Scanning method and system

Abstract

There is provided a system (10) and method of scanning by a camera (16) onboard an airborne or spaceborne platform (12) configured to move in a flight direction (14) around a celestial body such as the earth, a planet, moon or asteroid at a flight velocity parameter (Vgsat). A camera (16) may be configured and provided onboard the airborne or spaceborne platform (12). The camera (16) may be capable of scanning the ground or surface below at a scanning velocity parameter (Vgscan) extending generally in a direction across the flight direction (14). The camera (16) may have an associated imaging detector array. A yaw correction angle (Ψc) may be determined based at least on the flight velocity parameter (Vgsat) and the scanning velocity parameter (Vgscan). A yaw correction may be applied to yaw the camera imaging detector array's footprint by the determined yaw correction angle (Ψc).

Description

[0001]SCANNING METHOD AND SYSTEM CROSS-REFERENCES TO RELATED APPLICATIONS This application claims priority from United Kingdom patent application numbers 2310792.3 and 2310793.1, both filed on 13 July 2023, and both of which are incorporated by reference herein. FIELD This 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, 64 or 96 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 is intended only to facilitate an understanding of the present disclosure. 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 a celestial body at a flight velocity parameter, the method comprising: providing the camera and an imaging detector array thereof onboard the platform so as to be capable of scanning the ground or surface at a scanning velocity parameter extending generally in a direction across the flight direction; determining a yaw correction angle based at least on the flight velocity parameter and the scanning velocity parameter; and causing the camera imaging detector array’s footprint on the imaged surface to be yawed by the determined yaw correction angle. The camera may be provided about a roll axis so as to be pivotable thereabout, the roll axis being generally in line with the flight direction. The celestial body may, for example, be the earth, a planet, a moon, an asteroid, a sun, or any other object that is to be scanned. The method may include yawing the camera itself, or its components, or the platform by the determined yaw correction angle. Components of the camera may include a focal plane and / or one or more imaging detector chip(s) and / or one or more active pixel array(s) on the chip(s) and / or one or more optical elements such as mirrors. The determination of the yaw correction angle may include performing a vector sum of at least the flight velocity parameter and the scanning velocity parameter. The flight velocity parameter may be projected on the ground or surface below. The scanning velocity parameter may correspond to a footprint velocity of a field of view of the camera on the ground, e.g., due to the rotational action of the scanner. The step of determining the yaw correction angle may include determining a total velocity parameter of the camera’s footprint relative to the imaged ground or surface below based at least on the scanning velocity parameter and the flight velocity parameter. The camera’s surface or ground footprint may be the camera’s field of view projected on the ground or surface below. The yaw correction angle may be determined by calculating an angle between the total velocity parameter and the scanning velocity parameter. The yaw correction angle may be determined based on one or more of the following formulae: if ve is negligible, then: Ψ^^≈ atan furthermore, if ɣ = 0 i.e. scan axis is perfectly aligned with flight velocity, then: ^^ Ψ^^ ^^ ^^^≈ a^^^tan ( ) ^^^^ ^^ ^^ ^^ ^^wherein β is the angle between veand vgscan; wherein ɣ is an offset angle which may exist between the scanner axis and the flight velocity parameter; wherein vtis a total velocity parameter of the camera’s footprint relative to the imaged ground or surface below; wherein vgscanis the scanning velocity parameter; wherein vgsatis the flight velocity parameter; and wherein Ψcis the determined yaw correction angle. Earth’s rotational velocity may be neglected in the determination of the total velocity parameter vt, but it will be appreciated that this may not be the case for other celestial bodies. The camera may use 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 use 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 time delayed integration (TDI) imaging array detector may have a plurality of bands or channels, and each band of the detector may have a plurality of TDI stages arranged in rows and a plurality of pixels in each TDI stage arranged in columns. The plurality of bands or channels may be used to acquire a plurality of spectral bands and / or to enhance spatial resolution. The focal plane of the camera may contain a plurality of time delayed integration (TDI) imaging array detectors. The method may include configuring or mounting or fixing columns of the time delayed integration (TDI) imaging array detector, or the time delayed integration (TDI) imaging array detector, or the focal plane holding the time delayed integration (TDI) imaging array detector, or the camera, at the determined yaw correction angle so that the detector columns align with the total velocity vector in orbit. The camera, or its components, may be yawed with respect to the camera boresight axis (optical axis, yaw axis) by the determined yaw correction angle. The method may include providing a yaw actuator onboard the airborne or spaceborne platform. The yaw actuator may be arranged to yaw the camera, or its components, or the airborne or spaceborne platform, by the determined yaw correction angle. The method may include yawing the airborne or spaceborne platform, or yawing the camera, or yawing its components, 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 a celestial body at a flight velocity parameter, the scanning system comprising: a camera and an imaging detector array thereof configured to be provided onboard the airborne or spaceborne platform, the camera being capable of scanning the ground or surface at a scanning velocity parameter extending generally in a direction across the flight direction; a yaw correction angle determining component which is configured to determine a yaw correction angle based at least on the flight velocity parameter and the scanning velocity parameter; and a yaw actuating component which is arranged to cause the camera imaging detector array’s footprint on the imaged surface to be yawed by the determined yaw correction angle. The yaw actuating component or yaw actuator may be arranged to yaw the camera itself, or its components, or the platform by the determined yaw correction angle. The camera may be provided about a roll axis so as to be pivotable thereabout, the roll axis being generally in line with the flight direction. The celestial body may, for example, be the earth, a planet, a moon, an asteroid, a sun, or any other object that is to be scanned. The flight velocity parameter and / or the scanning velocity parameter and / or the total velocity parameter and / or the yaw correction angle may be determined or calculated prior to launch of an imaging mission. One or more imaging components of the system may be mounted or fixed permanently at a required yaw angle prior to launch. 