RESOURCE MANAGEMENT FOR SATELLITE-BASED OBSERVATION

Abstract

An observation system for observing a region of interest. The observation system has multiple mobile sensor carrier platforms and a resource allocation unit. The mobile sensor carrier platforms may be configured as satellites having a sensor signal emitter and/or a sensor signal receiver, for example. The resource allocation unit is configured to assign tasks to the sensor carrier platforms on the basis of various criteria in order to improve the efficiency for carrying out the tasks and the completion rate.

Claims

1. An observation system for observing a region of interest, comprising: a plurality of mobile sensor carrier platforms; and a resource allocation unit; wherein a first group of sensor carrier platforms from the plurality of mobile sensor carrier platforms contains a sensor arrangement having a sensor signal emitter; wherein a second group of sensor carrier platforms from the plurality of mobile sensor carrier platforms contains a sensor arrangement having a sensor signal receiver; wherein the observation system is configured to have the plurality of mobile sensor carrier platforms operated in at least one of three modes of operation at a predetermined instant; wherein each mobile sensor carrier platform is configured: to statically observe the region of interest in a first mode of operation; to dynamically move the region of interest in a second mode of operation, according to at least one object to be observed; to emit sensor signals into the region of interest in a third mode of operation, and reflections of the sensor signals are able to be received by at least one receiver that is spatially separate from the plurality of mobile sensor carrier platforms; wherein the resource allocation unit is configured to assign at least one mobile sensor carrier platform to a task linked to one of the three modes of operation, on a basis of one or more criteria of: relative position between the mobile sensor carrier platform and the region of interest, direction of movement of the sensor carrier platform, direction of movement of an object to be observed in the region of interest, number of sensor carrier platforms with line of sight to the region of interest, attitude of the sensor carrier platform, alignment of the sensor signal emitter and the sensor signal receiver, available resources of the sensor carrier platform for observation tasks, priority of the observation task.

2. The observation system of claim 1, wherein the resource allocation unit is configured to ascertain a total number of sensor carrier platforms with a line of sight to the region of interest of the task; wherein the resource allocation unit is configured so as, given multiple tasks, to initially make an assignment according to availability and to assign a sensor carrier platform to a task that has a lowest number of sensor carrier platforms with line of sight to a relevant region of interest.

3. The observation system of claim 2, wherein the resource allocation unit is configured to follow the assignment according to availability by making an assignment according to priority for the tasks, which involves initially assigning tasks with higher priority to sensor carrier platforms.

4. The observation system of claim 3, wherein further tasks are assigned to sensor carrier platforms according to priority in descending order.

5. The observation system of claim 1, wherein the resource allocation unit is configured to reassign a task to a mobile sensor carrier platform at regular or irregular intervals of time.

6. The observation system of claim 5, wherein the resource allocation unit is configured to reassign a task to a mobile sensor carrier platform when the resource allocation unit receives a further task.

7. The observation system of claim 1, wherein the resource allocation unit is configured to produce a command for adapting the attitude of a sensor carrier platform and to transmit the command to the sensor carrier platform.

8. The observation system of claim 1, wherein the resource allocation unit is configured to produce a command for adapting the alignment of the sensor signal emitter and/or the sensor signal receiver and to transmit the command to a sensor carrier platform.

9. The observation system of claim 1, wherein the plurality of mobile sensor carrier platforms is a plurality of satellites in an Earth orbit.

10. The observation system of claim 9, wherein the region of interest is observed in the first mode of operation and/or the second mode of operation by a first sensor carrier platform emitting a signal and the first sensor carrier platform and/or at least one other sensor carrier platform receiving the signals reflected by an observed object.

11. The observation system of claim 9, wherein a sensor carrier platform is configured to observe the region of interest in the first mode of operation and/or the second mode of operation on a basis of a timestamp of acquired data, with a result that the sensor signal emitter emits radar signals into an area in a region of interest whose observation data have an oldest timestamp in the region of interest.

12. The observation system of claim 9, wherein the region of interest is observed in the third mode of operation by a sensor carrier platform emitting a signal and the receiver being arranged aboard an aircraft that is in air and receiving signals reflected by an observed object.

