What is a Medium Earth Orbit Satellite?

Medium-orbit earth satellites mainly refer to earth satellites whose satellite orbit is 2,000 to 20,000 km from the surface of the earth. It belongs to the earth non-synchronous satellite, mainly as a supplement and extension of the land mobile communication system, and organically integrates with the ground public network to achieve global personal mobile communication. Therefore, it has great advantages in global personal mobile communications. ^[1]

The height of the satellite's orbit from the earth's surface. Satellite orbits can generally be divided into the following four categories. Figure 1-9 shows the general situation of these types of satellite orbits.

Representative medium-orbit (MEO) satellites include lnmarsat-P, Odyssey, and MAGSS-14. The Odyssey system was launched by TRW Space Technology Group. It consists of 12 satellites with a height of about 10000km distributed on 3 orbital planes with an inclination of 55 °. Using the L / S / Ka frequency band, each satellite has 19 beams and a total capacity of 2800 speech channels. The system can be One circuit is provided for 100 users, and 12 satellites can serve 2.8 million users worldwide. The system construction cost is about 2.7 billion U.S. dollars, and the design life of the satellite is 12-15 years. It was put into use in 1997. The Inmarsat-P system (the third generation of the International Maritime Satellite) is a project called "Project-21" developed by the International Maritime Satellite Organization. It plans to spend 1 billion U.S. dollars in the late 1990s to build a completely new 21st century global personal satellite mobile communication system. The first and second generation maritime satellites only used geostationary satellites. Can only cover areas above 70 ° latitude. The third generation of maritime satellites uses a combination of synchronous and medium-orbit satellites. It uses 4 synchronous satellites, with a large antenna of more than 10 meters, and has on-board processing functions, which can realize communication between handhelds in one hop. The middle orbit consists of 12 satellites with an altitude of about 10,000km, and also has on-board processing functions. The system service target is to be able to see satellites in 90% of the conditions in the various environmental conditions of the service area. At this time, the handheld with a transmission power of 0.25 to 0.4W, the communication reliability is 95%, and the size of the handheld It is 300 cm ³ , weighs 300 g, retails for $ 1,500, and the total system investment is $ 2.5 billion. Inmarsat-P's partners include Comsat, Hughes, Matra Marcom, Nokia, Ericssion, NEC, etc., which were put into use from 1998 to 2000, with a life of 11 years, and the telephone fee is 2 dollars per minute. MAGSS-14 is a medium-orbit global satellite mobile communication system developed by the European Space Agency. It consists of 14 satellites with a height of 10354 kilometers and is distributed on 7 orbital planes with an orbital inclination of 56 °. This angle is optimal for mid-latitude coverage, and it ensures that the minimum elevation angle at which a user sees a satellite is 28.5 °. At this altitude, the satellite orbits around for one quarter of a stellar day (23 hours and 56 minutes). This 4: 1 harmonic makes the satellite's ground orbit repeat every day, which provides some useful network coverage characteristics for Dynamic Constellation (DSC). When the user's elevation angle is 28.5 °, the maximum inclined path length is 12500km, and the satellite coverage area radius calculated from this is 4650km. The satellite movement makes the average visible time of an earth station and a star as long as 100min. Each star has 37 beams, which can cover the whole world. ^[2]

Medium orbit earth satellites are mainly used for global personal mobile communication functions. The following uses Odyssey satellite mobile communication system as an example to introduce its working principle.

Comparison table of three types of stationary, medium and low orbits

Track type	low	in	still
Number of satellites	Tens to hundreds	Fair 12	3
Space segment cost	highest	lowest	medium
Satellite life (years)	3 7	10 15	10 15
Ground access cost	highest	medium	lowest
Handheld work	can	can	Can't
Transmission delay	Unaware	Unaware	difference
Elevation angle	difference	the best	it is good
Link margin	10 16dB	7dB	6dB
operating	complex	medium	the easiest
Call forwarding	frequently	rare	no
Building penetration	limited	limited	no
Phased activation	no	can	can
Development time	long	short	long
Deployment time	long	medium	short
Technical risk	high	low	medium