High-resolution wide-swath SAR imaging

Abstract

A method of operating a Synthetic Aperture Radar ("SAR") to acquire image data for a swath including one or more sub-swath(s), the SAR being mounted on a platform moving along a flight direction and with an emitted beam directed toward the swath, the method including electronically steering the beam in azimuth along one sub-swath per burst and mechanically steering the beam in a direction opposite the flight direction during each burst. This method can improve the swath-to-resolution ratio.

Description

[Technical field]

[0001] The present invention relates to Synthetic Aperture Radar (SAR) imaging, and more particularly to the field of High-Resolution Wide Swath (HRWS) SAR imaging. [Background technology]

[0002] One of the main applications of Synthetic Aperture Radar (SAR) systems is to image and monitor the Earth's surface. In such applications, SAR systems are typically mounted on aircraft or space-based platforms. SAR systems are active radar systems in which pulses of radio waves are transmitted towards the area to be imaged, and an image is constructed by receiving and processing echoes from the pulse that are reflected or scattered from the target area. SAR systems are fundamentally different from optical imaging systems in that they use electromagnetic radiation of different wavelengths, and in addition provide their own radiation. An advantage over optical systems is that images can be obtained during the day or night, and even through cloud cover.

[0003] SAR systems are well known in the art, and since the invention of SAR in the 1950s, there have been continuous improvements in the ability of SAR systems to image the Earth, for example, in terms of the resolution achievable and the size of the area that can be imaged. Generally, in a "real aperture" radar imaging system, the longer the antenna, the higher the achievable resolution in the direction of travel of the platform carrying the antenna (known as the azimuth resolution). However, the length of the antenna required to achieve good azimuth resolution can be prohibitive in terms of size and weight, especially for space-borne systems. SAR solves this problem by utilizing the motion of the platform carrying the SAR system to create a "synthetic aperture" that can provide similar azimuth resolution as a long "real aperture" antenna, but using a much shorter and smaller antenna. However, conventional single aperture SAR systems are still constrained by a fundamental trade-off between the azimuth resolution that can be achieved and the width of the "strip" that can be imaged, called the swath width. Essentially, as finer azimuth resolution is achieved, the width of the swath that can be imaged decreases, resulting in a trade-off in that wider swaths cannot be imaged without degrading the azimuth resolution.

[0004] This trade-off applies to well-known prior art SAR scanning modes such as stripmap, ScanSAR (Scanning Synthetic Aperture Radar), TOPS (Topographic Propagation), etc. Due to this limitation, techniques that can simultaneously image larger scan swaths while achieving high azimuth resolution are desirable.

[0005] Considering the limitations of the prior art, it is believed that the technical problem underlying the present invention is to provide a SAR imaging method to obtain an improved swath-to-resolution ratio.

[0006] The embodiments described below are not limited to those that address any or all of the shortcomings of known methods discussed above. Summary of the Invention

[0007] This Summary is provided to introduce in a simplified form a selection of concepts 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. Modifications and alternative features used to facilitate the practice of the invention and / or to achieve a substantially similar technical effect are deemed to be within the scope of the invention(s) disclosed herein.

[0008] In a first aspect, there is provided a method of operating a synthetic aperture radar "SAR" to acquire image data for a swath including one or more sub-swaths, the SAR being mounted on a platform moving along a flight direction and a radiation beam being directed at the swath, the method comprising the steps of electronically steering the beam in azimuth along one sub-swath per burst, and mechanically steering the beam in an azimuth direction opposite the flight direction during each burst.

[0009] In a second aspect, there is provided a satellite for operation in Earth orbit including a synthetic aperture radar "SAR" that acquires image data for a swath including one or more sub-swaths, said satellite configured to move along a flight direction, said SAR configured to direct a radiation beam towards the swath, said SAR further configured to electronically steer the beam in azimuth along one sub-swath per burst and also mechanically steer the beam in a direction opposite the flight direction during each burst.

[0010] In a third aspect there is provided a ground station configured to control a satellite to perform the method of the first aspect, optionally according to the second aspect. The ground station may be configured to send control signals to the satellite.

[0011] The methods described herein may be performed, for example, by software on a tangible storage medium in machine-readable form. In the form of a computer program including computer program code means, the program is adapted to perform all steps of any of the methods described herein when executed on a computer and the computer program may be embodied on a computer-readable medium. Examples of tangible (or non-transitory) storage media include disks, thumb drives, memory cards, RAM, flash memory, etc., but do not include propagating signals. 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.

[0012] This application recognizes that firmware and software are separately tradable commodities of value. This is intended to encompass software and firmware 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 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.

[0013] The preferred features may be combined as appropriate and with any aspect of the invention as will be apparent to those skilled in the art. The method according to the first aspect may be described with features according to the satellite according to the second aspect. The satellite according to the second aspect may be configured with features according to the method according to the first aspect. Embodiments of the invention will now be described, by way of example, with reference to the following drawings, in which: [Brief description of the drawings]

[0014] [Figure 1] 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 to acquire swath image data. [Diagram 3] FIG. 3 is a schematic diagram of a satellite operating to acquire swaths of image data during an azimuth burst while performing a mechanical reverse scan. [Figure 4] Schematic diagram showing (a) the satellite heading, mechanical steering direction, and resulting effective ground speed of the satellite beam, and (b) the acquisition pattern. [Diagram 5] 4B is a series of graphs showing (a) electronic steering in azimuth, (b) electronic steering in elevation, (c) mechanical steering in azimuth, and (d) overlay of the acquisition pattern according to FIG. 4B. [Figure 6] 1 is a schematic diagram of components of a satellite. [Figure 7] FIG. [Figure 8] A plot of the signal and potential performance loss due to azimuth ambiguity is shown. [Figure 9] 1 shows a plot of range ambiguity ratio (RAR) versus time for a single burst. [Figure 10] 1 illustrates an exemplary algorithm that can be used to determine the parameters used in image acquisition.

[0015] Common numbers are used throughout the drawings to denote like features. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] In a first aspect, the disclosure provides a method of operating a synthetic aperture radar "SAR" to acquire image data for a swath including one or more sub-swaths, the SAR being mounted on a platform moving along a flight direction and a radiation beam being directed at the swath, the method comprising the steps of electronically steering the beam in an azimuth direction along one sub-swath per burst, and mechanically steering the beam in an azimuth direction opposite the flight direction during each burst.

[0017] First, the terminology used to describe SAR imaging is explained.

