The invention concerns a method of communication between stations or users on the ground via satellites.
The ever-increasing bit rate of telecommunications leads to the development of diverse types of communication, in particular communications relayed by satellites. A number of communications systems of this type are planned. Such systems generally use constellations of satellite in xe2x80x9clow Earth orbitxe2x80x9d or xe2x80x9cmedium Earth orbitxe2x80x9d. In such a constellation, a plurality of satellites follow the same orbit at an altitude in the range 1,000 km to 1,500 km above the surface of the Earth, for example, and the constellation includes a plurality of orbits, usually in the form of pairs of closely spaced orbits. The set of orbits or pairs of orbits covers most of the surface of the Earth where communications are useful.
An xe2x80x9cInternetxe2x80x9d type system has already been proposed for satellite communication between users virtually anywhere on the Earth in which information is transmitted between two distant users using satellite links. In the system concerned the number of satellites and their distribution are such that each user can communicate directly with a satellite at any time and information is transmitted between two users by means of the satellites with which they communicate directly. The satellites communicating directly with the users are linked by other satellites of the constellation. The choice of the intermediate satellites, in other words the routing of the information, is handled individually for each call using control means on board each satellite.
A telecommunications system of the above kind necessitates a high satellite density. The system is therefore costly. Controlling routing on board each satellite further increases the cost of the system.
There are other satellite telecommunications systems in which, within a particular terrestrial area, communications are effected via the only satellite passing over that area. When the satellite leaves the area concerned, another satellite takes over from it. In that system users in the same area communicate via a single satellite and users in different areas must communicate using means other than the satellites. The system is extremely simple but its capacity is limited.
Document US 4 912 656 is illustrative of the prior art and describes onboard processing which makes the communications system very costly. Document EP 0 780 998 is also illustrative of the prior art but does not provide for any simply processing of links between satellites.
The method of the invention enables communication between distant stations or users on the ground via satellites and uses a relatively low cost communications system.
In the method of the invention, to set up communication between two distant stations or users on the ground via a constellation of satellites, the link between the satellite communicating directly with a first station (or user) and the satellite communicating directly with a second station (or user) is at all times effected via at least one predetermined link.
In other words, the routing of information between satellites is predetermined. There is therefore no need to provide complex control means onboard each satellite.
In addition, in the method of the invention, communication between satellites is optical. Finally, to obtain a high information bit rate, there is provision for using more than one wavelength. The plurality of wavelengths can be exploited for addressing one satellite from another. For example, a message carried by a wavelength xcexn can be addressed to the nth satellite preceding the transmitter satellite.
The term xe2x80x9cpredetermined linkxe2x80x9d means that communication between the satellites communicating directly with the ground stations is effected via one or more predetermined satellites. If the satellites communicating directly with the ground stations are at a sufficiently short distance from each other, communication can be effected without any intermediary.
The links between satellites are pre-programmed, for example. The links can be controlled by a ground station, which further minimizes the onboard control means.
The links between satellites can be predetermined because the position of each satellite is known at all times. Which satellites can communicate with each other is therefore known at all times because their positions and the distances between them are known.
During a call, the link between two stations or users on the ground may be modified. Because of the movement of the satellite, each station continues to communicate directly with a satellite for a limited time. Another predetermined link is set up when another satellite takes over direct communication.
In a preferred embodiment of the invention each link between two satellites is associated with at least one other predetermined link so that the latter replaces the first link should it fail, the first link having priority. The second link is advantageously a complementary link, meaning that the combination of the first and second links forms a ring around the Earth.