High-resolution wide-band SAR imaging

Abstract

A method is provided for operating a synthetic aperture radar (SAR) to acquire image data of a mapping strip comprising one or more sub-mapping strips, wherein the SAR is carried on a platform moving along the flight direction and a radiated beam is directed to the mapping strip. The method includes: for each cluster, electronically manipulating the beam in the azimuth direction along the sub-mapping strip; and mechanically manipulating the beam in the opposite direction to the flight direction during each cluster. This method allows for improved mapping strip resolution.

Description

[0001] This invention relates to synthetic aperture radar (SAR) imaging. More specifically, this invention relates to the field of high-resolution wide-strip (HRWS) SAR imaging. Background Technology

[0002] One of the primary uses of Synthetic Aperture Radar (SAR) systems is for imaging and monitoring the Earth's surface. In this application, SAR systems are typically carried out on airborne or spacecraft platforms. SAR systems are active radar systems in which pulses of radio waves are emitted toward the area to be imaged, and an image is constructed by receiving and processing the echoes from the pulses as they are reflected or scattered back from the region of interest. A fundamental difference between SAR systems and optical imaging systems is that they use electromagnetic radiation of different wavelengths, and they provide their own radiation. They have advantages over optical systems in that they can acquire images both day and night, and also through cloud cover.

[0003] SAR systems are well known in the art, and improvements in their ability to image the Earth have continued since the invention of SAR in the 1950s, for example, regarding achievable resolution and the size of the imageable area. Generally, the longer antenna in a “real aperture” radar imaging system results in a higher achievable resolution (called azimuth resolution) in the direction of travel of the platform carrying the antenna. However, the antenna length required to achieve good azimuth resolution can make them impractical in terms of size and weight, especially for spacecraft systems. SAR addresses this problem by creating a “synthetic aperture” using the movement of the platform carrying the SAR system, which provides azimuth resolution similar to that of a longer “real aperture” antenna, but using a much shorter and smaller antenna. However, conventional single-aperture SAR systems are still limited by a fundamental trade-off between achievable azimuth resolution and the width of the imageable “strip” (called mapping strip width). Essentially, the trade-off is that achieving finer azimuth resolution reduces the width of the imageable mapping strip, and a wider mapping strip cannot be imaged without reducing azimuth resolution.

[0004] This trade-off applies to well-known existing SAR scanning modes, such as Stripmap, ScanSAR (Scanning Synthetic Aperture Radar), and TOPS (Topographic Observation Using Progressive Scanning). Due to this limitation, a technique is desired that allows for high azimuth resolution while imaging a wider mapping strip.

[0005] Considering the limitations of the prior art, the technical problem to be solved by the present invention can be seen in providing a SAR imaging method to obtain improved mapping strip resolution.

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

[0007] This invention is provided to introduce, in a simplified form, some concepts that will be further described in the following detailed description. This invention is not intended to identify key or essential features of the claimed subject matter, nor is it intended to define the scope of the claimed subject matter; variations and alternative features that contribute to the work of this invention and / or achieve substantially similar technical effects should be considered to fall within the scope of the invention disclosed herein.

[0008] In a first aspect, a method is provided for operating a synthetic aperture radar "SAR" to acquire image data of a mapping strip comprising one or more sub-mapping strips, wherein the SAR is carried on a platform moving along a flight direction and a radiated beam is directed to the mapping strip, the method comprising: for each cluster, electronically manipulating the beam in an azimuth direction along the sub-mapping strip; and mechanically manipulating the beam in a direction opposite to the flight direction during each cluster.

[0009] In a second aspect, a satellite for operation in orbit around the Earth is provided, comprising a synthetic aperture radar "SAR" to acquire image data of a mapping strip comprising one or more sub-mapping strips, wherein the satellite is configured to move along a flight direction and the SAR is configured to direct a radiated beam toward the Earth, wherein the SAR is further configured to electronically manipulate the beam along the sub-mapping strip in an azimuth direction for each cluster and to mechanically manipulate the beam in a direction opposite to the flight direction during each cluster.

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

[0011] The methods described herein can be executed by software in a machine-readable form on a tangible storage medium, for example, as a computer program comprising computer program code means that, when run on a computer, is adapted to perform all the steps of any method described herein, and wherein the computer program can be included on a computer-readable medium. Examples of tangible (or non-transitory) storage media include disks, thumb drives, memory cards, RAM, flash memory, etc., and do not include propagation signals. The software can be adapted to execute on a parallel or serial processor, such that the method steps can be executed in any suitable order or simultaneously.

[0012] This application acknowledges that firmware and software can be valuable, separately tradable goods. It is intended to include software and firmware that run on or control “dumb” or standard hardware to perform desired functions. It is also intended to include software that “describes” or defines the configuration of hardware, such as HDL (Hardware Description Language) software, for example, for designing silicon chips or configuring general-purpose programmable chips to perform desired functions.

[0013] Preferred features can be suitably combined, as will be apparent to those skilled in the art, and can be combined with any aspect of the invention. The method according to the first aspect can be described in conjunction with the features described in the satellite according to the second aspect. The satellite according to the second aspect can have features in conjunction with the features described in the method according to the first aspect. Attached Figure Description

[0014] Embodiments of the invention will be described by way of example with reference to the following figures, wherein:

[0015] Figure 1 This is a schematic perspective view of a satellite in orbit above the Earth.

[0016] Figure 2 is a schematic diagram of a satellite operating in ScanSAR mode to obtain image data of the mapping zone.

[0017] Figure 3 This is a schematic diagram of a satellite that operates during azimuth scattering while performing a mechanical backward scan to acquire image data of the mapping strip.

[0018] Figure 4 This is a schematic diagram showing (a) the satellite's direction of travel, the direction of mechanical maneuvering, and the combined effective ground speed of the satellite beam, and (b) the acquisition pattern.

[0019] Figure 5 It shows the basis Figure 4 b is a series of graphs showing (a) electronic control in azimuth, (b) electronic control in elevation, (c) mechanical control in azimuth, and (d) the superposition of acquisition patterns.

[0020] Figure 6 This is a schematic diagram of the satellite components.

[0021] Figure 7 This is a partial perspective view of the satellite.

[0022] Figure 8 The graph shows the signal and the potential performance loss due to ambiguity in the azimuth direction.

[0023] Figure 9 The graph shows the range ambiguity ratio (RAR) of a single cluster versus time.

[0024] Figure 10 An exemplary algorithm is shown that can be used to determine parameters for image acquisition.

[0025] The same reference numerals are used in all the accompanying drawings to indicate similar features. Detailed Implementation

[0026] In a first aspect, the present invention provides a method for operating a synthetic aperture radar "SAR" to acquire image data of a mapping strip comprising one or more sub-mapping strips, wherein the SAR is carried on a platform moving along a flight direction and a radiated beam is directed to the mapping strip, the method comprising: for each cluster, electronically manipulating the beam in an azimuth direction along the sub-mapping strip; and mechanically manipulating the beam in a direction opposite to the flight direction during each cluster.

[0027] First, the terminology used to describe SAR imaging will be explained:

[0028] To create a SAR image, continuous pulses of radio waves are transmitted to “illuminate” the target scene, and the echo of each pulse is received and recorded. A single beamforming antenna can be used to transmit pulses and receive echoes. The transmitted pulses are described as a radiating beam. In receive mode, the antenna receives radiation reflected and backscattered from this radiating beam. When the SAR is mounted on a mobile platform such as a satellite and thus moves relative to the target, the antenna position relative to the target changes over time, and the frequency of the received signal changes due to the Doppler effect. Signal processing of continuously recorded radar echoes allows combining recordings from multiple antenna positions to form a synthetic aperture, allowing the creation of higher resolution images.

[0029] The area on Earth that is momentarily illuminated by a SAR is called the coverage area. A mapping strip is a band of terrain swept by the coverage area as the SAR moves across the Earth. The direction along the SAR's flight / travel direction is usually called the azimuth or "along the track." The direction transverse to the flight direction is usually called the range or "cross track." The direction opposite to the flight direction corresponds to the rear azimuth direction.

