Multi-point imaging using synthetic aperture radar

Abstract

A method of operating a synthetic aperture radar (SAR) to acquire image data, comprising: steering a SAR beam in azimuth relative to a direction of travel during a first time period to acquire image data of a first area of ​​the Earth to be imaged; steering the SAR beam in azimuth during one or more additional time periods to acquire image data of one or more additional areas of the Earth to be imaged; and steering the SAR beam in azimuth in a rearward direction relative to the direction of travel during a time period including the first time period and the one or more additional time periods to reduce a speed of travel of the beam relative to the Earth.

Description

[0001] The invention belongs to the field of imaging using synthetic aperture radar. Background Art

[0002] Synthetic aperture radar (SAR) can be used to image areas on Earth by emitting radar waves and recording the returns from those transmitted beams. SAR systems can be mounted on airborne platforms such as aircraft, as well as in satellites operated from space. Various modes of operating SAR are available, such as strip chart, spotlight, and scanSAR.

[0003] In strip-map mode, the satellite uses its SAR system to image data along a swath in azimuth, or along the direction of travel relative to the Earth's surface. In scanSAR mode, multiple swaths are imaged along a swath by electronically steering the SAR beam in elevation (perpendicular to the satellite's direction of travel) to image different swaths as the satellite passes over the ground.

[0004] Classic spotlight imaging is a technique used to achieve high-resolution imaging. In this mode, the satellite beam is steered in azimuth to dwell on a point on the ground for a longer period of time than would normally be the case using strip chart or scanSAR modes. Spotlight mode can achieve high-resolution imaging by illuminating a ground station or target area on the Earth for a longer period of time and over a wider range of angles than would be possible without beam steering. However, due to the need to steer the beam over significant angles between spotlight images, conventional SAR systems require significant gaps between images.

[0005] One problem that arises is how to image multiple close points in space. Operators of SAR systems are constantly striving to improve the efficiency of image acquisition while maintaining the required resolution and accuracy, particularly in the field of satellite imaging, but also in other implementations of SAR. Since a typical spotlight mode generally only allows imaging one continuous point on the Earth at high resolution, imaging another close point would require completing another orbit (in the case of satellite-mounted SAR) and returning to the same area to image the second point at high resolution. This is expensive, inefficient, and results in delays in receiving data in a timely manner.

[0006] Some of these problems are addressed by some embodiments of the invention described below. However, the invention is not limited to solutions to these problems, and some embodiments of the invention address other problems. Summary of the Invention

[0007] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description.This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to determine the scope of the claimed subject matter.

[0008] Some embodiments of the present invention provide a satellite, ground station, satellite system, or method for processing raw SAR data, wherein the raw data is obtained using an extended dwell time. In the case of a satellite, the extended dwell time can be achieved by mechanical manipulation of the satellite.

[0009] In one aspect, a method of operating a synthetic aperture radar (SAR) to acquire image data is provided below, wherein the SAR is carried on a platform traveling relative to the surface of the Earth and is oriented toward the surface of the Earth. The method includes: steering a SAR beam in azimuth relative to the direction of travel during a first time period to acquire image data of a first area of ​​the Earth to be imaged; steering the SAR beam in azimuth during one or more additional time periods to acquire image data of one or more additional areas of the Earth to be imaged; and steering the SAR beam in azimuth in a rearward direction relative to the direction of travel during a time period including the first time period and the one or more additional time periods to reduce a speed of travel of the beam relative to the Earth.

[0010] The maneuvering during the first time period may be within a first angular range, and the maneuvering during the one or more additional time periods may be within the same angular range.The first time period and the one or more additional time periods may overlap or they may be consecutive.

[0011] The steering of the beam in azimuth may be periodic during a time period comprising the first time period and the one or more additional time periods.

[0012] The steering within the first angular range may be in a forward direction.The SAR beam may be electronically steered within the first angular range, for example using a phased array antenna.

[0013] The SAR beam can be steered in elevation between successive data acquisitions. This elevation steering can also be electronic, for example using a phased array antenna.

[0014] The SAR beam may be mechanically steered in a rearward direction, for example, by changing the orientation of the SAR relative to the platform or by changing the orientation of the platform relative to the Earth's surface.

[0015] Also provided below is a method for forming images of different regions on the Earth, comprising: receiving a request for images of a plurality of regions on the Earth; identifying a subset of the plurality of regions that are sufficiently close together to enable image data associated with those regions to be acquired during an extended dwell period over a larger area including the identified regions; and determining a sequence of maneuvering operations to be performed to enable image data associated with the subset of the plurality of regions to be acquired during the extended dwell period; and transmitting the determined sequence of maneuvering operations to a SAR control device. The SAR may then be operated according to any of the methods described herein.

[0016] Also provided is a computer-readable medium comprising instructions that, when implemented in a processor in a computing system, cause the computing system to operate a SAR according to any of the methods described herein.

[0017] There is also provided a satellite operating in orbit around the Earth according to any of the methods described herein, comprising a propulsion system, an attitude determination and control system (ADCS) configured to steer a SAR beam in a backward direction, one or more radar antennas or antenna arrays configured to steer the SAR beam in azimuth within a first angular range, a synthetic aperture radar (SAR) image data acquisition device, and a communication system configured to transmit and receive signals to and from one or more ground stations on the Earth.

[0018] Embodiments of the present invention also provide a computer-readable medium comprising instructions, for example in the form of an algorithm, which, when implemented in a computing system forming part of a satellite operating system, causes the system to perform any of the methods described herein.