13. The observation system of claim 1, wherein the resource allocation unit is arranged spatially separately from the plurality of mobile sensor carrier platforms; or wherein the resource allocation unit is structurally associated with a mobile sensor carrier platform.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Some details are described in more detail below with reference to the accompanying drawings. The representations are schematic and not to scale. Identical reference signs refer to identical or similar elements. In the drawings:

[0051] FIG. 1 shows a schematic representation of a satellite in an Earth orbit;

[0052] FIG. 2 shows a schematic representation of a satellite;

[0053] FIG. 3 shows a schematic representation of an observation system;

[0054] FIG. 4 shows a schematic representation of a satellite in an Earth orbit;

[0055] FIG. 5 shows a schematic representation of Earth's surface with two regions of interest;

[0056] FIG. 6 shows a schematic representation of different modes of operation of a satellite-based observation system.

DETAILED DESCRIPTION

[0057] FIG. 1 shows a satellite 100, which is equipped as a sensor carrier platform, in an orbit 14 and Earth 10. The same orbit may contain multiple satellites, which revolve around Earth in the same orbital plane. Depending on the position of a satellite in the orbit 14, different areas of Earth's surface can be seen.

[0058] The satellite 100 has line of sight to a point or area 16 on Earth's surface or in the atmosphere above Earth's surface if a straight line can be drawn from the satellite 100 to the area 16. If line of sight exists to the area 16, a satellite can observe the area 16. Generally, line of sight exists between a satellite and an area 16 on Earth's surface if the satellite is above the horizon line 12.

[0059] A satellite constellation contains a plurality of satellites that revolve around Earth in different orbital planes. Each orbital plane contains multiple satellites normally at the same relative distance from one another. It is therefore possible for almost every area on Earth's surface to be observed by at least one satellite almost at every instant.

[0060] As is not difficult to see from the schematic representation in FIG. 1, not every satellite is suitable for every observation task at every instant. Rather, at least the sight line criterion needs to be satisfied.

[0061] A resource allocation unit 200 is shown in FIG. 1. The resource allocation unit 200 is in data communication with some or multiple satellites in order to transmit and assign tasks to the satellites. The resource allocation unit 200 takes into consideration the parameters cited earlier on for this assignment.

[0062] FIG. 2 schematically shows the design of a satellite 100 with its relevant functional units. The satellite 100 contains a control unit 105, a data transmission interface 110, a sensor signal emitter 120, a sensor signal receiver 130 and a propulsion unit 140.

[0063] The data transmission interface 110 is designed to transmit data to other satellites and/or the resource allocation unit, or to receive data. The data transmission interface is designed in particular for wireless communication, for example using optical signals or radiofrequency signals.

[0064] The sensor signal emitter 120 is designed to emit radar signals in order to detect objects in the observed region of interest. By way of example, the sensor signal emitter 120 is an antenna or an antenna array, and can be controlled electronically. The sensor signal emitter 120 may be movably mounted in or on the satellite 100 using a suspension 122 in order to adapt the radiating direction and/or radiation characteristic of the sensor signal emitter 120, or may align itself with the entire satellite as a result of fixed suspension. By way of example, the suspension 122 can move the sensor signal emitter about at least one axis, preferably about two or three axes that are perpendicular to one another, in order to steer the sensor signal emitter in a desired direction.

[0065] The sensor signal receiver 130 is the counterpart of the sensor signal emitter 120. The sensor signal receiver 130 receives signals that the sensor signal emitter has emitted and that have been reflected by an object in the region of interest. The sensor signal receiver thus receives the reflected radar signals in order to take them as a basis for tracking an object and the movement thereof in the region of interest. Like the sensor signal emitter 120, the sensor signal receiver 130 may be movably mounted in or on the satellite 100 using a suspension 132, or may align itself with the entire satellite as a result of fixed suspension.

[0066] The sensor signal emitter 120 and the sensor signal receiver 130 may be arranged in or on the satellite 100 structurally separately from one another. In this case, the sensor signal emitter and the sensor signal receiver can be moved and pointed at a region of interest independently of one another. However, it is also conceivable for the sensor signal emitter 120 and the sensor signal receiver 130 to be movably arranged in or on the satellite 100 by way of a single suspension. In this case, the sensor signal emitter and the sensor signal receiver are always aligned in the same direction.

