|Title of Invention||
"SYSTEM FOR ACQUIRING AN IMAGE BY OVERSAMPLED PUSHBROOM SCANNING"
|Abstract||Device for acquiring an image by push-broom scanning from a satellite or aerial vehicle carrying at least one array of sensors of charge coupled type, characterised in that the attitude and the angular rates Of roll, pitch and yaw of the vehicle are controlled by means such that the longitudinal and lateral ground speeds of the array satisfy where Vr-1 = P/Ti, P being the footprint of the array, Ti the sampling period, in and 1 two integers.|
|Full Text||PROCESS FOR ACQUIRING AN IMAGE BY OVERSAMPLED PUSH-BROOM SCANNING
The present invention relates to a process for acquiring an image by "push-broom" scanning by at least one array of sensors of the charge coupled type CCD travelling past the observed area.
The invention advantageously finds application to
the satellite observation of the earth or else to
observation from aerial vehicles (airplane, drone, etc).
The Applicant has already proposed, in their
patent application FR 95 09263, a process for acquiring
images by means of arrays or matrices of CCD sensors,
according to which the array or matrix is oriented with
respect to the direction of movement in such a way as to
carry out an oversampling while attenuating the effects
of the spectral aliasing.
An object of the invention is to propose an acquisition which makes it possible to achieve finer samplings.
To this end, it proposes a process for acquiring an image by push-broom scanning from a satellite or aerial vehicle carrying at least one array of sensors of charge coupled type, characterized in that the attitude and the angular rates of roll, pitch and yaw of the vehicle are controlled so that. the longitudinal and lateral ground speeds of the array satisfy
where Vraf = P/Ti, P being the footprint of the array, Ti the sampling period, m and 1 tvo integers.
Other characteristics and advantages of the invention will emerge further from the description which follows. This description is purely illustrative and non-limiting. It should be read in conjunction with the appended drawings in which:
- Figure 1 is a diagrammatic representation of the ground image of an array of CCD detectors moving relative to a frame tied to the earth, and in which the angles of role, pitch and yaw of the vehicle which transports the said array have been marked;
Figure 2 is a diagrammatic representation illustrating a sampling obtained with an array in which the angle between the normal to the said array and the ground speed of the centre of the image of said array is cm;
Figures 3 and 4 are block diagram representations of the device means in accordance with two particular embodiments of the invention allowing acquisitions of the type illustrated in Figure 2;
- Figure 5 diagrammatically represents an array
arrangement which is advantageous for the implementation
of the invention;
- Figure 6 illustrates a possible embodiment with
such an arrangement.
In Figure 1, S denotes the satellite carrying the acquisition array, X, Y, Z the local orbital frame whose centre S coincides with the centre of mass of the satellite, Vsat and Vgnd the vectors corresponding to the absolute velocity of the point of aim of the array and to the relative velocity of this image of the array with respect to the ground.
In this example, the acquisition array (or arrays) are arranged in the conventional manner in the focal plane of an optic, with or without a change of aim mirror (if the instrument possesses a change of aim mirror, the latter will be stationary at the moment of image capture; if the whole instrument can rotate with respect to the satellite at the moment of image capture, this instrument will remain stationary at the moment of image capture).
The angles of roll, pitch and yaw have been respectively labelled , θ and Ψ; the corresponding angular velocities by
In most earth observation satellites, the body of the vehicle is locked on to the local orbital frame. A system of mirrors makes it possible to orient the axis of aim so as to provide access to the maximum possible areas .
The velocity Vgnd is the difference between the velocity Vsat and the velocity V.arth induced by the rotation of the earth (Ωearth A CV with C at the centre of the earth) .
where V denotes the point of aim on the earth and Ω the absolute angular velocity of the satellite S, assuming the axis of aim to be tied and fixed to the satellite.
According to the invention, the angular rates of roll, pitch and yaw of the satellite are controlled so as to produce a sampling of the type illustrated in Figure 2, in the case of an angle of ground slip of the array such that:
α1/m = Arctan 1/m,
the longitudinal and lateral ground speeds of the array being given by:
where P is the footprint of the array, T£ the sampling
period and m and 1 are integers. In Figure 2, the crosses represent the ground images of the centres of the detectors at each sampling instant t + kTi, with k an integer and t a given sampling instant.
In particular, if m = 1 and 1 = n, with n an integer, a so-called "integer supermode" sampling is obtained with ground images of the centres of detectors distributed over an orthonormal grid. Such a sampling has the advantage of having a reduced footprint and swish pan effect.
By way of example, the table below gives the characteristics of several possible "integer supermode" samplings.
* For a 60 km field in the normal mode of image capture under track (SPOT 1, 2,3 and 4).
When the case of integer supermode type sampling does not hold, we then speak of "fractional supermode" sampling.
This type of sampling has numerous advantages. It makes it possible not to use double arrays, the technology of which is specific to the need of earth observation by satellite. It is therefore less expensive. Likewise, it has great flexibility of use. In particular, for a satellite and a given optical observation instrument (fixed and/or already in orbit) it makes it possible to programme image captures which give improved image resolution (after appropriate processing). Likewise, it also makes it possible to programme "image capture strips" tilted with respect to the suborbital track of the satellite (or the direct ground track of the aeroplane or drone), which may be of benefit in the quasi-simultaneous capture of images which are greatly offset laterally.