[0019] Features of the different aspects and embodiments of the invention may be combined as appropriate, as will be apparent to those skilled in the art, and may be combined with any aspect of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Embodiments of the present invention will be described, by way of example only, with reference to the following drawings, in which:

[0021] Figure 1 is a schematic perspective illustration of a satellite in orbit above Earth.

[0022] FIG2 is a schematic diagram of a satellite operating in SCANSAR mode.

[0023] Figure 3 is a diagram of satellites operating to form images of multiple discrete areas on Earth.

[0024] Figure 4 is a schematic diagram showing the effect of mechanically maneuvering a satellite to slow its effective Earth speed.

[0025] Figure 5 is a series of graphs showing the superposition of mechanical control of azimuth with electronic control of azimuth and elevation.

[0026] Figure 6 It is a schematic diagram of the components of the satellite.

[0027] Figure 7 This is a partial perspective view of the satellite.

[0028] Figure 8 is a flow chart illustrating a method of forming images of different regions on the Earth according to some embodiments of the present invention.

[0029] The same reference numerals are used throughout the drawings to denote similar features. DETAILED DESCRIPTION

[0030] The following embodiments of the present invention are described by way of example only. These embodiments represent the best modes currently known to the applicant for carrying out the invention, although they are not the only modes in which the invention may be practiced.

[0031] Some embodiments of the present invention provide systems and methods for operating a SAR (Spectral Altitude Assist) system to obtain images of an area on Earth. For this purpose, the SAR system may be carried on a platform that travels relative to the Earth's surface. For example, SAR is commonly used onboard satellites. However, the methods and systems described herein are not limited to space and may be performed using an aircraft or any other suitable platform.

[0032] Some embodiments of the present invention use a combination of mechanical and electronic steering of the radar beam, as will be described further herein. Mechanical steering can be achieved by changing the orientation of the SAR antenna relative to the platform on which it is carried (e.g., a satellite or aircraft). However, for suitably agile satellites, mechanical steering can be achieved by changing the orientation of the entire platform relative to a target, such as an area on the Earth. This is described below with reference to satellites, but it will be understood that the same principles can be applied to other types of platforms.

[0033] Figure 1 1 is a perspective view of a satellite 100 in Earth orbit, an example of a platform that can be used in the methods and systems described herein. The satellite includes a body 110 and "wings" 160. One or more antennas can be mounted on the satellite's wings. Each antenna can include a phased array antenna, or in other words, multiple antenna assemblies that can be controlled to steer the direction of the antenna beam, thereby controlling the direction and shape of transmitted pulses or the direction and area from which radiation can be received. This is electronic beam steering and is well known in the art. References herein to beam steering or electronic steering are intended to refer to the control of radar beams for transmission or reception using phased array antennas.

[0034] Electron beam steering is highly precise and occurs rapidly. Therefore, it can manipulate small angles quickly. This is also known as fast small-angle electron steering. Electron beam steering can be performed in two dimensions (azimuth and elevation).

[0035] Satellite 100 can be configured to mechanically steer the antenna, and therefore the SAR beam, in addition to electronic steering. In this example, this is achieved by steering the entire satellite 100. This can be accomplished using the satellite attitude determination and control system (ADCS), which can be equipped with one or more reaction wheels, one of which is indicated by 170. Mechanical beam steering allows for wider steering angles than electronic steering, thus providing wide ground coverage, particularly for small and agile satellites that have the ability to slew quickly while consuming less power. Using this technique, even larger angles can be steered. This is also referred to as high-angle mechanical steering. In some of the methods described herein, mechanical steering can be fast enough to reduce the effective ground velocity of the radar beam, or even reduce it to near zero. However, mechanical steering is still relatively slow compared to electronic steering.

[0036] As is known in the art, a SAR is operated to periodically alternate between a transmit mode, in which pulses of radiation are steered toward the Earth's surface, and a receive mode, in which radiation reflected from the surface is received.

[0037] refer to Figure 6 and Figure 7 describe Figure 1 Further details on the satellite.

[0038] As is well known in the art, to create a SAR image, continuous pulses of radio waves are transmitted to "illuminate" the target scene, and the echoes from each pulse are received and recorded. A single beamforming antenna can be used to transmit the pulses and receive the echoes. When the SAR is mounted on a mobile platform, such as a satellite, and thus moves relative to the target, the antenna position relative to the target changes over time, and due to the Doppler effect, the frequency of the received signal varies. Signal processing of the continuously recorded radar echoes allows the recordings from multiple antenna positions to be combined, thereby forming a composite antenna aperture that allows the creation of higher-resolution images.

[0039] The area currently being captured by the SAR is the coverage area. Directions along the SAR's flight direction are often called azimuths or along-track directions. Directions transverse to the flight direction are often called ranges, elevations, or cross-track directions. Directions opposite the flight direction correspond to backward azimuth directions.

[0040] Figure 2 is a schematic diagram of satellite 100 operating in scanSAR mode. Satellite 100 is in a well-known side-looking configuration, in which it transmits and receives signals from an area to the side of the satellite, rather than directly below it. This area has a width, also known in the art as a swath. For each radar pulse transmitted from the satellite, signal data in the form of echoes may be received at different frequencies from different points along the swath due to the Doppler effect at the center of the SAR.

[0041] Satellite 100 is shown traveling from right to left relative to the Earth in its orbit, as indicated by the arrow. In scanSAR mode, the area from which data is collected, such as the 100 km × 100 km area indicated in FIG. 2 , is divided into sub-swaths, and each sub-swath is divided into what are referred to herein as "blocks" in the direction of travel, forming an offset checkerboard pattern in the example of FIG. 2 . The SAR beam can be steered in elevation to collect data from different sub-swaths. During this steer, echoes are received from different locations across the sub-swaths to provide a set of data for each square (or other shape) on the ground, such as Block A.