[0067] By way of example, the suspension 122, 132 may be a cardanic suspension provided with one or more drives in order to produce the desired movement.

[0068] A propulsion unit 140 is arranged in order to produce a necessary or desired movement for the satellite 100. The propulsion unit 140 can be used to align the satellite 100 in a desired direction.

[0069] A control unit 105 is arranged and designed to configure and actuate the functional units of the satellite 100. By way of example, the control unit 105 ascertains or receives a control command for the propulsion unit 140 via the data transmission interface 110. The control unit 105 then passes the necessary commands to the propulsion unit 140 so that an appropriate drive force is produced. Similarly, the control unit 105 can intercept control commands for aligning the sensor signal emitter 120 and the sensor signal receiver 130 or can determine the control commands on the basis of an observation task and can actuate the suspensions 122, 132 as appropriate. Depending on the relative position and/or alignment of a satellite in relation to the region of interest to be observed, it may be necessary not only to align the sensor signal emitter and/or the sensor signal receiver but also to alter the alignment of the satellite as a whole. Accordingly, the control unit then produces the necessary control commands on the basis of the assigned task.

[0070] FIG. 3 shows a schematic representation of an observation system 50. The observation system 50 contains multiple satellites 100A, 100B, . . . , 100n and a resource allocation unit 200. The number of satellites in the observation system 50 is not limited to a specific number, but rather multiple satellites in one or more orbital planes of a satellite constellation may be part of the observation system 50. Similarly, two or more resource allocation units 200 can be used to assign tasks to multiple satellites. In this case, the two or more resource allocation units may be connected to one another via a mobile radio network or a line-based network in order to be synchronized with one another and/or to assign the tasks to all of the satellites in a coordinated manner.

[0071] The resource allocation unit 200 has a data transmission interface 210. The data transmission interface 210 is designed to set up a data connection to the data transmission interface 110 of each individual satellite 100, with the result that data can be interchanged between the resource allocation unit and each individual satellite.

[0072] Each satellite 100, which can also be referred to generally as a sensor carrier platform, can be operated in at least one of three modes of operation, specifically the aforementioned modes of operation. These modes of operation can be freely selected on the basis of a task for the satellite. It is also possible for two modes of operation to be carried out in parallel, however, by virtue of the control unit of a satellite actuating the sensor signal receiver as appropriate.

[0073] The observation system 50 is designed to react flexibly to the requests of users. Almost any combinations of different observation modes (modes of operation) need to be produced in parallel from time to time. Surface areas and geometries of the region of interest in the first mode of operation may be freely selectable in order to provide maximum flexibility. The satellites that are in a satellite constellation (which can be referred to as transmit and receive satellites) are optimally employed with regard to different modes of operation and assigned to tasks by the resource allocation unit in accordance with the user requests. This assignment is made dynamically, for example, which means that each new user request also results in a different sensor combination being selected for carrying out the different modes of operationpossibly with a different prioritization than before the new request.

[0074] This technical challenge is met by applying a specific resource management in particular for the sensor signal emitters and sensor signal receivers. This sensor management can be performed using the resource allocation unit either on the ground or in dedicated satellites, which may be a part of the constellation and, as so-called processing nodes, may also carry out data processing steps. These processing node satellites or the ground stations that undertake management of the individual sensors should have direct contact, e.g. via optical links, with the satellites that need to be coordinated, in order to undertake the assigned tasks in a specific region.

[0075] The modes of operation can be activated either simultaneously or in succession, specifically depending on the current situational requirements of the users. Priorities can be assigned to the modes of operation and/or the requests. In addition, the position of the satellites in the orbit with respect to the region of interest and also the flight attitude are taken into consideration for ascertaining the suitability of all satellites to support the different modes of operation, followed by assignment of the satellites to the different tasks.

[0076] In the first mode of operation, objects to be tracked in a large area are detected. This involves using exclusively sensors of satellites, specifically the sensor signal emitter and the sensor signal receiver of the same satellite or of different satellites.

[0077] In the second mode of operation, a specific object to be tracked or a group of objects is tracked. A higher update frequency can be used for this tracking than in the first mode of operation. Exclusively sensors of satellites are used in the second mode of operation too.