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 including the scanning velocity parameter and the flight velocity parameter. The yaw correction angle determining component may be arranged to determine the yaw correction angle by determining an angle between the scanning velocity parameter and the total velocity parameter. In accordance with yet another exemplary 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 a celestial body at a flight velocity parameter, the scanning system comprising: a camera and an imaging detector array thereof configured to be provided onboard the airborne or spaceborne platform, the camera being capable of scanning the ground or surface at a scanning velocity parameter extending generally in a direction across the flight direction, wherein the camera imaging detector array’s footprint on the scanned surface is configured to be yawed by a determined yaw correction angle, and wherein the yaw correction angle is based at least on the flight velocity parameter and the scanning velocity parameter. The scanning system may include a yaw correction angle determining component. The yaw correction angle determining component may be configured to determine a yaw correction angle based at least on the flight velocity parameter and the scanning velocity parameter. The yaw correction angle determining component may be configured to determine the yaw correction angle in near real-time e.g., during flight of the airborne or spaceborne platform. The yaw correction angle may, e.g., be determined prior to launch of the airborne or spaceborne platform. The camera and / or its components may be fixed prior to launch of the airborne or spaceborne platform so as to yaw the camera’s footprint on the scanned surface during flight by the determined yaw correction angle. In other words, the yaw correction angle may be predetermined, prior to launch / flight, or it may be determined actively during flight (i.e., after launch of the airborne or spaceborne platform). Embodiments of the technology 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 pivotable or rotatable about a roll axis; Figures 2-3 are diagrammatic representations of a roll axis, a pitch axis, and a yaw axis of an airborne craft for explanatory purposes; Figure 4 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 or rotatable relative to nadir; Figure 5 is a high-level flow diagram of an exemplary method of scanning by a camera onboard an airborne or spaceborne platform; Figures 6-7 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; Figures 8-9 are diagrammatic representations of a yaw correction angle calculation and a yaw correction angle implementation of exemplary embodiments of the present disclosure; Figures 10-11 are similar to Figures 8-9, but illustrating a more general case where the scanner roll axis may be offset in yaw relative to the platform velocity vector; and Figures 12-14 are diagrammatic representations that illustrate the camera footprint on the ground or surface, yawed by the required correction angle, for three exemplary scanning configurations. 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 may 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. This required yaw correction angle may be determined and fixed prior to launch for the duration of the mission. Alternatively, the yaw correction angle may be determined or calculated in-flight by an electronic device associated with the airborne or spaceborne platform or craft, or it may be determined or calculated remotely. With reference to the technology of the present disclosure, the yaw correction angle may also be termed a required yaw angle, a yaw adjustment angle or a determined angle. With reference to Figures 6-7, the prior-art 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 of the camera’s footprint relative to the surface i.e., vector sum of satellite’s ground velocity and negative of Earth’s rotation velocity at given latitude. A first exemplary embodiment of the present disclosure is illustrated in Figure 1, and a second exemplary embodiment is illustrated in Figure 4. Figures 2-3 are explanatory illustrations to show the various axes of rotation of an airborne or spaceborne platform or craft in three-dimensional space. Figure 5 shows an exemplary method that may be implemented by aspects of the present disclosure. Figures 6-7 show a known technique for calculating a yaw steering angle for conventional pushbroom imaging. Figures 8-14 show exemplary techniques for determining a yaw correction 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 a celestial body such as the earth, a planet, moon or asteroid, 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. The flight velocity parameter may be the ground projection of the velocity vector of the craft or platform. In other words, the flight velocity parameter may be projected on the ground or surface below. An exemplary step of determining the yaw correction angle may include determining a total velocity parameter of the camera’s footprint relative to the imaged ground or surface below based at least on the scanning velocity parameter and the flight velocity parameter. The camera’s surface or ground footprint may be the camera’s field of view projected on the ground or surface below. The yaw correction angle may be determined by calculating an angle between the total velocity parameter and the scanning velocity parameter. 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), e.g., about a scanner roll axis or camera roll axis (19) so as to be pivotable thereabout. In the present embodiment, the camera roll axis may be generally in line with the flight direction (14), or offset at an arbitrary angle ɣ (926) therefrom (e.g., see Figures 10-11). 