Scanning method and system

Abstract

A camera 16 onboard an airborne or spaceborne platform 12, moving in a flight direction 14 around the earth at a flight velocity Vgsat, is pivotable about a roll axis 18 in line with the flight direct

Description

FIELD OF THE PRESENT DISCLOSURE The present disclosure relates to the field of space optics, and more particularly to airborne or satellite remote sensing performed with a scanning optical imaging sensor. BACKGROUND Mechanical scanning of Earth’s surface by remote sensing satellites were the norm just a few decades ago, but progressively dropped out of favour in the visual and near infrared regions with the advent of charge-coupled device (CCD) linear arrays. The failure of the Landsat-7 scanning mechanism in 2003 further weakened the case argued by proponents of mechanical scanning. Shortly afterwards, the NewSpace revolution was born into the era of large imaging arrays. This has brought about reductions in form factor, mass, and cost of spaceborne imaging systems. Pushbroom-related imaging techniques have become the workhorse for optical remote sensing by small satellites such as the CubeSat class. These techniques use the forward motion of the satellite in low or medium Earth orbit to scan the surface with a linear array of detectors directed in a cross-track direction of satellite motion, much like a document scanner scans a document. Different detectors may accomplish pushbroom-type imaging i.e., linear arrays, matrix (2D) detectors or time delay integration (TDI) detectors. The primary drawback of these techniques is the limited swath they afford. The swath width is constrained to the field of view of the optical system i.e., the optics and the length of the imaging detector in the cross-track direction determines swath width. With these pushbroom-type systems, the swath width is a constant width across the direction of travel of the satellite and it is “pushed” along the earth’s surface as the satellite travels. This has the drawback that it only covers a narrow strip of ground with each orbit of the satellite. High resolution satellite imaging has for decades been performed using a technique referred to as time delayed integration or TDI imaging. The imaging detector shifts accumulated charge from one pixel to the next in the scan direction, in synchronisation with the imaged ground surface below. The technique improves the signal to noise ratio (SNR) of the imagery. Tens of TDI stages are typically used, for example 32 or 64 stages. Conventional satellite TDI imaging, however, suffers image quality degradation in a near-polar orbit due to cross-track motion of the ground below as the Earth rotates during collective integration time of all the TDI stages. In other words, forward motion of the satellite platform provides pushbroom (linescan) motion to the imaging detector, but depending on the inclination of the orbit, the ground point being imaged moves sideways at the same time. This sideways motion becomes significant relative to the forward motion at high resolution and large number of TDI stages. The resultant smear degrades the MTF (modulation transfer function) of the image. MTF is a measure of image sharpness. During prolonged integration times, the image smears orthogonally to the scan direction and image quality (MTF) degrades. The applicant considers there to be room for improvement. The preceding discussion of the background to the invention is intended only to facilitate an understanding of the present invention. It should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was part of the common general knowledge in the art as at the priority date of the application. SUMMARY In accordance with an aspect of the present disclosure there is provided a method of scanning by a camera onboard an airborne or spaceborne platform configured to move in a flight direction around the earth at a flight velocity parameter, the method comprising: providing the camera about a roll axis so as to be pivotable thereabout, the roll axis being generally in line with the flight direction, so as to be capable of scanning the ground at a scanning velocity parameter extending generally in a direction across the flight direction; determining a yaw steering angle based at least on the flight velocity parameter and the scanning velocity parameter; and yawing the camera or the platform by the determined yaw steering angle. The determination of the yaw steering angle may include performing a vector sum of at least the flight velocity parameter and the scanning velocity parameter. The scanning velocity parameter may correspond to a footprint velocity of a field of view of the camera on the ground. The step of determining the yaw steering angle may include determining a total velocity parameter of the airborne or spaceborne platform based at least on the scanning velocity parameter and the flight velocity parameter. The yaw steering angle may be determined by calculating an angle between the total velocity parameter and the scanning velocity parameter. The yaw steering angle may be determined based on one or more of the following formulae: wherein vt is a total velocity parameter of the airborne or spaceborne platform, wherein vgScan is the scanning velocity parameter, wherein vgsat is the flight velocity parameter, and wherein HJs is the determined yaw steering angle. Earth’s rotational velocity may be neglected in the determination of the total velocity parameter vt. The camera may be a time delayed integration (TDI) imaging array detector that has a plurality of TDI stages and a plurality of pixels in each TDI stage. The camera may have a swath width. The camera may be provided about the roll axis so as to have its swath width extending generally in line with the flight direction. The camera may be arranged to move its swath width across the flight direction as it pivots about the roll axis. The camera may be a time delayed integration (TDI) imaging array detector, and it may have a plurality of TDI stages arranged in rows and a plurality of pixels in each TDI stage arranged in columns. The rows of TDI stages may extend generally in line with the flight direction. The columns of pixels may extend generally across the flight direction. The method may include providing a yaw actuator