[0078] The information acquired in the first and second modes of operation can be transmitted to a ground station or any receiver via a data connection. The receiver may be arranged outside the sensor carrier platform or may be part or a functional module of the sensor carrier platform that has acquired the information. In the latter case, the sensor carrier platform thus contains processing capacities, and performs the function of an aforementioned processing node. All data received by this processing node can then be collectively evaluated in order to ascertain the position of one or more objects to be tracked. The sensor carrier platform that contains the processing node can also contain the resource allocation unit.

[0079] In the third mode of operation, a satellite or multiple satellites provide/s a radar signal, the reflections of which from objects to be tracked are acquired and evaluated by other aircraft in order to draw conclusions about the position of the objects to be tracked.

[0080] FIG. 4 shows a satellite 100 with reference to Earth 10. Areas 150, 152 of different size can be observed with reference to Earth's surface 18. The first area 150 is larger than the second area 152. The first area 150 is that area to which the satellite 100 can have a line of sight and that the satellite 100 can observe according to the attitude control of the satellite. From time to time, the second area 152 is distinctly smaller than the first area 150 and can be observed by adjusting the sensor signal emitter and the sensor signal receiver.

[0081] User requests are converted into tasks in the observation system 50. This conversion process can be carried out by the resource allocation unit, for example. A task contains all the necessary information to attain the desired operational response of the observation system. In particular, a task contains the following information: coordinates and extent of the region of interest and also mode of operation in which the region of interest is intended to be observed. In addition to the aforementioned characterizing features of a mode of operation, other demands may also be made on the observation system, such as for example: the operational demands on the system (response: e.g. sequential coverage of large regions or target tracking, activation time t.sub.Ak: permissible delay for providing a resource for the task, availability period tv: minimum period for which the resources must be available after the activation time (e.g. has the effect of a restriction if a satellite will shortly reach the horizon)), the performance requirements to be met, e.g. position resolution, status of the task: active or inactive. This permits predefined start and end times to be taken as a basis for not beginning the task at the next possible instant, but rather performing it at a planned time and possibly repeatedly, e.g. in the case of routine tasks, priority, for resource allocation in the event of conflicts, and optionally the quantity of resources needed.

[0082] The system supports the simultaneous performance of different tasks and is therefore capable of monitoring large regions for flight movements and at the same time tracking single targets or target groups (e.g. pairs of fighter planes) with high accuracy and at a high update rate within the large region (Track While Scan). For this purpose, the system permits resources (satellites or sensor time) to be allocated flexibly to different tasks. The allocation can be made automatically, on the basis of a rating of the suitability of all satellites for the respective tasks, and/or on the basis of the priorities of the respective tasks.

[0083] An illustrative method for allocating resources that is able to be implemented by the resource allocation unit may be designed as follows: the assignment of the satellites to the tasks is determined for a given instant and renewed at suitable intervals of time in order to take account of the changes in the satellite positions relative to the region of interest, or in the visibility. Furthermore, the assignment can be renewed at any time in the event of changes to the task profile, or to the priorities. Flexible transitions between different groups of tasks and modes of operation are therefore possible.

[0084] By way of example, all satellites are reassigned at the beginning, when the priorities of tasks change or when active tasks are added. The assignments are updated for example provided that previously active tasks have ended, or become inactive.

[0085] All currently available satellites (not in maintenance mode or the like) are determined for the reassignment. Next, a test of suitability for all active tasks is carried out for each available satellite on the basis of the following conditions: the region of interest is within the horizon (visibility) of the satellite; the angle of incidence ? with respect to the reference coordinate of the region of interest (angle between the vertical and the direction of the satellite) is within a predetermined range ?.sub.min, ?.sub.max (e.g. 0.80 degrees), the satellite can contribute to the task within the maximum activation time (includes satellites that are currently already contributing to this task), the satellite complies with the minimum availability period in accordance with the orbit prediction.

