GB2145597A

GB2145597A - Space vehicle attitude sensor

Info

Publication number: GB2145597A
Application number: GB08421211A
Authority: GB
Inventors: Bernd Kunkel; Winfried Birner
Original assignee: Messerschmitt Bolkow Blohm AG
Current assignee: Airbus Defence and Space GmbH
Priority date: 1983-08-26
Filing date: 1984-08-21
Publication date: 1985-03-27
Also published as: GB2145597B; DE3330861A1; GB8421211D0; DE3330861C2; FR2551201A1; FR2551201B1

Abstract

The device, which is mounted on a space vehicle, comprises parallel rows of detectors 1, 2, 3 consisting of individual detector elements 11, 12, 13. By means of a suitable optical system an image of an object (e.g. the earth) is projected onto the rows of detectors 1, 2, 3. If the image is found to impinge on sensor 11', a block 8 of adjoining detector elements (e.g. 11'') is observed for variations in outputs, which are indicative of movement of the space vehicle. <IMAGE>

Description

SPECIFICATION Device for ascertaining changes in position of a space vehicle This invention relates to a device for ascertaining changes in position of a space vehicle which is positionally stabilised relative to a remote space object which serves as positional reference.

Space vehicles, more especially satellites, are currently used for a variety of purposes. They serve, for example, as communications satellites and/or for obtaining image information regarding the surface of certain space objects, for instance in the case of geostationary satellites of the earth. In the latter case, the satellites have to be aligned very accurately with their pick-up devices relative to the earth. The space object "earth" serves accordingly as positional reference relative to which the satellite has to be positionally stabilised. In order, upon the image pick-up, to achieve the highest image quality possible, the satellite must be prevented from being subjected to fluctuations with respect to its position.

It must be aligned with its optical axis as constantly as possible at the same point of the earth's surface.

Incipient changes of position, which frequently occur as a result of unavoidable external influences, must be registered at once and be compensated for by the position regulation system. For this it is first of all necessary to ascertain at once an incipient change of position, namely with high time resolution.

The task of the present invention is, therefore, to provide a device of the kind mentioned at the beginning hereof which is able to ascertain the changes in position of the space vehicle, more especially even only small and briefly occurring changes in position, with high sensitivity and the least possible time lag.

In accordance with the invention, this task is achieved in that there is provided a device for ascertaining changes in position of a space vehicle which is positionally stabilised relative to a remote space object serving as positional reference, said device being characterised by comprising at least three rows of detectors arranged parallel side-byside, for representing or image-forming the surface of the space object in the plane of the rows of detectors which consist in each case of detector elements which are lined up linearly one after the other and which register in measurable manner the intensity of incident electromagnetic radiation, and evaluation means which by repeated comparison of the measured values within a flat or twodimensional block of adjacent detector elements ascertains the shift of significant image structures between the detector elements.

In this way it is ascertained whether a change in position of the space vehicle in the form of a tilting of its axes occurs, more especially of the optical axis associated with the rows of detectors. The accuracy with which such a tilting can be ascertained depends on the areal extent of the individual detector elements in the plane of representation, as well as upon how ciosely adjacent the detector elements can be arranged. It is presupposed that the image section (fragment) taken in by the rows of detectors has significant image structures. This means that, for example, image elements having strong brightness differences are present which can be registered in measurable manner by the detector elements.It is, however, possible to use also other measurable properties, for instance different spectral composition ofthe radiation emanating from the individual surface elements of the space object. A change in position of the space vehicle leads to a shift of the image in the image plane, in which respect significant image structures migrate from one detector element to the adjacent one, if the degree of shift if large enough. In order to be able to register such a shift in all two surface dimensions, accordingly an areal arrangement of detector elements is necessary. With a linear arrangement, in accordance with a single row of detectors, merely a shift in the direction of this line could be registered.

Through the arrangement of at least three rows of detectors lying parallel to one another and closely adjacent, the result is achieved that the migration of a significant image structure, lying for example on a detector element of the central row of detectors, in any desired direction can be ascertained. It is, of course, possible to arrange more than three rows, for example five or seven rows of detectors, parallel to one another. This is advisable all the more so the less structured the image is with respect to the property that is to be measured.

Preferably charge-coupled semiconductor sensors (CCD's) are used as the detector elements, for example those which are sensitive in the infrared spectral region.

The arrangement of the rows of detectors can be so undertaken that a central row of detectors as well as several rows of detectors which are arranged on both sides thereof at a spacing from one another and which lie opposite one another are provided, in which respect the latter are to contain in each case only a fraction of the detector elements of the central row of detectors. This arrangement takes account of the fact that rows of CCD detectors have in the past been obtainable only in a limited length i.e., with a limited number of detector elements ranged side-by-side. The central row of detectors can then be composed of many partial rows arranged one after the other. On both sides there can then be provided at a spacing from one another and parallel to the central row of detectors shorter partial rows of detectors which then consist in each case of one piece.These partial rows of detectors are in each case to lie opposite one another, so that together with the central row of detectors lying therebetween in each case portions consisting of three parallel rows arise. Here, too, these portions can in each case consist of more than three parallel rows.

The evaluation can then be so effected that for the comparison, the measured values of areal blocks consisting each of three detector elements of the central row of detectors as well as of the respectively adjacent partial rows of detectors are utilised.

The invention is preferably applicable to satellites stabilised in three axes. Upon use of CCD detector elements, on account of the rapid readability thereof and the high time resolution bound up therewith, also the use of the case of revolving satellites is possible.

Particularly critical are small brief changes in position, for instance so-called jitter movements.