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 camera roll axis (19), for example by way of a rotatable actuator such as an electric motor. The camera may be continuously rotated about the camera roll axis ŝ (19), e.g., through multiple revolutions of 360°. Alternatively, the airborne or spaceborne platform (12) itself may be pivotable about the roll axis (18, 19), e.g., by 360° (and in such an embodiment the camera may be fixed in position onboard the platform (12) and a platform roll axis x̂ (18) becomes the camera roll axis ŝ (19), i.e., these axes are aligned). Of course, if the platform (12) is rotated the camera may also be rotated about the roll axis (18, 19) during rotation of the platform (12). The exemplary embodiment of Figure 4 is similar, however in the embodiment of Figure 4, 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 4 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 camera roll axis (119), a platform 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 4 for the sake of brevity. Figure 4 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, a yaw correction angle (Ψc) may be determined prior to launching the mission, based at least on the flight velocity parameter (Vgsat) and the scanning velocity parameter (Vgscan). The components of the camera, or the camera (16), may be configured to be yawed by a yaw correction angle (Ψc) about a camera yaw axis (23), in a fixed configuration for the duration of the imaging mission. Alternatively, the scanning system (10) may include a yaw correction angle (Ψc) determining component (24) which may be configured to determine a yaw correction angle (Ψc) 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 the camera (16), or its components, or the platform (12) by the determined yaw correction angle (Ψc). The yaw actuator may be any type of actuator that can yaw the components of the camera, or yaw the camera (16), about a camera yaw axis (23) 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 (x̂, 18), pitch axis (ŷ, 31) and yaw axis (ẑ, 30) is shown in Figure 2, for explanatory purposes. Also shown is the scanner roll axis (ŝ, 19) or camera roll axis, which may either be aligned with the platform roll axis (x̂, 18) as shown in Figure 2, or offset from the platform roll axis (18) as shown in Figure 3. It will be appreciated that when the yaw actuator is required to yaw the camera or its components, an electric motor or other actuator for moving the camera (16) or its components about the camera yaw axis (23) 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 yaw actuator may be arranged to cause the camera imaging detector array’s footprint on the imaged surface to be yawed by the determined yaw correction angle. The yaw actuator may be arranged to yaw the camera itself, or its components, or the platform by the determined yaw correction angle (Ψc 834, 1034, 1234, 1334, 1434, as the case may be) in accordance with various examples of the technology of the present disclosure. The yaw actuator may also be termed a yaw actuating component. The yaw correction angle may be calculated upfront and the detector or camera may be mounted (or fixed) at the required yaw correction angle (e.g., prior to launch). Alternatively, the yaw correction angle may be determined in near-real time, e.g., during flight, and a yaw actuator may implement it. The camera yaw axis (23) may also be termed a camera boresight or an optical axis. When yaw correction is applied, the camera or its components may be yawed or rotated about this camera yaw axis to align a column direction with a total velocity vector. The camera yaw axis may coincide with the platform yaw axis when the scanner (and camera) are nadir pointing. The yaw correction angle (Ψc) may be referred to in the present disclosure, e.g., where a TDI imaging detector array is mounted such that its columns align with the total velocity vector. One or more imaging components of the system may be mounted or fixed permanently at a required yaw angle prior to launch. The system (10, 100) and method (200) of the present disclosure may include configuring the camera or the components of the camera in a yawed rotation about the camera yaw axis (23, 123), with a reference angle taken when camera boresight aligns with the nadir axis (22, 122). This may be performed prior to launching the imaging mission and remain at a fixed angle for the duration of the mission. 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 components of the camera, or the camera, or the airborne or spaceborne platform by the determined yaw correction angle (Ψc). The present disclosure may include features of yawing the components of the camera, or yawing the camera, dynamically and in near-real time. In the exemplary embodiment of Figure 1 (and that of Figure 4), 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 4, 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 correction angle (Ψc) determining component (24) may be arranged to determine the yaw correction angle (Ψc) by determining an angle between the scanning velocity parameter (Vgscan) and the total velocity parameter (Vt). 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 4) e.g., in the range +60° to -60° or +45° to -45° or +30° to -30°. Oscillation may be symmetrical about the nadir point, or it may not be. The telescope or camera may be oscillated through a partial rotational arc (see Figure 4) e.g., in the range +60° to 0° or +45° to -15° or +30° to +20° or any other pair of angles. Scanning may be achieved by continuous rotation of the camera (16) about the camera’s roll axis (19), which may be generally aligned with the satellite’s velocity vector (X-axis), i.e., the roll axis (18) to scan the ground or surface (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 camera’s roll axis (19), which may be generally aligned with 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 platform roll axis (x-axis, 18) may be generally aligned with the orbital or flight velocity vector. Or the platform roll axis (x-axis, 18) may be offset from the orbital or flight velocity vector. The scanner roll axis or camera roll axis (ŝ, 19) may be aligned with the satellite’s velocity vector (x-axis, x̂, 18), as illustrated in Figures 1, 2, 4, 8 and 9. In this case, the scanning motion (727) acts orthogonally to the forward velocity of the satellite (Vgsat, 725). In a more general case, the scanner roll axis or pivot axis (19) may be offset by an angle from the satellite’s velocity vector (x- axis, 18), as illustrated in Figures 3, 10 and 11. This may be due to the scanner roll or pivot axis (19) mounted at an offset relative to the platform roll axis (18) and / or due to yaw of the platform relative to its flight velocity vector. An offset angle may be brought about deliberately or due to uncertainties and inaccuracies in manufacturing or platform attitude control. The yaw correction angle (Ψc) of the present disclosure may apply whether the scanner roll axis is perfectly aligned with the platform velocity vector (18) or if the scanner roll axis is offset from the platform velocity vector. In the case of the present disclosure, conventional yaw steering of the satellite platform (as performed in the case of pushbroom imaging) may be ineffective at reducing orthogonal smear, since the scan axis and therefore scanner velocity vector (927) yaws along with the platform. The direction of Vt therefore changes as the platform is yawed. Platform yaw may therefore be represented by offset angle ɣ. An effective means of aligning detector columns with Vt is yawing the camera or components thereof about the camera yaw axis rather than yawing the platform. In so doing the directions of Vgsat, Vgscan and Vt can remain unchanged. 