[0086] Satellites for which the suitability test has yielded only one task are assigned to the respective task. Satellites that would be suitable for multiple tasks are assigned to the associated possible tasks by way of conflict resolution using an iterative scheme, as follows:

[0087] Initially, all tasks still need to be attended to, expressed by way of binary values b.sub.k=1. [0088] 1. A resource component is determined for all tasks

[00001]$r_k=\left(P_k{b}_k\right)/{.Math.}_k\left(P_k{b}_k\right)$ [0089] 2. From the number M.sub.k of resources suitable for the current task k, the current task is allocated the first N.sub.k, where

[00002]$N_k=\mathrm{Min}\left(\mathrm{round}\left(r_k{M}_k\right);?L_k\right)$

[0090] where ?L.sub.k specifies the number of resources still needed before the maximum number for the respective task is reached. Note: for tasks without a maximum number, it holds that ?L.sub.k=?) [0091] 3. The task currently being processed is not considered further, i.e. b.sub.k=0 [0092] 4. The scheme is continued at 1. with the next task until all tasks with conflicts have been processed.

[0093] Example: 9 satellites are intended to be assigned that would be suitable for 3 tasks, P.sub.1=1; P.sub.2=0.5; P.sub.2=0.3, no upper limit being stipulated in each case: [0094] Task 1 is assigned Min(round(9.Math.1/1.8); ?)=5 satellites, i.e. there are still 4 satellites available. [0095] Task 2 is assigned Min (round(4.Math.0.5/0.8); ?)=3 satellites, i.e. there is still 1 satellite available. [0096] Task 3 is assigned the remaining satellite.

[0097] For the purpose of updating the assignments, (1.) satellites that continue to satisfy the respective conditions remain assigned to their previous tasks (minimizing the necessary manuevers or periods of inactivity), (2.) satellites that have not been assigned hitherto, or that are no longer suitable for the previous task, are retested in respect of suitability for the active tasks, and (3.) conflict resolution as described in the case of reassignment may be performed for the satellites that come under 2.

[0098] For a satellite to access a region of interest, it is necessary to align the antenna according to the coordinates of the region of interest and according to the orbital position of the satellite, this being able to be carried out mechanically and/or electronically in principle. In this context, it is assumed that the satellites have mechanical agility to allow pre-alignment with the specific assigned region of interest. The attitude control of the satellite maintains the alignment with the reference point despite the relative movement of the satellite, and the satellites' having electronic agility with respect to the radar sensors/antennas, to allow rapid fine alignment within the region of interest. The electronic actuation takes place according to the rules described in the sections that follow.

[0099] This combined approach limits the required electronically accessible swivel range of the antenna to a technically manageable degree, provides the high agility only where it is needed, restricts it to the region of interest, and nevertheless facilitates global coverage, as shown in FIG. 4 using the areas 150, 152.

[0100] The activation time is determined from the performance data of the attitude control (angular acceleration, angular rate), the present orientation of the satellite and the orientation required for the respective task (region of interest). The availability period is obtained on the basis of the test of future orbital positions with regard to the above conditions, minus the activation time.

[0101] The modes of operation of the satellites are explained in more detail with reference to FIG. 5.

[0102] By way of example, the observation generally targets a region of interest that, due to its size, cannot be covered using the available resources at every location at every instant. It is therefore necessary to move the antenna lobes of the sensor signal emitters over the region of interest 20 in accordance with a passage of time. The chosen method here is intended to ensure that every location in the region of interest is covered anew after as short an update time as possible.

[0103] The region of interest is scanned progressively by advancing the antenna lobes. A specific location is illuminated in each case by one or more transmitters (Tx-Multi-Beam On Target). The transmitting antenna patterns can optionally be expanded by way of beam shaping, e.g. to increase the size of the footprints for steep angles of incidence. To facilitate multilateration, detection must be successful in at least three transmission paths simultaneously. In order for this to occur with sufficient probability, digital beamforming is provided for the receiving satellites using multiple antenna lobes simultaneously. This allows every receiving satellite to receive signals from all illuminated locations in the region of interest simultaneously. Optionally, the size of the antenna lobe in the direction of reception can additionally be increased by way of digital beamforming, e.g. by generating additional antenna lobes.

[0104] The directions of the antenna lobes of the transmitters are determined at every instant using an optimization method that takes into consideration the shape of the respective antenna main lobe on the ground. The alignment of the receiving antennas follows the alignment of the transmitting antennas. The region of interest is initially divided into a grid. The current coverage of the antenna main lobes in this grid is ascertained as a binary result (grid point covered or not covered).