Here, too, the usability of COD detector elements advantageously makes itself noticeable. A further advantage of these elements lies in the fact that even relatively rapid movements on the surface of the observed space object, for example of clouds relative to the surface of the earth, are not disturbing. There it emerges, for instance in the case of a geostationary satellites with a resolution of 2 km per detector element, a relative speed of the cloud movement of 100 km/h as well as a read-out duration of 10 ms, that the cloud shift only 2.7 metres in this time, an amount which is insignificantly small in comparison with the resolution.

The invention will be described further, by way of example, with reference to the accompanying drawings in which: Fig. 1 illustrates schematically a central row of detectors with adjacent partial rows of detectors of a preferred embodiment of the device of the invention; and Fig. 2 illustrates schematically three parallel rows of detectors with an evaluation means also being part of the preferred device of the invention.

Shown in schematic manner in Fig. 1 is a central row 1 of detectors of the preferred device, consisting of individual detector elements 11 lined up one after the other. In the case of the latter it is to be a matter of infra-red CCD's, for instance hybrid HgCdTe-CCD's, which are sensitive in the spectral region of about 10 to 12 lim. The central row 1 of detectors can contain, order-of-magnitude-wise, about 5000 or more individual detector elements, in which respect this overall length is formed by the lining up, in a row, of rows which are currently obtainable on the market and which consist in each case of 256 or 512 individual detector elements.

Arranged on both sides of the central row 1 of detectors, at a spacing from one another and in parallel alignment, are partial rows 2, 3, 2a, 3a and so forth of detectors. These can in each case also have a length of, for example, 256 individual detector elements 12,13 (shown shortened in the illustration). Onto this arrangement of parallel rows of detectors, from a suitable optical system (not shown), for example a telescope, which is entrained by the satellite and which is directed for instance at the surface of the earth, there is projected an image fragment of this surface. Through the evaluation means, to be described in detail hereinafter, by comparison of the signals supplied by the adjacent detector elements it is ascertained on which detector element a significant image structure is depicted.In this respect it can be a matter, for example, of clouds or other structures which stand out from the surroundings, for instance with respect to their brightness. Once such a significant image structure has been made out, for example on a detector element 11', then the block 8 arranged around this detector element and consisting of altogether eight or nine quadratically-arranged detector elements is further observed constantly by the evaluation device, namely in the sense that the measured values arising at the detector elements are repeatedly compared with those of the preceding measurement. Such comparisons can, as has already been mentioned, be carried out in a time of only 10 ms.A change of position of the satellite in the sense of a tilting now leads to the fact that the significant image structure drifts away from the detector element 11', for example towards the diagonally adjacent detector element 11". The direction in which the significant image structure migrates or drifts is of course connected casually with the direction of the change of position. Thus from this, in a clear manner, correction impulses can be derived immediately which compensate for the incipient change of position.

Illustrated schematically in Fig. 2 is evaluation means 7 of the preferred embodiment, which means 7 is fed with information from three parallel rows of detectors.

Connected subsequent to the rows of detectors are in each case filters 31,32, 33 which serve for noise suppression. The filtered output signals thereof are in each case fed to analogue digital converters 21, 22, 23, which possess, for example, 6 to 8 bit resolution. Subsequently there follows an intermediate store 14, which is for example alternately during one ms written and in the next as read-out. Following on the intermediate store 14 there is a light/dark value corrector 15, which guarantees a uniform reproduction characteristic with respect to all the detector elements.

Finally the corrected image signals pass to a computer 10. This assesses the signals of the individual detector elements and ascertains at the start on which detector elements significant image structures are depicted. By repeated comparison of the contents of the blocks of detector elements, to be associated with the individual significant structures, with the values in each case stored away beforehand, for example in ms-timing it is ascertained whether shifts of the significant structures occur. If no shift is measurable, in which respect about 0.5% to 2% stages can be preset, then the relative position of the satellites has not changed measurably in this short time interval.

Critical are, for example, jitter movements of the satellite, which in the past was a problem to detect, and which occur with a frequency of about 100 Hz. If changes recognisable by the computer are ascertained, direction and amount of the change in position is established and passed on to the satellite position regulation system for the generation of correction impulses.

Claims

1. A device for ascertaining changes in position of a space vehicle which is positionally stabilised relative to a remote space object serving as positional reference, said device being characterised by comprising at least three rows of detectors arranged parallel side-by-side, means for representing or image-forming the surface of the space object in the plane of the rows of detectors which consist in each case of detector elements which are lined up linearly one after the other and which register in measurable manner the intensity of incident electromagnetic radiation, and evaluation means which by repeated comparison of the measured values within a flat or twodimensional block of adjacent detector elements ascertains the shift of significant image structures between the detector elements.

2. A device as claimed in claim 1, characterised by a central row of detectors as well as several rows of detectors which are arranged on both sides thereof at a spacing from one another and which lie opposite one another and which in each case contain only a fraction of the detector elements of the central row of detectors.

3. A device as claimed in claim 2, characterised in that the flat blocks, utilised for the evaluation, each contain three detector elements from the central row of detectors as well as the respectively adjacent rows of detectors in quadratic arrangement.

4. A device as claimed in one of the preceding claims, characterised in that charge-coupled semiconductor sensors are used as the detector elements.

5. A device as claimed in claim 4, characterised in that sensors which are sensitive in the infra-red spectral region are used.

6. A device as claimed in one of the preceding claims, characterised in that the evaluation means contains one analogueldigital converter each per row of detectors, a read-out electronics system which effects the successive read-in of the measured values of the individual detector elements into the associated analogue/digital converters as well as a computer which compares the digitised measured values of consecutive measurements with one another.

7. A device as claimed in claim 6, characterised in that an intermediate store is connected to the analogue/digital converters.

8. A device for ascertaining changes in position of a space vehicle substantially as herinbefore described with reference to and as illustrated in the accompanying drawing.