The GOES time code service, 1974-2004: a retrospective

Journal of Research of the National Institute of Standards and Technology, March-April, 2005 by Michael A. Lombardi, D. Wayne Hanson

NIST ended its Geostationary Operational Environmental Satellites (GOES) time code service at 0 hours, 0 minutes Coordinated Universal Time (UTC) on January 1, 2005. To commemorate the end of this historically significant service, this article provides a retrospective look at the GOES service and the important role it played in the history of satellite timekeeping.

Key words: broadcasting; Coordinated Universal Time (UTC); orbit prediction; satellites; timekeeping.

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1. Introduction

After nearly 30 years of continuous operation, NIST ended its Geostationary Operational Environmental Satellites (GOES) time code service at 0 hours, 0 minutes Coordinated Universal Time (UTC) on January 1, 2005. This event marked the end of an important chapter in the history of timekeeping. The GOES time code service is historically significant for at least two reasons: it was the first time code service ever broadcast via satellite, and was the first time code service of any type that provided transmitter position data in addition to the time. The position data made it possible for receivers whose position was also known to compute and remove the signal path delay and improve the timing accuracy.

NIST began preparing for the end of the GOES time code service in the mid-1990s, when it was clear that Global Positioning System (GPS) satellite timing receivers provided better accuracy and reliability than GOES at a lower cost. Nearly all GOES receivers were being replaced by GPS units, making the decision to eventually stop the service an easy one. However, during their heyday, GOES time code receivers were widely used by the electric power and aviation industries; and it is estimated that more than 10 000 receivers were sold, produced by at least three different manufacturers. This article provides a retrospective view of the GOES time code service, beginning with a look at the early days of radio time code broadcasts and the first satellite timing experiments.

2. Ground Based Time Code Broadcasts

Wireless time signals existed long before satellites; in fact, telegraphic time signals from ground based transmitters were broadcast beginning in 1903 by the United States Navy [1, 2]. NIST [then the National Bureau of Standards (NBS)] has participated in this arena for many years, beginning with standard frequency broadcasts from radio station WWV in 1923 [3], and later adding telegraphic [4], voice [5], and digital time codes [6] to its broadcasts. From the beginning, it was known that wireless time signals are delayed as they travel the path from the transmitter to the receiver, and that the accuracy of the received time signal can be no better than the knowledge of the path delay. Signals originating from a ground based transmitter have path delays that are difficult to estimate, since the delay continually changes due to changing ionospheric conditions. Some of these problems are reduced by signals that do not reflect off the ionosphere, such as line-of-sight signals with small coverage areas, and groundwave signals in the low frequency (LF) part of the radio spectrum below 300 kHz. However, it was clear even in the pre-satellite days that a time signal broadcast from the sky high above the Earth, where there was a clear, unobstructed path between the transmitter and receiver, would potentially be more accurate than any ground based signal.

3. The First Satellites and Early Satellite Timing Experiments

The Space Age began with the launch of the Russian satellite Sputnik 1 in October 1957, followed by the launch of the first American satellite, named Explorer 1, just four months later. The earliest satellites were used for solar and atmospheric studies, but the emphasis quickly turned to telecommunications. The U. S. Army's SCORE (Signal Communication by Orbiting Relay Equipment) was perhaps the first telecommunications satellite, broadcasting prerecorded Christmas wishes from President Eisenhower after its launch in December 1958. However, Echo 1, a 30.5 m diameter mylar balloon launched by the National Aeronautics and Space Administration (NASA) in August 1960 is generally credited with ushering in the age of satellite telecommunications [7]. A passive-relay satellite, Echo 1 simply served as a "mirror" that reflected radio signals back to Earth, albeit with an approximately 180 dB loss in signal strength [8]. Echo 1 enabled the first satellite telephone link in February 1962, and in April of that same year enabled the broadcast of a television program from California to Massachusetts [9].

Early propagation studies conducted using Echo 1 in 1960 [10] are sometimes identified as the first satellite timing experiments, but the first precise time experiment was probably performed via the active-relay satellite Telstar 1 in August 1962, roughly one month after that satellite was launched. This experiment allowed the United States Naval Observatory (USNO), and the United Kingdom's National Physical Laboratory (NPL) and Royal Greenwich Observatory (RGO) to perform a transatlantic clock comparison using a new technique called two-way satellite time transfer. The ground stations used for the time transfer experiment were located in Andover, Maine and Goonhilly Downs in the United Kingdom. The results were impressive. Uncertainties of just a few microseconds were reported, about 1000 times smaller than the uncertainties previously reported for transatlantic clock comparisons using ground based radio signals, which had been limited to about 2 ms [11].