[0018] To create a SAR image, successive pulses of radio waves are transmitted to illuminate a target scene, and the echoes of each pulse are received and recorded. A single beamforming antenna can be used to transmit the pulses and receive the echoes. The transmitted pulse can be described as a beam of radiation. In receive mode, the antenna receives reflected and backscattered radiation from this beam of radiation. 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 with time, causing the frequency of the received signal to change due to the Doppler effect. Signal processing of the successively recorded radar echoes allows recordings from multiple antenna positions to be combined to form a synthetic antenna aperture, producing a higher resolution image.

[0019] The area on the Earth illuminated by the SAR at any one instant is called its footprint. A swath is the strip of terrain through which the SAR's footprint passes as it moves across the Earth. The direction along the SAR's flight direction / heading is usually called the azimuth or "along track". The direction across the flight direction is usually called the range or "cross track". The direction opposite the flight direction is the astern heading.

[0020] A swath includes one or more sub-swaths. Each sub-swath may vary in extent (or elevation). As an example, to obtain a wide ground coverage, a swath may consist of at least two and up to five sub-swaths. Alternatively, a swath may include up to ten sub-swaths, or up to twenty sub-swaths, or up to fifty sub-swaths. In each sub-swath, the image data may be blocked into bursts in the azimuth direction. In other words, each sub-swath may be divided into blocks in the azimuth direction, and one block of image data may be collected during a burst. Each burst includes multiple pulses. Typically, a burst may consist of twenty to several hundred pulses. The image data may be acquired using a burst-by-burst approach.

[0021] Beam steering refers to the direction of the radiated antenna beam. For example, in a SAR system with a phased array antenna, the beam can be steered electronically by adjusting the phase of the RF signals going to and from the antenna elements. This changes the direction of the main lobe of radiation sent and received from the phased array antenna. Electronic beam steering can be done quickly with high precision. For example, electronic steering is used when the steering needs to be done quickly and the angle required for steering is not large. This can be called high speed small angle electronic steering.

[0022] Electronic beam steering can typically be performed in the azimuth or "along track" direction, or in the elevation or "cross track" direction, or both.

[0023] An acquisition cycle includes two or more azimuthal bursts, with the beam switched to point to different sub-swaths in the cross-track direction. By performing multiple acquisition cycles in succession, successive lengths of swaths can be imaged.

[0024] Mechanical beam steering refers to directing the beam of a SAR system by physically steering the antenna or the platform carrying the antenna. Mechanical beam steering can allow for wide steering angles and therefore provide wide ground coverage. Larger angles can also be steered, but the steering angular speed is usually much slower than can be achieved with electronic steering. This is also called large angle mechanical steering.

[0025] As indicated in the background, there is a trade-off between azimuth resolution and swath. For real aperture radar (RAR) systems, the azimuth resolution depends on the width of the radar beam (width of illumination) and the distance from the antenna to the target. The beam width is usually inversely proportional to the length of the antenna (also called the aperture), so generally the longer the antenna, the higher the azimuth resolution. However, if the distance to the target is very long (for example, if the radar system is mounted on a space-based platform), the azimuth resolution will be very coarse unless the antenna is very long. Depending on the required azimuth resolution and the distance to the target, the antenna length may need to be several kilometers. This is clearly not practical for aircraft or space-based systems, especially space-based systems.

[0026] SAR systems solve this problem by using the forward motion of the SAR platform and special processing of the echo data to create a very long synthetic antenna length (or aperture) using a much shorter actual antenna. For a SAR system with a focused beam, the azimuth resolution ρ az is independent of the range to the target and is related to the length of the antenna L by a formula well known to those skilled in the art of SAR as Equation 1 below.

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[0027] Because there is no relative movement of the satellites in range or cross-orbit, the factors of the driven range resolution are somewhat different from the azimuth resolution: the azimuth resolution depends on the antenna length, whereas the range resolution depends on the transmitted pulse bandwidth.

[0028] Recall that SAR systems operate in pulsed mode, sending out radar pulses in transmit mode and then turning off the transmit signal to receive the returning echoes. In some SAR systems, the transmit time is, for example, about 5% to 20% of the time required to complete one transmit / receive cycle. The pulses are sent out at a certain frequency, known as the Pulse Repetition Frequency (PRF). By the Nyquist sampling theorem, to avoid aliasing, the PRF must be greater than or equal to all the receive Doppler bandwidth B d As shown in Equation 2, the instantaneous field of view of the target is:

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[0029] In the classical strip map mode, the beam velocity on the ground, V ビーム , is essentially the same as the velocity of the ground-based SAR platform, and V g , Doppler bandwidth B D is expressed by the ground speed and azimuth resolution ρ of the SAR platform as shown in Equation 3 below. az It can be expressed as a function of

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[0030] This results in a azimuth resolution of ΔR, as shown in inequality 4. S and ground-based SAR platform ρ az As a function of slant range swath width V g A fundamental inequality arises that limits

number

[0031] In inequality 4, c is the speed of light and PRI is the pulse repetition interval, which is given by the inverse of the pulse repetition frequency (PRF). This inequality describes the fundamental trade-off between slant range swath width and azimuth resolution. Finer azimuth resolution requires a higher pulse repetition frequency and therefore a smaller pulse repetition interval, thereby resulting in a narrower slant range swath width.

[0032] In addition, the swath width of the slant range ΔR S is the difference between the distance from the antenna to the far end of the swath and the distance from the antenna to the near end of the swath, not the actual width of the swath along the ground. The ground swath width also depends on the angle at which the swath is imaged, and the actual value can be calculated from the slant range swath width using basic trigonometry and techniques well known in the art. In any case, a larger slant range swath width will result in a larger ground swath width for a particular slant angle.

[0033] To provide an example of how the slant range swath width is calculated based on the azimuth resolution, consider a satellite carrying a single aperture SAR system operating in low Earth orbit, approximately 550 km above the Earth's surface. At a distance of 550 km from the Earth's surface, in a rotating Earth-centered coordinate system, the satellite has a ground speed of approximately 7 km / s. g Inserting a speed of light of 7 km / s for a and approximately 300,000 km / s for c into inequality 4, the swath width of the slant range becomes as shown in inequality 5.

