Multispot imaging using synthetic aperture radar.

Abstract

A method of operating a synthetic aperture radar (SAR) to acquire image data, comprising the steps of 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 backwards relative to the direction of travel during a time period that includes the first and one or more additional time periods to reduce a speed of travel of the beam relative to the Earth.

Description

[Technical field]

[0001] The present invention is in the field of synthetic aperture radar imaging. [Background technology]

[0002] Synthetic Aperture Radar (SAR) can be used to image areas on Earth by transmitting radar waves and recording the reflected echoes from those transmitted beams. SAR systems can be installed on airborne platforms such as aircraft as well as satellites operated from space. Different modes of operating SAR can be used, including strip map, spotlight, and scanning SAR.

[0003] In stripmap mode, the satellite uses its SAR system to image data along a strip, either in azimuth or along-track direction relative to the Earth's surface. In scanSAR mode, multiple bands are imaged along the strip by electronically steering the SAR beam in elevation (perpendicular to the satellite's direction of travel), imaging different bands as the satellite moves across the ground.

[0004] Classical spotlight mode imaging is a technique used to achieve high-resolution imaging. In this mode, the satellite beam is steered in azimuth to remain on a spot on the ground for longer than it would normally stay on that area using stripmap or scanSAR modes. Spotlight mode achieves high-resolution imaging by illuminating a ground site, i.e., a target area on the Earth, over a wide range of angles for a longer period of time than would be possible without beam steering. However, traditional SAR systems require large gaps between spotlight images due to the need to steer the beam to a significant angle between the spotlight images.

[0005] One problem that arises is how to image multiple closely spaced spots. SAR system operators are constantly striving to improve the efficiency of image collection while maintaining the required resolution and accuracy, especially in the field of satellite imagery, but also in other implementations of SAR. In the classical spotlight mode, typically only one contiguous spot on the Earth can be imaged at high resolution, so imaging another spot nearby requires completing another orbit (in the case of a satellite-borne SAR) and returning to the same area to image the second spot at high resolution. This is costly, inefficient, and introduces delays in receiving data in a timely manner.

[0006] Some embodiments of the present invention described below address some of these problems, however, the present invention is not limited to addressing these issues and some embodiments of the present invention address other issues. 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 for use in determining 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 that uses an extended dwell time to acquire raw data. In the case of a satellite, the extended dwell time may be achieved by mechanical steering of the satellite.

[0009] In one aspect, a method of operating a Synthetic Aperture Radar (SAR) to acquire image data, the SAR being carried on a platform traveling over the surface of the Earth and pointed at the surface of the Earth, includes 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 a backward azimuth relative to the direction of travel during a time period that includes the first and one or more additional time periods to reduce a speed of travel of the beam relative to the Earth.

[0010] The steering during a first time period may span a first angular range and the steering during one or more additional time periods may span the same angular range, and the first time period and the one or more additional time periods may overlap or be contiguous.

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

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

[0013] The SAR beam may be elevation steered between successive data acquisitions, which may be electronically steered, for example, using a phased array antenna.

[0014] The SAR beam may be mechanically steered backwards, for example, by changing the orientation of the SAR relative to the platform, or by changing the orientation of the platform relative to the surface of the Earth.

[0015] A method is provided for forming images of different regions on the Earth, the method comprising: receiving a request for images of a plurality of regions on the Earth, identifying a subset of the plurality of regions that are close enough together so that image data relating to the identified regions may be acquired during an extended dwell over a larger region that includes the identified regions, determining a sequence of steering maneuvers to be performed to enable acquisition of image data relating to the subset of the plurality of regions during the extended dwell, and communicating the determined sequence of steering maneuvers to a SAR control device. The SAR may operate according to any of the methods described herein.

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

[0017] There is provided a satellite for operating in an Earth orbit according to any of the methods described herein, comprising a T-propulsion system, an attitude determination and control system ADCS for steering the SAR beam in a rearward direction, one or more radar antennas or antenna arrays for steering the SAR beam in azimuth through a first angular range, a synthetic aperture radar "SAR" image data acquisition device, and a communications system for transmitting signals to and receiving signals from one or more ground stations on Earth.

[0018] In some embodiments of the present invention, a computer readable medium is provided that includes instructions in the form of an algorithm that, when implemented in a computing system that forms part of a satellite operating system, causes the system to perform any of the methods or processes described herein.

[0019] The features of the various aspects and embodiments of the invention may be combined as appropriate and in any combination with any aspect of the invention, as will be apparent to the skilled artisan. Embodiments of the present invention will now be described, by way of example only, with reference to the following drawings, in which: [Brief description of the drawings]

[0020] [Figure 1] 1 is a schematic perspective view of a satellite in orbit above the Earth; [Diagram 2] FIG. 1 is a schematic diagram of a satellite operating in SCANSAR mode. [Diagram 3] 1 is a schematic diagram of a satellite operating to form images of multiple discrete regions on the Earth; [Figure 4] FIG. 2 is a schematic diagram illustrating the effect of mechanically steering a satellite to reduce its effective ground speed. [Diagram 5] 1 is a series of graphs illustrating the superposition of mechanical steering in azimuth and electronic steering in azimuth and elevation. [Figure 6] 1 is a schematic diagram of components of a satellite. [Figure 7] FIG. [Figure 8] 1 is a flow chart illustrating a method for forming images of different regions on the earth according to some embodiments of the present invention.Common reference numbers are used throughout the drawings to denote like features. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Embodiments of the present invention are described below by way of example only. These examples represent the best ways of practicing the invention currently known to applicant, but are not the only ways in which this may be accomplished.