[0042] Electronic steering can be achieved very quickly. In a development of ScanSar, often referred to as Topography Observation with Progressive Scanning, or TOPS, after the SAR beam has been steered to the edge of block A, the SAR beam is electronically steered rapidly in elevation to an adjacent sub-swath and back in azimuth to its starting azimuth to repeat the steering in azimuth to collect data from a second block B, and so on.

[0043] Each traverse of the beam in azimuth may correspond to several hundred pulses, referred to herein as a “burst.” Thus, a burst corresponds to a block on the ground, and data is collected over the length of the swath in a burst-by-burst approach.

[0044] The resolution of an image depends on a number of factors, including the amount of data acquired to generate the image. A technique for achieving high-resolution imaging is known as spotlight mode, in which the SAR beam is steered to dwell on a specific area. However, this technique is typically used to image only one point or area. Furthermore, this dwelling results in the need to "recover" the satellite by steering it back to its original orientation before it can take another image, during which time the satellite will travel in its orbit, making it impossible to form a high-resolution image of such a dense "point". In other words, significant spatial gaps are often required between spotlight images. In order to image another point or area close to the first point, the satellite may need to complete at least one full orbit to return to the origin, which is very costly in terms of time and efficiency.

[0045] In some methods and systems, described in greater detail below, mechanical steering of the SAR beam in a backward direction is superimposed on electronic steering of the beam in a forward direction. The effect of the mechanical steering is to reduce the beam's velocity relative to the Earth as the satellite travels in its orbit. This can allow for longer burst durations and, therefore, higher-resolution images. In some implementations, mechanical steering can be used to make the beam stationary or nearly stationary relative to the ground. This backward steering, combined with electronic steering in one or both of azimuth and elevation, can allow for acquisition of higher-resolution images from multiple closely spaced points, which was previously impossible.

[0046] exist Figure 3 This mechanical maneuver is shown in exaggerated form in , where satellite 100 is shown with a different orientation at each of three positions along its path 600. Between each position, the satellite has rotated in azimuth, as shown by arrow 601.

[0047] Now refer to Figure 4 and Figure 5 Describe some examples of superimposed electronic and mechanical manipulation. Figure 4 Depicted is an area on the ground divided into smaller areas or blocks, as might be used to operate a SAR in ScanSAR mode or TOPS mode. Thus, as previously described, the SAR beam can be electronically steered to collect data from blocks 1-15 sequentially in numerical order.

[0048] To this end, the beam can be periodically steered forward in the azimuth (along the track) direction, as Figure 5 (a) shows that the beam angle is steered from a negative angle through zero to an equal positive angle over the burst period, and this process is repeated for successive bursts over the same angular range. This periodic electronic forward steering of the azimuth angle is superimposed on mechanical backward steering of the azimuth angle over a larger angular range and for a longer period. Figure 5 (c) shows a simplified example of having 16 forward electronic scans during one relatively slow backward mechanical scan. In a practical implementation, the ratio of the mechanical azimuth steering period to the electronic steering period will be at least 2:1 to allow for at least two spots for each mechanical scan. In some implementations of the methods described herein, when the beam is mechanically steered in azimuth, the electronic steering in azimuth is repeated periodically, while the mechanical steering is not repeated periodically. It should be noted that although the graphs show the electronic and mechanical steering being linear, this is not necessary, particularly for backward mechanical azimuth steering.

[0049] Figure 5The electronic steering of azimuth shown in (a) is similar to the steering performed in TOPS, where the beam is quickly returned to the starting angle between each "sweep" over a range of azimuth angles. Such rapid returns are not necessary for the methods described herein, and in some implementations the beam can be slowly steered in the reverse direction while continuing to transmit pulses, for example, at the same rate as the forward steering. Furthermore, electronic azimuth steering is not required in the forward direction. The improvement in resolution and / or spacing between points results primarily from the combination of electronic azimuth steering and mechanical steering backward relative to the direction of travel of the satellite in orbit. The electronic steering of azimuth can be periodic for the duration of the mechanical steering, but this is not required, as explained further below.

[0050] Some implementations of the imaging methods described herein can be used to image different "points," or areas or patches, within the same sub-swath. In this case, no manipulation of elevation is required. The points can be continuous, overlapping, or discontinuous. In the case of overlapping points, some data from the first point can be used to image the overlapping point, without requiring additional manipulation.

[0051] Other implementations can be used to form images of points or blocks in different sub-swaths. For example, consider a satellite receiving a request to form an image such as Figure 4 Images or image data are transmitted for areas / blocks 5 (in sub-swath 1), 8 (in sub-swath 4), and 10 (in sub-swath 2) as shown. Figure 5 The graphs of show two ways this can be achieved. There are two graphs (b1) and (b2) for two different possible implementations, where the elevation manipulation is periodic sub-swath to sub-swath (b1) or one point at a time (b2), each pointing to a block in the first, fourth and then second sub-swath (e.g., Figure 4 These are two examples of the range of possibilities for how to image each individual block, which can be determined based on what "points" need to be imaged (e.g., their positions and desired resolution).

[0052] exist Figure 5 In the example of (b1), the beam is steered from one sub-swath to another (in the order of 1, 4, 2, 1, etc.) in elevation (cross-track) between each burst or over a complete scan in azimuth, so that data is collected periodically during the duration of the mechanical steer ( Figure 5(c)). In this figure, the ratio of bursts to mechanical manipulation duration is exaggerated and will in reality be much larger. Therefore, an alternative option is to have electronic manipulation occur once every n bursts, where n is an integer, while still achieving periodic data collection from different blocks during the duration of the mechanical manipulation.