[0030] A mapping strip comprises one or more sub-mapping strips. The extent (or elevation angle) of each sub-mapping strip can vary. As an example, to obtain large ground cover, a mapping strip can consist of at least five sub-mapping strips. Alternatively, a mapping strip can include up to 10, 20, or 50 sub-mapping strips. Within each sub-mapping strip, image data can be bursted in the azimuth direction. In other words, each sub-mapping strip can be divided into blocks in the azimuth direction, with image data from one block collected during bursting. Each burst consists of multiple pulses. Typically, a burst can include 20 to several hundred pulses. Image data can be acquired using a burst-by-burst method.

[0031] Beam control refers to the pointing of a radiating antenna beam. For example, in a SAR system with a phased array antenna, the beam can be electronically controlled by adjusting the phase of the RF signals going to and from the antenna elements. This changes the direction of the main lobe of the radiation transmitted and received from the phased array antenna. Electronic beam manipulation is highly precise and can occur rapidly. In one example, electronic manipulation is used when manipulation needs to occur quickly and the angle required for manipulation is small. This can be called fast small-angle electronic manipulation.

[0032] Electronic beam manipulation can typically be performed in the azimuth "along the track" direction or in the elevation or "cross track" direction, or both.

[0033] An acquisition cycle consists of two or more azimuth clusters, where the beam is switched to points on different sub-mapped strips along the intersecting track directions. Longitudinal mapped strips can be imaged by performing multiple acquisition cycles consecutively.

[0034] Mechanical beam maneuvering refers to guiding the beam of a SAR system by physically directing the antenna or the platform carrying the antenna. Mechanical beam maneuvering allows for a wide maneuvering angle, thus providing wide ground coverage. Larger angles can be maneuvered, but the rate of maneuvering angle is typically much slower than that achievable with electronic maneuvering. This is also known as large-angle mechanical maneuvering.

[0035] As described in the background section, there is a trade-off between azimuth resolution and mapping band width. For practical aperture radar (RAR) systems, azimuth resolution depends on the width of the radar beam (the width of the illuminated area) and the distance from the antenna to the target. Beam width is generally inversely proportional to antenna length (also known as aperture), so a longer antenna usually results in finer azimuth resolution. However, if the distance to the target is very large (e.g., if the radar system is carried on a spacecraft platform), the azimuth resolution will be very coarse unless the antenna is extremely long. Depending on the required azimuth resolution and the distance to the target, the antenna may need to be several kilometers long. This is clearly impractical for airborne or spacecraft systems, especially spacecraft systems.

[0036] SAR systems address this issue by using the forward motion of the SAR platform and special processing of echo data to create a very long synthetic antenna length (or aperture) using a much shorter real antenna. In the case of a SAR system with a focused beam, the azimuth resolution ρ... az It is independent of the distance to the target and is related to the length of the antenna L by an equation known to those skilled in the art of SAR, as shown in Equation 1 below:

[0037]

[0038] Since satellites do not have relative motion in the distance or cross-track directions, the factors driving range resolution differ somewhat from those driving azimuth resolution. Azimuth resolution depends on the antenna length, while range resolution depends on the transmitted pulse bandwidth.

[0039] Recall that SAR systems operate in pulse mode, emitting radar pulses in transmit mode and then shutting off the transmit signal to receive the returned echoes. In some SAR systems, the transmit time is, for example, about 5% to 20% of the time required to complete a transmit / receive cycle. The pulses are emitted at a certain frequency, called the pulse repetition frequency (PRF). Due to the Nyquist sampling theorem, and to avoid aliasing, the PRF needs to be greater than the received Doppler bandwidth Bd from all targets in the instantaneous field of view, as shown in Equation 2:

[0040] PRF>B D (Equation 2)

[0041] For the classic zonal map pattern, the beam velocity V on the ground... 波束 Basically related to the speed V of the SAR platform on the ground. g The same applies, and the Doppler bandwidth BD can be expressed as the grounding velocity and azimuth resolution ρ of the SAR platform. az The function is shown in Equation 3 below.

[0042] B D≈V g / ρ az (Equation 3)

[0043] This produces a fundamental inequality that will determine the width ΔR of the tilted surveying band. S Limited to azimuth resolution ρ az and the velocity V of the SAR platform relative to the ground g The function is shown in inequality 4:

[0044]

[0045] In inequality 4, c is the speed of light, and PRI is the pulse repetition interval given by the reciprocal of the pulse repetition frequency (PRF). This inequality describes the fundamental trade-off between the width of the tilt range mapping strip and the azimuth resolution: a finer azimuth resolution requires a higher pulse repetition frequency, and therefore a smaller pulse repetition interval, resulting in a narrower tilt range mapping strip width.

[0046] Note the width ΔR of the tilt range mapping zone. S The grounding mapping strip width is the difference between the distance from the antenna to the far edge of the mapping strip and the distance from the antenna to the near edge of the mapping strip, not the actual width of the mapping strip along the ground. The grounding mapping strip width also depends on the angle at which the mapping strip is imaged and can be calculated from the tilt range mapping strip width using basic trigonometry and techniques known in the art. In any case, for a given tilt angle, a larger tilt range mapping strip width results in a larger grounding mapping strip width.

[0047] To provide an example of how to calculate the width of a tilted mapping strip based on azimuth resolution, consider a satellite carrying a single-aperture SAR system operating in low Earth orbit at approximately 550 km above the Earth's surface. At this distance, the satellite would have a grounding velocity of approximately 7 km / s in a Earth-centered rotating coordinate system. g Given a speed of light of 7 km / s and c = approximately 300,000 km / s, inequality 4 is inserted, and the width of the surveying zone within the tilt range is shown in inequality 5:

[0048]

[0049] Inequality 5 can then be used to calculate the maximum tilt range mapping strip width achievable for a given resolution of that satellite. For example, if the desired azimuth resolution is 1.5m, this would be the maximum tilt range mapping strip width achievable at approximately 32km. In the example where the incident angle is between approximately 45° and 47.7° and the tilt range mapping strip width is approximately 32km, the grounding mapping strip width would be approximately 44km.

[0050] In an example according to the invention, a satellite carrying a single-aperture SAR system and a method for operating such a satellite are described, enabling higher tilt range mapping bandwidth to azimuth resolution without requiring multiple apertures. This is achieved by combining mechanical manipulation in the azimuth direction with electronic manipulation. According to this example, the beam is mechanically manipulated in the opposite direction to the flight direction during each azimuth burst. This is also known as (overlay) mechanical backscan.

[0051] For classic SAR zonal mapping and ScanSAR models, V 波束 Basically the same speed as the SAR platform on the ground, as shown in Equation 6:

[0052] V 波束 =V g (Equation 6)

[0053] Note that in the example of a satellite-borne SAR system, due to the Earth's rotation and the satellite's orbit with a semi-major axis larger than the Earth's radius, the satellite's velocity on the ground in the Earth's rotating coordinate system may differ from its inertial velocity within its orbital mapping zone. In airborne systems, this difference is negligible for practical purposes.

[0054] In an example according to the invention, the satellite is mechanically manipulated in the opposite direction to its flight direction to superimpose the velocity in the opposite direction onto the satellite's velocity on the ground, such that the effective velocity V of the beam to the ground is achieved. 波束 Less than V g As shown in the following inequality 7:

[0055] V 波束 <V g (Inequality 7)

[0056] In this example, V in inequality 4 g No longer equal to V 波束 Furthermore, mechanically manipulating the beam in the opposite direction of flight effectively decouples the beam grounding velocity from the satellite grounding velocity.