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[0034] Inequality 5 can be used to calculate the maximum slant range swath width achievable for a particular satellite resolution. For example, if the desired azimuth resolution is 1.5 m, then the maximum slant range swath width this can achieve is approximately 32 km. For an example where the incidence angle is between approximately 45° and 47.7° and the slant range swath width is approximately 32 km, the ground swath width is approximately 44 km.

[0035] In an embodiment of the present disclosure, a satellite equipped with a single aperture SAR system and a method of operation thereof are described that allows for higher swath widths of slant range to azimuth resolution without the need for multiple apertures. This is achieved by combining mechanical and electronic steering in the azimuth direction. According to this embodiment, during each azimuth burst, the beam is mechanically steered in the direction opposite to the flight direction. This is also called (superimposed) mechanical reverse scanning.

[0036] For the classical SAR strip map mode and ScanSAR mode, v ビーム is essentially the same speed as a ground-based SAR platform.

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[0037] Note that in the case of a satellite-based SAR system, due to the rotation of the Earth, the velocity of the satellite on the ground in the Earth's rotating coordinate system may differ from the inertial velocity of the satellite in orbit, and the satellite may travel in an orbit with a semi-major axis larger than the radius of the Earth. For airborne systems, this difference can be ignored for practical purposes.

[0038] In one example according to the present disclosure, a reverse velocity is superimposed on the satellite's velocity on the ground by mechanically steering the satellite in a direction opposite to the flight direction, thereby reducing the effective velocity of the beam on the ground, V, as shown in Inequation 7 below: ビーム is V g It will be less than.

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[0039] In this example, V in inequality 4 g is no longer V ビーム and mechanically steering the beam in a direction opposite to the flight direction, effectively decoupling the ground speed of the beam from the ground speed of the satellite.

[0040] By decoupling the ground speed of the beam from the ground speed of the satellite, inequality 4 no longer applies, making it possible to achieve a higher swath width to resolution ratio than inequality 4 allows. In one example, the beam ground speed V ビーム is selectable by mechanical steering, and V g to zero. A faster backward mechanical slew rate is achieved with a negative V ビーム may also lead to a decrease in overall residence time at a particular location.

[0041] Acquiring image data for one sub-swath may include acquiring image data in one or more bursts. Two successive bursts may illuminate the same section of one sub-swath, overlapping sections, different adjacent sections, or different sections spaced apart from one another. Thus, a single continuous strip corresponding to one sub-swath may be imaged. Acquiring image data for two or more sub-swaths may include acquiring image data in two or more bursts, at least two of the bursts illuminating different sub-swaths.

[0042] In one example, acquisition of image data within a sub-swath may include electronic steering during each azimuth burst. The beam can be electronically steered in the azimuth direction from back to front (forward azimuth burst). This improves radiation uniformity across the swath compared to conventional ScanSAR. In one example, electronic steering within each burst can also be performed in the azimuth direction from front to back (backward azimuth burst).

[0043] The beam may be mechanically steered by rotating the SAR relative to the platform and / or by moving or pivoting the platform containing the SAR. Alternatively, the SAR may be steered using a rotating reflector. If the platform, such as a satellite, is small enough to be mechanically steered and the antenna is rigidly attached to the platform to direct the beam to a specific target, the platform itself may rotate. The beam may be mechanically steered over a range of field angles of at least -10° to +10°, -23° to +23°, -30° to +30°, -45° to +45°, or -60° to +60°.

[0044] By mechanically steering the beam, the effective ground speed of the beam on the ground can be reduced. If the scanning speed of the mechanical backscan is less than the ground speed of the satellite, the effective ground speed of the beam on the ground can be reduced. If the speed of the mechanical backscan is equal to the ground speed of the satellite, the effective ground speed can be reduced to zero. In this embodiment, a spotlight mode is utilized, where the beam is directed towards a fixed point to illuminate / dwell on a specific area. Longer illumination times result in longer synthetic aperture lengths and improved resolution.

[0045] The steering angular rate for mechanically steering the beam may be lower than the steering angular rate for electronically steering the beam. In one example, the mechanical steering angular rate is at least two or three times lower than the electronic steering angular rate. The steering angular rate of the azimuth burst may be selected because of the better controllability of the electronic steering. For example, during one azimuth burst, the beam may be electronically steered at a steering angular rate of 17 s or more to achieve a long illumination time. Alternatively, per burst, the beam may be electronically steered at a steering angular rate of 27 s or more to perform two consecutive bursts on the same target to increase the number of viewing angles. This contributes to reducing speckle in the image by averaging the value of each pixel over two or more "looks" and to improving resolution by increasing the target illumination time. Between azimuth bursts, the beam may be electronically steered at a much faster rate, such as 1007 s or more. Typically, mechanical steering proceeds at a much slower and more continuous rate than electronic steering due to the inertia associated with mechanical systems. A mechanical reverse scan can be performed in, for example, 17 seconds or less, 27 seconds or less, or 57 seconds or less.

[0046] In one example, the steering angle range for mechanically steering the beam may be at least 5 times, at least 10 times, or at least 30 times higher than the steering angle range for electronically steering the beam. This implementation allows image acquisition to begin already while the target is outside the range accessible to electronic steering and / or continue while the target is no longer within the range accessible to electronic steering. Electronic steering in azimuth is typically performed in the range of ±1°, ±1.5°, or ±2°, while mechanical steering can be performed in the range of up to substantially ±45°. The range of mechanical scanning can be selected depending on the desired image size / swath length and the desired resolution. The limits of the angular range available for electronic steering vary depending on the physical device, but in the example, the limits are set to a maximum of ±25° or more in elevation and ±2° or more in azimuth.

[0047] To increase the total swath width, and therefore the image coverage area, the beam may be electronically steered in elevation between the two bursts. With two-dimensional electronic steering (elevation and azimuth), multiple sub-swaths can be imaged with the same SAR beam. Within a sub-swath, the imaging area can be scanned in one burst, or in two or more bursts of shorter duration. This can be achieved by electronically steering the azimuth of the beam from forward to backward between the bursts, and two or more bursts may each be run as a forward azimuth burst. Thus, the two bursts are run with the same azimuth antenna pattern covering the same area. Between the bursts, the beam can be steered very rapidly using small angle electronic steering in azimuth, elevation, or both, on a very fast virtually instantaneous timescale.

[0048] If a wider swath width is required, the beam can be continuously steered in elevation during a single acquisition cycle that includes multiple bursts, with each burst illuminating a different sub-swath. During a single acquisition cycle, the pattern is not fixed at one sub-swath, but is continuously steered to different elevations corresponding to two or more sub-swaths. Each sub-swath is illuminated during one or more bursts.