[0053] exist Figure 5 In the example of (b2), the beam is steered in the elevation (cross-track) direction so that the beams from blocks 5, 8, and 10 ( Figure 4 ). Thus, after 5 bursts or electronic scans in azimuth, the beam is steered from sub-swath 1 (to tile 5) to sub-swath 4 (to tile 8), and after another 5 scans in azimuth, the beam is steered from sub-swath 4 to sub-swath 2 (to tile 10).

[0054] like Figure 5 As shown, continuous collection of data corresponding to different regions is not required, and there may be gaps between bursts, e.g., between data acquisitions of different regions or "points." Furthermore, the angular ranges swept during each burst do not have to be the same. In fact, they may vary due to the movement and / or mechanical maneuvering of the satellite during each burst. For example, during the azimuth sweeps of Burst 1 and Burst 5, the angular ranges may be slightly different for each burst.

[0055] In these examples, data is collected from the corresponding blocks in the order in which they are collected along the direction of satellite travel, but since the effective ground velocity of the SAR beam may be close to stationary due to mechanical maneuvers, this is not required, and signals from different blocks can be collected in any order. Also in these examples, the time periods for collecting data from different blocks are equal, but this is not required, and the time periods can vary, for example, due to different requirements for different blocks.

[0056] exist Figure 5In the examples of (b1) and (b2), there are multiple electronic scans in azimuth corresponding to each block, such as the five scans shown. Imaging these blocks multiple times does not provide increased resolution, but rather provides a higher diversity of viewing angles. When combined, the pixels can be averaged so as to reduce the "dots" caused by bright returns in a particular pixel. This is called "multiple looks". In general, the electronic manipulation of azimuth can be varied and can be controlled to be fast or slow, while still faster than mechanical manipulation, so as to scan the block during the duration of the mechanical manipulation. For example, the electronic manipulation of azimuth can be slowed to the point where data for each block is collected in a single electronic azimuth scan. Slowing down the electronic azimuth scan and scanning each point in one long burst allows for higher resolution. In other words, mechanical manipulation in azimuth allows for longer burst durations (where the bursts correspond to electronic azimuth scans) or for collecting data relating to the same block over multiple bursts.

[0057] In all of the above examples, during a first time period, the SAR beam is steered in azimuth relative to the direction of travel over a first range of angles to acquire image data of a first area or block to be imaged, e.g., Figure 5 The azimuth scan shown in (a) is shown. The manipulation is repeated during one or more additional time periods to acquire image dates of one or more additional blocks to be imaged in the same or different azimuth ranges. Concurrently, during a time period including the first time period and the additional time period, the SAR beam is steered in a backward direction relative to the direction of travel, for example, within a second different angular range, to reduce the speed of travel of the beam relative to the Earth, for example, as shown in Figure 5 As shown in (c), due to the backward steering of the SAR beam, multiple high-resolution "points" can be acquired more closely spaced than has been possible so far. Figure 5 The solid line in (c) shows the azimuth angle changing linearly with time due to mechanical maneuvers. The dashed line shows an alternative implementation where the angle changes faster at the beginning of the maneuver cycle, changes slowest as it passes through zero azimuth, and changes faster toward the end in order to keep the beam velocity constant relative to the ground. The rate of change of the azimuth angle can vary in any manner, including a combination of linear and nonlinear schedules, depending on the specific implementation.

[0058] like Figure 5 As shown, it can be Figure 5 (b1) shows interleaving of time periods for collecting data of different blocks, where data of one block is collected in successive bursts interleaved between other bursts collecting data of one or more other blocks. Alternatively, the time periods may be continuous, e.g. Figure 5 As shown in (b2), during the time period corresponding to each block, there can be a slow scan, or as shown in Figure 5(a) shows that there are several consecutive scans in azimuth.

[0059] The steering of the beam in azimuth may be periodic during a backward steering cycle comprising a first time period and one or more additional time periods, during which data corresponding to different regions or blocks are collected, such as Figure 5 As shown in (a).

[0060] The SAR beam can be steered in elevation between successive data acquisitions, for example where different areas or patches from which data is to be collected are in different sub-swaths. It is worth noting that image data for different areas visible when the SAR beam is mechanically steered can be acquired in any order and not necessarily in the direction of travel or sequentially across the swath, although both are possible.

[0061] As noted elsewhere, the methods described herein are particularly, but not exclusively, suitable for implementation in conjunction with SAR carried on board a satellite. Figure 1 、 Figure 6 and Figure 7 A satellite suitable for implementing the present invention is described. It will be appreciated that the longer the beam dwells at a particular location, the greater the angular range required for mechanical maneuvering of the satellite, and the longer it will take to make up for the "gaps" in the path of travel along the orbit caused by the dwell. Consequently, there will be gaps between successive regions that can be imaged as the satellite rotates back to its original position, similar to a classic spotlight pattern. However, if all the points are closely spaced (e.g., within a 100 km x 100 km area), they can all be imaged in a single sweep, minimizing the effects of the gaps.

[0062] Figure 6 is a schematic representation of components of a satellite (e.g., a microsatellite) according to some embodiments of the present invention. Solid arrows between components are used to indicate power connections, heavier solid arrows are used to indicate RF signal connections, and dashed lines are used to indicate data connections.