[0022] In accordance with an embodiment of the present invention, a system and method are provided for operating a SAR to obtain images of an area on the Earth. For this purpose, the SAR may be mounted on a platform that travels relative to the surface of the Earth. For example, SARs are commonly used on board satellites. However, the methods and systems described herein are not limited to space and may be performed using aircraft or other suitable platforms.

[0023] An embodiment of the invention employs a combination of mechanical and electronic steering of the radar beam as further described herein. Mechanical steering may be achieved by reorienting the SAR antenna relative to the platform on which it is mounted, such as a satellite or aircraft. However, for a suitably agile satellite, mechanical steering may be achieved by reorienting the entire platform relative to a target, such as an area on the Earth. This will be described below with reference to a satellite, although it will be understood that the same principles can be applied to other types of platforms.

[0024] FIG. 1 is a perspective view of a satellite 100 in orbit above the Earth as an example of a platform that may be used in the methods and systems described herein. The satellite includes a body 110 and "wings" 160. The satellite wings may carry one or more antennas. Each antenna may comprise a phased array antenna, or in other words, multiple antenna components that may be controlled to steer the direction of the antenna beam, either to control the direction and shape of the transmitted pulse, or to control the direction and area of ​​reception of radiation. This is electronic beam steering, and is well known in the art. By beam steering or electronic steering herein is meant steering of a radar beam using a phased array antenna for transmitting and receiving.

[0025] Electronic beam steering is highly accurate and fast. It may be used to steer small angles quickly. This is also called high speed small angle electronic steering. Electronic beam steering can be performed in two dimensions (azimuth and elevation).

[0026] In addition to electronic steering, the satellite 100 may be configured to mechanically steer the antenna, and therefore the SAR beam. In this example, this is accomplished by steering the entire satellite 100. This may be accomplished using a satellite attitude determination and control system ADCS, which may include one or more reaction wheels, one of which is shown at 170. Mechanical beam steering allows for wider steering angles than electronic steering, and therefore can provide greater ground coverage, especially for small, agile satellites that can turn at sufficiently fast speeds with low power consumption. This technique allows for larger angles to be steered; this is also referred to as large angle mechanical steering. In some of the methods described herein, the mechanical steering may be fast enough to reduce the effective ground speed of the radar beam, or even to near zero. However, mechanical steering is still relatively slow compared to electronic steering.

[0027] As known to those skilled in the art, it periodically alternates between a transmit mode in which pulses of radiation are directed toward the Earth's surface, and a receive mode in which radiation reflected from the Earth's surface is received.

[0028] The satellites of FIG. 1 are described in more detail with reference to FIGS.

[0029] As is known in the art, a SAR image is created by transmitting successive pulses of radio waves to "illuminate" a target scene, and receiving and recording the echoes of each pulse. The pulses can be transmitted and the echoes received by a single beam-forming antenna. When a SAR is mounted on a mobile platform, such as a satellite, and moves relative to the target, the position of the antenna relative to the target changes over time, causing the frequency of the received signal to change due to the Doppler effect. Signal processing of the successively recorded radar echoes allows the recordings from multiple antenna positions to be combined to form a synthetic antenna aperture, producing a higher resolution image.

[0030] The area currently captured by the SAR is the footprint. The direction along the SAR's flight direction is usually called azimuth or along track. The direction across the flight direction is usually called range, elevation, or cross track. The direction opposite the flight direction corresponds to backward azimuth.

[0031] 2 is a schematic diagram of a satellite 100 operating in ScanSAR mode. The satellite 100 is in a well-known side-looking configuration that transmits and receives signals to the side of the satellite, rather than directly below it. This area has a width, also known in modern times as a swath. For each radar pulse transmitted by the satellite, signal data in the form of echoes can be received at different frequencies from different points across the swath, due to the Doppler effect, which is at the heart of SAR.

[0032] The satellite 100 is shown traveling from right to left, as indicated by the arrow, relative to the Earth in orbit. In ScanSAR mode, the area of ​​interest for data collection, e.g., the 100 km x 100 km area shown in Figure 2, is divided into sub-swaths, and each sub-swath is divided in the direction of travel into what are referred to herein as "blocks," forming an offset checkerboard pattern in the example of Figure 2. The SAR beam may be steered in elevation to collect data from different sub-swaths. During this steering, echoes are received from different locations throughout the sub-swath, providing a set of data for each square (or other shape), e.g., block A, on the ground.

[0033] Electronic steering can be achieved very quickly. In a ScanSar development commonly known as Topographical Surveying by Progressive Scanning (TOPS), the SAR beam is pointed to the edge of block A, then electronically steered rapidly in elevation to an adjacent sub-swath, then azimuthally back to the starting azimuth, and the azimuth steering is repeated to collect data from a second block B.

[0034] Each traversal of the beam in azimuth may correspond to several hundred pulses, referred to herein as a "burst." A burst thus corresponds to a block of ground, and data over a swath length is collected in a burst-by-burst approach.