[0049] Two or more acquisition cycles may be performed, with each first burst of each acquisition cycle illuminating the same sub-swath. The acquisition cycle may be repeated at different azimuth positions. The steering in elevation is repeated cyclically, allowing imaging of two or more consecutive sub-swaths. Once the last sub-swath is illuminated, the antenna electronically returns to the first sub-swath, leaving no gaps between bursts of the same sub-swath. This acquisition pattern allows for the acquisition of wide-swath SAR images. The electronic steering performed in this acquisition pattern may correspond to the electronic steering of the progressive scanning topographical observation (TOPS) imaging mode. The TOPS acquisition pattern allows for better radiation uniformity compared to regular SCANSARs that use only electronic steering in elevation. Electronic steering in both azimuth and elevation, combined with mechanical steering, allows for wide-swath imaging at high resolution, especially for small and agile satellites. A swath of at least 100km x 100km may be imaged with a resolution of around 5m by a small, agile satellite or microsatellite with a mass of only about 150kg.

[0050] Mechanical steering of the beam in a direction opposite to the flight direction may be performed continuously during two or more acquisition cycles. Thus, a slow backward azimuth scan is superimposed on a fast electronic steering in elevation and azimuth, allowing imaging of two or more consecutive sub-swaths. The mechanical steering during forward-to-aft acquisition may be at a constant rotational speed or at a varying speed, such as that required to maintain a constant effective ground speed. As the beam is mechanically steered from forward to aft, the Doppler frequency shift changes from >0 to 0 to <0. Because the mechanical scans are superimposed, the electronic scan is not always substantially perpendicular to the flight direction. Instead, the line of sight during acquisition may, for example, begin with a substantially non-vertical forward look, pass through a substantially vertical side look, and end with a substantially non-vertical aft look. Using a single continuous mechanical reverse scan increases acquisition time by reducing settling times caused by mechanical antenna positioning, such as moving and rotating / settling the satellite and the antenna on the satellite.

[0051] The parameters used for image acquisition may be determined by selecting an image size, selecting a resolution, selecting a maximum electronic steering angle, calculating a burst duration, selecting a beam speed, and deriving an image acquisition time.

[0052] In a second aspect, the present disclosure relates to a satellite operating in an Earth orbit equipped with a synthetic aperture radar "SAR" that acquires image data for a swath including one or more sub-swaths, the satellite configured to move along a flight direction, the SAR configured to direct a radiation beam towards the swath, the SAR further configured to electronically steer the beam in azimuth along one sub-swath per burst, and mechanically steer the beam in a direction opposite the flight direction during each burst.

[0053] The satellite may be equipped with an Attitude and Decision Control System (ADCS) that includes one or more reaction wheels that control the mechanical steering of the beam by rotating the satellite, including the SAR. In one example, the satellite may use three or more reaction wheels to allow it to rotate about all three axes. The ADCS may be used to control the orientation of the satellite and may be implemented in several ways.

[0054] The satellite may be configured to mechanically steer the beam by slewing in azimuth at up to 1° / sec. The total mass of the satellite may be less than 1000 kg, less than 500 kg, less than 250 kg, or less than 100 kg. A satellite with a small mass will have a much lower moment of inertia than a large conventional SAR satellite. Large satellites have a larger moment of inertia and therefore require more energy and time to accelerate and decelerate again to rotate at a given slew rate. The power requirements to slew a satellite at a given speed are much less onerous for a small satellite system with a smaller moment of inertia, which is advantageous since there is limited power available for satellites in space.

[0055] The SAR may include a small single aperture radar and / or a phased array that allows electronic beam steering in two dimensions. The SAR may comprise a single aperture phased array radar. In a single aperture radar, pulses are transmitted and echoes are received using a single beamforming antenna. Due to the limited space available for the sensor payload, single aperture radars, especially small radars, are desirable for designing compact, high resolution SAR systems on board (unmanned) mobile platforms such as satellites. According to the present disclosure, fine azimuth resolution and wide swath can be obtained simultaneously using a small single aperture radar, which is normally not possible due to the fundamental limit imposed by Inequality 4. Thus, single aperture radars can overcome limitations that could previously only be overcome by multi-aperture approaches at the expense of increasing antenna costs. Phased array antennas with antenna elements spatially distributed over two dimensions perpendicular to the radar range dimension may allow two-dimensional beam steering in azimuth and elevation.

[0056] The physical devices can be designed to provide different electronic steering ranges. In one example, the total range over which a phased array antenna can be electronically steered depends on the spacing between the antenna's elements in azimuth and elevation. The closer the element spacing, the greater the range that can be achieved. The antenna elements of a phased array antenna can be arranged in a two-dimensional grid pattern such that the spacing in one direction is significantly different than the spacing in the other direction. Thus, the angular range in azimuth can be significantly different than the angular range in elevation, even though electronic steering is achieved in a similar manner in both directions. In one example, a phased array antenna has 20 antenna elements spread over 3.2 m to steer a beam in azimuth, with antenna element spacing of about 160 cm, and an electronic steering range of about ±1° azimuth. In the same example, the same phased array antenna has 16 antenna elements spread over 40 cm to steer a beam in elevation, providing an antenna element spacing of about 2.5 cm. The closer spacing provides an even greater electronic steering range in elevation at ±25°. Higher angular coverage in the azimuth direction can be achieved by adding more antenna elements and spacing them closer together, but this may result in additional complexity, weight, cost, and other trade-offs. In a third aspect, the present disclosure provides a ground station for controlling a satellite according to the second aspect to perform the method of the first aspect. The ground station may be configured to send control signals to the satellite.

[0057] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention are described below by way of example only, as these embodiments represent the best patterns for carrying out the invention currently known to the applicants, although they are not the only ways possible.

[0058] 1 is a perspective view of a satellite 100 in orbit above the Earth as an example of a platform for use in the methods and systems for Earth observation described herein. A target area on the Earth to be imaged is indicated at 200. The satellite 100 comprises a body 110, solar panels 150, and "wings" 160. The satellite wings may carry one or more antennas. Each antenna may comprise a phased array antenna, or in other words, each antenna may comprise multiple antenna elements that may be controlled to electronically steer the direction of the antenna beam, control the direction and shape of the transmitted pulses, and / or control the direction and area from which echoes are received.

