Left Image: ECHO 1 First Passive Communication Satellite (NASA Image - GIF - 79 KB)
Right Image: TELSTAR First Active Communication Satellite (NASA Image - GIF - 88 KB) Click on each to enlarge.
In 1965, scientists from USNO got together with researchers from the Radio Research Laboratory (RRL) in Japan to perform the first transpacific clock comparisons using the communication satellite Relay II .
RELAY Communication Satellite (NASA Image - GIF - 99 KB)
Compared to today the early signal transfer methods and measurements were primitive. In most of these early experiments, a pulsed signal generated every second, known as a one-pulse-per-second (1pps), was modulated onto the carrier wave of a video signal for transmission. Oscilloscopes, polaroid cameras, and other primitive methods of measurement were employed. Precisions reached during the Telstar and Relay experiments were near the 10-to-100-nanosecond level. The accuracies were near the 100-to-1000-nanosecond level.
The ATS-1 Communication Satellite with engineer before launch (Unknown credit - GIF - 144 KB)
The next experiments with a significant impact on the development of TWSTT at USNO was a series of measures made using the Communication Technology Satellite (CTS/Hermes) during the late 1970s. The National Research Council (NRC) in Canada, National Bureau of Standards (NBS) in the United States, and USNO performed TWSTT experiments that reached the 0.2-nanosecond-precision levels. Accuracies at the 50 nanosecond level were attained [5,6]. This experiment tested phase-shift-keyed signals using normal COMSAT satellite modems, rather than pseudo-random-noise sequence modems, for transfer of the time ticks.
An experiment between the Deutsche Forschungs und Versuchsanstalt fur Luft und Raumfahrt (DFVLR), COMSAT, USNO and Institut fur Luft und Raumfahrt der Technischen Universitat Berlin (TU) was performed during 1983 using an Intelsat-V satellite . Precisions at the 300-picosecond level were attained in one of the first applications of the Mircrowave Time and Ranging Experiment (MITREX) modem.
CTS/Hermes Communication Satellite (NASA Image - GIF - 72 KB)
In 1987, routine (three times per week) experiments were begun using commercial Ku-band satellites. These experiments were begun by the National Institute of Standards and Technology (NIST, formerly known as NBS), USNO, and NRC. These routine experiments continue.
In 1993, eight timing centers and laboratories began routine (three times per week) transatlantic TWSTT experiments. Those participating include USNO, NIST, the Technical University of Graz, Austria (TUG), the National Physical Laboratory, United Kingdom (NPL), the NMi van Swinden Laboratory, The Netherlands (VSL), Deutsche Telekom AG, Germany (DTAG), Physikalisch-Technische Bundesandstalt, Germany (PTB), and the Observatoire de la Cote d'Azur, France (OCA) [8,9]. These experiments were restarted during January 1997.
Since 1996, we have used 24/7 a channel on the CONUS satellite located at 103.0W for TWSTT operations. Initially the satellite was GSTAR-1. Currently the satellite is GE-1.
Left image: The A2100 type communications satellite artists view
(Lockheed Martin Missiles & Space - GIF - 99 KB)
Right image: GE-1 an A2100 Hybrid (C- Ku-band) Communications Satellite
(Russ Underwood: Lockheed Martin Missiles & Space - GIF - 125 KB)