[0057] By decoupling the beam grounding velocity from the satellite grounding velocity, inequality 4 no longer applies, and it becomes possible to achieve a higher mapping bandwidth to resolution ratio than would otherwise be allowed by inequality 4. In one example, the beam grounding velocity V 波束 It can be selected via mechanical operation, and can be selected from V g It keeps dropping to zero. A faster rate of backward mechanical conversion can even lead to negative V. 波束 However, the total stay time at a specific point will be reduced.

[0058] Acquiring image data for a sub-mapped area can include acquiring image data using one or more clusters of light. Two consecutive clusters of light can illuminate the same portion, overlapping portion, different adjacent portions, or different portions separated from each other of a sub-mapped area. Thus, a single continuous strip-shaped area corresponding to a sub-mapped area can be imaged. Acquiring image data for two or more sub-mapped areas can include acquiring image data using two or more clusters of light, wherein at least two clusters of light illuminate different sub-mapped areas.

[0059] In one example, acquiring image data within a sub-mapping strip may include electronic manipulation during each azimuth burst. The beam can be electronically manipulated in the azimuth direction from back to front (forward azimuth burst). This can provide better radiometric uniformity across the entire mapping strip compared to conventional ScanSAR. In another example, electronic manipulation within each burst can also be performed in the azimuth direction from front to back (backward azimuth burst).

[0060] The beam can be mechanically manipulated by rotating the SAR relative to the platform and / or moving or transforming the platform including the SAR. Alternatively, a rotating reflector can be used to manipulate the SAR. To guide the beam to a specific target, the platform (such as a satellite) can perform its own rotation when it is small enough for mechanical manipulation and the antenna is rigidly attached to the platform. The beam can be mechanically manipulated within a viewing angle of at least -10° and +10°, -23° and +23°, -30° and +30°, -45° and +45°, or -60° and +60°.

[0061] Mechanically manipulating the beam reduces the effective ground velocity of the beam. Any mechanical backward scan speed less than the satellite ground velocity will reduce the effective ground velocity of the beam on the ground. When the mechanical backward scan speed equals the satellite ground velocity, the effective ground velocity can be reduced to zero. In this embodiment, a spotlight mode is used, where the beam is manipulated to a fixed point to illuminate / reside over a specific area. The longer illumination duration results in an increased synthetic aperture length and thus better resolution.

[0062] The angular rate used for mechanically manipulating the beam can be lower than the manipulation angular rate used for electronically manipulating the beam. In one example, the mechanical manipulation angular rate is at least 2 to 3 times lower than the electronic manipulation angular rate. The good controllability of electronic manipulation allows for selection of the manipulation angular rate for azimuth burst firing. For example, during an azimuth burst firing, the beam can be electronically manipulated at a manipulation angular rate of 17 s or greater to achieve a high illumination time. Alternatively, the beam can be electronically manipulated at a manipulation angular rate of 27 s or greater during each burst firing, while performing two consecutive burst firings on the same target to increase the number of viewing angles. This can help reduce speckle in the image by averaging the value per pixel across two or more “appearances”, or improve resolution by increasing the target illumination time. Between azimuth burst firings, the beam can be electronically manipulated at a faster rate, such as 1007 s or higher. Generally, mechanical manipulation is performed at a slower and more continuous rate than electronic manipulation due to the inertia associated with the mechanical system. Mechanical backward scanning can be performed, for example, in 17 seconds or less, 27 seconds or less, or 57 seconds or less.

[0063] In one example, the control angle range for mechanically manipulating the beam can be greater than that for electronically manipulating the beam, optionally at least 5, at least 10, or at least 30 times greater. This implementation allows image acquisition to begin while the target is still outside the range accessible by electronic manipulation, and / or to continue when the target is no longer within the range accessible by electronic manipulation. Electronic manipulation at the azimuth angle is typically performed within ±1°, ±1.5°, or ±2°, while mechanical manipulation can practically be performed within ±45°. The range of the mechanical scan can be selected based on the desired image size / map strip length and the desired resolution. Limitations on the available angular range for electronic manipulation vary from one physical device to another, but in one example, the limits can be set up to ±25° or higher at elevation and up to ±2° or higher at azimuth.

[0064] To increase the overall mapping band width and thus the coverage area of ​​the image, beam manipulation can be performed electronically at the elevation angle between two clusters. Two-dimensional electronic manipulation (at both elevation and azimuth) allows multiple sub-mapped bands to be imaged by the same SAR beam. Within each sub-mapped band, the imaging area can be scanned using a cluster, or two or more clusters with shorter durations. This can be achieved by electronically manipulating the beam at the azimuth angle from front to back between clusters, allowing two or more clusters to each act as forward azimuth clusters. Thus, two clusters are performed using the same azimuth antenna pattern covering the same area. Between clusters, small-angle electronic manipulation can be performed very quickly at the azimuth, elevation, or both on a very fast, practically instantaneous timescale.

[0065] If a wider mapping band is required, the beam can be continuously manipulated at the elevation angle during an acquisition cycle that includes multiple clusters, with each cluster illuminating a different sub-mapping band. During an acquisition cycle, the pattern is not fixed to a single sub-mapping band, but is continuously manipulated to different elevation angles corresponding to two or more sub-mapping bands. Each sub-mapping band is illuminated during one or more clusters.

[0066] Two or more acquisition cycles can be executed, where each first cluster of light in each acquisition cycle illuminates the same sub-mapped swath. Acquisition cycles can be repeated at different azimuth positions. Elevation manipulation is repeated cyclically to allow imaging of two or more consecutive sub-mapped swaths. When the last sub-mapped swath is illuminated, the antenna is electronically manipulated back to the first sub-mapped swath, leaving no gaps between clusters within the same sub-mapped swath. This acquisition style allows for the acquisition of wide-swath SAR imagery. The electronic manipulation performed in this acquisition style can correspond to terrain observation via the progressive scan “TOPS” imaging mode. Using the TOPS acquisition style, better radiometric uniformity can be achieved compared to conventional SCANSAR, which uses electronic manipulation only at elevation. When electronic manipulation at both azimuth and elevation is combined with mechanical manipulation, high-resolution and wide-swath imaging becomes possible, especially for small and agile satellites. In one example, a mapping swath of at least 100 km * 100 km can be imaged at a resolution of 5 m by an agile satellite or microsatellite with a mass of only about 150 kg.

[0067] During two or more acquisition cycles, the beam can be mechanically manipulated continuously in the direction opposite to the flight direction. Thus, a slow backward azimuth scan is superimposed on rapid electronic manipulation at both elevation and azimuth angles to allow imaging of two or more consecutive sub-mapping strips. The forward-to-back mechanical manipulation throughout the acquisition process can be at a constant rotation rate or at a varying rate, such as the rate required to maintain a constant effective ground velocity. When the beam is mechanically manipulated from front to back, the Doppler shift varies from >0 to 0 to <0. Due to the superimposed mechanical scan, the electronic scan is not always substantially perpendicular to the flight direction. Instead, the direction of observation during acquisition can, for example, begin with a substantially non-perpendicular forward look, transition through a substantially perpendicular side look, and end with a substantially non-perpendicular backward look. Using a single, continuous mechanical backward scan allows for increased acquisition time because it reduces setup time caused by mechanical antenna positioning (such as moving or rotating / positioning the satellite or antennas on the satellite).

[0068] The parameters used for image acquisition can be determined by: selecting the image size, selecting the resolution, picking the maximum electronic control angle, calculating the burst duration, selecting the beam velocity, and exporting the image acquisition time.

[0069] In a second aspect, the present invention provides a satellite for operation in orbit around the Earth, comprising a synthetic aperture radar "SAR" to acquire image data of a mapping strip comprising one or more sub-mapping strips, wherein the satellite is configured to move along a flight direction and the SAR is configured to direct a radiated beam toward the Earth, wherein the SAR is further configured to electronically manipulate the beam along the sub-mapping strip in an azimuth direction for each cluster and to mechanically manipulate the beam in a direction opposite to the flight direction during each cluster.