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 or other celestial body with an optical scanner. Cross-track scanning may be achieved at very high angular rotational speeds, e.g., in the order of about 45° / s, or about 90° / s, or about 135° / s, or about 180° / s, or more. High- speed scanning may require very high speed TDI imaging. TDI line frequencies of about hundreds of kHz or over a MHz 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 correction 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, 927) (see Figures 8 and 10). Figures 8 and 10 are described in more detail below. In Figure 5 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 a celestial body such as the earth, a planet, moon or asteroid, at a flight velocity parameter (Vgsat). The method may include providing (210) the camera and an imaging detector array thereof onboard the platform 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 correction angle (Ψc) 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 components of the camera(or components associated with the camera), or the platform, by the determined yaw correction angle (Ψc). Alternatively, or in addition, the method may include causing (235) the camera imaging detector array’s footprint on the imaged surface to be yawed by the determined yaw correction angle. It will be appreciated that steps (230, 235) may be performed as alternative steps in the method (200) (e.g., with step 235 replacing step 230). The imaging detector array is not shown in the drawings, but it will be appreciated that the imaging detector array may be associated with the camera (e.g., 16, 116, 1216, 1316, 1416). The determination (220) of the yaw correction angle (Ψc) 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) due to the rotational scanning action. Referring to Figures 1 and 4, 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 (42.2) on the ground as the camera (16) rotates about the camera roll axis (19) or the platform (12) rotates about the platform roll axis (18). Referring to Figure 4, the camera may move its field of view across the ground to cover an area as indicated by the directional arrow (142) in Figure 4. In the present embodiment of Figure 4, 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 (18,19), i.e., pivoting from nadir (122) to opposite angles. In embodiments of the present disclosure, the camera (16, 116) may be provided about the camera roll axis (19) which may be generally aligned with the platform roll axis (18) 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 camera roll axis (19, 119) which may be generally aligned with the platform roll axis (18). Referring now to Figures 6-7, there is shown a known technique for a yaw steering angle calculation (Figure 6) and a yaw steering angle implementation (Figure 7) 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. The yaw steering angle (Ψs), may be referred to in reference to prior art i.e. yaw steering of a conventional TDI satellite. In Figure 6, 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) of the camera footprint relative to the surface can be taken as the vector sum of satellite ground velocity (Vgsat) (510) and negative of Earth’s rotational velocity (Ve) (540). I.e., Vt = Vgsat - Ve. The opposite or negative of Earth’s rotational velocity can be used in the vector sum since Earth’s rotation acts to move the ground surface away under the camera footprint, as opposed to the satellite ground velocity which moves the camera footprint itself (in the positive total ground footprint direction) over the surface. 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. Generally, the satellite platform carrying the imaging payload (camera) is steered by the attitude control system of the platform, in yaw, relative to its flight velocity vector. This yaw steering acts to align the columns (or column direction 531) of the TDI imaging array detector with the total velocity vector Vt (550). Alignment of the imaging detector array’s columns with the total velocity vector acts such that photon charge accumulated by the TDI imaging array detector remains within the same column as it passes from one TDI stage (row) to the next. Retaining charge originating from one ground pixel within the same column, prevents orthogonal smearing or MTF degradation. In so doing, the yaw steering eliminates cross-track smearing due to Earth’s rotation. The column direction (ĉ, 531) of the imaging detector array is illustrated by shading of one column (532). The columns and rows are exaggerated in size for purposes of illustration. 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 of 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 6. 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 8-9, there is shown diagrammatic representations of a yaw correction angle calculation and a yaw correction angle implementation of exemplary embodiments of the present disclosure (e.g., those of Figures 1 and 4). In embodiments of the present disclosure, a yaw correction angle (Ψc) 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 8, 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, with footprint moving at ground / surface velocity Vgscan in the leftward direction in Figure 8. This may be termed the scanning velocity parameter (Vgscan) (727). The satellite (12) travels forward (downward in Figure 8) 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) possibly being across the direction of flight (25) (see, e.g., Figures 1 and 4) in a near-polar orbit, 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 scanner footprint ground / surface velocity (Vgscan, 727), the satellite ground / surface velocity (Vgsat, 725) and the negative of Earth’s rotational velocity (Ve, 737). I.e., Vt = Vgscan + Vgsat - Ve. The opposite or negative of Earth’s rotational velocity is used in the vector sum since Earth’s rotation acts to move the ground surface away under the camera footprint, as opposed to the scanner ground velocity and satellite ground velocity which move the camera footprint itself (in the positive total ground footprint direction). 