[0063] Some components are part of the satellite "bus" 610, Figure 6 Some components may be part of the "payload" 660, as indicated by rectangles in FIG. Figure 6 The other components are part of the antenna module 670 and are also represented by Figure 6 The rectangle in the . Figure 6The satellite components shown in FIG. 1 include a power supply 101 and a power distribution system 102. Power supply 101 and power distribution system 102 provide power to propulsion system 190, propulsion controller 109, attitude determination and control system (ADCS) 131, computing system 103, buffer 135, and communication system 104. Power supply 101 and power distribution system 102 also provide power to components within payload 660, such as pulse generator 620 and power amplifier 623. Buffer 135, although shown as a separate item, may be included in computing system 103. Propulsion controller 109 is shown herein as a separate component, but in practice it may form part of computing system 103. The propulsion controller may be controlled using control software implemented in one or more processors included in propulsion controller 109 or in response to instructions received, for example, from computing system 103. When instructions are transmitted from computing system 103, the computing system may be considered to include the propulsion controller. One of the functions of propulsion controller 109 may be to output control signals to the ion and electron sources of the thrusters in propulsion system 190.

[0064] The satellite bus 610 may typically be located in the body 110 of the satellite 100, such as Figure 1 As shown. Power distribution system 102 may include control logic as is known in the art. Communication system 104 may include, for example, one or more communication antennas located on satellite fuselage 110. Alternatively, communication system 104 may send and receive signals via one or more communication antennas located on wings 160 of the satellite.

[0065] In the case of an Earth observation satellite, the satellite payload 660 may include one or more radar antenna arrays, which may be located at one or more wings 160 of the satellite. Figure 6 A single antenna element 625 is shown, which may be part of a phased array antenna for SAR imaging. Antenna element 625 transmits and receives signals 626. Antenna element 625 is shown with an associated power amplifier 623 and phase shifter 624 for transmitting radar signals, and an associated low-noise amplifier 628 and phase shifter 627 for receiving return signals. Together, these form antenna module 670. The phased array antenna may include multiple antenna modules 670. In one example, a satellite-mounted phased array antenna includes 320 antenna elements and associated amplifiers and phase shifters. Different phased array antennas will have different numbers of antenna elements depending on their design and intended purpose. As is well known in the art, electronic steering of the antenna is achieved by phase shifting the individual antenna elements via phase shifters 624 and 627.

[0066] The pulse generator 620 generates an RF signal, which is transmitted to the radar transmitter 621. The radar signal is transmitted to the RF divider 622, and it divides the RF signal and transmits it to the plurality of antenna modules 670. Figure 6 6. A single antenna module 670 is shown, but multiple antenna modules may be present. RF combiner 629 receives the combined signals from multiple antenna modules 670 and transmits the received RF signal to radar receiver 630. Data is stored in memory 631. Memory 631 may be the same as or separate from memory 108. Pulser 620, radar transmitter 621, radar receiver 630, RF divider 622, and RF combiner 629 may be located in satellite fuselage 110 or on satellite wing 160. Figure 6 The additional arrows extending from the RF divider 622 represent one or more additional RF outputs from the RF divider 622 to one or more additional antenna modules, and the additional arrows pointing to the RF combiner 629 represent one or more additional RF inputs entering the RF combiner 629 from the one or more additional antenna modules.

[0067] The methods and systems described herein relate to the manipulation of a single antenna or a single aperture. However, they can be easily extended to systems including multiple antennas or multiple apertures.

[0068] As known to those skilled in the art, antenna modules 670 multiplied by the number of antenna modules together form an image acquisition device for a satellite, which can perform functions other than acquisition of image data.

[0069] In a typical satellite, the antenna may include a phased array antenna as described above.A phased array antenna having antenna elements spatially distributed in two dimensions perpendicular to the radar range dimension may allow two-dimensional beam steering in azimuth and elevation.

[0070] The available electronic steering of a phased array antenna may be limited by the range and spacing of the phase centers of the physical antenna in azimuth, resulting in reduced gain / grating lobes if excessive steering is attempted. The limits on the available angular range will vary from one physical device to another, but for a typical satellite designed for low Earth orbit, the limits may be set at ±25° in elevation and ±2° in azimuth.

[0071] Payload 660 receives power from power distribution system 102 and instructions from computing system 103. Data from payload 660, such as received radar signals, also flows back to computing system 103 and may be stored in memory 108. The data may be processed by computing system 103, for example, to generate an image as described elsewhere herein, which may then be output to communication system 104 for onward transmission. Figure 6 In the system shown, raw data may also be output by computing system 103 to communication system 104, which further transmits it for processing by a remote computing system. Figure 6 SAR processor 133 can be located, for example, at ground station 600, or at another processing location. Computing system 103 can send operating instructions to other components located in payload 660, such as radar transmitter 621, radar receiver 630, and / or phase shifters 624 and 627, as will be familiar to those skilled in the art. Raw SAR data can be stored in memory 108 or 631 in the satellite. Memories 108 and 631 can be the same or different memory modules, or can also be part of computing system 103.

[0072] The raw SAR data stored in the buffer 135 can be transmitted to the ground station 600 or the remote SAR processor 133. In one example, 30 seconds of image data can be stored in the buffer 135 at full resolution (bandwidth). More data can be stored at a lower resolution (e.g., 60 seconds at half resolution). In one example, the microsatellite has a 150 MB download link. At this data rate, downloading 30 seconds of full-resolution image data takes approximately 3 minutes.

[0073] The communication system 104 may communicate with earth stations or other satellites using radio frequency communications, optical (eg, laser communications), or any other form of communication known in the art.

[0074] Satellites, e.g. Figure 1 The satellite 100 is usually provided with a propulsion system 190 for maneuvering the satellite with the generated thrust. Figure 1 1 is shown as being mounted on a surface of the body 110 opposite to the solar cell panel 150 .

[0075] like Figure 1 As shown, propulsion system 190 includes a plurality of thrusters 105 that generate thrust for maneuvering satellite 100 when needed.

[0076] The thrusters 105 are typically operated to keep the satellite in a particular orbit. For example, the thrusters may be used to propel the satellite in a particular direction relative to the Earth's surface.

