MOBILE SATELLITE COMMUNICATION SYSTEM CAN BE BROADLY CLASSIFIED BY ORBIT PERIOD

MOBILE SATELLITE COMMUNICATION SYSTEM CAN BE BROADLY CLASSIFIED BY ORBIT PERIOD

1. Mobile Satellite Communication System by the Geostationary Satellite
The geostationary satellite is the artificial satellite which looks stationary from the ground. 3-4 geostationary satellites can cover almost the entire surface of the earth. Most of the artificial satellites actually used for communications or broadcasting are geostationary satellites.
• i. Altitude: about 36,000km
• ii. Orbit: the circle orbit cycle on the equator is the same as the earth's autorotation time.
• iii. Number of Satellites: four (service areas are duplicated.)
• iv. Principle Satellite System: Inmarsat Communication System, N-STAR Communication System, Omunitrucks Communication System
2. Mobile Satellite Communication System by the Quasi-Zenith Satellite
The quasi-zenith satellite is an artificial satellite of the satellite system where one satellite always stays near the zenith in Japan by positioning at least three satellites synchronously on the orbit inclined at 45 degrees from the geostationary orbit. As the ground surface orbit draws the shape of number 8, it's also called "Number 8 Orbit Satellite". It can obtain a high elevation angle to reduce the influence of buildings and so forth (blocking.)
• i. Altitude: about 36,000km
• ii. Orbit: circle orbit crossing with the equator by the angle of 45 degrees
• iii. 3 as the minimum
• iv. The research and development of the satellite communication system is in progress
3. Mobile Satellite Communication System by the Non-Geostationary Satellite
This is roughly divided into three kinds of orbits: highly elliptic orbit, medium earth orbit, and low earth orbit. The medium and low earth orbits have lower satellite altitudes to shorten the radio transmission delay, enabling more speedy and smooth communication. Specifically, the highly elliptic orbit can obtain a higher elevation angle. It is currently being researched and developed.


i. Highly Elliptic Orbit (HEO)
1. Altitude: about 40,000km
2. Orbit: about 5-6 hours
3. Number of Satellites: 2-3 as the minimum
4. The system planning is in progress.
ii. Medium Earth Orbit (MEO)
1. Altitude: several thousand - 20,000km (about 10,000km)
2. Orbit: about 5-6 hours
3. Number of Satellites: 8-10 (for the entire world)
4. The system planning is in progress.
iii. Low Earth Orbit (LEO)
1. Altitude: 500km - several thousand km (about 1,000km)
2. Orbit: about 5-6 hours
3. Number of Satellites: several dozen (for the entire world)
4. Principle Satellite System: Globalstar Mobile Satellite Communication System, Orbcomm Mobile Satellite Communication System (IRIDIUM Mobile Satellite System (abolished))
Satellites in GSO

GSO satellites orbit the Earth in the equatorial plane with the same angular velocity as the Earth at a height of about 36 000 km above the equator. Geostationary satellites therefore appear stationary to an earth-bound observer and a single satellite can provide continuous service to roughly one third of the Earth's surface (but excluding
polar regions above ± 75 degrees of latitude). The maximum distance the satellite can "see" on the Earth's surface is about 42 000 km and means the propagation delay for a single hop via the satellite (once up and down) can be up to 280 ms. Geostationary satellites also move about their nominal positions causing a small but noticeable Doppler shift on both the feeder and mobile links.For personal and vehicle terminals, handover during a call between GSO satellites is unnecessary because the coverage is static and wide. However handover might be contemplated for aircraft terminals between different spot beams of the same satellite. In the latter case there is practically no difference in path length to consider. Within Europe, GSO satellites appear at low elevation angles. For the geographical latitude of 50°North (e.g.Luxembourg), the satellites reach approximately 31° elevation as a maximum when the satellite is due South: either East or West of this position the elevation slowly reduces. Frequent blocking of the line-of-sight signal therefore occurs from trees, buildings and hills. GSO satellites can work in such a shadowed environment but the satellite Equivalent Isotropic Radiated Power (EIRP) would have to be increased by 15 dB to 20 dB or more depending on the coverage required.This could be achieved but has a serious impact on the size and cost of the satellite. In addition, assuming that the mobile EIRP is limited, the satellite receive sensitivity also has to increase and this can only be done with very large spacecraft dish antennas. For this reason, only very low bit rate services (i.e. paging, alerting, etc.) might be viable under such circumstances until the user moves to a more favourable position to receive a voice call.

