Introduction to Small Satellite Communications

Introduction

The concept of modern trunk satellite communications was first conceived by Arthur. C Clarke in 1945, proposing to carry radio transponders on satellites in Geostationary orbit where they would appear in a fixed position in the sky. This would permit communication links beyond the line of sight, and three of these satellites can give near global coverage. All the early communication satellites would now be considered in the small satellite class, but soon bigger and more powerful satellites were deployed to service the rapidly growing demand. Small satellites have found a niche in providing specialised communication services.

First Space Communications

The first satellite broadcast was a Christmas greeting by president Eisenhower in 1958, using the SCORE satellite, and a number of experiments with passive reflectors such as ECHO in 1960 proved the concepts of beyond-line of sight communications. in 1962 active transponders were placed in low Earth orbit with the TELSTAR and RELAY satellites, so that signals were amplified on their way back to Earth. The first Geostationary satellite to carry a transponder was SYNCOM, launched in 1963, which provided real time coverage of the Tokyo Olympics to the US. It was followed by the launch of commercial services via Early Bird, or INTELSAT-F1 in 1965, and the first commercial communications satellite Telstar-2 in the same year. Also in this year, Russian communication satellites were deployed in highly elliptical MOLNYA orbits, providing up to eight hours of communications coverage at high latitudes via a quasi-stationary transponder.

Store and Forward Communications

A satellite in low Earth orbit has only limited coverage, with a footprint diameter near 6000km [try it!]. However, with a suitable choice of orbital inclination, it can cover each part of the globe within 24 hours. Store and Forward communications is a concept different from that employed in trunk communications. It stores signals received from the uplink, recording the message transmitted to the satellite. The message can then later be broadcast or retransmitted it on demand. A single satellite in low Earth orbit can therefore provide a true global messaging service. This concept was first proposed by Brandon in 1957 [Bra 73], and implemented on the COURIER satellite in 1960 using on board tape recorders.

A digital Store and Forward transponder was first pioneered on UoSAT-2 in 1984 sponsored by VITA (Volunteers in technical assistance) and AMSAT (Amateur Satellite Organisation), soon followed by a similar US military experiment on GLOMR in 1985. This led to subsequent Store and Forward satellites in the same series. UoSAT-3 launched in 1990, which became HealthSat-1 in 1991. HealthSat-2 procured by the US medical aid organisation Satelife in 1992 was the culmination of the development of this style spacecraft.

References

We would appreciate if you could use the SSHP in your references: "Small Satellites Home Page", Surrey Space Centre, "http://www.smallsatellites.com/"