[0105] The lines of vision are optimized using a map of the information age of the region of interest. That is to say that for each grid point 21 (some of which are shown in the region 20 by way of illustration) in the region of interest 20 the time at which the point was picked up is stored. If the grid point is picked up at the present time, the information age is zero (seconds), whereas for example a point that was most recently picked up by an antenna lobe N seconds ago is assigned the information age N. Therefore, all grid points initially have an applicable age gain applied at all times in accordance with the difference from the previous time step, whereas at the end of the optimization in this step the points that are currently picked up are assigned the information age zero.

[0106] The optimization is carried out by a global minimum search, e.g. particle swarm, using a cost function. So that the parameter space does not become too large for the optimization, it is optionally possible to preselect N (e.g. 250) grid points with the greatest information age.

[0107] If two or more transmitters are each intended to pick up a location simultaneously (Tx-Multi-Beam On Target), groups of transmitting satellites are formed in accordance with the desired number of simultaneous antenna lobes. The selection for this is made on the basis of the greatest possible similarity of the shape of the antenna lobes. The test for similarity between two specific antenna lobes, represented in binary form in the grid, is carried out e.g. by way of logic XOR comparison. Logic ORing produces a combined antenna lobe for the group. The further optimization is then carried out on the basis of the combined antenna lobes. The new positions of the antenna lobes are obtained as an optimization by way of the cost function, which represents a weighting for the following aspects: (1) information age: the antenna lobes are intended to be aligned such that locations with great information age are preferred as far as possible. The information age of the youngest grid point within the antenna lobe therefore determines the costs, which become higher the younger the youngest point is; (2) efficiency in terms of avoiding overlapping antenna lobes. Grid points that are picked up by multiple (combined) antenna lobes result in ever higher additional costs as the overlap increases; and (3) efficient in terms of attitude within the region of interest: for antenna lobes that point in the direction of the edge of the region, the costs become ever higher the more surface area outside the region of interest is picked up.

[0108] A position in the region of interest divided into a grid is found for each (combined) antenna group as the result of the optimization. The extent and shape of the antenna lobes divided into a grid are taken as a basis for setting the information age for all grid points picked up to zero and repeating the described method in the next time step.

[0109] FIG. 5 shows how this observation by grid points 21 may appear for the region 20 in the first mode of operation. In addition, the delimited region 22 can be observed in another mode of operation.

[0110] Multiple targets (or groups of targets) can be dynamically tracked with high temporal and spatial resolution in the second mode of operation, shown by way of illustration with reference to the region 22, which is distinctly smaller than the region 20.

[0111] The region of interest 22 is illuminated (i.e. radar signals are transmitted into the region of interest) by one or optionally by multiple transmitting satellites simultaneously (Tx-Multi-Beam On Target). In order to facilitate multilateration, reception is effected by as many satellites as are required in order to have a high probability of achieving successful detection in at least three bistatic transmission paths. A distinction is drawn between two states: target(s) acquired and targets not acquired. At the beginning, the system monitors a predetermined coordinate (capture range), with which the antenna lobes of the assigned transmitters and receivers are continuously aligned for this purpose. The first target detected after the beginning of the task can be defined as a reference target in an automated manner. Alternatively, the selection can be influenced by a user. The results of the multilateration (position and speed) are taken as a basis for repositioning the antenna lobes at every instant, i.e. continually updating the directions, in such a way that the reference target remains in the center. The fluctuating target backscatter means that failures in the detection can arise in the meantime, of which the system is tolerant owing to the extent of the antenna lobe, i.e. despite movement the target remains within the antenna lobes for a certain time, even if the antenna lobes are not repositioned. In the event of a loss of detection, the system can hold the antenna lobes in the last position of successful detection. Alternatively, the system can take the last ascertained speed vector of the target as a basis for making a prediction about the expected current position of the target. Another possibility is for the system to determine the current position of the target on the basis of another mode of operation carried out in parallel. The selection of the respective strategy can be supported by the history of the previous detections (e.g. target acts in a highly agile manner, or is on a longer, steady approach).