onboard the airborne or spaceborne platform. The yaw actuator may be arranged to yaw steer the camera or the airborne or spaceborne platform by the determined yaw steering angle. The method may include yawing the airborne or spaceborne platform, or yawing the camera, dynamically and in near-real time. In accordance with another aspect of the present disclosure there is provided a scanning system for an airborne or spaceborne platform configured to move in a flight direction around the earth at a flight velocity parameter, the scanning system comprising: a camera configured to be provided onboard the airborne or spaceborne platform about a roll axis so as to be pivotable thereabout, the roll axis being generally in line with the flight direction, the camera being capable of scanning the ground at a scanning velocity parameter extending generally in a direction across the flight direction; a yaw steering angle determining component which is configured to determine a yaw steering angle based at least on the flight velocity parameter and the scanning velocity parameter; and a yaw actuator which is arranged to yaw steer the camera or the platform by the determined yaw steering angle. The system may include a flight velocity parameter determining component arranged to determine or sense the flight velocity parameter of the airborne or spaceborne platform. The system may include a scanning velocity parameter determining component arranged to determine or sense the scanning velocity parameter. The system may include a total velocity parameter determining component arranged to determine a total velocity parameter of the airborne or spaceborne platform by performing a vector sum of the scanning velocity parameter and the flight velocity parameter. The yaw steering angle determining component may be arranged to determine the yaw steering angle by determining an angle between the scanning velocity parameter and the flight velocity parameter. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Figure 1 is a diagrammatic view of a scanner system onboard a spaceborne platform illustrating its path of flight relative to the ground, and with a camera (or the platform itself) being rotatable about a roll axis; Figure 2 is a diagrammatic representation of a roll axis, a pitch axis, and a yaw axis of an airborne craft for explanatory purposes; Figure 3 is another diagrammatic view of a scanner system onboard a spaceborne platform illustrating its path of flight relative to the ground, with a camera of the scanner system being pivotable relative to nadir; Figure 4 is a high-level flow diagram of an exemplary method of scanning by a camera onboard an airborne or spaceborne platform; Figures 5-6 are diagrammatic representations of a yaw steering angle calculation and a yaw steering angle implementation for a conventional pushbroom time delayed integration (TDI) imaging technique; and Figures 7-8 are diagrammatic representations of a yaw steering angle calculation and a yaw steering angle implementation of exemplary embodiments of the present disclosure. DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS The present disclosure may enable cross-track scanning by a satellite or airborne craft. In other words, scanning or imaging may be performed across a flight direction of the satellite or airborne craft. A scanner or camera may be provided onboard the craft, and it may have a field of view onto the ground. The camera may be rotated or pivoted, and its field of view of the ground my move. This may be referred to as the footprint of the camera. A telescope may also be used in conjunction with the camera. The present disclosure extends to a scanner module for a satellite or for an aircraft or drone or unmanned aerial vehicle (UAV). The scanner module may, for example be retrofitted to an existing platform or craft. A required rotational angle or yawing angle may be calculated, estimated or determined by embodiments of the present disclosure. The yaw steering angle may be determined or calculated by an electronic device associated with the airborne or spaceborne platform or craft, or it may be determined or calculated remotely. The problem of image smearing as referred to in the background may be mitigated by either progressively rolling the satellite off-nadir or alternatively by a technique referred to as yaw steering. The latter is the preferred means of compensating for Earth’s rotation. The yaw steering angle may be calculated as the angle between the forward ground velocity of the satellite and total ground velocity i.e., vector sum of satellite’s ground velocity and Earth’s rotation velocity at given latitude (however, earth’s rotation velocity may be neglected in embodiments of the present disclosure). A first exemplary embodiment of the present disclosure is illustrated in Figure 1, and a second exemplary embodiment is illustrated in Figure 3. Figure 2 is an explanatory illustration to show the various axes of rotation of an airborne or spaceborne platform or craft in three-dimensional space. Figure 4 shows an exemplary method that may be implemented by aspects of the present disclosure. Figures 5-6 show a known technique for calculating a yaw steering angle for conventional pushbroom imaging. Figures 7-8 show exemplary embodiments for determining a yaw steering angle according to aspects of the present disclosure. In Figure 1 is shown an exemplary embodiment of a scanning system (10) for an airborne or spaceborne platform (12) configured to move in a flight direction (14) around the earth at a flight velocity parameter (Vgsat) (25) which is shown diagrammatically. It will be appreciated that “Vgsat” may be used in the present disclosure to designate the flight velocity parameter of a satellite (12), however a similar velocity parameter may be applicable for an airborne platform such as a drone, so Vgsat is to be construed to refer to the flight (or space travel) velocity parameter of the airborne or spaceborne platform irrespective of whether the system (10) is onboard a satellite or another flying platform such as a un-manned aircraft, drone, other aircraft or the like. Components of the scanning system (10) may be provided onboard the airborne or spaceborne platform (12), or one or more components of the scanning system may