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 for imaging the earth. The yaw correction angle (Ψc) 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 prior to launching the imaging mission or it may be determined during the mission by the total velocity parameter (Vt) determining component (38). An exemplary implementation of the yaw correction angle (Ψc) by the system (10, 100) is diagrammatically illustrated in Figure 9. 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) extending in the along-track direction of the satellite (see Figure 8), 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 6). The linear imaging detector array’s longer axis of pixels (730) may therefore be aligned with the satellite velocity vector (Vgsat) (see Figure 8) instead of the conventional cross-track orientation (i.e., cross-flight direction orientation) (see Figure 6). It will be appreciated that the columns (730) of pixels in Figure 8 extend horizontally in the figure as exemplified by the shaded column (732) in Figure 8, 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 column direction (ĉ, 731) of the imaging detector array is illustrated by shading of one column (732). 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 in direction (731, 732) generally across the flight direction (Vgsat, 725). The columns of pixels (730) may extend (732) generally in (align with) the scan direction (733). The rows of pixels (720) may extend in the swath direction (700). Components of the camera may include a focal plane and / or one or more imaging detector chip(s) and / or one or more active pixel array(s) on the chip(s). The camera may use 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 time delayed integration (TDI) imaging array detector may have a plurality of bands or channels, and each band of the detector may have a plurality of TDI stages arranged in rows and a plurality of pixels in each TDI stage arranged in columns. The plurality of bands or channels may be used to acquire a plurality of spectral bands and / or to enhance spatial resolution. The focal plane of the camera may contain a plurality of time delayed integration (TDI) imaging array detectors. The technology of the present disclosure may include configuring or mounting or fixing columns of the time delayed integration (TDI) imaging array detector, or the time delayed integration (TDI) imaging array detector, or the focal plane holding the time delayed integration (TDI) imaging array detector, or the camera, at the determined yaw correction angle so that the detector columns align with the total velocity vector in orbit. The camera, or its components, may be yawed with respect to the camera boresight axis (optical axis, yaw axis) by the determined yaw correction angle. The yaw correction is, e.g., applied by rotating the camera or components thereof about the camera yaw axis (23) by the yaw correction angle (Ψc, 834, 1034). This yaw correction acts to align the columns (or column direction ĉ, 731, 931) of the TDI imaging array detector with the total velocity Vt (750, 950). Alignment of the imaging detector array’s columns with the total velocity vector acts such that photon charge accumulated by the TDI imaging array detector remains within the same column as it passes from one TDI stage (row) to the next. Retaining charge originating from one ground pixel within the same column, may prevent orthogonal smearing or MTF degradation. In so doing, the yaw correction may eliminate or alleviate orthogonal smearing due to the platform ground velocity. The column direction (931) of the imaging detector array is illustrated by shading of one column (932). The columns and rows are exaggerated in size for purposes of illustration. The yaw correction angle (Ψc) may be calculated as the angle between the ground velocity (Vgscan, 727) of the scanner’s footprint on the ground due to the rotational scanning action, and a total velocity vector (Vt) of the scanner’s footprint relative to the ground. The total velocity vector (Vt) may be taken as the vector sum of the scanner’s footprint velocity due to the rotational scanning action (Vgscan, 727) plus the camera’s footprint due to satellite’s ground velocity (Vgsat, 725) plus movement of the ground surface under the camera’s footprint due to 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 may be 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). In the case of other celestial bodies such as planets, moons or asteroids, the celestial body’s rotational velocity (Ve) may or may not be negligible compared to the total velocity parameter (Vt). The scanning velocity parameter may correspond to a footprint velocity of a field of view of the camera on the ground, e.g., due to the rotational action of the scanner. Turning now to Figures 10-11, there is shown more general diagrammatic representations of a yaw correction angle calculation and a yaw correction angle implementation of exemplary embodiments of the present disclosure (e.g., those of Figures 1 and 4). Figures 8-9 illustrate a simpler case of perfect alignment between the scanner roll axis (ŝ, 19, 119) and platform roll axis (x̂, 18) and flight velocity parameter (25). Figures 10-11 illustrate the more general case where the scanner roll axis (19, 119) is offset from the flight velocity direction (18, 925) by angle ɣ (926). In this case, the scan velocity parameter is also offset from the cross-track direction (orbit-normal direction, ŷ, 31) by angle ɣ (926). The satellite’s ground speed may be calculated prior to launching the mission or calculated or sensed by the system (10) during the mission. 