[0035] The resolution of an image depends on many factors, including the amount of data acquired to generate the image. The technique used to achieve high-resolution imaging is the known spotlight mode, where the SAR beam is directed to dwell on a specific area. However, this technique is usually used to image only one spot or area. Moreover, this dwell requires the satellite to "recover" by returning to its original orientation before taking another image. Meanwhile, the satellite is progressing in its orbit, so it is not possible to form such high-resolution images of closely spaced "spots". In other words, large spatial gaps between spotlight images are often required. To image another spot or area close to the first spot, the satellite may need to complete at least one full orbit to return to the original point, which is very costly in terms of time and efficiency.

[0036] In some of the methods and systems described in 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 traveling speed of the beam relative to the Earth as the satellite travels in orbit. This can result in longer burst durations and higher image resolution. In some implementations, mechanical steering may be used to bring the beam to a stop or near a stop relative to the ground. Combining this backward steering with electronic steering in azimuth and / or elevation allows for high-resolution imagery to be obtained from multiple closely spaced spots that was not previously possible.

[0037] This mechanical steering is shown in exaggerated form in Figure 3, where satellite 100 is shown to have a different orientation at each of three positions along its path 600. Between each position, the satellite rotates through an azimuth angle indicated by arrow 601.

[0038] Figure 4 shows an area on the ground divided into small regions or blocks that may be used for operation of the SAR in ScanSAR or TOPS mode. In this manner, the SAR beam may be electronically steered to collect data from blocks 1 through 15 in numerical order.

[0039] To do this, the beam may be periodically steered forward in the azimuth (along track) direction, as shown in FIG. 5(a), where the beam angle is steered from a negative angle through zero to an equal positive angle over a burst period, and this is repeated for successive bursts over the same angle range. This periodic forward azimuth electronic steering is superimposed with rearward azimuth mechanical steering over a wider range of angles and for a longer period of time. FIG. 5(c) shows a simplified example for illustrative purposes of 16 forward electronic scans during one relatively slow rearward mechanical scan. In practical implementations, the ratio of mechanical azimuth steering periods to electronic steering periods will be at least 2:1 to allow for at least two spots per mechanical scan. In some implementations of the methods described herein, the electronic steering in azimuth is periodically repeated and the mechanical steering is not periodically repeated, as the beam is mechanically steered in the azimuth direction. Note that while the graphs show the electronic and mechanical steering to be linear, this need not necessarily be the case, especially for rearward azimuth mechanical steering.

[0040] The electronic steering in azimuth shown in FIG. 5(a) is similar to the steering done in TOPS where the beam is rapidly returned to the starting angle between each "sweep" through the azimuth range. This rapid return is not essential to the method described here, and in some implementations the beam may be slowly steered in the opposite direction, for example at the same speed as the forward steering, while pulses continue to be transmitted. Furthermore, it is not essential that the electronic azimuth steering be in forward azimuth. Improvements in resolution and / or spot-to-spot spacing are primarily obtained from azimuth electronic steering combined with aft mechanical steering relative to the direction of travel of the satellite in orbit. The electronic steering in azimuth may be periodic within the duration of the mechanical steering, but this is not essential as will be further explained below.

[0041] Some implementations of the imaging methods described herein may be used to image different "spots", i.e., regions or blocks, within the same sub-swath. In this case, no steering to elevation is required. The spots may be contiguous, overlapping, or non-contiguous. In the case of overlapping spots, data from the first spot can be used to image the overlapping spots, and no additional steering is required.

[0042] Other implementations may be used to image spots, or blocks, in different sub-swaths. For example, consider a satellite receiving a request to transmit imaging or image data for areas / blocks 5 (in sub-swath 1), 8 (in sub-swath 4), and 10 (in sub-swath 2) as shown in FIG. 4. The graphs in FIG. 5 show two ways to accomplish this. There are two graphs (b1) and (b2) that represent two different possible implementations, where the elevation steering is cyclical from sub-swath to sub-swath (b1), or one spot at a time (b2), each directed to the first, fourth, then second sub-swath block (e.g., blocks 5, 8, 10 in FIG. 4). These are two examples of a range of possibilities for how each individual block is imaged, which may be determined according to which "spots" (e.g., their location and desired resolution) are required for imaging.

[0043] In the example of Figure 5(b1), the beam is steered in elevation (cross-track) from one sub-swath to another during each burst, a complete sweep in azimuth (1, 4, 2, 1, etc.), such that data is collected periodically for the duration of the mechanical steering (Figure 5(c)). The ratio of burst to mechanical steering duration is exaggerated in this illustration and will be much larger in reality. Therefore, an alternative option is to perform electronic steering every n bursts (n is an integer) while achieving periodic collection of data from the various blocks for the duration of the mechanical steering.

[0044] In the example of Figure 5(b2), the beam is steered in the elevation (cross-track) direction such that data from each of blocks 5, 8, and 10 (Figure 4) are collected during successive periods within the mechanical steering duration. Thus, after five bursts or azimuth electronic sweeps, the beam is directed from sub-swath 1 (to image block 5) to sub-swath 4 (to image block 8), and after five more sweeps in azimuth, from sub-swath 4 to sub-swath 2 (to image block 10).