Satellites in HEO

Satellites in HEO constellations orbit the Earth in planes that are inclined nominally 63,4° against the equatorial plane.This is necessary in order to keep the apogees in the most northern (southern) positions within their elliptical orbits.Typically HEO orbital periods are between 8 and 24 hours. HEO satellites are normally active only about their apogees where they appear nearly stationary to an earth observer for about eight hours, and then have to hand over to a following satellite.The satellites belonging to one particular system appear in time shift in the same celestial region. In the HEO track is sketched in profile showing at every point the true distance to the Earth's surface. In this specially depicted case, the orbital period is 12 hours and the satellites appear
alternatively at the opposite sides of the rotating globe. Therefore the illustrated HEO track reaches a maximum height at both ends above the geographical latitude of 63,4° North. At both upper ends (solid line), the satellite payloads are active. The dotted line constitutes the part where the satellite payloads are (typically) switched off. For comparison, see figure 2, where two HEO loops are indicated corresponding
to the two ends in profile in figure 1.Under the above conditions, the HEO apogee (maximum height above the Earth's surface) can be up to 42 000 km.However the maximum range to the Earth's surface is in the order of 47 000 km resulting in a maximum propagation delay of the order of 310 ms. HEO satellites reach high relative speeds during their active phase (order of magnitude:2 km/s), so that the Doppler shift (1.3 x 10-5 of radio frequency and bit rate) cannot be neglected: the radio frequency
shift is mainly due to the microwave feeder link and is of the order of 50 kHz for C-band feeder links. The satellite motion is mainly radial relative to the user community, so that common compensation of the Doppler main component is feasible.Irrespective of any user roaming, HEO systems require handover from the descending to the ascending satellite typically every eight hours. Depending on the specific system design, the distance to the two satellites at handover could be significant and a jump in path length cannot be excluded. However, a large Doppler jump will always happen.Within Europe HEO satellites can appear near the zenith. Therefore the user can work under vertical line-of-sight condition for most of the time, with blockage only being experienced in tunnels or under bridges, trees, etc. However vertical propagation is not very good within multi-storey buildings and hence paging, alerting, etc. may not be satisfactory.
Because vertical propagation can be in principle multipath-free, high data rate services are possible for outdoor operation.A number of HEO orbits have been studied extensively and given names such as "Molnya", "Tundra", and "Loopus".




Satellites in MEO

MEO satellites are in principle the same as LEO satellites. The differences are that MEO systems cause more propagation delay (80 ms to 120 ms), their Doppler shift is smaller, and handover happens less frequently and is less problematic. MEOs also need to work in a multipath environment as the number of satellites is usually smaller than for LEO but the average margins can be lower since many calls will be at a continuously high
elevation angle.The typical MEO altitude is between 10 000 km and 20 000 km, just outside the Van Allen belts with an orbital period of around 6 to 12 hours. A complete MEO constellation would probably require between 10 to 15 satellites. MEO satellites are used to provide current global radio navigation services and are optimum for such services.

Satellites in LEO

LEOs are typically circular orbits where satellites fly low above the Earth's atmosphere typically 700 to 1 500 km, bounded by outer atmospheric drag and the Van Allen radiation belts with an orbit time of about 90 minutes. For orbits near 1 500 km, inclinations near 50 degrees reduce the risk of debris collisions. Whereas polar orbits provide a whole Earth coverage including the poles themselves, inclined orbits can provide improved coverage over the populated areas located between latitudes -75 to +75 degrees. One proposed system is known to stay 700 km above the surface (see figure 1; LEO) where the coverage area at any point in time may measure up to 3 000 km in radius for about 10 degree elevation. This implies a maximum propagation delay of 20 ms and while higher altitude LEO systems would have higher propagation delays, they will never approach the values associated with GSO or HEO systems for a single satellite hop. However, on-board processing and Inter-Satellite Links (ISL) can increase delays considerably.LEO satellites move at very high speeds relative to the Earth's surface (7 km/s) and produce large Doppler frequency shifts (4,7x10-5 of radio frequency and bit rate). As the velocity is tangential to the Earth, Doppler compensation may need to be applied individually for each user.LEO systems, in common with HEO systems, also require to handover between adjacent satellites, but at a much more frequent rate of about ten minutes. Although the two LEO satellites are widely spaced, the individual path lengths can be similar and it is possible to minimise any path length jump. However, the Doppler shift jump will still always happen. As LEO satellites orbit very close to Earth, they can be considered as moving base stations. For the user the satellites appear most of the time below 30 degree elevation. Therefore LEO satellites work much of the time in a multipath environment. The additional satellite EIRP and receive sensitivity to compensate for multipath losses are achieved witha much smaller antenna on a LEO spacecraft (compared to GSO) because of the much shorter range (roughly 1/12th).
Diversity techniques may offset some of these multipath effects.The total number of satellites required to give total global coverage depends on many factors including quality of service and system capacity but the total could be as high as 70. Lower numbers are possible using special orbits or by using a mixture of LEO and GSO (for example). The cost for large numbers of LEO satellites is offset to some extent by their lower complexity and easier launch requirements. However their orbital life tends to be half that of typical GSO satellites (10 - 13 years). Another factor in LEO design is the required battery capacity and solar panel size to allow operation for nearly 50% of time in eclipse.