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, ω 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 6378.14km. The two vectors, namely satellite ground track velocity (Vgsat) and the scanner’s (i.e., camera’s) (16) ground footprint velocity (Vgscan) due to the rotational scanning action may operate generally in orthogonal direction in embodiments of the present disclosure (e.g., see Figure 1). For cross-track scanning, the scanning action may be generally perpendicular to forward velocity of the satellite projected on the ground. In the exemplary corrected case, the scan direction is not changed but merely the orientation of the imaging detector relative to the scan direction. Referring to Figures 8 and 10, the yaw correction angle (Ψc) may be calculated as an angle between the scanner’s ground velocity vector (Vgscan, 727, 927) 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 i.e. due to scanner rotation; Vgsat is the satellite’s ground velocity; and Ve is rotational velocity of the earth at given latitude. The required yaw correction angle (Ψc) may then be calculated as the angle between the scan and total velocity vectors: The yaw correction angle may be determined based on one or more of the following formulae: if veis negligible, then: Ψ^^≈ atan furthermore, if ɣ = 0 i.e. scan axis is perfectly aligned with flight velocity, then: ^^ Ψ ≈ ata^^ ^^ ^^ ^^^^n ( ) ^^^^ ^^ ^^ ^^ ^^wherein β is the angle between veand vgscan. wherein ɣ is an offset angle which may exist between the scanner axis and the flight velocity parameter wherein vtis a total velocity parameter of the camera’s footprint relative to the imaged ground or surface below, wherein vgscanis the scanning velocity parameter, wherein vgsatis the flight velocity parameter, and wherein Ψcis the determined yaw correction angle. Earth’s rotational velocity may be neglected in the determination of the total velocity parameter vt, but it will be appreciated that this may not be the case for other celestial bodies. 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 ωt. The required yaw correction angle (Ψc) may therefore change with increasing off-nadir angles. It is envisaged that it may not be simple or practical to vary the yaw correction angle of the platform at very rapid rates. Even varying the yaw correction angle of the payload or components thereof alone may be challenging on small satellites. An example will, however, illustrate that yaw correction at an optimal but fixed angle for the duration of scan may be useful in increasing TDI integration time and therefore SNR. Consider a high-speed cross track scanner orbiting at 500km altitude, and rotating at π rad / s = 180° / s = 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 correction angle may range from 0.257° at nadir to 0.185° when rolled 30° off-nadir. As discussed above, an optimal and fixed yaw correction angle (Ψc) 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 correction 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 correction 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 correction 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 correction as described in the present disclosure. In the above example, the full pixel’s smear occurring, with no yaw correction, at 222 TDI stages, may be reduced to 14% of a pixel’s smear by yaw correction. This may improve MTF degradation due to cross-track smear from 0.637x for a full pixel’s smear 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 or constant-angular-speed oscillating scanner. However, in certain situations, for example an oscillating scanner (e.g., Figure 4), the angular scan speed may be varied so that the ground scan speed remains constant during image acquisition. In such a case, the required yaw correction angle (Ψc) may remain constant for all off-nadir angles; improved yaw correction 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 correction angle (Ψc) 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 4), the fixed yaw correction angle (Ψc) 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 120° / s, or about 50° / s and over, or about 180° / s and over. A very wide swath may be scanned by the high-speed scanner, and a high spatial resolution (such as 10m or 1m or 0.1m or better) may be achieved (e.g., by implementing the features of the present disclosure e.g., as described in relation to Figures 8-11), 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 camera yaw axis) the imaging scanner or components thereof relative to the scanning velocity vector or scan velocity parameter (Vgscan) of the airborne or spaceborne platform. In so doing, motion of the imaging detector’s footprint relative to the imaged ground pixel(s) may remain aligned with the imaging detector’s column of pixels integrating the charge. By yawing the scanning imager or its components relative to the scanning velocity vector, the undesired smearing effect is reduced significantly. Referring again to Figure 5, the step (220) of determining the yaw correction angle (Ψc) 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 correction angle (Ψc) 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 relative to nadir scanning speed may also facilitate yaw correction according to aspects of the present disclosure. Figures 12 to 14 illustrate exemplary camera footprints on the ground or surface, yawed by the required correction angle, for three different scanning configurations. A ground surface projection (1243, 1343, 1443) of the TDI imaging array detector, or camera footprint or footprint on the imaged surface are shown. Scanning mirrors (1317, 1417) are also shown for alternative configurations in Figures 13 and 14. These may also be considered components of the camera according to various aspects of the present disclosure. Figure 12 shows an oscillating or rotating scanning system (1210) with camera (1216) to illustrate the cross-track scanning action projected on the ground. Yaw correction (1234) may be applied. Other known configurations exist for scanning the imaged surface in the cross-track direction. These techniques have not, to our knowledge, been used for the type of high-speed cross-track scanning described in the present disclosure, but may be used in future on small or large platforms. Yaw correction may equally be applied in these configurations to increase the integration time without excessive smear, as illustrated in the case of a scanning camera (1210, 1216 in Figure 12). Features shown in Figure 12 may be similar to those of Figure 4, such as the airborne or spaceborne platform (1212), the platform roll axis (1218), the camera roll axis (1219), the surface / ground (1220), and the camera boresight axis (1223). Alternative configurations include, but are not limited to the following examples: 1. Continuous rotation of the camera, e.g. Figure 1. 2. Continuous rotation of the entire platform, or large part thereof carrying the camera instead of the camera as such, e.g. variation of Figure 1. 3. Figure 13 shows another exemplary scanning system (1310) with an optical axis of a camera aligned with a platform (1312) roll axis (1318). Diagrammatic front and side views of the scanning system (1310) are shown. A scanning mirror (1317) is placed in the optical path externally to the camera (1316) aperture and typically centered on the optical axis (1323), redirecting the full field of view of the camera. The scan mirror (1317) is often elliptical in shape and nominally inclined at 45°, redirecting the camera field of view to the ground (1320) below. The mirror (1317) and camera (1316) are fixed together to rotate as a single unit about the platform roll axis (1318). This rolling action scans the camera footprint (1343) in the cross-track direction. The camera (1316) and mirror (1317) may a. rotate continuously (not shown in the drawing), or b. oscillate back and forth as shown in the drawing. The optical axis (1323), of the camera (1316) (via the mirror (1317)) is also shown in Figure 13, as well as the camera roll axis (1319), and the yaw correction angle (1334). The optical axis (1323), of the camera (1316) (via the mirror (1317)) is also shown in Figure 13, as well as the camera roll axis (1319), and the yaw correction angle (1334). Alternatively, the entire platform or large part of the platform carrying the camera may rotate about the roll axis (1318) to perform the scanning action. 