[0045] As shown in Figure 5, it is not necessary to collect data corresponding to different regions consecutively, and there may be gaps between bursts, e.g., between data acquisition of different regions or "spots." Also, the ranges of angles swept during each burst need not be the same; in fact, they may differ due to the satellite's progression and / or mechanical steering during each burst. For example, the ranges of angles during the azimuth scans of burst 1 and burst 5 may differ slightly from burst to burst.

[0046] In these examples, data is collected from each block in sequence along the satellite's direction of travel, although the effective ground speed of the SAR beam may be nearly stationary due to mechanical steering. This is not required and signals from different blocks may be collected in any order. Also, in these examples, the times to collect data from different blocks are equal, but this is not required and the times may differ, for example due to different requests for different blocks.

[0047] In the example of Figure 5(b1) and (b2), there are multiple electronic sweeps at the azimuth angle corresponding to each block, with five sweeps as shown. Imaging a block multiple times does not improve resolution, but rather increases the diversity of viewing angles. When combined, pixels can be averaged to reduce "speckle" due to bright returns of certain pixels. This is known as "multi-looking". In general, electronic steering can vary the azimuth angle and can be controlled to be faster or slower while still being faster than mechanical steering to scan the block during the duration of the mechanical steering. For example, electronic steering in azimuth can be slowed to the extent that data for each block is collected in a single electronic azimuth sweep. Higher resolution can be achieved by slowing down the electronic azimuth sweep and scanning each spot in one long burst. In other words, mechanical steering in azimuth allows for longer burst durations (if the burst corresponds to an electronic azimuth sweep) and collection of data related to the same block over multiple bursts.

[0048] In all the above examples, the SAR beam is steered in azimuth with respect to the direction of travel over a first angular range during a first time period to acquire image data of a first region or block of the imaged object, for example as shown by the azimuth sweep shown in FIG. 5(a). This steering is repeated during one or more additional time periods to acquire image data of one or more additional blocks to be imaged over the same or different azimuth range. At the same time, the SAR beam is steered backward with respect to the direction of travel, for example over a second different angular range, during a time period including the first time period and the additional time period, thereby reducing the traveling speed of the beam relative to the Earth, for example as shown in FIG. 5(c). Thus, the backward steering of the SAR beam may allow the acquisition of multiple high-resolution "spots" more closely spaced than previously possible. The solid lines in FIG. 5(c) show that the azimuth angle changes linearly with time as a result of the mechanical steering. The dotted lines show an alternative implementation where the angle changes faster at the beginning of the steering period, slowest as the azimuth crosses zero, and faster towards the end to keep the beam speed relative to the ground constant. The rate of change of the azimuth angle may be changed in any manner, combining linear and non-linear progressions, depending on the specific implementation.

[0049] As shown in Figure 5, the time periods during which data is collected for one block in successive bursts between other bursts that collect data for one or more other blocks may be interleaved as shown in Figure 5(b1), or during the time period corresponding to each block, there may be one slow sweep as shown in Figure 5(b2), or there may be multiple successive sweeps in azimuth as shown in Figure 5(a).

[0050] As shown in FIG. 5(a), azimuth steering of the beam may be performed periodically during a backward steering period that includes a first and one or more additional periods.

[0051] The SAR beam may be steered in elevation between successive data acquisitions, for example, when the areas or blocks from which data are collected are in different sub-swaths. In particular, image data for different areas visible as the SAR beam is mechanically steered can be acquired in any order, not necessarily sequentially along the axis or across the swath, although both are possible.

[0052] As mentioned elsewhere, the method described herein is particularly suited to, but not limited to, implementations related to SARs onboard satellites. A satellite suitable for implementing the invention will now be described with reference to Figs. 1, 6 and 7. It will be appreciated that the longer the beam dwells at a particular location, the greater the range of angles required for mechanical steering 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 this dwell. As with the conventional spotlight mode, there are gaps between successive areas that can be imaged while the satellite rotates back to its original position. However, if all the spots are closely spaced (e.g., within a 100km x 100km area), they can all be imaged in one pass, minimizing the effect of the gaps.

[0053] 6 is a schematic diagram illustrating components of a satellite, e.g., a microsatellite, in accordance with some embodiments of the present invention. Solid arrows between components are used to indicate power connections, thick solid arrows are used to indicate RF signal connections, and dashed lines are used to indicate data connections.

[0054] Some components may be part of the satellite "bus" 610, shown as a rectangle in FIG. 6, and some may be part of the "payload" 660, shown as a rectangle in FIG. 6. Other components are part of the antenna module 670, also shown as a rectangle in FIG. 6. The satellite components shown in FIG. 6 include a power supply 101 and a power distribution system 102. The power supply 101 and the power distribution system 102 provide power to the propulsion system 190, the propulsion controller 109, the attitude determination and control system ADCS 131, the computing system 103, the buffer 135, and the communication system 104. The power supply 101 and the power distribution system 102 also provide power to components such as the pulse generator 620 and the power amplifier 623 in the payload 660. The buffer 135 is shown as a separate item, but may be included in the computing system 103. The propulsion controller 109 is shown here as a separate item, but may in fact be part of the computing system 103. The propulsion controller may be controlled using control software implemented in one or more processors configured in the propulsion controller 109 or in response to instructions received from, for example, the computing system 103. If instructions are sent from the computing system 103, the computing system may be considered to comprise the propulsion controller. One of the functions of the propulsion controller 109 may be to output control signals to the ion and electron sources of the thrusters in the propulsion system 190.