[0070] The satellite may include an attitude determination and control system (ADCS) comprising one or more reaction wheels configured to control the mechanical manipulation of the beam by rotating the satellite, including the SAR. In one example, the satellite may use three or more reaction wheels, allowing them to rotate about all three axes. The ADCS can be used to control the satellite's orientation and can be implemented in a variety of ways.

[0071] Satellites can be configured to mechanically manipulate beams by rotating in the azimuth direction at rates up to 1° / second. Satellites can have a total mass of less than 1000 kg, less than 500 kg, less than 250 kg, or less than 100 kg. Satellites with lower masses have a much lower moment of inertia than conventional, larger SAR satellites. Larger satellites require greater energy consumption and time to accelerate to a given conversion rate due to their higher moments of inertia, and also require them to decelerate again. For smaller satellite systems with lower moments of inertia, the power requirements for rotating the satellite at a given rate are less cumbersome, which is an advantage because the power available to satellites in space is limited.

[0072] SAR can include small single-aperture radars and / or phased arrays that allow electronic beam manipulation in two dimensions. SAR can include single-aperture phased array radars. With a single-aperture radar, pulses are transmitted and the echo is received using a single beamforming antenna. Due to the limited available space for sensor payloads, single-aperture radars, especially smaller ones, are ideal for the design of compact, high-resolution SAR systems on (unmanned) mobile platforms such as satellites. According to the invention, fine azimuth resolution and a wide mapping band can be simultaneously obtained using a small single-aperture radar, which is typically impossible due to the fundamental limitations of Inequality 4. Therefore, single-aperture radar can overcome limitations previously overcome only through multi-aperture methods with increased antenna costs. A phased array antenna with antenna elements spatially distributed in two dimensions perpendicular to the radar range dimension allows for two-dimensional beam manipulation in both azimuth and elevation.

[0073] Physical devices can be designed to provide different electronic control ranges. In one example, the total possible range for an electronically controllable phased array antenna depends on the spacing between antenna elements in both azimuth and elevation. The closer the spacing of the elements, the wider the range that can be achieved. Antenna elements in a phased array antenna can be arranged in a two-dimensional grid pattern, such that the spacing in one direction can be completely different from the spacing in another direction. Thus, the angular range in azimuth can be completely different from the angular range in elevation, even if electronic control is achieved in a similar manner in both directions. In one example, a phased array antenna with 20 antenna elements distributed over 3.2m for beam steering in the azimuth direction provides an electronic control range of approximately ±1° azimuth for an antenna element spacing of approximately 160cm. In the same example, the same phased array antenna could have 16 antenna elements spread over 40cm for beam steering in the elevation direction, providing an antenna element spacing of approximately 2.5cm. Closer spacing allows for a wider electronic control range in the elevation direction of ±25°. A higher angular range can be achieved in the azimuth direction by adding more antenna elements and spacing them closer together, but this leads to additional complexity, weight, cost, and other trade-offs. In a third aspect, the invention provides a grounding station configured to control a satellite, optionally according to the second aspect, to perform the methods of the first aspect. The grounding station can be configured to transmit control signals to the satellite.

[0074] Embodiments of the invention are described below by way of example only. These embodiments represent the best mode known to the applicant for practicing the invention, although they are not the only way to implement the invention.

[0075] Figure 1 This is a perspective view of a satellite 100 in orbit around the Earth, serving as an example of a platform that can be used in the methods and systems for Earth observation described herein. The target area on Earth to be imaged is indicated by 200. Satellite 100 includes a body 110, solar panels 150, and "wings" 160. One or more antennas may be mounted on the satellite wings. Each antenna may include a phased array antenna; in other words, each antenna may include multiple antenna elements that can be controlled to electronically manipulate 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 can be received.

[0076] Satellite 100 can be configured to mechanically manipulate the antenna, and therefore manipulate the beam in transmit and / or receive modes in addition to electronic manipulation. In this example, mechanical manipulation is achieved by manipulating the entire satellite 100. This can be achieved using a satellite attitude determination and control system (ADCS), which may be equipped with one or more reaction wheels, one of which is represented by 170. The ADCS can be used to mechanically manipulate the satellite to keep the target area 200 within the radar aperture, in other words, within the satellite's line of sight for a longer period than it would be visible without mechanical manipulation when the satellite 100 is traveling in its orbit.

[0077] refer to Figure 6 and Figure 7 describe Figure 1 Further details about the satellite.

[0078] Figure 2 is a schematic diagram of a satellite operating in scanning imaging mode according to the prior art. Satellite 100 is a well-known side-looking configuration, in which it transmits signals from a target area 200 to the side of satellite 100 rather than directly below it. In this side-looking configuration, the bottom of wing 160 (where the antenna elements are located) points towards the imaging area. Satellite 100 travels in a flight direction 120. The target area 200 has a width also referred to in the art as a mapping strip. In this example, the mapping strip includes sub-mapping strips 200A, 200B, and 200C. For each radar pulse transmitted from satellite 100, signal data in the form of echoes can be received at different frequencies from different points on the mapping strip 200 due to the Doppler effect at the SAR center.

[0079] Satellite 100 is shown traveling from right to left relative to the Earth in its orbit, as indicated by the arrows. In ScanSAR mode, the area from which data is collected, such as the 100km x 100km area shown in Figure 2, is divided into an appropriate number of sub-mapped bands, for example, three sub-mapped bands 200A, 200B, and 200C, where each sub-mapped band is divided into portions referred to herein as “blocks” in the azimuth direction. Thus, in each sub-mapped band, the received data is clustered as radar echoes in the azimuth direction. In the example of Figure 2, the blocks form an offset pattern. The wide map 200, consisting of several sub-mapped bands 200A, 200B, and 200C, is imaged by alternately illuminating each sub-mapped band. The radar antenna beam sweeps across the sub-mapped bands 200A, 200B, and 200C at different elevation angles to image the wide beam 200. The available illumination time is shared among multiple clusters of light in different areas or "blocks" of the covered ground, thus compensating for a wider coverage area with azimuth resolution. Electronic control of elevation angles is performed very quickly to switch between different sub-mapping strips 200A, 200B, and 200C.

[0080] In the conventional scanning radar depicted in Figure 2, the beam velocity to the ground is the same as the satellite's ground velocity. Therefore, during each burst, no electronic beam manipulation is performed, and the beam slides at the satellite's ground velocity. Consequently, the azimuth resolution is reduced because the per-resolution mapping band constraint applies according to Equation 4.

[0081] In some methods and systems described in more detail below, mechanical beam manipulation is performed in the direction opposite to the flight direction, while electronic beam manipulation in the azimuth angle is also performed during each burst to increase the uniformity of radiation measurements.

[0082] Figure 3 This schematically illustrates how image data of the mapping strip 200 is acquired while operating in cluster mode during a mechanical backward scan. The mechanical manipulation is... Figure 3 The image is shown in enlarged form, with satellite 100 shown as having a different orientation at each of three positions (a), (b), and (c) along its mapping strip. Between each position along the flight direction 120, satellite 100 has been rotated as indicated by the arrows to point further backward in the azimuth direction. The direction of rotation is opposite to the flight direction 120. At the first position (a), satellite 100 looks forward relative to the flight direction 120; at the second position (b), the satellite beam is perpendicular to the satellite's flight direction; and at the third position (c), satellite 100 looks backward relative to the flight direction 120. Thus, satellite 100 performs a mechanical backward scan. Due to the superimposed mechanical scan, the direction of observation is substantially not perpendicular to the flight direction, with the sole exception at position (b). This implementation allows image acquisition to potentially begin before position (a) while the target 200 is still outside the electronically accessible range, and / or to continue after position (c) while the target is no longer within the electronically accessible range.

[0083] In addition to mechanical backward scanning, the beam is electronically manipulated in the azimuth angle during each burst phase, which... Figure 3 The arrows at each location indicate the beam pattern. Electronic azimuth manipulation is performed in the forward direction indicated by the arrow. In another example, electronic azimuth manipulation can be performed in the backward direction. Due to azimuth beam manipulation, each point is illuminated by an omnidirectional beam. Compared to ScanSAR shown in Figure 2, the azimuth rotation achieved through electronic manipulation throughout the acquisition process achieves the same mapping strip coverage but allows for better radiometric uniformity.