[0077] Return Reference Figure 6 , ADCS131 is usually located in the satellite body 110 and is used to control the orientation of the satellite. ADCS can be implemented in many ways. The figure shows that ADCS131 includes a set of reaction wheels, Figure 1 The reaction wheels are typically, but not necessarily, located in the satellite body 110 . Figure 7is a partial perspective view of a satellite and shows a set of three reaction wheels 41, 42, 43 located in the satellite body 110. Reaction wheels are sometimes also called momentum wheels.

[0078] In the satellites described herein, ADCS can be used to mechanically steer the satellite to keep a target area on Earth within the radar aperture, in other words, within the satellite's line of sight, as the satellite travels in its orbit, for a period of time longer than the target would be visible without mechanical steering. In principle, the angular range for mechanical steering is limited only by the horizon in each direction, but the larger the angle, the greater the distance to the target area, and therefore the weaker the returned signal. In the methods described herein, a suitable range for mechanical steering angles is -45° to +45°, but higher ranges, such as -60° to +60°, are also possible.

[0079] The reaction wheels 41, 42, 43 function by using electric motors to rotate wheels within the spacecraft body 120. By maintaining angular torque, rotating the wheels in one direction causes the spacecraft to rotate in the opposite direction, as there are no external forces in space. The use of reaction wheels is a well-known way of orienting spacecraft such as satellites.

[0080] In one example, three reaction wheels are positioned within the spacecraft fuselage, one for orienting the satellite in each axis. Thus, reaction wheels 41, 42, 43 are shown with orthogonal axes.

[0081] In another example, four or more reaction wheels can be used to provide better control over various aspects of satellite dynamics, such as slew rate (how fast the satellite can turn) and fine positioning control, particularly for satellites with higher moments of inertia. This technique can facilitate the ability to stay on one point on the Earth's surface, as discussed further elsewhere herein, but is not required.

[0082] Currently, the various satellites in orbit around Earth are generally defined by weight ranges, although the boundaries between these classes are somewhat fluid and arbitrary:

[0083] CubeSat: 1kg-10kg

[0084] Microsatellite: 50kg-250kg

[0085] Small satellite: 500kg-800kg

[0086] Regular satellite: 800-1200kg

[0087] Large satellite: >1200kg.

[0088] Reaction wheels are rated according to their "momentum capacity," which has units of nm (Newton-meter-second). The rate of rotation is related to the speed of the wheel and the inertia of the satellite system. Satellites with particularly low mass have a much lower moment of inertia than conventional larger SAR satellites. Suitable low masses can be below 1000 kg, for example, below 500 kg, below 250 kg, between 50 kg and 250 kg, or below 100 kg.

[0089] Currently, very small CubeSats do not have the capacity to carry current SAR payloads. Heavier satellites are generally less maneuverable due to their higher inertia. Embodiments of the satellites and operating methods described herein have been successfully implemented in microsatellites.

[0090] Embodiments of the present invention are particularly applicable to a class of satellites known as microsatellites.

[0091] Some of the methods described further herein benefit from reaction wheels within a specific size range. For example, a suitable range for microsatellites may be 0.5 to 2.5 nm.

[0092] Reaction wheels with a 1 nm rating have been successfully tested. This enables slewing in the 1° / second range, which is sufficient to track a point on the ground and implement any of the methods described herein without consuming too much power. Thus, in any of the satellites described herein, the ADCS can be configured to slew the satellite in azimuth at speeds of up to 1 degree / second using mechanical steering. Additionally or alternatively, the ADCS can be configured for dwell times of up to 60 seconds.

[0093] Larger satellites are known to use reaction wheels on the order of 10 nm, but due to the large mass of the satellite and the resulting high moment of inertia they currently cannot achieve sufficient rotation rates for the dwell times discussed further herein, and they also consume much more power than smaller reaction wheels.

[0094] In one example, a satellite orbits the Earth in a low Earth orbit. A low Earth orbit may be between 160 km and 1000 km above the Earth's surface. An example of a SAR-based Earth observation satellite may have an orbit between 450 km and 650 km above the Earth. In an example according to the present invention, a satellite has an orbit 550 km above the Earth's surface. For example, in an orbit 550 km above the Earth, the satellite effectively traverses the Earth at approximately 7.5 km / s, or 27,000 km / h. Most satellites in this orbit will pass through the Earth at speeds in the range of 7-8 km / s.

[0095] In some embodiments, to stay on a SAR antenna and keep the SAR antenna pointed at a point on Earth, a microsatellite can be designed to rotate with a slew rate capability of approximately 17 seconds. This is not mechanically achievable in conventional satellites. However, according to some embodiments of the present invention, a satellite such as a microsatellite can slew at the speed necessary to maintain a horizon-to-horizon pointing direction on Earth for approximately 10 minutes. However, at the extreme end of this range, the distance to the imaged point or target is too great to obtain a good SAR image, so there is a smaller practical dwell time.

[0096] As described elsewhere herein, embodiments of the present invention are not limited to changing the orientation of an entire satellite, which is convenient in the case of small, lightweight, agile satellites as described above. For example, in some embodiments, mechanical steering can be achieved by changing the orientation of an antenna relative to the satellite it is carrying.

[0097] In the foregoing, only one SAR beam was considered. However, it will be appreciated that the methods and systems described herein can be extended to use multiple SAR beams. For example, a platform can carry apparatus for multiple SARs, each of which can operate according to any of the methods described herein.