4. Variations of the above configuration may be found where the platform may remain for example nadir pointing, the camera is fixed relative to the platform, the mirror is inclined relative to the camera at required angle and only the mirror rotates or oscillates to redirect and scan the footprint on the ground. 5. Figure 14 shows an exemplary scanning system (1410) with a camera (1416) fixed to the satellite platform (1412), with its optical axis (1423) aligned with a platform pitch axis (31). An external scanning mirror (1417) is placed in the optical path externally to the camera (1416) aperture and typically centred on the optical axis, spanning the full field of view of the camera. The scan mirror (1417) is often nominally inclined at 45° in a resting position, directing the camera field of view towards nadir. The mirror is configured to roll about the platform roll axis (1418), in front of the camera (1416) aperture, while the camera remains fixed relative to the platform. This rolling action of the mirror scans the camera footprint (1443) in the cross-track direction. A disadvantage of this configuration is the relatively large mirror (1417) often required to intercept the camera field of view at large off-nadir angles. Nonetheless, it remains an effective means of cross-track scanning. The scan mirror may a. continuously rotate in front of the aperture, either with one (front) or two (front and back) reflecting surfaces, or b. oscillate in front of the aperture as shown in the drawing. The optical axis (1423), of the camera (1416) (via the mirror (1417)) is also shown in Figure 14, as well as the yaw correction angle (1434). 6. Two-dimensional scanning of the camera footprint may be performed for example by a two-axis gimbal or mechanism with similar purpose. If one of the scan directions extends generally in a direction across the flight direction, a yaw correction as disclosed here may be applied to the camera footprint too. Still other configurations may be envisaged that scan the camera footprint on the ground below. For example, a fixed focal plane with rotating telescope. The fixed focal plane may have a nearby fixed optical assembly. Corrections may be applied, such as derotation. Various configurations also exist with scanning corrections or optimizations, which may be applied internally to the camera optical path, for example to trace parallel scan tracks on the ground instead of zigzag tracks. In all these cases the camera footprint (1243, 1343, 1443) may be yawed relative to the scan direction to reduce or eliminate the smear caused by platform forward velocity at prolonged integration times. It will be appreciated that all the above examples, and more, may be addressed by the technology of the present disclosure, not only the illustrative example of a scanning camera rolling about the platform roll axis as in Figures 8-12. It will also be appreciated that any of the features of each of the examples may be used in conjunction with any one or more of the features of the other embodiments. The yaw correction formulae of the present disclosure may facilitate the camera and / or the platform and / or components of the camera to be yawed to accomplish image smear correction or image smear alleviation. Yawing of the platform relative to Vgscanmay act simply as an offset angle (ɣ), and it may also alleviate smear. However, the yaw correction angle of the present disclosure and its implementation may significantly reduce smear and / or correct or improve it. Yawing the camera may of course include yawing of components thereof (such as focal plane, imaging detector chip, or active pixels array on the chip) instead of the whole camera. The present disclosure extends to a scanning system for an airborne or spaceborne platform configured to move in a flight direction around a celestial body at a flight velocity parameter. The scanning system may include a camera and an imaging detector array thereof configured to be provided onboard the airborne or spaceborne platform. The camera may be capable of scanning the ground or surface at a scanning velocity parameter extending generally in a direction across the flight direction. The camera imaging detector array’s footprint on the scanned surface may be configured to be yawed by a determined yaw correction angle. The yaw correction angle may be based at least on the flight velocity parameter and the scanning velocity parameter. The scanning system may include a yaw correction angle determining component. The yaw correction angle determining component may be configured to determine a yaw correction angle based at least on the flight velocity parameter and the scanning velocity parameter. The yaw correction angle determining component may be configured to determine the yaw correction angle in near real- time e.g., during flight of the airborne or spaceborne platform. The yaw correction angle may, e.g., be determined prior to launch of the airborne or spaceborne platform. The camera and / or its components may be fixed prior to launch of the airborne or spaceborne platform so as to yaw the camera’s footprint on the scanned surface during flight by the determined yaw correction angle. In other words, the yaw correction angle may be predetermined, prior to launch / flight, or it may be determined actively during flight (i.e., yawing by the determined yaw correction angle may occur after launch of the airborne or spaceborne platform). The foregoing description has been presented for the purpose of illustration; it is not intended to be exhaustive or to limit the technology 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 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 present disclosure be limited not by this detailed description, but rather by any claims that issue on an application based hereon. Accordingly, the present disclosure is intended to be illustrative, but not limiting, of the scope of any accompanying claims. Finally, throughout the specification and any 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.