be provided remotely, for example on earth and forming part of a server system in data communication (at least intermittently) with the airborne or spaceborne platform. In the exemplary embodiment in Figure 1, exemplary components of the system (10) are illustrated in the block diagram, but it will be understood that these components may be provided onboard the platform (12), or remotely, e.g., on earth. The scanning system (10) may include a camera (16) which may be arranged to be provided onboard the airborne or spaceborne platform (12) about a roll axis (18) so as to be pivotable thereabout. In the present embodiment, the roll axis may be generally in line with the flight direction (14). The camera (16) may be capable of scanning the ground (20) at a scanning velocity parameter (Vgscan) (27) which may extend generally in a direction across the flight direction (14). An exemplary nadir angle (22) of the camera (16), or downward direction is also shown in the diagrammatic illustration in Figure 1. Figure 1 illustrates an exemplary embodiment that may implement continuous rotational scanning of Earth’s surface by a spaceborne camera (16). The camera (16) may continuously rotate through a full 360°. The camera (16) is shown rotating relative to the host satellite platform (12). In the exemplary embodiment in Figure 1, the camera (16) may be rotatable about the roll axis (18), for example by way of a rotatable actuator such as an electric motor. The camera may be continuously rotated about the roll axis (18), e.g., through multiple revolutions of 360°. Alternatively, the airborne or spaceborne platform (12) itself may be pivotable about the roll axis (18), e.g., by 360° (and in such an embodiment the camera may be fixed in position onboard the platform (12)). Of course, if the platform (12) is rotated the camera may also be rotated about the roll axis (18) during rotation of the platform (12). The exemplary embodiment of Figure 3 is similar, however in the embodiment of Figure 3, a camera (116) of a scanning system (100) may be pivotable from nadir (122) to opposite angles, for example in an oscillatory movement or in a to-and-fro movement. Further features of the embodiment of Figure 3 may be similar to those of Figure 1, and like reference numerals may be used to designate like components or features. For example, the flight direction (114), a nadir direction (122), a camera (116), a roll axis (118), a flight velocity parameter (125), and a scanning velocity parameter (127). It will be appreciated that the embodiment of the scanning system (100) may include the further components of the scanning system of Figure 1, even though some of the components (e.g., of the block diagram of Figure 1) are omitted from Figure 3 for the sake of brevity. Figure 3 shows an exemplary embodiment in which oscillatory scanning of Earth’s surface is implemented by a spaceborne camera (116). The camera (116) is shown oscillating relative to the host satellite platform (112). Referring again to Figure 1, the scanning system (10) may include a yaw steering angle (^Ps) determining component (24) which may be configured to determine a yaw steering angle (^Ps) based at least on the flight velocity parameter (Vgsat) and the scanning velocity parameter (Vgscan). The system (10) may further include a yaw actuator which may be arranged to yaw steer the camera (16) or the platform (12) by the determined yaw steering angle (^Ps). The yaw actuator may be any type of actuator that can yaw the camera (16) or that can yaw the airborne or spaceborne platform (12) about a yaw axis (30). A diagrammatic representation of the airborne or spaceborne platform and its roll axis, pitch axis and yaw axis (30) is shown in Figure 2, for explanatory purposes. It will be appreciated that when the yaw actuator is required to yaw the camera, an electric motor or other actuator for moving the camera (16) about the yaw axis (30) may be implemented. Alternatively, or in addition an actuator such as a steering device for space travel may be used to yaw the platform if it is in space (e.g., a gimbaled moving device, gimbaled thrust system or other thrust systems). Yet further alternatively, or in addition, control surfaces (such as a rudder or the like) or other control apparatus(es) may be used as a yaw actuator if the platform is an airborne platform. The system (10, 100) and method (200) of the present disclosure may include providing a yaw actuator onboard the airborne or spaceborne platform. The yaw actuator may be arranged to yaw steer the camera or the airborne or spaceborne platform by the determined yaw steering angle (^Ps). The present disclosure may include features of yawing the airborne or spaceborne platform, or yawing the camera, dynamically and in near-real time. In the exemplary embodiment of Figure 1 (and that of Figure 3), the system (10, 100) may further include a flight velocity parameter (Vgsat) determining component (26) which may be arranged to determine or sense the flight velocity parameter (Vgsat) of the airborne or spaceborne platform (12). The system (10) may further include a scanning velocity parameter (Vgscan) determining component (28) arranged to determine or sense the scanning velocity parameter (Vgscan). In the case of the camera (16) being rotatable or pivotable by an electric motor, the scanning velocity parameter (Vgscan) determining component (28) may include or be in data communication with a rotary encoder which generates encoder data relating to rotation or pivotal movement of the camera. This encoder data may be used by the system (10) to determine or estimate the scanning velocity parameter (Vgscan). However, in the embodiment in which the entire satellite or spaceborne platform is rotated, the scanning velocity parameter may be determined in another way, for example by accessing data relating to the rotational speed of the satellite. In the exemplary embodiments (10, 100) of Figures 1 and 3, the scanning system may further include a processor (32) for executing the functions of components described in the present disclosure, which may for example be provided by hardware or by software units executing on the processor (32). The software units may be stored in a memory component (34) and instructions may be provided to the processor (32) to carry out the functionality of the described components. In some cases, for example in a cloud computing implementation, software units arranged to manage and / or process data on behalf of the processor (32) may be provided remotely. In an exemplary embodiment, the transmitting / receiving component (36) may be in data communications with a server, e.g., on the earth. Referring again to Figure 1, the system may further include a total velocity parameter (Vt) determining component (38) arranged to determine a total velocity parameter of the airborne or spaceborne platform (12). This may for example be done by way of performing a vector sum of the scanning velocity parameter (Vgscan) and the flight velocity parameter (Vgsat). Further features of the total velocity parameter (Vt) determining component (38) will be described in more detail below. In exemplary embodiments of the present disclosure, the yaw steering angle (^Ps) determining component (24) may be arranged to determine the yaw steering angle (^Ps) by determining an angle between the scanning velocity parameter (Vgscan) and the flight velocity parameter (Vgsat). This will also be described in more detail below. The present disclosure may implement cross-track scanning of an optical imaging payload onboard a satellite, and this may be achieved by various rotational means. Cross-track scanning may be referred to as scanning across the direction of flight of the airborne or spaceborne platform. Cross-track scanning may, for example, be implemented by continuous rotation of the satellite and telescope / camera. Alternatively, or in addition, continuous rotation of the telescope or camera may be implemented (e.g., as shown in Figure 1). Further alternatively, or in addition, the telescope or camera may be oscillated through a partial rotational arc (see Figure 3) e.g., in the range +60° to -60° or +45° to -45°. Scanning may be achieved by continuous rotation of the camera (16) about the satellite’s velocity vector (X-axis), i.e., the roll axis (18) to scan the ground (20) below. This may typically be accomplished by rotation at a nominally constant rotational speed. Oscillatory scanning may, on the other hand, be accomplished by partial rotation (roll) or pivoting of the imaging payload about the satellite’s velocity vector (X-axis) (i.e., the roll axis (18)). The camera (16) may for example rotate (roll) from -45° to +45° about the nadir point (22) in the cross-track direction. Scanning may therefore be achieved in the cross-track direction, which for example traces a zig-zagged line-scanned pattern on the ground. Scan speed and off-nadir roll angles may be matched to satellite forward velocity to ensure continuous coverage (imaging) as long as the scanning imager is activated. The present disclosure may provide a means for increasing the signal to noise ratio (SNR) attainable during high-speed cross-track scanning of the earth with an optical scanner. Crosstrack scanning may be achieved at very high angular rotational speeds, e.g., in the order of about 907s, or about 1357s, or about 1807s, or more. High-speed scanning may require very high speed TDI imaging. TDI line frequencies of about hundreds of kHz may be implemented by the present disclosure, e.g., so as to enable TDI capability during these high-speed cross-track rotational scanning speeds. The present disclosure may implement a yaw steering technique to compensate for forward ground speed (Vgsat) of the satellite which smears the TDI image created by high-speed cross-track scanning. Orthogonal smear may degrade MTF (modulation transfer function) of the image, or it may reduce image sharpness. In an attempt at improving SNR by increasing total integration time, the MTF degrades due to smear resulting from orthogonal motion of the ground below during the prolonged integration time. The origin of the smear, in this case, may be the forward velocity of the satellite (Vgsat, 725) which may act orthogonally to the cross-track scanning motion (727) (see Figure 7). Figure 7 is described in more detail below. In Figure 4 is shown an exemplary method (200) of scanning by a camera onboard an airborne or spaceborne platform configured to move in a flight direction around the earth at a flight velocity parameter (Vgsat). The method may include providing (210) the camera about a roll axis so as to be rollable or pivotable thereabout, the roll axis being generally in line with the flight direction, so as to be capable of scanning the ground at a scanning velocity parameter (Vgscan) extending generally in a direction across the flight direction. The method may further include determining (220) a yaw steering angle (^Ps) based at least on the flight velocity parameter (Vgsat) and the scanning velocity parameter (Vgscan). The method may yet further include yawing (230) the camera or the platform by the determined yaw steering angle (^Ps). The determination (220) of the yaw steering angle (^Ps) may include performing a vector sum of at least the flight velocity parameter (Vgsat) (25, 125) and the scanning velocity parameter (Vgscan) (27, 127). In embodiments of the present disclosure, the scanning velocity parameter (Vgscan) (27, 127) may correspond to a so-called “footprint” velocity of a field of view of the camera on the ground (20, 120). Referring to Figures 1 and 3, the camera (16, 116) may have a swath width (40, 140). The swath width may, for example, correspond to a width on the ground that can be scanned by the field of view of the camera. The camera may move its field of view across the ground to cover an area as indicated by the directional arrow (42) in Figure 1. In the present embodiment of Figure 1, this area (42) may extend from a first position (42.1) to a second position (42.1) on the ground as the camera (16) or platform (12) rotates about the roll axis (18). Referring to Figure 3, the camera may move its field of view across the ground to cover an area as indicated by the directional arrow (142) in Figure 3. In the present embodiment of Figure 3, this area (142) may extend from a first position (142.1) to a second position (142.2) on the ground as the camera (16) or platform (12) pivots from one side to the other about the roll axis (18), i.e., pivoting from nadir (122) to opposite angles. In