[0084] The beam-to-ground velocity during acquisition corresponds to the sum of mechanical and electronic manipulation within a cluster, thus decoupling the beam-to-ground velocity from the satellite grounding velocity. In other words, the beam grounding velocity depends on the scan rate during azimuth clustering and the mechanical backward scan rate. Therefore, the beam grounding velocity can be selected via mechanical manipulation. Mechanically manipulating the beam in the direction opposite to flight direction 102 reduces the effective grounding velocity of the beam on Earth, allowing for greater illumination and thus better resolution. The effect of slowing down the beam grounding velocity is a longer clustering duration and a longer acquisition time, which results in finer resolution. Therefore, azimuth resolution is not compromised because the mapping band constrained by each resolution according to Inequality 4 is not applicable.

[0085] In addition to mechanical backscanning and electronic manipulation in the azimuth angle during each acquisition period, the beam is continuously manipulated in the elevation angle during multiple acquisition cycles. Figure 3 In the example, mapping band 200 includes three sub-mapping bands 200A, 200B, and 200C, each of which comprises multiple blocks. At the end of a cluster in, for example, the first sub-mapping band 200A, the viewing angle is changed to illuminate the second sub-mapping band 200B, which is pointing backward again. When the third / final sub-mapping band is imaged, the beam is pointed back to the first sub-mapping band, leaving no gaps between clusters within the same sub-mapping band. Beam manipulation between clusters is achieved through rapid small-angle electronic manipulation.

[0086] Figure 3 The mapping strip 200, depicted as a 100km x 100km area, can be imaged at a 5m resolution. The satellite travels at a speed of 7.5km / s along its flight direction, with mechanical maneuvers adding a speed of 3.75km / s to the satellite's actual grounding speed, resulting in an effective grounding speed of 3.25km / s. The total mechanical maneuvering angle in this example is 35°.

[0087] Now refer to Figure 4 and Figure 5 Other examples describing the superposition of electronic and mechanical manipulation.

[0088] Figure 4 a shows how to select the beam grounding speed via mechanical manipulation. Figure 4 The top arrow 124 in 'a' indicates the velocity of the satellite pointed to along the flight direction 120. Arrow 126 indicates the direction of mechanical manipulation, which is opposite to the flight direction 120 and less than the velocity of the satellite 124. Arrow 202 depicts the final effective ground velocity of the beam guided along the flight direction 120. Therefore, by using superimposed mechanical backward scanning, the effective ground velocity of the beam is reduced and decoupled from the satellite ground velocity. The effective ground velocity of beam 202 can be selected via mechanical manipulation 206. Figure 4The scan velocity caused by electronic manipulation during each azimuth angle is not shown in diagram a, which can be oriented along or opposite to the flight direction. For completeness, manipulation in the elevation direction is not shown, as it does not affect the beam grounding velocity.

[0089] Figure 4 b shows the offset style of the cluster image after processing the raw synthetic aperture radar data. Figure 4 b depicts areas on the ground divided into smaller regions or blocks in an offset pattern, as this can be used to operate the SAR in ScanSAR or TOPS mode. Therefore, as previously mentioned, the SAR beam can be electronically manipulated to collect data from blocks 1-15 in numerical order from left to right and along the flight direction 120 (see [link to data]). Figure 4 a). Figure 4 In diagram b, the intersecting shaded rectangles within the dashed lines correspond to a 100km x 100km imaging area or mapping zone. Figure 3 In contrast, the sample includes four sub-maps: 200A, 200B, 200C, and 200D.

[0090] Reference Figure 5 Time Chart Explanation Figure 4 The acquisition style is described in b. Figure 4 The blocks marked 1-10 in b correspond to Figure 5 Clusters 1-10 are shown in diagram d. Therefore, each block is imaged in a single cluster. Instead of imaging each block in a single cluster, more than one cluster can be performed on each block to increase the number of views per block, but at the cost of lower fine resolution. When imaging each block in a single cluster, the acquisition cycle consists of four consecutive clusters, corresponding to blocks in four adjacent sub-mapped areas 200A, 200B, 200C, and 200D at different elevation angles. Therefore, to image a mapped area, blocks 1-4 are imaged in the first acquisition cycle, blocks 5-8 are imaged in the second consecutive acquisition cycle, and so on.

[0091] Therefore, such as Figure 5 As shown in diagram a, the beam is periodically forward-engineered electronically in the azimuth angle, where, during the clustering period, the beam angle in the azimuth angle changes from a negative angle through zero to an equal positive angle. At the end of the clustering period, the beam is rapidly manipulated back to a negative angle. This is... Figure 5 In 'a', it is represented as a basically vertical line because it can occur within microseconds. For continuous clusters, this repeats within the same angular range.

[0092] To switch between sub-maps, such as Figure 5As shown in b, the beam is periodically electronically manipulated in terms of elevation angle. During each burst period, the beam is pointed to a constant elevation angle corresponding to each burst period of the respective sub-mapping bands 200A, 200B, 200C, and 200D, which is determined by... Figure 5 The horizontal line in b represents this. At the end of the acquisition cycle, the beam is rapidly (gradually) maneuvered to the next sub-mapped zone in terms of elevation angle and cross-track direction. At the end of each acquisition cycle, the beam is rapidly maneuvered back to the first sub-mapped zone. This is in Figure 5 In b, it is represented as a basically vertical line because it occurs within microseconds.

[0093] like Figure 5 As shown in c, the periodic electronic control superimposed on the azimuth and elevation angles is mechanical rearward control in the azimuth angle over a larger angular range and a longer time period. Figure 5 c shows two simplified examples of mechanical manipulation for illustrative purposes. Figure 5 The solid line shown in c corresponds to a constant angular rate of manipulation used for mechanical control, such as the constant rotation of a satellite. Figure 5 The dashed line in 'c' corresponds to the nonlinear control angular rate that can be used for mechanical maneuvering to maintain a constant effective ground speed. The nonlinear control angular rate provides a higher rate at the beginning and end (when the control angle is at its maximum), and a lower rate when the control angle is approximately zero (when the direction of observation is perpendicular to the flight direction). Mechanical rearward maneuvering at the azimuth angle can also correspond to a combination of linear and nonlinear control angular rates.

[0094] The beam angle in the azimuth angle can be mechanically manipulated from a positive angle to an equal negative angle within the acquisition period (in... Figure 5 (Only the beginning of the acquisition cycle is depicted in the text). Regarding... Figure 3 The exemplary rotations shown correspond to a positive angle for the first (looking forward) position (a), zero for the second (perpendicular to the flight direction) position (b), and a negative angle for the third (looking backward) position (c). Mechanical rearward maneuvering can also be performed within an asymmetrical range of control angles, such as, for example, only for control angles <0° or >0°.

[0095] like Figure 5 As shown, especially when comparing Figure 5 a and Figure 5At time c, the angular rate of manipulation for mechanically manipulated beams is lower than that for electronically manipulated beams. The superior controllability of the electronic control rate allows for the selection of specific electronic scan rates to increase the illumination time for each block. Furthermore, the angular range for mechanically manipulated beams is significantly greater than that for electronically manipulated beams. For electronically manipulated beams, the typical angular range is approximately ±1° in azimuth and ±25° in elevation, while the mechanical angular range can reach up to ±45°, or even ±60°. While larger azimuth manipulation angles can theoretically be achieved with electronic beam manipulation, it requires more complex, larger, and more expensive antennas. By combining fast, small-angle electronic manipulation with large-angle mechanical manipulation, an improved mapping bandwidth to resolution ratio can be achieved, which is impossible with other known single-aperture radar imaging techniques.

