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

[FIGURE 1 OMITTED]

For customers who required high accuracy time-keeping, NBS designed a special purpose delay "computer" in the form of a circular slide rule [23]. This slide rule could be printed on paper, cut out, and assembled; and was later printed on laminated plastic and distributed free of charge to those who requested it (Fig. 2). Beginning in 1972, the voice announcements included the satellite's longitude and latitude and a radius correction. Using this information, along with the longitude and latitude of their receiver, the customer could manipulate the slide rule and obtain a path delay estimate. Delays estimated with the slide rule had a standard deviation of < 25 [micro]s when compared to actual delays measured from Boulder over a period of several months, and the combined uncertainty of the signal was < 50 [micro]s. This was confirmed at monitoring stations installed in Colorado, Massachusetts, Peru, and Brazil, where the WWVS broadcast was received and compared to UTC synchronized clocks [24,25].

[FIGURE 2 OMITTED]

Although it represented an important step forward in satellite timekeeping, WWVS never became an NBS service. The experiment was discontinued in August 1973, about 2 years after it began, but plans to develop a service continued. WWVS was placed into NBS budget requests for several years, was granted a frequency allocation of 400.1 MHz from the International Telecommunications Union (ITU), and NBS and NASA developed a Memorandum-of-Agreement (MOA) to provide the service. However, the possibility of WWVS formally ended in 1977 when the Director of NBS terminated the MOA, indicating that the "satellite service would place a significant additional financial burden on NBS....", and that the "satellite service would depend on the continued availability of suitable satellites which are primarily dedicated to other services [26]". By this time, the GOES satellite service had already been launched.

7. The GOES Service Begins

While the WWVS experiments were still in progress, NBS had begun work on another type of one-way satellite broadcast service, publishing a feasibility study in 1973 [27]. This proposed service would use the Synchronous Meteorological Satellites (SMS), the first of which was to be launched by NASA in May 1974. Once these satellites were functioning properly they were to be turned over to the National Oceanic and Atmospheric Administration (NOAA), and future satellites launched by the program would be renamed GOES (Geostationary Operational Environmental Satellites).

The SMS/GOES satellites (Fig. 3) were designed to collect information about the weather. Their objective was to sense meteorological conditions from a fixed location above the Earth and to deliver this data to operational forecasters and private interests on the ground. Part of the GOES mission was to take pictures of storm patterns, frontal systems and the like. Since it was necessary to accurately locate these pictures with respect to Earth longitude and latitude, the position of the satellite had to be precisely known. To accomplish this, a ranging system was developed for NOAA under a NASA contract. The ranging system had a theoretical precision of about 1 m, and an accuracy limited mainly by the real time knowledge of the effects of the ionosphere and the troposphere. This ranging system was based on a concept termed trilateration. It worked by making ranging, or time delay measurements from the Earth to the satellite from three widely separated ground stations. The primary tracking station, called the Control and Data Acquisition Station or CDA, was located at the NOAA facility at Wallops Island, Virginia, near the city of Chincoteague (Fig. 4).