embodiments of the present disclosure, the camera (16, 116) may be provided about the roll axis (118) so as to have its swath width (40,140) extending generally in line with the flight direction (14, 114). The camera may be arranged to move its swath width (40, 140) across the flight direction (14, 114) as the camera pivots about the roll axis (18, 118). Referring now to Figures 5-6, there is shown a known technique for a yaw steering angle calculation (Figure 5) and a yaw steering angle implementation (Figure 6) for a conventional pushbroom time delayed integration (TDI) imaging technique. Pushbroom imaging is a scanning method in which the swath direction (500) of the scanner extends across the direction of flight (510), and in which the scanner is typically fixed relative to the satellite. In other words, the scanner “pushes” its field of view over the ground like a broom in a straight line. The detector then scans pixels in a first TDI stage or row of the detector or scanner. These rows are referred to as TDI stages (520). The pixels are typically arranged in columns along the swath width / swath direction (500). In the present example, there may be N TDI stages (520) and M columns (530) of pixels. In Figure 5, a TDI imaging array detector is shown, with N TDI stages (or rows) extending in the scan direction and M columns of pixels extending in the swath direction (500). The detector array is, for example, mounted onboard a satellite, as part of an imaging payload. Typically, the imaging array is mounted on the focal plane of an imaging telescope. The satellite travels forward at a velocity Vgsat (510). Earth below rotates at a speed Ve (540), depending on latitude of the imaged ground point. A total ground velocity (Vt) (550) can be taken as the vector sum of satellite ground velocity (Vgsat) (510) and Earth’s rotational velocity (Ve) (540). I.e., Vt = Vgsat + Ve. A yaw steering angle required to mitigate cross-track smear is then conventionally calculated as the angle between the ground velocity of the satellite (Vgsat) and the total ground velocity (Vt). In Figure 6, the yaw steering angle implementation is diagrammatically illustrated for conventional pushbroom TDI imaging. The satellite or imaging payload is yawed at the calculated (required) yaw steering angle to mitigate the smearing notion of Earth’s surface below. The TDI detector or scanner may generally have more pixels in the swath direction (500), e.g., about 12 thousand pixels which provides the swath width om the imager. These may be referred to as the M columns (530) of the imager. The TDI stages direction may generally have a smaller pixel count e.g., 64 pixels for a 64-stage TDI detector as shown in the example of Figure 5. These TDI stages may be referred to as the rows (520) of the imager or scanner. It will be appreciated that embodiments of the present disclosure may implement a scanner having one or more of these features. As an example, for a satellite orbiting at 500km in a sun-synchronous orbit, the ground speed may be taken as 7,059m / s, Earth’s rotational speed at the equator may be 465m / s and an exemplary yaw steering angle may be calculated as 3.71°. Turning now to Figures 7-8, there is shown diagrammatic representations of a yaw steering angle calculation and a yaw steering angle implementation of exemplary embodiments of the present disclosure (e.g., those of Figures 1 and 3). In embodiments of the present disclosure, a yaw steering angle (^Ps) may be calculated or determined for the camera or scanner, e.g., for TDI imaging. An exemplary TDI imaging array detector is shown in Figure 7, with N TDI stages (rows) (720) in a scan direction (733), and M columns (730) of pixels extending in a swath direction (700). The detector array may be mounted onboard a satellite (or airborne) platform, as part of a rotating imaging payload. Typically, the imaging array may be mounted on the focal plane of an imaging telescope. In the exemplary embodiment, the payload (camera) rotates to scan the ground below, at velocity Vgscan in the leftward direction in Figure 7. This may be termed the scanning velocity parameter (Vgscan) (727). The satellite (12) travels forward (downward in Figure 7) at velocity Vgsat which may also be termed a flight velocity parameter (725). The Earth below rotates at a speed Ve (737), depending on latitude of the imaged ground point, but the velocity of the Earth (Ve 737) may be negligible compared to other velocity components Vgscan (727) and Vgsat (725). Earth’s rotational velocity Ve may be neglected, or negligible in quantity due to the scanning motion of the camera (16) being across the direction of flight (25) (see, e.g., Figures 1 and 3), and / or due to the scanning velocity (Vgscan) being orders of magnitude greater than Ve. Total ground velocity (Vt) (750) may be calculated as the vector sum of satellite ground velocity (Vgsat, 725) and Earth’s rotational velocity (Ve, 737). I.e., Vt = Vgscan + Vgsat + Ve. However, Ve may be negligible as stated above, so a simpler formula such as Vt = Vgscan + Vgsat may be implemented to estimate or approximate Vt with acceptable accuracy. The yaw steering angle (^Ps) required to mitigate along-track smear due to satellite forward velocity may then be calculated as the angle between the scanning ground velocity (Vgscan, 727), and the estimated total ground velocity (Vt, 750). The total ground velocity (Vt) may for example be determined by the total velocity parameter (Vt) determining component (38). An exemplary implementation of the yaw steering angle (^Ps) by the system (10, 100) is diagrammatically illustrated in Figure 8. This may significantly reduce or alleviate image smearing. The camera (16, 116) may have a telescope associated therewith. The camera and / or the telescope may be oriented with its swath (40) (column direction of the detector) extending in the along-track direction of the satellite (see Figure 7), or generally in line with the direction of flight, instead of the conventional orientation where swath extends in the cross-track direction for pushbroom imaging (see Figure 5). The linear imaging detector array’s longer axis of pixels (730) may therefore be aligned with the satellite velocity vector (Vgsat) (see Figure 7) instead of the conventional cross-track