciated that the computing device may be located remotely and accessed via a network or other communications link (eg, using a communications interface).

[0087] It is understood that the benefits and advantages described above may relate to one or several embodiments. The embodiments are not limited to those that solve any or all of the problems mentioned or that have the benefits and advantages mentioned. Variations are to be considered within the scope of the present invention.

[0088] A reference to "a" or "an" item refers to one or more of that item. The term "comprising" is used herein to mean including the identified method steps or elements, but these steps or elements do not include an exclusive list and the method or device may include additional steps or elements.

[0089] As used herein, the terms "component" and "system" are intended to include a computer-readable data store comprised of computer-implementable instructions that, when executed by a processor, cause a particular function to be performed. Computer-executable instructions may include routines, functions, etc. It should also be understood that a component or system may be located on a single device or distributed across multiple devices.

[0090] Moreover, as used herein, the word "exemplary" is intended to mean "serving as an example or example of something."

[0091] The accompanying figures illustrate exemplary methodologies. Although the methodologies are shown and described as a series of operations performed in a particular order, it is understood and should be understood that the methodologies are not limited by the order. For example, some operations may occur in a different order than described herein. Furthermore, some acts may occur simultaneously with other acts. Furthermore, in some cases, not all acts may be required to perform the methodologies described herein.

[0092] Additionally, the acts described herein may include computer-executable instructions implemented by one or more processors and / or stored on a computer-readable medium. Computer-executable instructions include routines, subroutines, programs, threads of execution, etc. Additionally, the results of the operations of these methods may be stored in a computer-readable medium, displayed on a display device, and / or displayed on a similar device.

[0093] Although the ordering of steps of the methods described herein is exemplary, the steps may be performed in any suitable order, or simultaneously where appropriate. Furthermore, steps may be added or substituted to any method, or single steps may be deleted from any method, without departing from the scope of the subject matter described herein. Aspects of any of the examples described above may be combined with aspects of any other example to form further examples without losing the desired effect.

[0094] The above description of the preferred embodiment is given by way of example only, and it should be understood that various modifications may be made by those skilled in the art. What has been described above comprises one example of one or more embodiments. Of course, it is not possible to describe all possible modifications and variations of the above-described apparatus or method for purposes of describing the above aspects, but one skilled in the art will recognize that many more modifications and combinations of the various aspects are possible. Accordingly, the described aspects are intended to include all such modifications, variations, and variations that fall within the scope of the appended claims.

Claims

1. 1. A method of generating assistance data for controlling a satellite orbiting the Earth, comprising the steps of: receiving tracking data for the satellite; applying an orbit determination algorithm, estimating a satellite's orbit based on the tracking data; and predicting future ephemeris data for the satellite based on the estimated orbit; generating assistance data including the predicted future ephemeris data; transmitting assistance data to the satellite for use in attitude and orbit control of the satellite.

2. The method of claim 1 , wherein the tracking data comprises Global Navigation Satellite System "GNSS" sensor data, Satellite Laser Ranging "SLR" measurements, or satellite radar measurements.

3. The method of claim 1 or 2, wherein the tracking data indicates a series of times and corresponding positions and velocities of the satellites.

4. 2. The method according to claim 1, further comprising determining a current position of the satellite based on the aiding data if GNSS or GPS sensor data on board the satellite is unavailable or unreliable.

5. 2. The method according to any one of the preceding claims, wherein at least the steps of applying the orbit determination algorithm and generating the assistance data are performed on Earth.

6. 2. The method of claim 1, wherein estimating the orbit of the satellite is further based on a previously estimated orbit.

7. 10. A method according to any one of the preceding claims, wherein the tracking data corresponds to a predetermined time interval, optionally corresponding to the time between two ground station passes of the satellite.

8. The method of claim 7 , wherein the tracking data includes one or more gaps corresponding to one or more tracking cessations during a predetermined time interval.

9. 10. The method according to any one of the preceding claims, wherein the method is repeated for each pass of the satellite, and optionally the tracking data is received for each pass of the satellite and the generated assistance data is transmitted for each successive pass of the satellite.

10. 10. A method according to any one of the preceding claims, wherein the method is repeated for a second satellite, and optionally for multiple satellites in parallel.

11. 2. The method according to any one of the preceding claims, wherein the auxiliary data comprises predicted future ephemeris at time intervals of up to 1 minute, optionally 2 minutes, and further optionally 5 minutes, over a period of at least 6 hours, optionally at least 12 hours, and further optionally at least 24 hours.

12. 2. A method according to any one of the preceding claims, wherein the assistance data comprises predicted future ephemeris covering at least one orbital period of the satellite.

13. 2. The method of any one of the preceding claims, further comprising the step of verifying the accuracy of the results of the orbit determination algorithm before generating and transmitting the assistance data.

14. 2. A method according to any one of the preceding claims, wherein estimating an orbit comprises filtering the tracking data to provide filtered ephemeris for the satellites.

15. The method of claim 14 , further comprising sequentially processing the tracking data using a Kalman filter.

16. 16. A method according to claim 14 or 15, wherein predicting future ephemeris data for the satellite comprises propagating forwards in time from the filtered ephemeris.

17. 2. A method according to any one of the preceding claims, wherein a dynamic model is used to predict future ephemeris data of said satellites.

18. 13. The method of any one of the preceding claims, wherein the method is computer-implemented.

19. 13. A ground segment for generating assistance data for controlling a satellite orbiting the Earth, the ground segment comprising a ground station system configured to perform the method of any of the preceding claims.

20. 1. A ground segment for generating assistance data for controlling a satellite moving in an earth orbit, said ground segment comprising: a receiving module configured to receive tracking data for the satellite; estimating the orbit of the satellite based on the tracking data; and predicting future ephemeris data for the satellite based on the estimated orbit; an orbit determination toolkit "ODTK" configured to apply an orbit determination algorithm; an assistance data module configured to generate assistance data including the predicted future ephemeris data; a transmission module configured to transmit assistance data to the satellite for use by an Attitude Determination and Control System "ADCS" unit of the satellite.

21. 21. The ground segment of claim 20, wherein the ground segment components are distributed across multiple locations, optionally different ground locations.

22. Use of a ground segment according to claims 19 to 21 for generating assistance data for controlling one or more satellites moving in orbit around the Earth, each of said one or more satellites being configured to determine its position based on said assistance data.

23. A satellite moving in Earth orbit, A receiving module configured to receive assistance data generated for a satellite according to a method according to any one of claims 1 to 18; an attitude determination and control system "ADCS" unit for controlling the satellite in orbit, said ADCS unit configured to determine a current position of the satellite based on said assistance data.

24. an on-board computer configured to select the most recent predicted ephemeris data from the assistance data based on a current on-board time of the satellite; 24. The satellite of claim 23, further comprising: an orbit propagator forming part of said ADCS unit and configured to numerically propagate predicted ephemeris data selected based on said current on-board time to determine a current position of said satellite.

25. further comprising a tracking module for tracking a satellite, optionally said tracking module being a GPS sensor module; 25. A satellite as claimed in claim 23 or 24, wherein the ADCS unit is further configured to determine a current position of the satellite based on sensor data of the tracking module, and wherein if the tracking module is not functioning, the ADCS unit is configured to switch from the sensor data to the assistance data.

26. The satellite according to any one of claims 23 to 25, wherein the satellite is a microsatellite and / or a radar satellite for earth observation.

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