Observation or telecommunication satellites

Details Observation or telecommunication satellites Observation or telecommunication satellites

1998-02-03

2001-04-10

Le, Hoanganh (Department: 2821)

Communications: radio wave antennas

Antennas

Antenna components

C343S7000MS, C343SDIG002

Reexamination Certificate

active

06215458

ABSTRACT:

The present invention concerns a space satellite.
In the remainder of the text, the invention will principally be described in the case of a radar remote sensing satellite.
As will be readily understood the description, applies with equal advantage to telecommunication satellites.
Similarly, the invention will be described in the case of an orbit around the Earth. Other heavenly bodies would of course be possible.
DESCRIPTION OF THE PRIOR ART
As shown in
FIGS. 1
to
3
, a radar satellite usually comprises a rectangular plane antenna
1
, an equipment module
2
and solar panels
3
.
The solar panels
3
are oriented towards the sun S and the antenna
1
is oriented towards the earth T and images laterally relative to the speed vector V of the satellite.
The various parameters of a satellite of this kind are as follows.
Dimensions of the Radar Antenna
The dimension of the antenna
1
in the direction of the speed vector V of the satellite, i.e., its length L in
FIG. 3
, is directly related to the resolution of the image along this same axis (azimuth or Doppler resolution), in a ratio between 1.1 and 2.
Perpendicularly to the speed vector V, the height H of the antenna
1
increases in direct proportion to the ground swath of the image (the width of the image on the ground transversely to the speed vector), the maximum incidence (the angular difference between the boresight and a vertical line through the imaged point on the ground) and the altitude, and in inverse proportion to the length L. Moreover, for given values of the preceding parameters, the height is directly proportional to the wavelength of the radar.
Consequently, a low-resolution radar (<10 m) uses an elongate antenna along the speed vector (L=15 m and H=1.5 m in the case of RADARSAT), whereas a medium or high resolution (<5 m) can lead to antennas having H much greater than L, especially at low frequency (L or S band) or with a plurality of frequencies with juxtaposed antennas along the height H.
Roll Inclination
As shown in
FIG. 3
, the adjustment about the roll axis of the boresight of the antenna
1
beam locates the image a greater or lesser distance away from the vertical line through the satellite (coverage of the incidence range). This adjustment is now obtained by electronic scanning between two incidences i
min
and i
max
, but in order to limit the scanning range and the height of the antenna
1
, the latter is oriented so that its normal N is aimed in a median direction in the incidence range. The roll angle r is typically 30°.
Power and Local Orbital Time
Unlike optical remote sensing, radar remote sensing does not require any particular conditions of solar illumination of the scene. On the other hand, it consumes satellite electrical power. All this leads to the adoption of a heliosynchronous orbit of local time 6 H or 18 H, enabling the solar panels
3
of the satellite to remain exposed to the sun and generating energy virtually continuously (few eclipses, unlike a diurnal local time such as is used in optical remote sensing) (see FIG.
1
).
The solar generator constituted by the panels
3
is generally insufficient to power the radar. The satellite also carries batteries from which the radar draws power. These batteries are charged when the radar is not operating.
Note that this approach is also a result of the fact that radar satellites use equipment modules that are not specially designed for this purpose, and are therefore compatible with diurnal orbits which require the provision of large batteries to cater for long eclipse times.
Dimensional and Attitude Stability
To function correctly the antenna
1
must remain flat and accurate pointing of the axis N normal to its surface must be maintained. The conventional approach is to impose a rigorous mechanical dimensional stability on the antenna assembly
1
and equipment module
2
and to have the attitude control system of the equipment module
2
handle pointing requirements.
It has already been proposed that, when electronic scanning antennas
1
are used, requirements in respect of the flatness and the attitude of the antenna panel should be relaxed and the phase-shifters of the antenna elements that constitute the antenna
1
should be controlled to reconstitute a correctly oriented perfect wave plane. This relaxes the structural constraints for the combination of the satellite and the antenna, and the attitude control system of the module
2
then has only a relatively coarse role.
This principle of decentralized adaptation at the level of the antenna
1
is essentially based on the ability to measure its deformations from flatness and the attitude of its median plane.
Until now, however, the proposed applications based on deformation or flatness sensors (especially optical sensors) have not been entirely satisfactory. Moreover, they do not allow measurement of the attitude of the frame of reference of the antenna
1
, which must remain entirely the responsibility of the equipment module or be effected by means of absolute attitude sensors on the antenna
1
.
Regardless of how these decentralized adaptation techniques may evolve, the attitude system of the equipment module
2
remains responsible for maintaining the reference position of the antenna
1
. In particular, the axis of the greatest dimension (lowest inertia) must be kept aligned with the speed vector (with the length L for a low-resolution radar), or normal to the speed vector with a roll angle as previously mentioned (dimension H for a high-resolution radar), such that the equipment module must compensate for gravity torques continuously. This compensation imposes continuous torques from the attitude control system and a minimal mechanical rigidity of the satellite as a whole and of the mechanisms for deploying the antenna in order to transmit these torques. Note also that the presence of the equipment module
2
introduces its own inertia constraints and, with the solar pressure exerted on the solar panels
3
, another disturbing torque.
SUMMARY OF THE INVENTION
The invention consists in a new type of satellite, in particular a radar remote sensing or telecommunication satellite.
One object of the invention is to propose a satellite of improved sensitivity compared to prior art satellites, allowing better operational performance in terms of accessibility and repeatability.
Another object of the invention is to propose a satellite of simplified construction, in particular allowing a large reduction in the cost of the antenna, the launch system and the equipment module, together with an increase in reliability and durability.
To this end, the invention proposes a low Earth orbit remote sensing or telecommunication satellite including a generally plane antenna forming member, wherein the antenna forming member lies substantially in a plane passing through the center of the Earth, for example in its orbital plane.
In accordance with another, independent aspect, the invention proposes a satellite that includes a solar generator and the solar generator cells are carried by the antenna forming member.
The satellite proposed by the invention is advantageously such that the height of the antenna forming member defined by the dimension along the gravity axis is greater than its dimension perpendicular to this axis, so that the satellite is naturally stabilized about the roll and pitch axes by the gravity gradient.
In particular, the antenna forming member may advantageously include an optionally partly hollow part with no antenna function which contributes to natural stabilization of the satellite about the roll and pitch axes by the gravity gradient.
In accordance with one independent aspect, the invention proposes a remote sensing or telecommunication satellite including a generally plane antenna forming member, wherein the antenna forming member has a deformable geometry and includes transmitted or received wave control means distributed over its surface, and it further includes a plurality of position and/or deformation and/or misalignment sensors distribute

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