Claims

CLAIMS:

1. A method of scanning by a camera onboard an airborne or spaceborne platform configured to move in a flight direction around a celestial body at a flight velocity parameter, the method comprising: providing the camera and an imaging detector array thereof onboard the platform so as to be capable of scanning the ground or surface at a scanning velocity parameter extending generally in a direction across the flight direction; determining a yaw correction angle based at least on the flight velocity parameter and the scanning velocity parameter; and causing the camera imaging detector array’s footprint on the imaged surface to be yawed by the determined yaw correction angle.

2. The method as claimed in claim 1, wherein the determination of the yaw correction 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 due to rotational scanning action.

4. The method as claimed in any one of claims 1 to 3, wherein the step of determining the yaw correction 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 correction 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 correction angle is determined based on one or more of the following formulae:Ψ^^= atan if ve is negligible, then: Ψ^^≈ atanfurthermore, if ɣ = 0 (scan axis is perfectly aligned with flight velocity), then: Ψ^^≈ atanwherein β is the angle between ve and vgscan; wherein ɣ is an offset angle which may exist between the scanner axis and the flight velocity parameter; wherein vt is a total velocity parameter of the camera’s footprint relative to the imaged ground or surface below; wherein vgscan is the scanning velocity parameter; wherein vgsat is the flight velocity parameter; wherein Ψc is the determined yaw correction angle; 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 Ψc is the determined yaw correction angle.

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

7. The method as claimed in any one of the preceding claims, wherein the camera uses 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 uses 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 the camera, or its components, or the airborne or spaceborne platform by the determined yaw correction 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, or yawing the components of 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 a celestial body at a flight velocity parameter, the scanning system comprising: a camera and an imaging detector array thereof configured to be provided onboard the airborne or spaceborne platform, the camera being capable of scanning the ground or surface at a scanning velocity parameter extending generally in a direction across the flight direction, wherein the camera imaging detector array’s footprint on the scanned surface is configured to be yawed by a determined yaw correction angle, and wherein the yaw correction angle is based at least on the flight velocity parameter and the scanning velocity parameter.

13. The scanning system of claim 12, wherein the scanning system comprises a yaw correction angle determining component which is configured to determine a yaw correction angle based at least on the flight velocity parameter and the scanning velocity parameter.

14. The scanning system of claim 13, wherein the yaw correction angle determining component is configured to determine the yaw correction angle in near real-time during flight of the airborne or spaceborne platform.

15. The scanning system of claim 12, wherein the yaw correction angle is determined prior to launch of the airborne or spaceborne platform, and wherein the camera and / or its components are fixed prior to launch of the airborne or spaceborne platform so as to yaw the camera’s footprint on the scanned surface during flight.

16. 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.

17. The system as claimed in claim 12 or claim 16, wherein the system includes a scanning velocity parameter determining component arranged to determine or sense the scanning velocity parameter.

18. The system as claimed in any one of claims 12, 16 or 17, 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.

19. The system as claimed in claim 18, wherein the yaw correction angle determining component is arranged to determine the yaw correction angle by determining an angle between the scanning velocity parameter and the flight velocity parameter.

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