[0096] In the actual implementation, by Figure 4 The total acquisition time for the area represented by the rectangle in b (including three or four sub-mapping strips) is approximately 40 seconds, achieving a resolution of 5m. In this example, each cluster requires approximately 2.5-3 seconds. For comparison, it is mentioned that the total acquisition time for the same-sized area in the scanning mode (as shown in Figure 2) is approximately 15 seconds, achieving a resolution of 15m. The longer acquisition time according to the invention is made possible by mechanically backscanning superimposed on the satellite's velocity on the ground, resulting in a slower effective grounding velocity for the beam. In an alternative example, a 60km × 60km area including two sub-mapping strips can be imaged with a resolution of 3m and a total acquisition time of 35 seconds.

[0097] As noted elsewhere, the methods described herein are particularly, but not exclusively, suitable for implementation in conjunction with satellite-borne SAR. SAR can, for example, be carried on other platforms such as aircraft. Reference will now be made to... Figure 1 , Figure 6 and Figure 7 The description applies to satellites used to implement this invention.

[0098] Figure 6 This is a schematic representation of components of a satellite (e.g., a microsatellite) according to some embodiments of the present invention. Solid arrows between components indicate power connections, heavier solid arrows indicate RF signal connections, and dashed lines indicate data connections.

[0099] Some components are part of the satellite "bus" 610, by Figure 6 The rectangle in the diagram indicates that some components can be part of the "payload" 660, by... Figure 6 The rectangle is represented in the image. Other components are part of the antenna module 670, in... Figure 6 The middle part is also represented by a rectangle. Figure 6The satellite components shown include a power supply 101 and a power distribution system 102. Power supply 101 and power distribution system 102 supply power to the propulsion system 190, propulsion controller 109, attitude determination and control system “ADCS” 131, computing system 103, buffer 135, and communication system 104. Power supply 101 and power distribution system 102 also supply power to components within the payload 660, such as pulse generator 620 and power amplifier 623. Buffer 135, although shown as a separate item, may be included in computing system 103. Propulsion controller 109 is shown as a separate component herein, but it can actually form part of computing system 103. The propulsion controller can be controlled by using control software implemented in one or more processors included in propulsion controller 109 or in response to instructions received, for example, from computing system 103. In the case of transmitting instructions from computing system 103, computing system can be considered to include propulsion controller. One function of propulsion controller 109 may be to output control signals to the ion and electron sources of the thrusters in propulsion system 190.

[0100] The satellite bus 610 may typically be located within the main body of the satellite 110. The power distribution system 102 may include control logic known in the art. The communication system 104 may include, for example, one or more communication antennas located on the main body of the satellite. Alternatively, the communication system 104 may transmit and receive signals via one or more communication antennas located on the wings of the satellite.

[0101] In the case of an Earth observation satellite, the satellite payload 660 may include one or more radar antenna arrays, which may be located on one or more wings 160 of the satellite. Figure 6 A single antenna element 625, which may be part of a phased array antenna for SAR imaging, is shown. Antenna element 625 transmits and receives signals 626. Antenna element 625 is shown having an associated power amplifier 623 and phase shifter 624 for transmitting radar signals, and an associated low-noise amplifier 628 and phase shifter 627 for receiving returned signals. These together form antenna module 670. A phased array antenna may include multiple antenna modules 670. In one example, a satellite-mounted phased array antenna includes 320 antenna elements and associated amplifiers and phase shifters. Different phased array antennas will have different numbers of antenna elements depending on their design and intended purpose. As is known in the art, electronic manipulation of the antenna is achieved by phase shifting the individual antenna elements via phase shifters 624 and 627.

[0102] Pulse generator 620 generates RF signals that are sent to radar transmit and receive module 621. The radar signals are sent to RF divider 622, which divides the RF signals and sends them to multiple antenna modules 670. Figure 6An antenna module 670 is shown, but multiple antenna modules may exist. An RF combiner 629 receives combined signals from multiple antenna modules 670 and transmits the received RF signals to a radar transmit and receive module 630. Data is stored in a memory 631. Memory 631 may be the same as or separate from memory 108. The pulse generator 620, radar transmitter 621, radar receiver 630, RF divider 622, and RF combiner 629 may be located in the satellite fuselage 110 or on the satellite wing 160. Figure 6 The additional arrows extending from the RF divider 622 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 one or more additional antenna modules into the RF combiner 629.

[0103] The methods and systems described in this paper relate to the manipulation of a single antenna or a single aperture. However, they can be readily extended to systems that include multiple antennas or multiple apertures.

[0104] The antenna modules 670, multiplied by the number of antenna modules, collectively form the satellite's image acquisition equipment, as is known to those skilled in the art. They can perform functions different from image data acquisition.

[0105] In a typical satellite, the antenna can include a phased array antenna as described above. A phased array antenna having antenna elements spatially distributed in two dimensions perpendicular to the radar range dimension allows for two-dimensional beam manipulation in both azimuth and elevation angles.

[0106] The available electronic manipulation of a phased array antenna may be limited by the range and spacing of the phase centers of the physical antenna in the azimuth angle. Excessive control attempts can lead to reduced gain and increased grating lobes. The limitations on the available angular range will vary from one physical device to another, but typical limitations can be set to ±25° elevation and ±2° azimuth.

[0107] Payload 660 receives power from power distribution system 102 and commands from computing system 103. Data from payload 660, such as received radar signals, also flows back to computing system 103 and can be stored in memory 108. The data can be processed by computing system 103, for example, to generate an image as described elsewhere herein, which can then be output to communication system 104 for forward transmission. Figure 6 In the system shown, raw data can also be output from computing system 103 to communication system 104, which further transmits it for processing by a remote computing system. Figure 6In this system, the SAR processor 133 may be located, for example, at a grounding station, or in another processing location. The computing system 103 may send operational commands 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 are known to those skilled in the art. Raw SAR data may be stored in the satellite in memory 108 or 631. Memory 108 and 631 may be the same or different memory modules, or they may be part of the computing system 103.

[0108] Raw SAR data is stored in buffer 135 and transmitted to ground station 600 or remote SAR processing 133. In one example, 30 seconds of image data can be stored at full resolution (bandwidth). More data can be stored at a lower resolution (e.g., 60 seconds at half resolution). In one example, the microsatellite has a 150MB download link. At this data rate, downloading 30 seconds of full-resolution image data would take approximately 3 minutes. During operation, approximately 5000 pulses per second can be transmitted. This means that 27 pulses could be in the air at any given time. A burst typically consists of 500-1000 pulses and takes 2-3 seconds.

[0109] The communication system 104 can communicate with earth stations or other satellites using radio frequency communication, optical (e.g., laser communication) or any other form of communication known in the art.

[0110] Satellites, for example Figure 1 Satellite 100 is typically equipped with a propulsion system 190 for maneuvering the satellite with generated thrust. The propulsion system 190... Figure 1 It is shown as being mounted on the surface of the body 110 opposite to the solar panel 150.

[0111] like Figure 1 As shown, the propulsion system 190 includes a plurality of thrusters 105 that generate thrust for maneuvering the satellite 100 when needed.

[0112] Thruster 105 is typically operated to keep a satellite in a specific orbit. For example, a thruster can be used to propel a satellite in a specific direction relative to the Earth's surface.

[0113] Return to reference Figure 6 The ADCS131 is typically located within the satellite body 110 and is used to control the satellite's orientation. The ADCS can be implemented in several ways. The ADCS131 is shown in the figure as comprising a set of reaction wheels, one of which is... Figure 1 The reaction wheel is schematically shown in the diagram. It is usually, but not necessarily, located in satellite body 110. Figure 7This is a partial perspective view of the satellite, showing a set of three reaction wheels 41, 42, and 43 located in the satellite fuselage 110. Reaction wheels are sometimes also referred to as momentum wheels.