[0055] The satellite bus 610 may be generally located within the body 110 of the satellite 100, as shown in Figure 1. The power distribution system 102 may comprise control logic as known to those skilled in the art. The communication system 104 may comprise, for example, one or more communication antennas located on the satellite body 110. Alternatively, the communication system 104 may transmit and receive signals via one or more communication antennas located on the wings 160 of the satellite.

[0056] In the case of an Earth Observation satellite, the satellite payload 660 may include one or more radar antenna arrays located on one or more wings 160 of the satellite. FIG. 6 shows a single antenna element 625 that may be part of a phased array antenna used for SAR imaging. The antenna element 625 transmits and receives signals 626. The antenna element 625 is shown to have an associated power amplifier 623 and phase shifter 624 for transmitting the radar signal, and an associated low noise amplifier 628 and phase shifter 627 for receiving the return signal. These together form an antenna module 670. A phased array antenna may comprise multiple antenna modules 670. In one example, a satellite-borne phased array antenna comprises 320 antenna elements and associated amplifiers and phase shifters. The number of antenna elements varies depending on the design and purpose of the phased array antenna. Electronic steering of the antenna is achieved by phase shifting the individual antenna components via the phase shifters 624, 627, as will be appreciated by those skilled in the art.

[0057] The pulse generator 620 generates an RF signal that is sent to the radar transmitter 621. The radar signal is sent to the RF divider 622, which splits the RF signal and sends it to the multiple antenna modules 670. Although one antenna module 670 is shown in FIG. 6, there can be multiple antenna modules. The RF combiner 629 receives the combined signal from the multiple antenna modules 670 and sends the received RF signal to the radar receiver 630. Data is stored in a memory 631. The memory 631 may be the same as the memory 108 or may be separate. The pulse generator 620, the radar transmitter 621, the radar receiver 630, the RF divider 622, and the RF combiner 629 can be located in either the satellite body 110 or the satellite wings 160. The additional arrows extending from RF divider 622 in FIG. 6 indicate one or more additional RF outputs from the RF divider 622 to one or more additional antenna modules, and the additional arrows pointing to RF combiner 629 indicate one or more additional RF inputs from the one or more additional antenna modules to the RF combiner 629.

[0058] The methods and systems described herein refer to steering a single antenna or a single aperture, but can be easily extended to systems involving multiple antennas or multiple apertures.

[0059] Antenna modules 670, multiplied by the number of antenna modules, synthesize satellite image capture devices as known to those skilled in the art, and may perform functions other than capturing image data.

[0060] A typical satellite may be equipped with a phased array antenna, as described above, with antenna elements spatially distributed across two dimensions perpendicular to the radar range dimension, allowing for two-dimensional beam steering in azimuth and elevation.

[0061] The available electronic steering of a phased array antenna may be limited by the range of the physical antenna and the spacing of the phase centres in azimuth, and attempting too much steering will result in reduced gain / grating lobes. The available angular range limits vary with the physical device, but limits for a typical satellite designed for low Earth orbit may be set at ±25° in elevation and ±2° in azimuth.

[0062] The payload 660 receives power from the power distribution system 102 and instructions from the computing system 103. Data from the payload 660, such as received radar signals, may also be returned to the computing system 103 and stored in the memory 108. The data may be processed by the computing system 103, for example to generate images as described herein, and then output to the communication system 104 for onward transmission. In the system shown in FIG. 6, the computing system 103 may also output raw data to the communication system 104 for further processing and transmission to a remote computing system. In FIG. 6, the SAR processor 133 may be located, for example, in the ground station 600 or at another processing location. The computing system 103 may send operational instructions to other components located in the payload 660, such as the radar transmitter 621, the radar receiver 630, and / or the phase shifters 624 and 627, as is well known to those skilled in the art. The raw SAR data may be stored on the satellite in the memory 108 or 631. Memories 108 and 631 may be the same or different memory modules and may be part of computing system 103.

[0063] The raw SAR data stored in the buffer 135 may be communicated to a ground station 600 or a remote SAR processor 133. In one example, the buffer 135 may store 30 seconds of image data at full resolution (bandwidth). A lower resolution allows more data to be stored (e.g., 60 seconds at half resolution). In an example, the microsatellite has a 150MB download link. At this data rate, it takes approximately 3 minutes to download 30 seconds of full resolution image data.

[0064] The communication system 104 may communicate with earth stations or other satellites using radio frequency communications, optical, eg, laser, communications, or other forms of communication known in the art.

[0065] A satellite, such as satellite 100 of Figure 1, typically includes a propulsion system 190 that generates thrust to steer the satellite. Propulsion system 190 is shown in Figure 1 mounted on the body 110 on the side facing the solar panels 150.

[0066] As shown in FIG. 1, the propulsion system 190 includes a number of thrusters 105 that generate thrust for steering the satellite 100 as needed.

[0067] The thrusters 105 generally operate to maintain the satellite in a particular orbit. For example, the thrusters may be used to propel the satellite in a particular direction relative to the surface of the Earth.