[0059] In addition to electronic steering, satellite 100 may be configured to mechanically steer the antenna and therefore the beam when in transmit and / or receive mode. In this example, the mechanical steering is accomplished by steering the entire satellite 100. This may be accomplished using a satellite attitude determination and control system ADCS, which may be provided with one or more reaction wheels, one of which is shown at 170. The ADCS may be used to mechanically steer the satellite 100 as it progresses through its orbit to keep the target area 200 within the radar aperture, i.e., in view of the satellite, for a longer period of time than the target area 200 would be visible without mechanical steering.

[0060] The satellites of FIG. 1 are described in more detail with reference to FIGS.

[0061] 2 is a schematic diagram of a satellite operating in ScanSAR imaging mode according to the prior art. The satellite 100 is in a well-known side-looking configuration, transmitting and receiving signals from a target area 200 to the side of the satellite 100, rather than directly below it. In this side-looking configuration, the bottom of the wings 160 (where the antenna elements are located) is pointed toward the area to be imaged. The satellite 100 travels toward a flight direction 120. The target area 200 has a width, also known in the art as a swath. In this example, the swath includes sub-swaths 200A, 200B, 200C. For each radar pulse transmitted from the satellite 100, signal data in the form of echoes may be received from different points at different frequencies across the swath 200, due to the Doppler effect, which is at the heart of SAR.

[0062] The satellite 100 is shown moving in its orbit from right to left as indicated by the arrow relative to the Earth. In ScanSAR mode, the area to be collected, e.g., the 100 km*100 km area shown in FIG. 2, is divided into an appropriate number of sub-swaths, e.g., three sub-swaths 200A, 200B, 200C, and each sub-swath is divided in azimuth into what we call "blocks". Thus, in each sub-swath, the received data is blocked into bursts of radar echoes in azimuth direction. In the example of FIG. 2, the blocks form an offset pattern. A wide swath 200 consisting of several sub-swaths 200A, 200B, 200C is imaged by alternately illuminating each sub-swath. The radar antenna beam sweeps through the different elevation sub-swaths 200A, 200B, 200C to image the wide swath 200. The available illumination time is shared among many bursts covering different areas or "blocks" of the ground, so azimuth resolution is sacrificed for greater coverage. Electronic steering in elevation is performed very quickly to switch between the different sub-swaths 200A, 200B, 200C.

[0063] In classical ScanSAR as shown in Fig. 2, the beam velocity on the ground is the same as the ground velocity of the satellite. Therefore, during each burst, no electronic beam steering is performed and the beam slides with the ground velocity of the satellite. Therefore, the swath-per-resolution constraint according to Eq. 4 applies, which results in a loss of azimuth resolution.

[0064] In some methods and systems described in more detail below, mechanical beam steering in the direction opposite to the flight direction is performed, while electronic beam steering in azimuth during each burst is also performed to enhance radiation uniformity.

[0065] FIG. 3 shows diagrammatically how image data of a swath 200 is acquired when operating in burst mode while performing a mechanical reverse scan. The mechanical steering is shown exaggerated in FIG. 3, and the satellite 100 is shown to have a different orientation at each of three positions (a), (b), and (c) along the path. Between each position along the flight direction 120, the satellite 100 rotates to point its azimuth direction further aft, as shown by the arrows. The rotation is counter-rotating to the flight direction 120. In a first position a, the satellite 100 looks forward relative to the flight direction 120, in a second position b, the satellite beam is perpendicular to the satellite's flight direction, and in a third position c, it looks backward relative to the flight direction 120. Thus, the satellite 100 performs a mechanical reverse scan. Due to the superimposed mechanical scans, the line of sight direction is not substantially perpendicular to the flight direction, except at position (b). This implementation makes it possible to start image acquisition already before position (a) while the target 200 is still outside the range accessible by electronic steering, and / or to continue after position (c) while the target is not within the range accessible by electronic steering.

[0066] In addition to the mechanical backward scanning, the beam is electronically steered in the azimuth direction during each burst, as indicated by the arrows between the beams at each position in Figure 3. The azimuth electronic steering is performed in the forward direction as indicated by the arrows. In other examples, the azimuth electronic steering can be performed in the reverse direction. With azimuth beam steering, all points are illuminated with a full azimuth beam. Compared to the ScanSAR shown in Figure 2, the azimuth rotation achieved by electronic steering throughout the acquisition achieves the same swath coverage, but with better radiation uniformity.

[0067] The ground beam speed during acquisition corresponds to the sum of mechanical and electronic steering within one burst, thus decoupling the ground beam speed from the satellite ground speed. In other words, the ground speed of the beam depends on the scanning speed during the azimuth burst and the speed of the mechanical reverse scan. Thus, the beam ground speed is selectable by mechanical steering. Mechanically steering the beam in the direction opposite to the flight direction 102 reduces the effective ground speed of the beam on the Earth, thereby allowing greater illumination and therefore improved resolution. Slowing the ground speed of the beam increases the burst duration, which allows longer acquisition times and improved resolution. Thus, the azimuth resolution is not compromised, since the swath-per-resolution constraint according to Inequality 4 does not apply.

[0068] In addition to mechanical backward scanning and electronic steering in azimuth for each burst, the beam is continuously steered in elevation during multiple acquisition cycles. In the example of FIG. 3, swath 200 includes three sub-swaths 200A, 200B, 200C, each of which includes multiple blocks. For example, at the end of a burst in the first sub-swath 200A, the line of sight is changed to illuminate the second sub-swath 200B, which again points backward. Once the third / last sub-swath is imaged, the beam returns to the first sub-swath, ensuring that no gaps are left between bursts of the same sub-swath. Beam steering between bursts is achieved by fast small angle electronic steering.

[0069] Swath 200, an area of ​​100 km x 100 km shown in Figure 3, can be imaged with a resolution of 5 m. The satellite moves at a speed of 7·5 km / s along the flight direction and the mechanical steering superimposes a speed of -3.75 km / s on the satellite's actual ground speed, resulting in an effective ground speed of 3.25 km / s. The total mechanical steering angle in this example is 35°.

[0070] A further example of the superposition of electronic and mechanical steering is described with reference to FIGS.