Overall, these "super-modes" make it possible to increase the performance in terms of resolution and access of an existing earth observation satellite or one under development; they provide additional possibilities and system flexibility.
For example, in the SPOT1 satellite in orbit since February 1986, the resolution of which is 10 m x 10 m, it is possible with the solution proposed by the invention to perform acquisitions with footprints of 4.5m x 2.25 m (with 1 = 4 and m = 2) or even 2.4 m x 2.4m (with 1=4 and m = 1).
By way of example, the angular rates of roll and
pitch ( and θ) can be computed with a constraxnt
imposing a zero angle of yaw (Ψ = 0) , these rates being
calculated from the velocities vlongi and Vlafcaral
corresponding to the desired sampling.
As a variant, an angle of slip which differs from the value corresponding to the supermode carried out at zero yaw can be obtained with a yaw rotation.
By way of example, in the case of a satellite with geocentric pointing, means of the type illustrated in Figure 3 can be used to carry out sampling at zero angle of yaw.
These means comprise means 1 for measuring
attitude which consist, for example, of star trackers, earth trackers, gyroscopes or sun trackers.
They also use the on-board computer 2 of the satellite which, on the basis of the measurements made by the aforesaid means, makes an estimate of the attitude of the satellite and determines the actual values of the angles and angular rates of pitch, yaw and roll.
By setting prescribed values, for example on the attitude angles and/or the angular velocities, the onboard computer calculates commands intended to be applied to the attitude actuators 3.
The actuators 3 are reaction wheels on which
torques Cx, Cy and Cx are imposed, these wheels
corresponding to the three axes of the satellite. These
wheels must be de-glazed continuously or regularly by
Figure 4 illustrates the general case of the controller of a manoeuvring satellite.
Attitude settings in the form of a guidance quaternion or absolute angular velocity of guidance of the satellite are calculated on the ground from attitude
or velocity settings (Φc, θc, Ψc, θc, dc and Ψ/c) calculated by the ground segment in order to carry out the desired samplings. The attitude control therefore executes these settings after downloading "attitude profiles" to the satellite, these taking the form of time-dependent quaternions or a time-dependent satellite absolute angular velocity setting (in general these are time-dependent polynomials).
Illustrated in Figure 5 is an arrangement of arrays allowing advantageous implementation of the invention.
According to this arrangement, a plurality of arrays B of charge-coupled type is distributed in a sawtooth pattern in the focal plane of the earth observation instrument. These various arrays B are identical and parallel and distributed in line in the said focal plane.
The satellite is guided in terms of attitude in
such a way that the inclination of the arrays with respect to the normal to the velocity of the satellite on the ground is equal to:
αℓ/m = Arctg ℓ/m
the ground speed satisfying the formulae given on page 3 ( (see Figure 5), Vlongi and Vlat.ral being parallel and
perpendicular to the direction of the arrays B.
With such an arrangement it is only necessary to
implement pitching control of the attitude of the
satellite, and also possibly weak roll control in order
to compensate for the effect of the rotation of the
The acquisition carried out is therefore less
expensive than that described with reference to the preceding figures.
Moreover, it will be noted that this arrangement allows greater efficiency of acquisition. In particular, for the same number of elementary detectors, it is possible with a plurality of parallel arrays to achieve images of larger rectangular areas on the ground than with a single array of detectors.
Moreover, it will be noted, as was illustrated in Figure 6, that this technique makes it possible to use, to construct the arrays, arrays of integrated-circuit type, that is to say electronic chips, a technology which it is not possible to use in the case in which a one-piece array (with no special optic, of the DIVOLI type for example) must be arranged in the focal plane.
1. Process for acquiring an image by push-broom scanning from a satellite or aerial vehicle carrying at least one array of sensors of charge coupled type, characterized in that the attitude and the angular rates of roll, pitch and yaw of the vehicle are controlled so that the longitudinal and lateral ground speeds of the array satisfy
where Vref = P/Ti, P being the footprint of the array, Ti the sampling period, m and 1 two integers.
2. Process according to Claim 1, characterized in
that the attitude of the vehicle is controlled by
imposing a constant angle of yaw.
3. Process according to Claim 1, characterized in
that the attitude of the vehicle is controlled by varying
the pitch angle and the angle of yaw.
4. Process according to Claim 1, characterized in
that the vehicle comprises a plurality of arrays arranged
in parallel in a saw toothed pattern in the focal plane
of an optic, the vehicle being guided in such a way that
the inclination of the arrays with respect to the normal
to the velocity of the satellite on the ground is equal
α1/m = Arctan 1/m.
5. Device substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.
|Indian Patent Application Number||1247/DEL/1998|
|PG Journal Number||36/2008|
|Date of Filing||12-May-1998|
|Name of Patentee||CENTRE NATIONAL D'ETUDES SPATIALES|
|Applicant Address||2, PLACE MAURICE QUENTIN 75001 PARIS, FRANCE.|
|PCT International Classification Number||G01C 11/02|
|PCT International Application Number||N/A|
|PCT International Filing date|