[0098] To give a specific example for illustrative purposes only, for a satellite traveling at 7.5 km / s at 550 km above the Earth, ignoring the curvature of the Earth, this means that in 30 seconds the satellite will be 225 km directly above the target. In order to remain pointed directly at the same point on the Earth throughout the entire 30-second period, an angular range of approximately 23 degrees is required. Different implementations may use different angular ranges. This may depend on factors such as, but not limited to, the ability of the mechanical steering and the amount of onboard storage in the satellite, as data is typically downloaded in batches as it passes through the ground station.

[0099] As can be understood from the above, all the methods described herein benefit from the use of agile microsatellites. Microsatellites of appropriate size can be rotated to observe targets for extended periods of time (up to 60 seconds). This provides them with the unprecedented ability to capture many image frames over a period of time at the same resolution as the scope resolution.

[0100] A satellite suitable for implementing any of the methods described herein has been described above. For a satellite or other platform already in orbit, the methods described herein can be implemented by appropriately controlling the satellite from the ground using a suitable computing system, such as ground station computing system 600. In other words, the SAR can be operated from the ground, and some of the methods described herein can be implemented in software. Thus, in one aspect, the present invention can provide a computer-readable medium comprising instructions that, when implemented by a processor in a computing system, cause the computing system to operate the SAR according to any of the methods described herein.

[0101] The acquisition of SAR image data described herein can have many different practical applications. The end-to-end process can begin with a request to image a specific point in an area, which can be requested by a customer or identified as interesting by an algorithm, for example. A series of moves in azimuth and optionally elevation can then be designed to, for example, use Figure 5 Any combination of (a), (b1), and (b2) will be used to optimally collect image data. Then, when the satellite is next over the total area, it will "park," perform the maneuver, and collect image data.

[0102] For example Figure 8 The method shown in may include receiving a request for images of a plurality of regions on the Earth at 801, identifying a subset of the plurality of regions that are sufficiently close together to enable image data associated with those regions to be acquired during an extended dwell period over a larger area that includes the identified regions at 803, and determining a sequence of steering operations as described herein to enable image data associated with the subset of the plurality of regions to be acquired during the extended dwell period at 805. Thus, the steering operations may include steering a SAR beam in azimuth relative to a direction of travel during a first time period to acquire image data of a first region on the Earth to be imaged, steering the SAR beam in azimuth during one or more additional time periods to acquire image data of one or more additional regions on the Earth to be imaged, and steering the SAR beam in azimuth in a rearward direction relative to the direction of travel to reduce a speed of travel of the beam relative to the Earth during a time period including the first time period and the one or more additional time periods, the first time period and the one or more additional time periods corresponding to the extended dwell period.

[0103] Operations 801-805 may occur at the ground computing system 600. Alternatively, one or more of operations 801-805 may occur at the onboard computing system 103. Either way, the sequence of maneuvering operations may be communicated 807 to SAR control equipment, such as the phase shifter 111 and ADCS 131. The SAR may then be operated according to any of the methods described herein to obtain image data.

[0104] Some embodiments of the invention described herein provide the following:

[0105] A ground station computing system configured to operate a SAR according to any of the methods described herein.

[0106] A satellite with an antenna capable of pointing at an observation area and maintaining its pointing position for an extended period of time (60 seconds). This is typically much longer than can be achieved with larger satellites (nominally 2 seconds) due to its low mass and low moment of inertia, allowing antenna and beam pointing to be achieved without consuming fuel and using only internal momentum wheels.

[0107] An imaging mode that keeps a target scene within the antenna reception window even if the target scene has significantly varying distances.

[0108] In any of the embodiments of the present invention, the satellite may be traveling in a low Earth orbit, or be configured to travel in a low Earth orbit.

[0109] A satellite according to any embodiment of the present invention may be configured for side-viewing, as is known in the art. It may have left-viewing and right-viewing capabilities.

[0110] Satellites according to any embodiment of the present invention may employ X-band radar.

[0111] Any computing system described herein can be combined in a single computing system having multiple functions. Similarly, the functions of any computing system described herein can be distributed across multiple computing systems.

[0112] Some operations of the method described herein can be performed by software in a machine-readable form, for example in the form of a computer program including computer program code. Therefore, some aspects of the present invention provide a kind of computer-readable medium, which, when implemented in a computing system, causes the system to perform some or all operations of any method described herein. The computer-readable medium can be a temporary or tangible (or non-transient) form, such as a storage medium including a disk, a thumb drive, a memory card, etc. The software can be suitable for execution on a parallel processor or a serial processor so that the method steps can be performed in any suitable order or simultaneously.

[0113] This application recognizes that firmware and software may be valuable, separately tradable commodities. It is intended to include software that runs on or controls "dumb" or standard hardware to perform a desired function. It is also intended to include software that "describes" or defines the configuration of hardware, such as HDL (Hardware Description Language) software, such as used in designing silicon chips, or for configuring general-purpose programmable chips to perform a desired function.

[0114] The above-described embodiments are substantially automatic. In some examples, a user or operator of the system may manually instruct some steps of the method to be performed.

[0115] In this embodiment of the invention, the system can be implemented as any form of computing and / or electronic system described elsewhere herein. For example, a ground station may include such a computing and / or electronic system. Such a system may include one or more processors, which may be microprocessors, controllers, or any other suitable type of processor, for processing computer-executable instructions to control the operation of the device so as to collect and record routing information. In some examples, such as where a system-on-chip architecture is used, the processor may include one or more fixed function blocks (also referred to as accelerators) that implement a portion of the method in hardware (rather than software or firmware). Platform software including an operating system or any other suitable platform software may be provided at a computing-based device to enable application software to be executed on the device.