[0112] FIG. 6 shows a summarizing schematic representation of three modes of operation. In the first or second mode of operation, the satellite 100A transmits a signal in the direction of an object 500 to be observed. The reflected radar signals are received by the same satellite 100A or a second satellite 100B. In the first or second mode of operation, only satellite-based sensors are involved in the observation. The first and second modes of operation have been described by way of illustration with reference to FIGS. 3 to 5.

[0113] In the third mode of operation, which is described in more detail by way of illustration below, the signals reflected by the tracked object 500 are reflected to a receiver 300, which is a sensor signal receiver aboard an aircraft, for example. In this third mode of operation, the satellites 100A and 100B assist an observation process in which a region of interest is observed by receivers aboard aircraft (or by ground-based receivers).

[0114] The third mode of operation facilitates passive radar operation of the aircraft by illuminating a region of interest by the transmitting satellites 100A, 100B.

[0115] The transmitting satellites 100A, 100B are equipped with an antenna and instrumentation suitable for two frequency bands so that radar signals that can be received by systems aboard aircraft are transmitted in the third mode of operation. Alternatively, two types of transmitting satellites can be used. The third mode of operation is produced by switching one or more transmitting satellites to the radar frequency of the aircraft. There is normally not provision in the satellites for a reception option for the frequency used for the third mode of operation, since this would increase the complexity of the receiving satellites without significant added value.

[0116] The observed object 500 is detected and located in the aircraft 300 separately by way of information transmitted via a data link. If an adequate number of transmitters are used, position determination can take place in the individual aircraft by way of multilateration. Since the system has no satellite-based receivers in the reception band of the aircraft radars, if there is no other information available about the required alignment of the antenna lobes then other resources of the constellation can be used to also carry out the first or second mode of operation in parallel with the third mode of operation. The detections and locations obtained for reference targets can then be taken as a basis for continually updating the directions of the antenna lobes of the transmitters in the X-band mode.

[0117] Optionally, the aircraft 300 can be assisted further by sending the detections/locations additionally obtained by the satellite constellation to the aircraft via suitable data transmission paths for assistance.

[0118] Optionally, suitable communication paths (e.g. line-of-sight laser link) can be used to transfer control information, e.g. for controlling the antenna lobes, from the aircraft to the satellites 100A, 100B of the constellation.

[0119] The observation system described here is independent of aircraft-based or ground-based sensors (this applies in all modes of operation at the transmitter; it applies to the first and second modes of operation at the receiver). A region of interest is observed from a satellite orbit. By way of example, this can involve using a multi-static radar system, the frequency range of which can be adapted for the respective application. This also allows the visibility of aircraft camouflaged against radar reconnaissance to be increased, because such aircraft are typically and predominantly protected against monostatic detection from the ground or from the air.

[0120] It should additionally be pointed out that comprising or having does not exclude other elements or steps and a(n) or one does not exclude a plurality. Furthermore, it will be pointed out that features or steps that have been described with reference to one of the exemplary embodiments above can also be used in combination with other features or steps of other exemplary embodiments described above. Reference signs in the claims should not be regarded as a limitation.

[0121] While at least one example embodiment of the present invention(s) is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the example embodiment(s). In addition, in this disclosure, the terms comprise or comprising do not exclude other elements or steps, the terms a, an or one do not exclude a plural number, and the term or means either or both. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise. This disclosure hereby incorporates by reference the complete disclosure of any patent or application from which it claims benefit or priority.

LIST OF REFERENCE SIGNS

[0122] 10 Earth [0123] 12 horizon line [0124] 14 orbit [0125] 16 area on Earth's surface or in the atmosphere [0126] 18 Earth's surface [0127] 20 region of interest for extensive observation [0128] 21 grid point [0129] 22 region of interest for detailed observation [0130] 50 observation system [0131] 100 satellite, sensor carrier platform [0132] 105 control unit [0133] 110 data transmission interface (transmit, receive) [0134] 120 sensor signal emitter, antenna, electronically controlled [0135] 122 suspension [0136] 130 sensor signal receiver, antenna [0137] 132 suspension [0138] 140 propulsion unit [0139] 150 first area, accessible by controlling attitude [0140] 152 second area, accessible by adjusting the antennas [0141] 190 data transmission link [0142] 200 resource allocation unit [0143] 210 data transmission interface [0144] 300 receiver [0145] 500 observed object