[0114] In the satellite described in this article, ADCS can be used to mechanically maneuver the satellite to keep a target area 200 on Earth within the radar aperture, in other words, within the satellite's line of sight, for a longer period of time as the satellite travels in its orbit than without mechanical maneuvering. In principle, the angular range used for mechanical maneuvering is limited only by the horizontal line in each direction; however, the larger the angle, the greater the distance to the target area, and therefore the weaker the returned signal.

[0115] The reaction wheels 41, 42, and 43 function by using electric motors to rotate wheels within the spacecraft fuselage 110. Through the conservation of angular torque, rotating the wheel in one direction causes the spacecraft to rotate in the opposite direction. The use of reaction wheels is a well-known method for orienting spacecraft such as satellites.

[0116] In one example, three reaction wheels are positioned within the spacecraft fuselage, each reaction wheel used to orient the satellite along each axis. Therefore, reaction wheels 41, 42, and 43 are shown with orthogonal axes.

[0117] In another example, four or more reaction wheels can be used to provide better control over various aspects of satellite dynamics, such as the rate of change (how fast the satellite can rotate) and fine positioning control, especially for satellites with high moments of inertia.

[0118] Currently, various satellites in orbit around the Earth are typically defined by weight ranges, although the boundaries between these classes are somewhat fluid and arbitrary:

[0119] CubeSats: 1kg-10kg

[0120] Microsatellites: 50kg-250kg

[0121] Small satellite: 500kg-800kg

[0122] Standard satellite: 800-1200kg.

[0123] Large satellite: >1200kg.

[0124] Reaction wheels are rated based on their "momentum capacity," which is measured in nm (Newton-meter-second). The conversion rate is related to the wheel's speed and the satellite system's inertia. Satellites with particularly low masses have much lower moments of inertia than conventional, larger SAR satellites. Suitable low masses can be below 1000 kg, for example, below 500 kg, below 250 kg, between 50 kg and 250 kg, or below 100 kg.

[0125] Currently, very small CubeSats are not capable of carrying current SAR payloads. Heavier satellites are generally less flexible due to their higher inertia. The embodiments of the satellites and operational methods described in this paper have been successfully implemented in microsatellites.

[0126] The embodiments of the present invention are particularly applicable to satellite categories known as microsatellites.

[0127] Some methods described further in this paper benefit from reaction wheels within a specific range. For example, a suitable range for microsatellites could be 0.5 to 2.5 nm. Reaction wheels with a range of 1 nm have been successfully tested. This allows for switching within a 17-second range, sufficient to track points on the ground and implement any of the methods described herein without consuming too much power. Therefore, in any satellite described herein, the ADCS can be configured to switch the satellite at speeds up to 1 degree / second in the azimuth direction using mechanical manipulation.

[0128] Larger satellites are known to use reaction wheels on the order of 10 nm, but due to the large mass of the satellites and the resulting high moment of inertia, they cannot currently achieve sufficient conversion rates, and they also consume more power than smaller reaction wheels.

[0129] In one example, the satellite orbits the Earth in a low Earth orbit. A low Earth orbit can range from 160 km to 1000 km above the Earth's surface. An example of a SAR-based Earth observation satellite could have an orbit between 450 km and 650 km above the Earth. In the example according to the invention, the satellite has an orbit 550 km above the Earth's surface. For example, in an orbit 550 km above the Earth, the satellite effectively traverses the Earth at approximately 7.5 km / s or 27,000 km / h. Most satellites in this orbit will traverse the Earth at speeds in the range of 7-8 km / s.

[0130] In some embodiments, a satellite, such as a microsatellite, can rotate at a speed required to keep its point on Earth pointed at for approximately 10 minutes from horizon to horizon. However, at the extremes of this range, the distance to the point or target being imaged may be too great to obtain a good SAR image, resulting in a relatively short actual dwell time.

[0131] As described elsewhere herein, embodiments of the invention are not limited to changing the orientation of the entire satellite, which is convenient in the case of the aforementioned small, lightweight, and flexible satellites. For example, in some embodiments, mechanical manipulation can be achieved by changing the orientation of the antenna relative to the satellite it carries.

[0132] In the preceding text, only a single SAR beam was considered. However, it will be recognized that the methods and systems described herein can be extended to use multiple SAR beams. For example, a platform can host equipment for multiple SARs, each of which can operate according to any of the methods described herein.

[0133] As can be understood from the above, all the methods described in this paper benefit from the use of agile microsatellites. Appropriately sized microsatellites can rotate to observe targets over extended time periods. This provides them with an unprecedented ability to achieve many image frames at the same resolution as the range resolution over a period of time.

[0134] The foregoing describes a satellite suitable for implementing any of the operational methods described herein. For satellites or other platforms already in orbit, the methods described herein can be implemented by using a suitable computing system, such as a ground station computing system 600, to appropriately control the satellite, for example, from the ground. In other words, the SAR can be operated from the ground, and some of the methods described herein can be implemented in software. Therefore, in one aspect, the present invention can provide a computer-readable medium comprising instructions that, when implemented by a processor in a computing system, cause the computing system to operate the SAR according to any of the methods described herein.

[0135] The acquisition of SAR image data described in this paper can have many different practical applications. End-to-end processing can begin with a request for imaging a specific area, for example, it can be requested by a client or identified as of interest by an algorithm. Depending on the size of the area and the desired resolution, an appropriate number of sub-maps can be selected. Based on the number of sub-maps, a clustering sequence at azimuth angles can then be designed to optimally acquire image data. Before acquisition begins, the satellite can be rotated to an initial position, such as... Figure 3 Position (a) in the middle. From the initial position, image data can be collected by mechanically manipulating the beam opposite to the flight direction and electronically manipulating the beam according to the clustering sequence.

[0136] In one example, a request is received from a customer to image a relatively large area of ​​100km x 100km. Using the existing TOPS (Topographic Observation with Progressive Scan) mode, where electronic manipulation is used in both azimuth and elevation angles, for example, three sub-mapped strips can be used to image a 100km wide mapping strip to achieve a resolution of 15 meters. Using TOPS mode, acquisition would take approximately 15 seconds as the satellite travels 100km along its orbit over the region of interest.

[0137] In one example, an agile satellite weighing approximately 150 kg can image a 100 km × 100 km area with a resolution of 5 m using the devices, methods, and techniques described herein, by combining electronic manipulation and the satellite's mechanical manipulation capabilities to reduce the effective ground velocity of the SAR beam. In another example, by dividing the area into four sub-mapping strips and having an extended acquisition time of approximately 42 seconds, a resolution of 5 m can be achieved over the 100 km × 100 km area. This represents a three-fold increase in resolution compared to the example TOPS case used for imaging this area size.

[0138] Figure 8 The performance of this 100km × 100km example is illustrated by plotting the signal and the potential performance loss due to ambiguity in the azimuth direction. The vertical axis is given in decibels (dB), and the horizontal axis is the azimuth in degrees. Typically, the minimum degradation of the signal and the low azimuth ambiguity value are desired. Figure 8 As can be seen, the signal degradation at each cluster edge does not exceed approximately -2.5 dB. The total ambiguity trajectory (AmbTot) shows a value of approximately -17 dB at the very edge of the cluster in a worse case. This is considered to be within the acceptable degradation range at the edges. The threshold value of the azimuth ambiguity can be fed into an algorithm for determining the possible imaging area and resolution, as referenced below. Figure 10 As described.

[0139] Figure 9 The graph shows the performance loss over time for a single cluster due to Amb Total and Amb Total Ratio (RAR). Figure 8 and Figure 9The trajectories shown are obtained from a mathematical model that calculates azimuth and range ambiguity values ​​through numerical integration of appropriate portions of the antenna pattern. In this example, covering a 100km × 100km area imaged at a resolution of 5m, the highest value for the total ambiguity trajectory (Amb Total) is approximately -28dB, resulting in a worst-case RAR of approximately -24dB. This is considered within acceptable limits. Similar to the azimuth ambiguity values, thresholding can be used to determine the possible resolution and imaging area achievable using high-resolution wide-strip mapping techniques.