orientation (i.e., cross-flight direction orientation) (see Figure 5). It will be appreciated that the columns (730) of pixels in Figure 7 extend horizontally in the figure, whereas the rows of TDI stages (720) extend vertically in the figure. The rows of TDI stages may be referred to as a first pixel direction (720), and the columns of pixels may be referred to as a second pixel direction (730). In the present embodiment, the first pixel direction (720) may extend generally in the flight direction (725), whereas the second pixel direction (730) may extend generally across the flight direction (725). The camera (16, 116) may be a time delayed integration (TDI) imaging array detector that has a plurality of TDI stages (720) and a plurality of pixels in each TDI stage. The plurality of TDI stages (720) may be arranged in rows and a plurality of pixels (730) in each TDI stage may be arranged in columns. The rows of TDI stages (720) may extend in line with the flight direction (Vgsat, 725), and in line with the swath width (700). The columns of pixels (730) may extend generally across the flight direction (Vgsat, 725). The columns of pixels (730) may extend generally in (align with) the scan direction (733). The rows of pixels (720) may extend in the swath direction (700). The yaw steering angle (^Ps) may be calculated as the angle between the ground velocity (Vgscan, 727) of the scanner’s footprint on the ground and a total velocity vector (Vt). The total velocity vector (Vt) may be taken as the vector sum of the scanner’s footprint velocity (Vgscan, 727) plus satellite’s ground velocity (Vgsat, 725) plus Earth’s rotational velocity (Ve) at given latitude. That said, Earth’s rotational speed (Ve) is an order of magnitude lower than the satellite’s ground speed (Vgsat, 725), and it may act in a direction more aligned with the cross-track scanning. Ve (737) may be neglected in embodiments of the present disclosure, or included if desired. However, embodiments of the present disclosure may neglect Earth’s rotation for the sake of simplifying and clarifying the derivations, determinations, estimations, or calculations. Ve may, for example, be neglected seeing as it is generally in line with the scanning direction (Vgscan, 727), and seeing as Ve may be orders of magnitude smaller than the scanning velocity parameter (727). Earth’s rotational velocity (Ve) may be neglected in a determination of the total velocity vector (Vt). The satellite’s ground speed may be calculated or sensed by the system (10). As an example, ground speed for a satellite orbiting at 500km may be taken as Vgsat = 7.059km / s. The ground footprint’s cross-track scan speed [km / s] may be derived as follows: where, co is the (preferably) constant rotational speed of the scanner, in rad / s; H is the satellite’s orbital altitude, in km; and Re is the equatorial radius of Earth, generally taken as 6 378.14km. The two vectors, namely satellite ground track velocity (Vgsat) and the scanner’s (i.e., camera’s) (16) ground footprint velocity (Vgscan) may operate orthogonally in embodiments of the present disclosure (e.g., see Figure 1). For uncorrected cross-track scanning, the scanning action may be perpendicular to forward velocity of the satellite projected on the ground. Referring to Figure 7, the yaw steering angle (^Ps) may be calculated as an angle between the scanner’s ground velocity vector (Vgscan, 727) and the total velocity vector (Vt). The total velocity vector may, for example, be calculated as a simple vector sum: Where, Vgscan is the scanner’s footprint ground velocity; Vgsat is the satellite’s ground velocity; and Ve is rotational velocity of the earth at given latitude. The required yaw steering anglers) may then be calculated as the angle between the scan and total velocity vectors: since Vgscan and Vt may be generally perpendicular to one another under an approximation of Ve being negligible; and where ^Ps is the yaw steering angle. It will further be appreciated that the abbreviations Vgsat, Vt, Vgscan and Ve used in the present disclosure may be vector quantities even though they are not written in bold. One or more of the formulae represented in the present disclosure may indicate these quantities with a line above the abbreviation to designate that a vector quantity is referred to. However, in some circumstances these abbreviations may be scalar quantities (e.g., referring to the speed, or size of the vector). In the current illustrative example, the angular scan speed may be maintained as constant. The ground scan velocity may therefore increase as the imager footprint progresses from nadir outwards in the roll direction. Nadir imaging occurs, for example, at time t = 0 and the off-nadir angle increases with time as the product cot. The required yaw steering angle (^Ps) may therefore change with increasing off-nadir angles. It is envisaged that it may not be simple or practical to vary the yaw steering angle of the platform at very rapid rates. Even varying the yaw steering angle of the payload alone may be challenging on small satellites. An example will, however, illustrate that yaw steering at an optimal but fixed angle for the duration of scan may be useful in increasing TDI integration time and therefore SNR. Consider a highspeed cross track scanner orbiting at 500km altitude, and rotating at tt rad / s = 1807s = 0.5Hz or one full revolution every two seconds. The satellite ground speed (Vgsat, 725) may e.g., be 7.06km / s. The ground scanning speed (Vgscan, 727) may range from 1571km / s at nadir (22) to 2183km / s at 30° off nadir roll. The required yaw steering angle may range from 0.257° at nadir to 0.185° when rolled 30° off-nadir. As discussed above, an optimal and fixed yaw steering angle (^Ps) may be required for a simple implementation on a small satellite. Another consideration is associated smear and image degradation. The question is, how many TDI stages may be supported before smearing the image excessively. Choosing a yaw steering angle of 0.2215° means that 1,600 TDI stages result in one full pixel’s smear due to residual (uncorrected) yaw error. Note that typical pushbroom imaging results in one full pixel’s smear in the Vgsat direction, with associated MTF degradation of 0.637x, which is quite acceptable to most applications. If