[0071] FIG. 4a shows how the beam ground speed can be selected by mechanical steering. The top arrow 124 in FIG. 4a represents the satellite's velocity along the flight direction 120. The arrow 126 represents the direction of mechanical steering, which is directed against the flight direction 120 and is less than the velocity of the satellite 124. The arrow 202 shows the effective ground speed resulting from the beam directed along the flight direction 120. By using a superimposed reverse mechanical scan, the effective ground speed of the beam is reduced and decoupled from the ground speed of the satellite. The effective ground speed of the beam 202 can be selected via mechanical steering 206. Although not shown in FIG. 4a, the scan speed induced during electronic steering within each azimuth burst can be directed along the flight direction or against the flight direction. For completeness, the elevation steering is not shown as it does not affect the ground speed of the beam.

[0072] Figure 4b shows the offset pattern of the burst image after processing the raw synthetic aperture radar data. Figure 4b shows an area on the ground divided into small areas or blocks with an offset pattern, similar to the operation of the SAR in ScanSAR or TOPS mode. Thus, as mentioned before, the SAR beam can be electronically steered to collect data from blocks 1-15 in numerical order and from left to right in the flight direction 120 (see Figure 4a). The hatched rectangle within the dashed line in Figure 4b corresponds to the imaged area or swath of 100km*100km. In comparison to Figure 3, the swath includes four sub-swaths 200A, 200B, 200C, 200D.

[0073] The acquisition pattern shown in FIG. 4b is explained with reference to the time diagram in FIG. 5. The blocks labeled 1-10 in FIG. 4b correspond to bursts 1-10 shown in FIG. 5d. Thus, each block is imaged in one (single) burst. Instead of imaging each block in a single burst, multiple bursts can be performed for each block, increasing the number of look angles for each block, but at the cost of lower resolution. If each block is imaged in one burst, then one acquisition cycle includes four successive bursts corresponding to blocks in four adjacent sub-swaths 200A, 200B, 200C, 200D at different elevations. Thus, to image a swath, in the first acquisition cycle, blocks 1-4 are imaged, in the second successive acquisition cycle, blocks 5-8 are imaged, and so on.

[0074] So, as shown in Figure 5a, the beam is periodically electronically steered forward in azimuth, steering the azimuth beam angle from a negative angle, through zero, to an equal positive angle over the duration of the burst. At the end of the burst period, the beam quickly returns to the negative angle, which can happen within a microsecond, and is therefore shown as a substantially vertical line in Figure 5a. For successive bursts, this is repeated over the same angle range.

[0075] To switch between sub-swaths, the beam is electronically steered in elevation periodically, as shown in FIG. 5b. During each burst, the beam points to a constant elevation in each burst period corresponding to a respective sub-swath 200A, 200B, 200C, 200D, shown as a horizontal line in FIG. 5b. At the end of the burst period, the beam is rapidly (stepwise) steered in elevation and cross-track direction to the next sub-swath. At the end of each acquisition cycle, the beam quickly returns to the first sub-swath. This occurs within microseconds, and is therefore shown as a substantially vertical line in FIG. 5b.

[0076] Superimposed on the periodic electronic steering in azimuth and elevation is mechanical rear steering in azimuth over a wider range of angles and for a longer period of time, as shown in FIG. 5c. FIG. 5c shows two simplified examples of mechanical steering for illustration purposes. The solid line shown in FIG. 5c corresponds to a constant steering angular velocity of the mechanical steering, such as a constant rotation of the satellite. The dashed line in FIG. 5c corresponds to a non-linear steering angular velocity of the mechanical steering, which can be used to maintain a constant effective ground speed. The non-linear steering angular velocity has a high start and end velocity when the steering angle is at its maximum, and a low velocity when the steering angle is near zero. The mechanical rear steering in azimuth may correspond to a combination of linear and non-linear steering angular velocities.

[0077] The azimuth of the beam angle may be mechanically steered from a positive angle, through zero, to an equal negative angle over the acquisition period (only the beginning of the acquisition period is shown in FIG. 5). For the example rotation shown in FIG. 3, a positive angle corresponds to the first (forward looking) position (a), zero corresponds to the second (perpendicular to the flight direction) position (b), and a negative angle corresponds to the third (rear looking) position (c). Mechanical aft steering may also be performed at asymmetric steering angle ranges, e.g., only for steering angles <0 or >0.

[0078] As shown in Figure 5, especially comparing Figures 5a and 5c, the steering angular velocity for mechanically steering the beam is lower than the steering angular velocity for electronically steering the beam. Due to the high controllability of the electronic steering rate, a specific electronic scanning rate can be selected to increase the exposure time of each block. In addition, the steering angle range for mechanically steering the beam is much larger than the steering angle range for electronically steering the beam. A typical steering angle range for electronically steering the beam is about ±1° in azimuth and up to ±25° in elevation, while the mechanical steering angle range can be up to ±45°, or even up to ±60°. Although a larger azimuth steering angle can theoretically be achieved with electronic beam steering, it would require a more complex, larger, and more expensive antenna. By combining high-speed small-angle electronic steering with large-angle mechanical steering, an improved swath width to resolution ratio can be achieved that is not possible with other known single aperture radar imaging techniques.

[0079] In a practical implementation, the total acquisition time for the area shown by the rectangle in Figure 4b (including three or four sub-swaths) is about 40 seconds, and a resolution of 5 μm can be achieved. In this example, each burst requires about 2.5 to 3 seconds. For comparison, the total acquisition time for the same size area in ScanSAR mode (as shown in Figure 2) is about 15 seconds, with a resolution of 15 m. The longer acquisition time according to the present invention is due to the mechanical backscanning superimposed on the satellite speed on the ground, which allows for a slower effective ground speed of the beam. In another example, a 60 km x 60 km area including two sub-swaths may be imaged with a resolution of 3 m and a total acquisition time of 35 seconds.

[0080] As noted elsewhere, the methods described herein are particularly, but not exclusively, suitable for implementation in conjunction with a SAR mounted on a satellite. The SAR may also be mounted on other platforms, such as an aircraft. A satellite suitable for implementing the invention will now be described with reference to Figures 1, 6 and 7.

[0081] 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.

[0082] Some components are 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, 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 in the payload 660, such as the pulse generator 620 and the power amplifier 623. 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 form part of the computing system 103. The propulsion controller may be controlled through the use of control software implemented in one or more processors included in the propulsion controller 109 or in response to instructions received, for example, from the computing system 103. When instructions are sent from the computing system 103, the computing system may be considered to constitute 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.