[0068] Returning to FIG. 6, the ADCS 131 is typically located within the satellite body 110 and is used to control the orientation of the satellite. The ADCS may be implemented in a number of ways. The ADCS 131 is shown in the figure as constituting a set of reaction wheels, one of which is shown diagrammatically in FIG. 1. The reaction wheels are typically, but not necessarily, located within the satellite body 110. FIG. 7 is a partial perspective view of a satellite, showing a set of three reaction wheels 41, 42, 43 located within the satellite body 110. The reaction wheels are sometimes also referred to as momentum wheels.

[0069] In the satellites described herein, the ADCS may be used to mechanically steer the satellite as it progresses in its orbit, to keep a target area on the Earth within the radar aperture, or in other words within the satellite's field of view, for a longer period of time than the target would be visible without mechanical steering. In principle, the angular range of the mechanical steering is limited only by the horizon in each direction, but the greater the angle, the greater the distance to the target area and the weaker the returned signal. In the methods described herein, the mechanical steering angle ranges from -45° to +45°, although a higher range, for example from -60° to +60°, is also possible.

[0070] The reaction wheels 41, 42, 43 work by using electric motors to spin wheels inside the spacecraft body 120. By holding an angular momentum, spinning the wheel in one direction will cause the spacecraft to spin in the opposite direction since there are no external forces in space. The use of reaction wheels is a well-known method for orienting spacecraft such as satellites.

[0071] In one example, three reaction wheels are positioned inside the spacecraft body, one for orienting the satellite on each axis, and thus reaction wheels 41, 42, 43 are shown with orthogonal axes.

[0072] In another example, four or more reaction wheels may be used to better control various aspects of the satellite's dynamics, such as slew rate (how fast the satellite can rotate) and precise position control, especially if the satellite has a large moment of inertia. This technique may contribute to the ability to stay at a specific point on the Earth's surface, as discussed in more detail elsewhere, but is not required.

[0073] There are currently many different types of satellites in orbit around the Earth, generally defined by weight ranges, although the boundaries between each type of satellite are somewhat arbitrary and not free.

[0074] Cuboid satellite: 1kg~10kg

[0075] Microsatellite: 50km~250km

[0076] Small satellite: 500kg~800kg

[0077] Normal satellite: 800-1200kg.

[0078] Large satellite:>1200kg

[0079] Reaction wheels are rated in terms of their "momentum capacity", which has units of nms (Newton-meter-second). The slew rate is related to the speed of the wheel and the inertia of the satellite system. A satellite with a particularly low mass will have a much lower moment of inertia than a conventional large SAR satellite. A suitable low mass may be less than 1000 kg, e.g. less than 500 kg, less than 250 kg, between 50 kg and 250 kg, or less than 100 kg.

[0080] Very small cube satellites are currently capable of carrying current SAR payloads. Heavier satellites generally have less agility due to their greater inertia. Embodiments of the satellites and methods of operation described herein have been successfully implemented on microsatellites.

[0081] Embodiments of the present invention are particularly applicable to a class of satellites known as micro-satellites.

[0082] Some of the methods described further herein benefit from reaction wheels within a certain rated range. A range suitable for microsatellites and the like can be 0.5-2.5 nm. Reaction wheels rated at 1 nms have been successfully tested. This allows for a 1° / sec range of turning, which is sufficient to track a spot on the ground and implement any of the methods described herein without consuming significant power. Thus, in any of the satellites described herein, the ADCS may be configured to use mechanical steering to turn the satellite in azimuth at up to 1° / sec. Additionally or alternatively, the ADCS may be configured for a dwell time of up to 60 seconds.

[0083] Larger satellites are known to use reaction wheels on the order of 10 nms, but due to the large mass of the satellite and the resulting high rotational inertia, they currently cannot achieve slew rates sufficient for the dwell times described further herein, and consume much more power than smaller reaction wheels.

[0084] In one example, the satellite orbits the Earth in a low Earth orbit. A low Earth orbit is between 160 km and 1000 km above the Earth's surface. An example of a SAR Earth observation satellite may have an orbit between 450 km and 650 km above the Earth. In one example according to the present invention, the satellite has an orbit that is 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 per second, or 27,000 km per hour. Most satellites in this orbit traverse the Earth at a speed of 7-8 km per second.

[0085] In some embodiments, a microsatellite may be designed to rotate with a slew rate capability of around 17 seconds to keep the SAR antenna stationed at a point on the Earth and pointed in that direction. This is not mechanically achievable with conventional satellites. However, in accordance with some embodiments of the present invention, a satellite such as a microsatellite can rotate at the speed required to keep pointing at a point on the Earth from horizon to horizon for about 10 minutes. However, at the extremes of this range, the actual dwell time is shorter because the distance to the spot or target to be imaged is too far to obtain good SAR images.

[0086] The present invention is not limited to reorienting the entire satellite as described above, which is convenient for such small, lightweight and agile satellites. For example, in some embodiments, mechanical steering may be achieved by reorienting the antenna relative to the satellite on which it is mounted.

[0087] In the above, only one SAR beam has been considered, however, it will be appreciated that the methods and systems described herein may be extended to the use of multiple SAR beams, e.g., a platform may include equipment for multiple SARs, each of which may operate according to any of the methods described herein.

