Study on feasibility of SATCOM for railway communication

Final Report
6.2.4.2 System architecture
This MEO solution is based in a constellation of satellites, even when only a limited coverage area is
needed. This is because only GEO satellites (located at ~36000 km from Earth surface) rotate at the
same speed than Earth, revolving in a circular orbit at a constant speed of once per day over the
equator.
Since the satellite footprint decreases in size as the orbit gets lower, MEO (and also LEO) systems
require larger constellations than GEO satellites in order to achieve global coverage. To provide global
communications with a MEO satellite system it is estimated that (at least) a constellation of 10-15
satellites are required.
Regarding the system architecture, several design aspects that shall be considered in a MEO
constellation are explained in Table 16:
Altitude
Visibility
Coverage
Latency
Complexity
Capacity
Routing
Usually around 5,000 – 20,000 km, although MEO is considered the region space
above LEO (~2,000 km) and below GEO (~36,000 km).
Very good satellite visibility, augmented by the use of satellite diversity techniques.
It depends on the orbits design and number of satellites used.
Low latency compared with GEO satellites. Final values of latency will depend on the
orbit radius, but it could be around 130 ms (for example, current O3B system, with an
orbit of 8,000 km, has a roundtrip delay of 120 ms
Since satellites are not in a fixed location from the Earth point of view, additional
complexity to GEO constellations is added for management, satellite handovers (and
seamless continuity when it occurs), etc.
It depends on the frequency band (the higher the band the higher the bandwidth) and
the techniques used to take advantage of it such as frequency reused.
In LEO and MEO constellations where satellites are in movement from the Earth point
of view, how to connect satellites with ground segment shall be considered:
- Using a minimum number of GES assuring that each satellite always has a
visible GES
- Using Inter-Satellite Links (ISL)
In a LEO constellation, where the number of satellites is bigger with reduced coverage
areas, the best option is to implement ISLs among satellites in order to reduce the
number of GES around the Earth (it is the case of Iridium).
But in a MEO constellation, where the number of satellites can be reduced
considerably, it is possible to design a satellite network around the Earth with a
reasonable number of GES, avoiding this way the implementation of ISL.
Doc.Nº: SRAIL-FNR-010-IND
Edit./Rev.: 1/1
Date: 16/02/2017
Page 74 of 285