[0083] The satellite bus 610 may generally be located within the body of the satellite 110. The power distribution system 102 may comprise control logic as known in the art. The communication system 104 may include, for example, one or more communication antennas located on the satellite body. Alternatively, the communication system 104 may transmit and receive signals via one or more communication antennas located on the wings of the satellite.

[0084] In the case of an Earth Observation satellite, the satellite payload 660 may include one or more radar antenna arrays that may be 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. A phased array antenna on board a satellite 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 phase shifters 624 and 627, as known in the art.

[0085] The pulse generator 620 generates an RF signal that is sent to the radar transmit / receive module 621. The radar signal is sent to the RF divider 622, which splits and sends the RF signal to the multiple antenna modules 670. Although one antenna module 670 is shown in FIG. 6, there may 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 transmit / receive module 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 may be located in either the satellite body 110 or the satellite wings 160. The additional arrows extending from the 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 the RF combiner 629 indicate one or more additional RF inputs from the one or more additional antenna modules to the RF combiner 629.

[0086] The methods and systems described herein refer to steering of a single antenna or a single aperture, however, they can be easily extended to systems with multiple antennas or multiple apertures.

[0087] As known to those skilled in the art, antenna modules 670, multiplexed with a number of antenna modules, collectively form a satellite image capture system, which may perform functions other than capturing image data.

[0088] In a typical satellite, the antenna may include a phased array antenna as described above. A phased array antenna, with antenna elements spatially distributed across two dimensions perpendicular to the radar range dimension, may enable two-dimensional beam steering in azimuth and elevation.

[0089] The available electronic steering of a phased array antenna can be limited by the range of the physical antenna and the spacing of the phase centres in azimuth, and attempting too much steering will reduce gain and increase grating lobes. The limits of the available angular range vary depending on the physical device, but typical limits are set at ±25° in elevation and ±2° in azimuth.

[0090] 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 outputs raw data to the communication system 104, which may also send it out for further processing at a remote computing system. In FIG. 6, the SAR processor 133 may be located, for example, at a ground station or another processing location. As is well known to those skilled in the art, the computing system 103 may send operational instructions to other components located within the payload 660, such as the radar transmitter 621, the radar receiver 630, and / or the phase shifters 624 and 627. The raw SAR data may be stored in the satellite's memory 108 or 631. Memories 108 and 631 may be the same or different memory modules and may be part of computing system 103.

[0091] The raw SAR data is stored in a buffer 135 and communicated to a ground station 600 or remote SAR process 133. In an example, 30 seconds of image data can be stored at full resolution (bandwidth). More can be stored at lower resolution (e.g. 60 seconds at half resolution). For example, a microsatellite has a 150MB download link. At this data rate, it takes about 3 minutes to download 30 seconds of full resolution image data. During operation, about 5000 pulses per second can be transmitted. This means that there can be 27 pulses in the air at any one time. A burst typically consists of 500-1000 pulses and takes 2-3 seconds.

[0092] The communication system 104 may communicate with earth stations or other satellites using radio frequency communications, optical, eg, laser, communications, or any other type of communications known in the art.

[0093] A satellite, such as satellite 100 of Figure 1, is typically provided with a propulsion system 190 for steering the satellite with the thrust generated by the propulsion system 190. Propulsion system 190 is shown in Figure 1 as being mounted on the surface of body 110 opposite solar panels 150.

[0094] 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.

[0095] Thrusters 105 generally operate to keep the satellite in a particular orbit. The thrusters may be used to propel the satellite in a particular direction relative to the surface of the Earth.

[0096] Referring back 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. It is shown in the figure to comprise 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.

[0097] The satellites described herein may use the ADCS to mechanically steer the satellite as it progresses in orbit to keep a target area 200 on the Earth within the radar aperture, and thus within the satellite's field of view, for a longer period of time than the target would be visible without the satellite's mechanical steering. In principle, the angular range of 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.

[0098] 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.

[0099] 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.

[0100] 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.

[0101] 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. Cuboid satellite: 1kg~10kg Microsatellite: 50kg~250kg Small satellite: 500kg~800kg Normal satellite: 800-1200kg. Large satellite:>1200kg

[0102] 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.

[0103] 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 operating methods described herein have been successfully implemented in microsatellites.

[0104] Embodiments of the present invention are particularly applicable to a class of satellites known as micro-satellites.

[0105] Some of the methods described further herein benefit from reaction wheels within a certain rated range. A suitable range 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 turning range of 17° / sec, 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.

[0106] 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.

[0107] 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.

[0108] In some embodiments, a satellite, such as a microsatellite, can orbit at a speed necessary to keep it pointed at a point on the Earth from horizon to horizon for about 10 minutes, although at the extreme ends of this range, the actual dwell time is shorter because the spot or target being imaged is too far away to obtain good SAR images.

[0109] 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.

[0110] 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. For example, a platform may include equipment for multiple SARs, each of which may operate according to any of the methods described herein.

[0111] From the above, it will be appreciated that all of the methods described herein benefit from the use of agile nanosatellites. Nanosatellites of suitable size can rotate to observe a target over an extended period of time. This allows for an unprecedented ability to achieve many frames of imagery at the same resolution as the range resolution over a period of time.

[0112] The above describes a satellite suitable for performing any of the operating 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 operates from the ground and some 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.

[0113] 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. Depending on the size of the area and the desired resolution, an appropriate number of sub-swaths may be selected. Based on the number of sub-swaths, a sequence of azimuth bursts may be devised to optimally acquire image data. Before acquisition begins, the satellite may be rotated to an initial position, such as position (a) in FIG. 3. From the initial position, image data may be collected by mechanically steering the beam opposite the flight direction and electronically steering the beam according to a sequence of bursts.

[0114] In one example, a request is received from a customer to image a relatively large area of ​​100km x 100km. Using the conventional Topographical Observation with Progressive Scanning (TOPS) mode, where electronic steering is used in both azimuth and elevation, a 100km wide swath may be imaged using three sub-swaths to achieve a resolution of, for example, 15 meters. Using the TOPS mode, the acquisition takes approximately 15 seconds as the satellite orbits 100km over the area of ​​interest.

[0115] In one example, a small agile satellite weighing approximately 150 kg can image a 100 km x 100 km area with 5 m resolution using the apparatus, methods, and techniques described herein, combining the satellite's mechanical steering capabilities with electronic steering to slow down the effective ground speed of the SAR beam. In one example, 5 m resolution can be achieved by dividing the 100 km x 100 km area into four sub-swaths and extending the acquisition time by approximately 42 seconds. This represents a three-fold improvement in resolution compared to the TOPS example for imaging this area.

