Timing and Frequency Considerations in the Worldwide Testing of a Spread Spectrum Communication System

D.G. Woodring, S.A. Nichols, Roger Swanson

Abstract:	During 1978 and 1979, an Air Force C-135 test aircraft was flown to various locations in the North and South Atlantic and Pacific Oceans for satellite communications experiments by AFAL. A part of the equipment to be tested on the aircraft was the SEACOM spread spectrum modem developed for NRL by Raytheon. Test results achieved in the program will be presented. The SEACOM modem operated at X band frequency from the aircraft via the DSCS II satellite to a ground station located at NRL. This modem incorporated the concepts of wide bandwidths, autonomous operation, high frequency multiplication factor and design-to-cost. For data to be phased successfully, it was necessary to maintain independent time and frequency accuracy over relatively long periods of time (up to two weeks) on the aircraft and at NRL. To achieve this goal, two Efratom atomic frequency standards were used. One of these has been in service at NRL since 1973. One standard was used as a portable clock and the other was used as the modem frequency standard. This paper will discuss the performance of these frequency standards as used in the spread spectrum modem, including the effects of high relative velocity, synchronization and the effects of the frequency standards on data performance. The aircraft environment, which includes extremes of temperature, as well as long periods of shutdown followed by rapid warmup requirements, will also be discussed. The limitations of maintained time in remote locations such as Thule, Greenland, Ascension Island, Lima, Peru, Hawaii and Dayton, Ohio will also be addressed.
Published in:	Proceedings of the 11th Annual Precise Time and Time Interval Systems and Applications Meeting November 27 - 29, 1979 Goddard Space Flight Center Greenbelt, Maryland
Pages:	65 - 78
Cite this article:	Woodring, D.G., Nichols, S.A., Swanson, Roger, "Timing and Frequency Considerations in the Worldwide Testing of a Spread Spectrum Communication System," Proceedings of the 11th Annual Precise Time and Time Interval Systems and Applications Meeting, Greenbelt, Maryland, November 1979, pp. 65-78.
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