Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Patent
1996-08-01
1998-06-30
Le, Que
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
2502032, 244 318, 35613901, H01J 4014
Patent
active
057738130
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention concerns an angular position finding system for an observation instrument, and in particular may be incorporated into an artificial satellite, a space vehicle or a space station.
2. Description of the Background
In particular, observation instruments may need to take pictures that have to be located with very high precision. The position and orientation of the instrument on the element that acts as a support are frequently not known sufficiently precisely. Furthermore, it is often necessary or useful to provide sighting change systems, in other words instruments that modify the orientation of the line of sight of the observation instrument. These instruments are required whenever it is disadvantageous or impossible to rotate the entire instrument support machinery, or when it is impossible to wait until the instrument has reached its required orientation by natural means, which occurs periodically on unstabilized satellites. Sighting change equipment acts on the observation instrument either by displacing the instrument itself, or by moving its line of sight (this solution is frequently used on satellites). The instrument then consists of a rotating mirror.
Any sighting change equipment introduces an additional uncertainty in the orientation of the line of sight, due to uncertainty about its mounting position or about the precision of its control, which for example can produce an uncertainty in the orientation of the sighting change mirrors. Finally, expansions caused by temperature changes can deform instrument support structures, particularly on satellites exposed to very large temperature differences between the surface illuminated by the sun and the surface in the shade.
All these circumstances explain why it is impossible to adjust the line of sight of observation instruments with a precision better than about two hundred seconds of arc, which corresponds to a positioning uncertainty of eight hundred meters on the ground for a satellite at an altitude of eight hundred kilometers, although the satellite itself can be located within a few tens of meters. This difference shows that improving the location of a view on the ground depends above all on the quality of orientation of the instrument or its line of sight.
SUMMARY OF THE INVENTION
This invention is designed specifically to eliminate this problem of directional inaccuracy by means of a system which, in its most general form, includes a first light pattern source rigidly attached to the instrument close to the focal plane of the instrument and consisting of at least two dots; a light sensor fixed at a know direction on the space vessel to which the instrument and the system are fixed; and a reflection mirror positioned to reflect light from the source to the sensor the mirror being rigidly attached either to the sensor or to the sighting change mirror (if there is one).
The position of the image of the light pattern on the sensor expresses the instrument orientation. If the system does not include any other particular elements, the orientation of the reflecting mirror (which must not be confused with the sighting change mirror) must be accurately known, which for example is possible if it is mounted carefully on a support rigidly attached to the sensor.
In particular, the light sensor may consist of a star sensor which is fairly frequently used in satellites. It then provides its own directional reference by detecting the image of reference stars at the same time as it detects the image of the light pattern, and compares the position of the two images on its screen. The directional reference may also be completed by a gyroscopic assembly, which is then rigidly attached to the light sensor.
Other uncertainties can arise if the direction of the reflecting mirror is not known accurately. This is why it may be necessary to use a second source rigidly attached to the light sensor and which outputs a second light pattern, the light from which is returned by the reflecting mirror to t
REFERENCES:
patent: 4173414 (1979-11-01), Vauchy et al.
patent: 4349838 (1982-09-01), Daniel
patent: 4740682 (1988-04-01), Frankel
Duchon Paul
Otrio Georges
Centre National d'Etudes Spatiales
Le Que
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