by Donald Martin, Paul Anderson, Lucy Bartamian
Although the performance of communication satellites could be predicted theoretically, until 1962 or 1963 there was considerable doubt concerning whether their actual performance would match the theory. This was one of the basic motivations for the early communication satellite experiments. Two other important factors were the desire to prove the satellite hardware (since space technology in general was still in its infancy) and the need to test operational procedures and ground equipment. Whereas the first few experiments (SCORE, Courier, and Echo) were very brief beginnings, the Telstar, Relay, and Syncom satellites laid definite foundations for the first operational satellites.
Communication satellites have been in commercial operation and military service since 1965 and 1967, respectively. However, there was, and still is, the need for additional experimental satellites. These are used to prove new technologies for later introduction into operational satellites. Some satellites combine experimental objectives with preoperational demonstrations. Discussions of such satellites are included in this chapter if their emphasis is primarily experimental; those directly continued by operational satellites are described in later chapters.
The Telstar experiment grew out of the Bell Systems' interest in overseas communication. Bell Telephone Laboratories was a major participant in communication experiments using Echo 1. The positive results of those experiments strengthened the interest in satellite communications generated by earlier analytical papers. Therefore, American Telephone and Telegraph Company (AT&T) decided to build an experimental active communication satellite. The objectives of the Telstar program were to:
- look for the unexpected
- demonstrate transmission of various types of information via satellite
- build a large ground antenna and learn how to use it
- gain experience in satellite tracking and orbital predictions
- study Van Allen radiation belt effects
- face the design problems required for a spaceborne repeater
Two Telstar satellites were produced. The satellites were 34.5-inch diameter spheres with solar cells covering most of the outer surface. The solar array output alone could not support operation of the communication subsystem, so batteries were used to supply the peak power requirements. The batteries were recharged during the periods when the satellite was not in view of the ground terminals and the communication subsystem was turned off. This subsystem had a single channel with a 50 MHz bandwidth. The program details are as follows.
- Sphere, 34.5 in. diam
- 170 lb in orbit (Telstar 1), 175 lb in orbit (Telstar 2)
- Solar cells and NiCd batteries, 15 W
- Spin-stabilized, 200 rpm
- One 50 MHz bandwidth double-conversion repeater
- 600 one-way voice circuits or one TV channel
- 60 two-way voice circuits (tests limited to 12 circuits by ground equipment)
- 4170 MHz
- All solid state except TWT (traveling wave tube)
- TWT operated linear at 3.3 W (saturated power: 4.5 W)
- 6390 MHz
- All solid state
- 12.5 dB noise figure
- Transmit: 48 small ports equally spaced around satellite waist
- Receive: 72 small ports
- Uniform pattern around waist and ±30 deg from waist plane
- Circular polarization
- Telemetry: 136.05 MHz, 200 mW transmitter
- Command: approximately 123 MHz
- Four-element helical antenna
- Two-year goal
- Telstar 1: 514 x 3051 nmi, 45 deg inclination
- Telstar 2: 525 x 5830 nmi, 43 deg inclination
- Telstar 1: launched 10 July 1962, operated until 23 November 1962, and 4 January to 21 February 1963
- Telstar 2: launched 7 May 1963, operated until May 1965
- Delta launch vehicle
- Telstar was developed by Bell Telephone Laboratories for AT&T
After Telstar 1 was launched, in the following 6 months, about 400 transmission sessions were conducted with multichannel telephone, telegraph, facsimile, and television signals. In addition, more than 250 technical tests and measurements had been performed. Stations in the United States, Britain, and France participated in these activities. In November 1962, the command subsystem on the satellite failed. The cause was later established as degradation of transistors due to Van Allen belt radiation. Various operations effected a recovery that allowed the satellite to be used for another month and a half early in 1963, after which the command subsystem failed again.
Telstar 2 was nearly identical to Telstar 1. The only significant design change was the use of radiation-resistant transistors in the command decoders. The Telstar 2 satellite orbit had a higher apogee than Telstar 1, which increased the time in view of the ground stations and decreased the time in the Van Allen belts. Telstar 2 was launched in May 1963 and operated successfully for 2 years.
Donald H. Martin is a senior engineering specialist in The Aerospace Corporation’s Architectures and Spectrum Management Office. Martin joined the Communications Department in the Engineering Group at Aerospace in 1968 after receiving B.S. and M.S. degrees in engineering from the University of California, Los Angeles. He has been collecting information on satellite communications since 1972, when his manager offered him a choice of assignments: of the three options, he chose to write a description of communication satellites then in orbit. The assignment grew the next year to include a report describing satellites being built, and gradually expanded to the first edition of “Communication Satellites in 1986”, with the book now in its Fifth Edition.
- Space Communications and Navigation 1958–1964, NASA SP-93 (1966). Special Telstar Issue, Bell Systems Technical Journal, Vol. 42, No. 4 (July 1963). Reprinted as Telstar I, NASA SP-32, Vols. 1–3 (July 1963) and Vol. 4 (including Telstar II supplement) (December 1965).
- K. W. Gatland, Telecommunication Satellites, Prentice Hall, New York (1964).
- I. Welber, "TELSTAR," Astronautics and Aerospace Engineering, Vol. 1, No. 8 (September 1963).
- I. Welber, "Telstar Satellite System," Paper 2618-62, ARS 17th Annual Meeting and Space Flight Exposition (November 1962).
- "Project Telstar," Spaceflight, Vol. 4, No. 5 (September 1962).
- J. Holahan, "Telstar, Toward Long-Term Communications Satellites," Space/Aeronautics, Vol. 37, No. 5 (May 1962).
- D. R. Glover, "NASA Experimental Communications Satellites," http://sulu.lerc.nasa.gov/dglover/satcom2. html (10 June 1999).
- D. R. Glover, "NASA Experimental Communications Satellites, 1958–1995," in Beyond the Ionosphere: Fifty Years of Satellite Communication, A. J. Butrica, ed., NASA History Office, Washington, D.C. (1997), ch. 2.
- M. B. Punnett, "The Building of the Telstar Antennas and Radomes," IEEE Antenna's and Propagation Magazine, Vol. 44, No. 2 (April 2002).