Satellite Systems

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, for instance

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

Frequency Allocations

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 Multiple Access. Other allocations exist in the S-Band around 2.5 GHz mainly for CDMA transmission.

System Overview

Geostationary Systems

INMARSAT
MSAT

Non-geostationary

Big Low-Earth Orbiting (LEO) Systems
- ARIES
- ELLIPSO
- IRIDIUM
- ODYSSEY
Little LEO Systems
- Orbcomm
- LEOSAT
- STARNET
- VITASAT

Selected Features of Digital Land Mobile Satellite Communication Systems Offering Voice, Data and Fax Services.

Compiled by Miquele Dlodlo

	GEO	ICO	MEO	Big LEO			HEO/MEO
	INMARSAT B,C & M	INMARSAT P	Odyssey	Iridium	Globalstar	Aries	Ellipso
Orbit (km above ground)	35 800	10 335	10354	780	1 401	1 018	520-7 846 ell(e); 8 040 circular(c)
No., Active Satellites	4	10	12	66	48	48	10 e; 6c
Spare Capacity	5	2	3	6	8	4	2 e; 1c
Orbital Planes	1	2	3	6	8	4
Inclination deg.	0	45	55	86.4	52	90	116.5 e; 0 c
Min. Single Hop Delay (ms)	240	68.95	69.07	5.20	9.35	6.79	3.47
Terminal Modes	1	2	2	2	2		2
Data Rates (kbps)		2.4	9.6	2.4	2.4	2.4	0.3-9.6
Voice Rate (kbps)		4.8	4.8	2.4/4.8	2.4/4.8/9.6	4.8	4.15
Multiple Access	SCPC		CDMA / FDMA / FDX	TDMA / FDMA	CDMA / FDX
Modulation	BPSK / OQPSK + FEC		QPSK + FEC	QPSK + FEC	QPSK + FEC		OQPSK
Voice Circuits per Satellite		4 500	2 300	3 840(1100)	2 808(1354)
Downlink Freq. GHz (traffic)	1.5	1.980-2.010	2.4835-2.500	1.616-1.6265	2.4835-2.500	2.4835-2.500	2.4835-2.500
Uplink Freq. GHz. (traffic)	1.6	2.17-2.20	1.61-1.6265	1.61-1.6265	1.61-1.6265	1.61-1.6265	1.61-1.6265
Additional Services	messaging	paging	paging messaging position location	paging messaging position location	paging		paging messaging position location

Notes: Blank spaces indicate cases where accurate information was unavailable at compilation time.

VSAT Satellites

Satellite data communication systems show a similar development as telephone systems.

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 kbit/s).

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.

JPL's Wireless Communication Reference Website

Chapter: Network Concepts and Standards Section: Satellite Systems