[0088] To give a concrete example, for a satellite traveling 7.5 km / s 550 km above the Earth, ignoring the curvature of the Earth, this means that within 30 seconds the satellite will be traveling 225 km away from directly above the target. To keep pointing directly at the same point on the Earth for the entire 30 seconds requires an angular range of about 23 degrees. Different implementations may use different angular ranges. This is determined by factors such as, but not limited to, mechanical steering capabilities and the capacity of the satellite's onboard memory, as data is typically downloaded in batches as it passes over the ground station.

[0089] From the above, it can be seen that all the methods described here benefit from the use of agile nanosatellites. A nanosatellite of suitable size can rotate and observe a target for a long period of time (up to 60 seconds). This allows an unprecedented ability to achieve many frames of imagery at the same resolution as the range resolution over a period of time.

[0090] The above describes a satellite suitable for carrying out any of the operational methods described herein. In the case of a satellite or other platform already in orbit, the methods described herein may be implemented, for example from the ground, by appropriately controlling the satellite using a suitable computing system, for example the ground station computing system 600. In other words, the SAR may be steered from the ground, and parts of the methods described herein may be implemented in software. Thus, the present invention may be provided with a computing readable medium including 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.

[0091] The acquisition of SAR image data as described herein may have many practical applications. The end-to-end process begins with a request to image specific spots in an area. For example, those spots may be requested by a customer or identified as interesting by an algorithm. A sequence of movements in azimuth and at any elevation may then be devised to optimally acquire the image data using any combination of Figures 5(a), (b1), and (b2). The satellite is then "parked" the next time it is over the general area, steering is performed, and image data is collected.

[0092] 8 includes receiving 801 a request for images of a plurality of regions on the Earth, identifying 803 a subset of the plurality of regions that are close enough to allow image data associated with the identified regions to be acquired during an extended dwell over a larger region including the identified regions, and determining a sequence of steering operations described herein to be performed to enable image data associated with the subset of the plurality of regions to be acquired during the extended dwell 805. Thus, the steering operations include steering the SAR beam in azimuth relative to the direction of travel during a first time period to acquire image data for 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 for one or more additional regions on the Earth to be imaged, and steering the SAR beam in a backward direction relative to the direction of travel during a time period that includes the first and one or more additional time periods to reduce a speed of travel of the beam relative to the Earth.

[0093] Operations 801-805 may be performed by the terrestrial computing system 600. Alternatively, one or more of operations 801-805 may be performed by the on-board computing system 103. In either case, the sequence of steering operations 807 may be communicated to a SAR control device, such as the phase shifter 111 or the ADCS 131. The SAR may then be operated according to any of the methods described herein to obtain image data.

[0094] The embodiments of the invention described herein are described as follows.

[0095] A ground station computing system configured to operate a SAR according to any of the methods described herein.

[0096] A satellite with an antenna that can be pointed at an observation area and keep it pointed in the same place for an extended period of time (60 seconds), much longer than what larger satellites can typically achieve (nominal 2 seconds) due to its small mass and low moment of inertia, allowing antenna and beam pointing to be achieved without consuming fuel and using only internal momentum wheels.

[0097] An imaging mode that allows the target scene to remain within the antenna reception window even when the distance to the target scene varies significantly.

[0098] In any of the embodiments of the present invention, the satellite may travel in low Earth orbit or be configured to travel in low Earth orbit.

[0099] A satellite according to any of the embodiments of the present invention may be configured for side-looking, as is known in the art, and may have both left-looking and right-looking capabilities.

[0100] A satellite according to any of the embodiments of the present invention may use X-band radar.

[0101] The computing systems described herein may be combined into a single computing system having multiple functions. Similarly, the functionality of any computing system described herein may be distributed across multiple computing systems.

[0102] Some operations of the methods described herein may be performed by software, for example in a machine-readable form, for example in the form of a computer program including computer program code. Thus, some aspects of the present invention provide a medium readable by a computing system that, when implemented in a computing system, causes the system to perform some or all of the operations of any of the methods of the present invention. The computer-readable medium may be in a transitory or tangible (or non-transitory) form, such as a storage medium, such as a disk, thumb drive, memory card, etc. The software may be adapted to run on a parallel or serial processor such that the method steps can be performed in any suitable order or simultaneously.

[0103] This application recognizes that firmware and software are separately tradable commodities of value. It is designed to include software that operates or controls on "dumb" or standard hardware to perform a required function. It is also intended to include software that "describes" or defines a hardware configuration, such as HDL (Hardware Description Language) software for designing silicon chips or configuring general purpose programmable chips to perform a desired function.

[0104] The above embodiments are largely automated: in some instances, a user or operator of the system may manually indicate some steps of the method to be performed.

[0105] In embodiments of the present invention, the system may be implemented as any form of computing and / or electronic system, as described elsewhere herein. For example, a ground station may comprise such a computing and / or electronic system. A device such as herein may comprise one or more processors, which may be microprocessors, controllers, or any other suitable type of processor, for processing computer-executable instructions for controlling the operation of the device to collect and record routing information. In some examples, for example when using a system-on-chip architecture, the processor may include one or more fixed function blocks (also called accelerators) that implement parts of the method in hardware (rather than in software or firmware). Platform software, including an operating system or any other suitable platform software, may be provided in the computing-based device to enable the application software to run on the device.