[0140] In another example, high-resolution wide-strip mapping technology can be used to image a smaller area of ​​60km × 60km at a resolution of even finer, 3m, within a 35-second acquisition period by using two sub-strips superimposed on top of electronic manipulation and mechanical manipulation. In one example, even finer resolutions, such as 1m or smaller, are possible.

[0141] Figure 10 An example of an algorithm that can be used to determine the parameters to be used for a particular acquisition is shown. In the first step 1101, the image size is selected, for example, 100km × 100km or 60km × 60km. In the second step 1102, the resolution is selected, for example, 5m. In step 1103, the maximum electronic control angle in the azimuth is selected, and this determines the patch size on the ground. The maximum electronic control angle is limited by the antenna design, and more specifically, by how far the antenna can scan before the grating lobe becomes a problem due to the spacing of the antenna elements in the azimuth.

[0142] Then the resolution drives the burst duration, which is calculated in step 1104 according to Equation 9:

[0143] τ=λR / (2V s ρ az (Equation 9)

[0144] In Equation 9, τ is the burst duration, λ is the wavelength, R is the tilt range, W is the satellite velocity, and ρ az This refers to the azimuth resolution. In step 1105, the beam velocity V is selected. g This allows the beam to slide on the patch within the required time τ. Then, in step 1106, the image acquisition time is derived from the image size along the orbit. According to this method, the parameters required for satellite task allocation to acquire images can be calculated. Any image size and resolution can be achieved, subject to acquisition time constraints and oblique angle limitations.

[0145] Any computing system described herein can be combined into a single computing system with multiple functions. Similarly, the functions of any computing system described herein can be distributed across multiple computing systems.

[0146] Some operations of the methods described herein can be performed by software in a machine-readable form, such as a computer program including computer program code. Therefore, some aspects of the invention provide a computer-readable medium that, when implemented in a computing system, causes the system to perform some or all of the operations of any of the methods described herein. The computer-readable medium can be in a transient or tangible (or non-transient) form, such as storage media including disks, thumb drives, memory cards, etc. The software can be adapted to execute on a parallel or serial processor, such that the method steps can be performed in any suitable order or simultaneously.

[0147] The above embodiments are largely automated. In some examples, the system user or operator may manually instruct some steps of the method to be performed.

[0148] In this embodiment of the invention, the system can be implemented as any form of computing and / or electronic system described elsewhere herein. For example, a grounding station may include such a computing and / or electronic system. Such a system may include one or more processors, which may be microprocessors, controllers, or any other suitable type of processor, for processing computer-executable instructions to control the operation of the device in order to collect and record routing information. In some examples, such as when using a system-on-a-chip architecture, the processor may include one or more fixed functional blocks (also referred to as accelerators) implemented in hardware (rather than software or firmware) as part of a method. Platform software, including an operating system or any other suitable platform software, may be provided at the computing-based device to enable application software to execute on that device.

[0149] The term "computing system" is used herein to refer to any device that has processing power so that it can execute instructions. Those skilled in the art will recognize that such processing power can be incorporated into many different devices, and therefore the term "computing system" includes PCs, servers, smartphones, personal digital assistants, and many other devices.

[0150] It should be understood that the above benefits and advantages may apply to one embodiment or several embodiments. The embodiments are not limited to those that solve any or all of the problem, or those that have any or all of the benefits and advantages.

[0151] Unless otherwise stated, any reference to “a” or “a portion” means one or more of those items. The term “comprising” is used herein to mean including the identified method step or element, but such step or element does not include an exclusive list, and a method or apparatus may include additional steps or elements.

[0152] Furthermore, with regard to the use of the term "includes" in the detailed description or claims, this term is intended to be inclusive in a manner similar to the term "comprising," since "comprising" is interpreted as a transitional word in the claims.

[0153] The accompanying drawings illustrate exemplary methods. While these methods are shown and described as a series of actions performed in a specific order, it should be understood and appreciated that these methods are not limited by the order of the actions. For example, some actions may occur in a different order than that described herein. Furthermore, one action may occur simultaneously with another. Moreover, in some cases, not all actions may be required to implement the methods described herein.

[0154] The order of steps in the methods described herein is exemplary, but these steps may be performed in any suitable order, or simultaneously where appropriate. Furthermore, steps may be added to or substituted in any method, or individual steps may be removed from any method, without departing from the scope of the subject matter described herein. Aspects of any of the above examples may be combined with aspects of any of the other described examples to form further examples.

[0155] It should be understood that the above description of preferred embodiments is given by way of example only, and various modifications can be made by those skilled in the art. The foregoing includes examples of one or more embodiments. Of course, it is impossible to describe every conceivable modification and alteration of the above-described apparatus or method in order to describe the foregoing aspects, but those skilled in the art will recognize that many further modifications and substitutions of the various aspects are possible. Therefore, the described aspects are intended to cover all such changes, modifications, and variations that fall within the scope of the appended claims.

Claims

1. A method for operating a synthetic aperture radar (SAR) to acquire image data of a mapping strip including one or more sub-mapping strips, wherein, The SAR is carried on a platform that moves along the flight direction, and the radiated beam is directed to the mapping strip. The method includes: During each burst, the beam is electronically manipulated along a sub-mapping strip in the direction of flight or in the opposite direction of flight; and During each burst, the beam is mechanically manipulated in a direction opposite to the flight direction.

2. The method according to claim 1, wherein, The beam is mechanically manipulated by rotating the SAR relative to the platform and / or moving or transforming the platform including the SAR.

3. The method according to any one of the preceding claims, wherein, Mechanically manipulating the beam reduces the effective grounding speed of the beam on Earth.

4. The method according to claim 1 or 2, wherein, The manipulation angular rate used to mechanically manipulate the beam is lower than, or at least three times lower than, the manipulation angular rate used to electronically manipulate the beam.

5. The method according to claim 1 or 2, wherein, The range of manipulation angles used for mechanically manipulating the beam is greater than, or at least 30 times greater than, the range of manipulation angles used for electronically manipulating the beam.

6. The method according to claim 1 or 2, further comprising: The elevation angle of the beam is electronically manipulated between the two clusters.

7. The method according to claim 1 or 2, further comprising: The beam is continuously manipulated at the elevation angle during an acquisition cycle comprising multiple clusters, wherein each cluster illuminates a different sub-mapping strip.

8. The method according to claim 7, further comprising: Perform two or more acquisition cycles, wherein each first cluster of light in each acquisition cycle illuminates the same sub-mapped area.

9. The method of claim 7, further comprising: During one or more acquisition cycles, the beam is continuously mechanically manipulated in a direction opposite to the flight direction.

10. The method according to claim 1 or 2, wherein, The parameters used for image acquisition are determined in the following way: a. Select image size; b. Select resolution; c. Select the maximum electronic control angle; d. Calculate the duration of the cluster of hairs; e. Select beam rate; and f. Image acquisition time.

11. A satellite for operation in orbit around the Earth, comprising a synthetic aperture radar "SAR" to acquire image data of a mapping strip including one or more sub-mapping strips, wherein, The satellite is configured to move along its flight direction, and the SAR is configured to direct its radiated beam toward the mapping strip, wherein the SAR is further configured to During each burst, the beam is electronically manipulated along a sub-mapping strip in the direction of flight or in the opposite direction; and During each burst, the beam is mechanically manipulated in a direction opposite to the flight direction.

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

13. The satellite according to claim 11 or 12, wherein, The satellite is configured to mechanically manipulate the beam by rotating it up to 1° / second in the azimuth direction.

14. The satellite according to claim 11 or 12, wherein, The satellite has a total mass of less than 1000 kg, or less than 100 kg.

15. The satellite according to claim 11 or 12, wherein, The SAR includes small single-aperture radar and / or phased arrays that allow electronic beam manipulation in two dimensions.

16. A grounding station configured to control a satellite according to any one of claims 11 to 15 to perform the method according to any one of claims 1 to 10.

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