[0116] FIG. 8 illustrates the performance of this 100km x 100km example by plotting the potential performance loss due to signal and azimuth ambiguity. The vertical axis is shown in decibels (dB) and the horizontal axis is azimuth in degrees. In general, it is desired that signal degradation be minimized with low azimuth ambiguity values. From FIG. 8, it can be seen that the signal at the edge of each burst shows degradation of about -2.5 dB or less. The total ambiguity trace (AmbTot) shows a worse value of about -17 dB at the very edge of the burst. This is considered to be within the acceptable range of edge degradation. As described below with reference to FIG. 10, the azimuth ambiguity threshold can be input into an algorithm for determining possible imaging area and resolution.

[0117] Figure 9 shows a plot of performance loss due to range ambiguity (AmbTotal) and range ambiguity ratio (RAR) versus time for a single burst. The traces shown in both Figures 8 and 9 are obtained from a mathematical model in which azimuth ambiguity and distance ambiguity values ​​are calculated by numerical integration of appropriate parts of the antenna pattern. In this example imaging a 100km x 100km area with 5m resolution, the highest value of the total ambiguity trace (AmbTotal) is about -28dB, leading to a worse case RAR of about -24dB. This is considered to be within acceptable limits. As with the azimuth ambiguity values, thresholds can be used to determine the achievable resolution and imaging area using high resolution wide swath techniques.

[0118] In another example, a high-resolution wide-swath technique can be used to image a small area of ​​60km x 60km at an even finer resolution of 3m by using two sub-swaths and mechanical steering superimposed on top of electronic steering over a 35 second acquisition period. In one example, even finer resolutions are possible, such as sub-1m resolution.

[0119] Figure 10 shows an example of an algorithm that can be used to determine the parameters to use for a particular acquisition. In a first step 1101, an image size is selected, for example 100kmx100km, 60kmx60km. In a second step 1102, a resolution is selected, for example 5m. In step 1103, a maximum electronic steering angle in azimuth is selected, which determines the patch size on the ground. The maximum electronic steering angle is constrained by the antenna design, and more specifically, the azimuth spacing of the antenna elements, how far the antenna can scan before grating lobes become a problem.

[0120] The resolution then determines the burst duration, which is calculated in step 1104 according to Equation 9.

number

[0121] In Equation 9, τ is the burst duration, λ is the wavelength, R is the slant range, W is the satellite velocity, ρ az is the azimuth resolution. In step 1105, the beam velocity, V g , and derive the image acquisition time from the along-track image size in step 1106, so as to slide the patch for a given time τ. This method allows the calculation of the parameters required to task the satellite with image acquisition. Any image size and any resolution can be implemented, subject to acquisition time constraints and squint angle limitations.

[0122] Any of the computing systems described herein may be combined into a single computing system with multiple functions. Similarly, the functionality of any computing system described herein may be distributed across multiple computing systems.

[0123] 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.

[0124] 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.

[0125] 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.

[0126] 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.

[0127] 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.

[0128] 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.

[0129] 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.

[0130] The accompanying figures illustrate exemplary methodologies. Although the methodologies are shown and described as a series of computations performed in a particular order, it is understood and should be understood that the methodologies are not limited by the order. For example, some actions may occur in a different order than described herein. Also, some actions may occur simultaneously with other actions. Furthermore, in some cases, not all computations may be required to implement the methodologies described herein.

[0131] 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.

[0132] The above description of the preferred embodiment is given by way of example only, and it should be understood that those skilled in the art may make various modifications. 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 encompass all such changes, modifications, 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 for a swath including one or more sub-swaths, the SAR being mounted on a platform moving along a flight direction and a beam of radiation being directed at the swath, the method comprising: electronically steering the beam azimuthally along one sub-swath per burst; and mechanically steering the beam in a direction opposite to the direction of flight on a burst-by-burst basis during each burst.

2. 2. The method of claim 1, wherein the step of mechanically steering the beam is performed by rotating the SAR relative to the platform and / or by moving or pivoting the platform containing the SAR.

3. The method of claim 1 , wherein mechanically steering the beam reduces the effective ground speed of the beam above the Earth.

4. The method of claim 1 , wherein a steering angular velocity for mechanically steering the beam is lower, optionally at least three times lower, than a steering angular velocity for electronically steering the beam.

5. 10. The method of claim 1, wherein a steering angle range for mechanically steering the beam is higher, optionally at least 30 times higher, than a steering angle range for electronically steering the beam.

6. The method of claim 1 further comprising the step of electronically steering the beam in elevation between the two bursts.

7. 10. The method of claim 1, further comprising the step of sequentially steering the beam in elevation during an acquisition cycle comprising several bursts, each burst illuminating a different sub-swath.

8. The method of claim 7 , further comprising the step of performing two or more acquisition cycles, wherein each first burst of each acquisition cycle illuminates the same sub-swath.

9. The method of claim 7 , further comprising continuously mechanically steering the beam in a direction opposite the flight direction during the one or more acquisition cycles.

10. The parameters used for image acquisition are: a. Selecting an image size; b. Selecting a resolution; c. Selecting a maximum electronic steering angle; d. Calculating the burst duration; e. Selecting a beam speed; f. deriving the image acquisition time.

11. 1. A satellite for operating in orbit around the Earth, comprising a synthetic aperture radar "SAR" for acquiring image data of a swath including one or more sub-swath(s), said satellite configured to move along a flight direction, said SAR configured to direct a beam of radiation towards the swath, said SAR further comprising: electronically steering the beam azimuthally along one sub-swath per burst; and A satellite configured to mechanically steer a beam in a direction opposite to the direction of flight during each burst.

12. 12. The satellite of claim 11, wherein the satellite includes an attitude determination and control system "ADCS" including one or more reaction wheels configured to control mechanical steering of the beam by rotating the satellite including the SAR.

13. 12. The satellite of claim 11, wherein the satellite is configured to mechanically steer the beam by slewing in azimuth at a rate of up to 1 / second.

14. A satellite according to claim 11, having a total mass of less than 1000 kg, optionally less than 100 kg.

15. A satellite according to any one of claims 11 to 14, wherein the SAR comprises a small single aperture radar and / or a phased array allowing electronic beam steering in two dimensions.

16. A ground station configured to control a satellite according to claim 15, and optionally to carry out the method according to claim 10.

Images