[0116] The term "computing system" is used herein to refer to any device that has processing capabilities so that it can execute instructions. Those skilled in the art will recognize that such processing capabilities can be incorporated into many different devices, and thus the term "computing system" includes PCs, servers, smart mobile phones, personal digital assistants, and many other devices.

[0117] It should be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those that solve any or all of the problems or those that have any or all of the benefits and advantages.

[0118] Unless otherwise stated, any reference to "an" item or "a portion" refers to one or more of those items. The term "comprising" is used herein to indicate including the identified method steps or elements, but such steps or elements do not comprise an exclusive list and the method or apparatus may include additional steps or elements.

[0119] Furthermore, to the extent that the term "includes" is used in either the detailed description or the claims, this term is intended to be inclusive in a manner similar to the term "comprising" as it is interpreted when used as a transitional term in the claims.

[0120] The accompanying drawings illustrate exemplary methods. Although these methods are shown and described as a series of actions performed in a particular order, it should be understood and appreciated that these methods are not limited by the order of the sequence. For example, some actions may occur in a different order than described herein. In addition, an action may occur simultaneously with another action. Furthermore, in some cases, not all actions may be required to implement the methods described herein.

[0121] The order of the steps of the methods described herein is exemplary, but the steps may be performed in any suitable order, or concurrently where appropriate. In addition, steps may be added or substituted in any method, or individual steps may be deleted from any method, without departing from the scope of the subject matter described herein. Aspects of any of the above examples may be combined with aspects of any of the other examples described to form additional examples.

[0122] It should be understood that the above description of the preferred embodiments is provided by way of example only, and that various modifications may be made by those skilled in the art. What has been described above includes examples of one or more embodiments. Of course, it is not possible to describe every conceivable modification and variation of the above-described apparatus or method for the purposes of describing the above-described aspects, but one of ordinary skill in the art will recognize that many further modifications and permutations of various aspects are possible. Accordingly, the described aspects are intended to encompass all such variations, modifications, and variations that fall within the scope of the appended claims.

Claims

1. A method of operating a synthetic aperture radar (SAR) to acquire image data, wherein: The SAR is carried on a platform traveling relative to the surface of the Earth and is directed toward the surface of the Earth, the method comprising: electronically steering the SAR beam in azimuth relative to the direction of travel during a first time period to acquire image data of a first area on Earth to be imaged; electronically steering the SAR beam in azimuth during one or more additional time periods to acquire image data for one or more additional areas of the Earth to be imaged; and During a time period including the first time period and one or more additional time periods, the SAR beam is mechanically steered in azimuth in a backward direction relative to the direction of travel to reduce a speed of travel of the beam relative to the Earth.

2. The method according to claim 1, wherein The electronically steering during the first time period is within a first angular range, and the electronically steering during the one or more additional time periods is within the same angular range.

3. The method according to claim 2, wherein: The electronically steering within the first angular range is in a forward direction.

4. The method according to claim 3, wherein: The electronic steering is performed using a phased array antenna.

5. A method according to any one of the preceding claims, wherein The first time period and the one or more additional time periods overlap.

6. The method according to any one of the preceding claims 1 to 3, wherein: The first time period and the one or more additional time periods are consecutive.

7. A method according to any one of the preceding claims, wherein The steering of the beam in azimuth to acquire image data is periodic during a time period comprising the first time period and one or more additional time periods.

8. The method of any preceding claim, further comprising steering the SAR beam in elevation between consecutive data acquisitions.

9. The method according to claim 8, wherein The SAR beam is electronically steered in elevation.

10. The method according to claim 1, wherein Mechanical manipulation is performed by changing the orientation of the SAR relative to the platform.

11. The method according to claim 1, wherein Mechanical maneuvers are performed by changing the orientation of the platform relative to the Earth's surface.

12. The method according to any one of claims 2 to 4, wherein: The first angle ranges from -5 degrees to +5 degrees, or from -8 degrees to +8 degrees.

13. A method according to any one of the preceding claims, wherein The steering in the rearward direction is within an angular range from -01 degrees to +1 degrees, from -10 degrees to +10 degrees, from -23 degrees to +23 degrees, or from -30 degrees to +30 degrees.

14. A method of forming images of different regions on the Earth, the method comprising: Receive requests for imagery of multiple regions on Earth, identifying a subset of the plurality of regions that are sufficiently proximate to acquire image data associated with those regions over an extended dwell period over a larger area including the identified regions, and determining a sequence of manipulation operations to be performed to enable acquisition of image data relating to said subset of said plurality of regions during said extended dwell period, The manipulation operations include: electronically steering the SAR beam in azimuth relative to the direction of travel during a first time period to acquire image data of a first area on Earth to be imaged; electronically steering the SAR beam in azimuth during one or more additional time periods to acquire image data for one or more additional areas of the Earth to be imaged; and mechanically steering the SAR beam in azimuth in a backward direction relative to the direction of travel to reduce a speed of travel of the beam relative to the Earth during a time period that includes the first time period corresponding to the extended dwell and one or more additional time periods; The determined sequence of maneuvering operations is transmitted to a SAR control device.

15. A computer readable medium comprising instructions which, when implemented in a processor in a computing system, cause the computing system to operate a SAR according to the method of any preceding claim.

16. A satellite operating in orbit around the Earth according to the method of any one of claims 1 to 14, comprising: Propulsion system, an attitude determination and control system "ADCS" configured to steer the SAR beam in a backward direction, one or more radar antennas or antenna arrays configured to steer the SAR beam in azimuth within a first angular range, Synthetic Aperture Radar (SAR) image data acquisition device, and A communication system is configured to send and receive signals to and from one or more ground stations on Earth.

17. A system comprising a satellite according to claim 16 and a ground station, wherein: The ground station is configured to implement the method according to claim 14.

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