- until recently there were no effective, flexible and cheap delivery systems of small KA to low orbits. The PEGASUS technology, i.e. group removal of low-orbital KA into an orbit allowed to solve this problem with use of the plane;
Systems from KA in elliptic orbits which can also have a set of options of construction, are applied as addition to systems with geostationary KA to service of subpolar and polar areas now.
The space segment, systems at this stage, includes 8 relay satellites (4 main and 4 reserve) located in a geostationary orbit. The network "Inmarsat" organized in 4 ocean regions covers practically all surface of the Globe, except for subpolar areas.
Such creation of space and land segments of COURSE system will allow to use the means and communication channels existing technical to the maximum and to minimize operational costs.
So far some types of the satellite communication systems differing, first of all, in creation of a space segment are known already. Systems with spacecrafts on geostationary concern them, elliptic and circular orbits, each of which has many versions.
Geostationary satellites are capable to provide a row from the listed above advantages, but, as a rule, at the expense of higher expenses. So, for example, it would be possible to render services from geostationary satellites when the task of economy of means at the center would not be set. Besides, use of several beams with the narrow directional pattern on the geostationary satellite increases power stocks by lines, but in this case only at the expense of increase in capital expenses.
For work within Inmarsat system mobile objects are equipped with the terminal terminal equipment. Such equipment has to meet certain technical and operational requirements "Inmarsat" and, known as Standards.
The third type of terminals - regional stations will be intended for transfer of big arrays of information during the work as a part of regional communication centers and will provide transfer information with a speed of 64 Kbps.
- request of the message. The message is transferred by the user's terminal on command from the satellite. This team arrives on the satellite from Control center of a network which serves the user terminal (modes DataNet, MapNet);
It is originally planned to launch two satellites in a polar orbit and further to expand a space segment to group from 36 low-orbital artificial satellites. 4 artificial satellites will be in a polar orbit and on 8 artificial satellites in 4 orbital planes with an inclination of 45 degrees and 785 km high. Weight of each satellite approximately 36,5 kg. Satellites will consist of transmitters, receivers and onboard processors. They will also contain GPS receivers, defining location of the satellite in an orbit. It is necessary for system of position control which supports satellites in orbits. It is planned that satellites will be calculated on operation within four years. Start of satellites is planned to carry out from the plane on Pegasus system that will allow to reduce the cost of system considerably.
As for structure of groups, from the point of view of ensuring communication between all terrestrial and space elements of system (connectivity of system) groups with symmetric structure at which all KA are on the relation to each other in identical situation are more favorable. For the best ensuring connectivity in system it is required to bring still some redundancy in relation to the minimum number of KA necessary for providing a continuous covering of the served territory.
It is supposed that the MESSENGER system will include the space segment consisting of 36 KA and the terrestrial segment including subscriber terminals of three types. Communication between subscribers can be made without use of land communication networks.
Time of delivery of the message from the terminal on the satellite, and also from the satellite through Terrestrial station and Control center of a network to the addressee makes 1-2 min. (provided that the terminal and Terrestrial station at the same time are in a visibility range the satellite. In a case when in a visibility range of the satellite there is only a terminal, the message is transferred at once from the terminal to the satellite, and from the satellite on Terrestrial station only when it gets to its visibility range. Respectively time of delivery of messages increases.