no yaw steering were applied, a full pixel’s smear may occur at 222 TDI stages. The imaging detector’s signal can therefore be improved more than seven-fold, which is very significant. Assuming operation in the shot noise dominated region of the detector, the subsequent SNR may improve with the square root of the signal, i.e., 2.7 times. Improvement in the read noise dominated region may be significantly higher. To illustrate the significance of this improvement in SNR, the aperture of the imaging optics needs to increase by a factor of the square root of 7 = 2.7 times to accomplish the same improvement in SNR. It is therefore evident from above example that this yaw steering mechanism may support very large numbers of TDI stages and it may therefore have tremendous value in improving SNR of a high speed cross track scanner. Today’s TDI detectors do not yet support such high number of TDI stages and synchronisation of motion in the scan direction becomes increasingly challenging with increasing number of TDI stages. Nonetheless, instead of increasing the TDI stages to improve SNR, the cross-track smear may be reduced by yaw steering as described in the present disclosure. In the above example, the full pixel’s smear occurring, with no yaw steering, at 222 TDI stages, may be reduced to 14% of a pixel’s smear by yaw steering. This may improve MTF degradation due to cross-track smear from 0.637x for a full pixel to 0.992x for 14% smear. This improvement in MTF may also be significant in terms of imager performance, as the alternative SNR improvement is, depending on application. The above illustrative example considered a constant angular velocity, typically envisaged for a continuously rotating camera. However, in certain situations, for example an oscillating scanner (e.g., Figure 3), the angular scan speed may be varied so that the ground scan speed remains constant. In such a case, the required yaw steering angle (^Ps) may remain constant for all off-nadir angles; improved yaw steering may be accomplished; and image smear may be significantly reduced or alleviated. In an intermediate case, the angular scan speed may also be partially reduced to give a more constant ground scan speed (Vgscan). In this case, the performance improvement may be better than the above illustrative example based on constant angular scan speed. For a continuously rotating scanner (e.g., that of Figure 1), a fixed yaw steering angle (^Ps) may be valid throughout the rotation cycle of the camera (16), despite not being perfectly accurate throughout the off-nadir sweep. For an oscillating scanner on the other hand (e.g., that of Figure 3), the fixed yaw steering angle (^Ps) may, e.g., only be valid (usable) in one scan direction. That said, in many cases imaging is not critical on the return path of the oscillation and the improvement in MTF or SNR on the forward sweep is highly desirable. One such scenario occurs when the returning sweep is at higher speed, with simple objective of returning the scanner to the starting position for the forward imaging sweep. Referring again to Figure 1, a rotary encoder component (44) may typically monitor a position and angular speed of the payload (i.e., telescope and camera (16)) relative to the satellite platform (12). Encoder data may also enable geolocation and synchronisation of TDI clocking by the system (10). The platform may be a small remote sensing satellite that carries a cross-track scanner. The telescope and camera may be arranged to scan at a very high speed, in the order of 1507s, or about 907s and over, or about 1807s and over. A very wide swath may be scanned by the high-speed scanner, and a high resolution may be achieved (e.g., by implementing the features of the present disclosure e.g., as described in relation to Figures 7-8), or by implementing additional pixels in the TDI stages, rows, or columns. A long integration time (exposure time) may be required to achieve good signal to noise ratio (SNR) in the scanned image. This may be done by TDI imaging during high-speed scanning by implementing aspects of the present disclosure. Long integration times may be associated with orthogonal smearing of the image. The scanner scans sideways, but at the same time the satellite travels forwards. This forward motion may smear the image, but the smearing effect can be mitigated by yawing (in other words rotating in the yaw axis) the satellite (or imaging scanner) relative to the velocity vector or velocity parameter (Vgsat) of the airborne or spaceborne platform. In so doing, the imaged ground pixel(s) may remain aligned with the imaging detector’s column of pixels integrating the charge. By yaw steering the host satellite (or the scanning imager) relative to the forward velocity vector, the undesired smearing effect is reduced significantly. Referring again to Figure 4, the step (220) of determining the yaw steering angle (^Ps) may optionally also include determining a total velocity parameter (Vt, 750) of the airborne or spaceborne platform based at least on the scanning velocity parameter (Vgscan, 727) and the flight velocity parameter (Vgsat, 725). The yaw steering angle (^Ps) may be determined by calculating an angle between the total velocity parameter (Vt) and the scanning velocity parameter (Vgscan). Reduced off-nadir ground scanning speeds may also facilitate yaw steering according to aspects of the present disclosure. The foregoing description has been presented for the purpose of illustration; it is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above disclosure. The language used in the specification has been principally selected for readability and 5 instructional purposes, and it may not have been selected to delineate or circumscribe the inventive subject matter. It is therefore intended that the scope of the invention be limited not by this detailed description, but rather by any claims that issue on an application based hereon. Accordingly, the disclosure of the embodiments of the invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims. 10 Finally, throughout the specification and accompanying claims, unless the context requires otherwise, the word ‘comprise’ or variations such as ‘comprises’ or ‘comprising’ will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. 15

