Mobile satellite communication has the potential to provide
an infrastructure independent of terrestrial systems.
Particularly, international fleet management systems employ satellite links for the exchange of
messages because satellites can also be used for continent-wide paging and navigation on
board the vehicle. The absence of a pan-European earth-based infrastructure for packet data
communication has been an important stimulus for satellite communication links between
vehicles and their home bases.
However, for systems with larger traffic loads, with
more local applications, satellite
communication systems may experience the disadvantage that local possibilities of frequency
re-use are limited, except for satellites with very low orbits and highly directional antennas.
Expected coverage of cellular telephone systems by the year 2000.
In due course, communication may thus need to be transferred to ground-
based cellular mobile networks. Satellite technology is particularly suited to cover rural, sparsely populated areas.
Moreover satellite systems have the
important ability to offer an early service which only later becomes available in the standard
fixed infrastructure. Earlier examples of this have been international satellite television
programme distribution and video conferencing, now also possible by broadband optical
cables, intercontinental digital leased lines and 'hot lines' for journalists and statesmen
travelling in less developed regions.
Cellular Satellite Networks
Mobile satellite systems work like terrestrial
cellular systems, except that the base stations
(i.e., satellites) move as will as mobile devices.
Satellite coverage attractive for areas of world not well served by
existing terrestrial infrastructure: ocean areas, developing countries.
Market predictions vary between 2 and 15 Million subscribers worldwide.
Selection of Appropriate Orbits
The altitude of the earth orbit has a substantial effect of the system performance,
- low altitude means lower path loss, smaller transmit power and smaller antennas
- low altitude means that more satellites and more ground stations are needed
- low orbit satellites have a larger speed, so more handovers are needed.
The average time between handovers ranges from 5 minutes to 1 hour, depending on orbit altitude.
- low orbit gives smaller round trip delays
A portion of the L-band around 1.6 GHz has been allocated for satellite telephony.
5.15 MHz of bandwidth can be used for Iridium using TDMA
Other allocations exist in the S-Band around 2.5 GHz mainly for CDMA transmission.
Big Low-Earth Orbiting (LEO) Systems
Little LEO Systems
Features of Digital Land Mobile Satellite Communication Systems
Offering Voice, Data and Fax Services.
Compiled by Miquele Dlodlo
B,C & M
|Orbit (km above ground)||
|520-7 846 ell(e); |
8 040 circular(c)
|No., Active Satellites|
10 e; 6c
||4||2 e; 1c|
||90||116.5 e; 0 c|
|Min. Single Hop Delay (ms)|
|Data Rates (kbps)||
|Voice Rate (kbps)|
||CDMA / FDMA / FDX
||TDMA / FDMA||CDMA / FDX
|Modulation||BPSK / OQPSK + FEC
||QPSK + FEC
||QPSK + FEC||QPSK + FEC
|Voice Circuits per Satellite||
|Downlink Freq. GHz (traffic)|
|Uplink Freq. GHz. (traffic)|
Notes: Blank spaces indicate cases where accurate information
was unavailable at compilation time.
Satellite data communication systems show a similar development as telephone
The tendency here is towards smaller and transportable ground stations. This
is shown by the popularity of the very small aperture terminals (VSATs).
VSATs are small satellite groundstations, normally linked within a
VSAT network. The size of a VSAT is limited to a maximum antenna diameter of
approximately 240 cm. VSATs are suitable for low rate data traffic (approximately 64
Slides on Mobile Satellite Systems
An Acrobat review
of mobile satellite systems has been compiled by Randy Katz.
Research in Europe
Europe aptly captured the mood of the communications research, development and deployment sectors in the industrialised world when naming one of their programs RACE. In a race, winning dominates virtually everything for the duration of the race. In this race with a difference, though, a lot of thought has gone into minimising costs to the end-user. Low-cost equipment has the potential of opening up mass markets and mass production on a global scale. The main result of the foregoing thought process is the quest for the small hand-held terminal with only the minimum sophistication necessary for operation in the DLMSC environment involved. A subsidiary result is the intended availability of advanced services to any user based on individualised demand rather than on some universal service objective.
The actual research and development carried out so far into non-GEOS systems (the best prospect for isolated rural people and the global PCS market) has been very expensive for the companies involved. As a result, the initial cost of services is quite likely to be out of reach of the isolated rural communities of the developing world so that the gap between the developed and the underdeveloped might at the outset widen instead of closing as desired. Hence, serious thought needs to be given to the question of how the pricing of basic services will be structured in order to avert the possible widening of the gap.