[0106] As used herein, the term "computing system" is used to refer to any device having processing capability capable of executing instructions. Those skilled in the art will appreciate that such processing capability may be incorporated into many different devices, and thus the term "computing system" includes PCs, servers, smart cell phones, personal digital assistants, and many other devices.

[0107] It should be understood that the above benefits and advantages may relate to one embodiment or to several embodiments, and the embodiments are not limited to those that solve any or all of the problems mentioned or that have the benefits and advantages mentioned.

[0108] Any reference to an "item" or "piece" refers to one or more of those items unless otherwise specified. The term "comprising" is used herein to mean including the specified method steps or elements, but that such steps or elements do not constitute an exclusive list and that the method or apparatus may include additional steps or elements.

[0109] Further, to the extent the term "comprising" is used in the detailed description or claims, it is intended that the term have the same inclusiveness as the term "comprising," as the term "comprising" is interpreted as a transitional term within the claims.

[0110] The accompanying figures illustrate exemplary methodologies. Although the methodologies are shown and described as a series of operations performed in a particular order, it is understood and should be understood that the methodologies are not limited by the order. For example, some operations may occur in a different order than described herein. Also, some actions may occur simultaneously with other actions. Furthermore, in some cases, not all operations may be required to implement the methodologies described herein.

[0111] Although the ordering of steps of the methods described herein is exemplary, the steps may be performed in any suitable order, or simultaneously where appropriate. Additionally, steps may be added or substituted to any method, or single steps may be deleted from any method, without departing from the scope of the subject matter described herein. Aspects of any of the above embodiments may be combined with aspects of any of the other embodiments described above to form further embodiments.

[0112] The above description of the preferred embodiment is given by way of example only, and it should be understood that various modifications may be made by those skilled in the art. The above includes one or more exemplary embodiments. Of course, for the purposes of describing the above aspects, it is not possible to describe each possible modification and variation of the above device or method, but those skilled in the art will recognize that many further modifications and arrangements of the various aspects are possible. Therefore, the described aspects are intended to include all such modifications, variations, and variations that fall within the scope of the appended claims.

Claims

1. 1. A method of operating a synthetic aperture radar (SAR) to acquire image data, said SAR being carried on a platform traveling relative to the surface of the Earth and directed toward the surface of the Earth, said 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 region on the 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 regions on the Earth to be imaged; and steering the SAR beam in a backward azimuth angle relative to the direction of travel during a period that includes the first and one or more additional periods to reduce a speed of travel of the beam relative to the Earth.

2. The method of claim 1 , wherein the steering during the first period spans a first range of angles and the steering during the one or more additional periods spans the same range of angles.

3. The method described in claim 2, wherein the steering over the first angle range is forward.

4. The method described in claim 3, wherein the electronic steering is performed using a phased array antenna.

5. The method of claim 1 , wherein the first period and the one or more additional periods overlap.

6. The method of claim 1 , wherein the first period and the one or more additional periods are consecutive.

7. The method described in claim 1, wherein steering in the azimuth angle of the beam to acquire image data is periodic during a period including the first period and the one or more additional periods.

8. The method of claim 1 , further comprising steering the SAR beam to elevation during successive data acquisitions.

9. The method of claim 8 , wherein the SAR beam is electronically steered to altitude.

10. The method of claim 1 , wherein the SAR beam is mechanically steered in a rearward direction.

11. The method of claim 10 , wherein the mechanical steering is performed by changing the orientation of the SAR relative to the platform.

12. The method of claim 10 , wherein the mechanical steering is performed by changing the orientation of the platform relative to the surface of the Earth.

13. The method of claim 1 , wherein the first angular range is at least +5 degrees to −5 degrees, optionally +8 degrees to −8 degrees.

14. 14. The method of any one of claims 1 to 13, wherein the rearward steering spans an angular range of at least -01 degrees to +1 degree, -10 degrees to +10 degrees, -23 degrees to +23 degrees, or -30 degrees to +30 degrees.

15. 1. A method for forming images of different regions on Earth, comprising: receiving a request for imagery of multiple regions on the Earth; identifying a subset of the plurality of regions that are sufficiently close that image data relating to those regions may be acquired during an extended dwell over a larger region that includes the identified region; and determining a sequence of steering operations to be performed to enable acquisition of image data relating to a subset of the plurality of regions during an extended dwell, the steering operations 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 region on the 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 regions on the Earth to be imaged; steering the SAR beam in a backward azimuth angle relative to the direction of travel during a period corresponding to the extended dwell, the period including the first and one or more additional periods, to reduce a speed of travel of the beam relative to the Earth; and communicating the determined sequence of steering maneuvers to a SAR control device.

16. A computing readable medium comprising instructions that, when implemented in a processor of a computing system, cause said computing system to operate a SAR according to the method of any one of claims 1 to 15.

17. A satellite for operation in Earth orbit according to the method of any one of claims 1 to 15, comprising: a propulsion system; an attitude determination and control system (ADCS) configured to steer the SAR beam in a rearward direction; one or more radar antennas or antenna arrays configured to steer the SAR beam in azimuth over the first angular range; Synthetic Aperture Radar (SAR) image data acquisition device; a communications system configured to transmit and receive signals to and from one or more ground stations on Earth.

18. 18. A system comprising a satellite according to claim 17 and a ground station configured to perform one or more of the operations according to claim 15.

Images