Claims

1. A method of scanning by a camera onboard an airborne or spaceborne platform configured to move in a flight direction around the earth at a flight velocity parameter, the method comprising:providing the camera about a roll axis so as to be pivotable thereabout, the roll axis being generally in line with the flight direction, so as to be capable of scanning the ground at a scanning velocity parameter extending generally in a direction across the flight direction;determining a yaw steering angle based at least on the flight velocity parameter and the scanning velocity parameter; andyawing the camera or the platform by the determined yaw steering angle.

2. The method as claimed in claim 1, wherein the determination of the yaw steering angle includes performing a vector sum of at least the flight velocity parameter and the scanning velocity parameter.

3. The method as claimed in claim 1 or claim 2, wherein the scanning velocity parameter corresponds to a footprint velocity of a field of view of the camera on the ground.

4. The method as claimed in any one of claims 1 to 3, wherein the step of determining the yaw steering angle includes determining a total velocity parameter of the airborne or spaceborne platform based at least on the scanning velocity parameter and the flight velocity parameter, and wherein the yaw steering angle is determined by calculating an angle between the total velocity parameter and the scanning velocity parameter.

5. The method as claimed in any one of the preceding claims, wherein the yaw steering angle is determined based on one or more of the following formulae:..... 4-; andwherein vt is a total velocity parameter of the airborne or spaceborne platform, wherein vgScan is the scanning velocity parameter, wherein vgsat is the flight velocity parameter, and wherein HJs is the determined yaw steering angle.

6. The method as claimed in claim 5, wherein earth’s rotational velocity is neglected in the determination of vt.

7. The method as claimed in any one of the preceding claims, wherein the camera is a time delayed integration (TDI) imaging array detector that has a plurality of TDI stages and a plurality of pixels in each TDI stage.

8. The method as claimed in any one of the preceding claims, wherein the camera has a swath width, and wherein the camera is provided about the roll axis so as to have its swath width extending generally in line with the flight direction and the camera is arranged to move its swath width across the flight direction as it pivots about the roll axis.

9. The method as claimed in claim 8, wherein the camera is a time delayed integration (TDI) imaging array detector that has a plurality of TDI stages arranged in rows and a plurality of pixels in each TDI stage arranged in columns, wherein the rows of TDI stages extend generally in line with the flight direction, and wherein the columns of pixels extend generally across the flight direction.

10. The method as claimed in any one of the preceding claims, wherein the method includes providing a yaw actuator onboard the airborne or spaceborne platform, wherein the yaw actuator is arranged to yaw steer the camera or the airborne or spaceborne platform by the determined yaw steering angle.

11. The method as claimed in any one of the preceding claims, wherein the method includes yawing the airborne or spaceborne platform, or yawing the camera, dynamically and in near-real time.

12. A scanning system for an airborne or spaceborne platform configured to move in a flight direction around the earth at a flight velocity parameter, the scanning system comprising:a camera configured to be provided onboard the airborne or spaceborne platform about a roll axis so as to be pivotable thereabout, the roll axis being generally in line with the flight direction, the camera being capable of scanning the ground at a scanning velocity parameter extending generally in a direction across the flight direction;a yaw steering angle determining component which is configured to determine a yaw steering angle based at least on the flight velocity parameter and the scanning velocity parameter; anda yaw actuator which is arranged to yaw steer the camera or the platform by the determined yaw steering angle.

13. The system as claimed in claim 12, wherein the system includes a flight velocity parameter determining component arranged to determine or sense the flight velocity parameter of the airborne or spaceborne platform.

14. The system as claimed in claim 12 or claim 13, wherein the system includes a scanning velocity parameter determining component arranged to determine or sense the scanning velocity parameter.

15. The system as claimed in any one of claims 12 to 14, wherein the system includes a total velocity parameter determining component arranged to determine a total velocity parameter of the airborne or spaceborne platform by performing a vector sum of the scanning velocity parameter and the flight velocity parameter.

16. The system as claimed in claim 15, wherein the yaw steering angle determining component is arranged to determine the yaw steering angle by determining an angle between the scanning velocity parameter and the flight velocity parameter.

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