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

[FIGURE 6 OMITTED]

The GOES time code service was designed to operate in two modes, known as uncorrected mode and corrected mode. In uncorrected mode, the satellite ephemeris included in the time code was ignored by the receiver. It was known that the average free space propagation delay from the CDA through the satellite to any given point in the coverage area would be near 260 ms. Therefore, the TCGs sent the time out 260 ms early, reducing the maximum possible time error anywhere in the coverage area to [+ or -]16 ms [34]. For example, the delay from the CDA to Boulder through GOES-East was near 252 ms, resulting in the uncorrected time arriving about 8 ms early; through GOES-West the delay was near 261.5 ms, resulting in the uncorrected time arriving about 1.5 ms late. Corrected mode required the receiver to have a microprocessor so it could read the ephemeris data code, and compute the free space delay from the satellite to the receiver's antenna (the receiver's coordinates were entered by the user and stored in memory). This improved the timing uncertainty to [+ or -]100 [micro]s [30], and allowed GOES to become the first time code service of any type that allowed receivers (Sec. 8) to automatically correct for path delay.

The coverage area for the GOES time service was larger than other time code services in the pre-GPS era (Fig. 7). It included nearly all of North and South America, and most of the United States received coverage from both satellites. Unlike reception of the ground based NBS time stations, GOES reception was equally good during the day or night, and since the receiving antennas were wide beam, pointing the antenna was not that critical. Receivers in the United States could usually automatically switch to the East satellite if West was not available, and vice versa, without moving the antenna.

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8. The NBS GOES Receiver Leads to Commercial GOES Receivers

Once the time code service had begun, it was obvious that GOES time code receivers must be made available before the new service would acquire any customers. To promote the service, NBS designed a receiver intended to stimulate the private sector into producing their own. The NBS unit could be built for less than $200 in parts, and represented a significant technical achievement in the days prior to the advent of the personal computer. It used the 4 bit Intel 4004 (1) as its central processing unit, now generally regarded as the first microprocessor ever developed. The extremely efficient assembly language firmware resided in just 512 bytes of read only memory (ROM), and the random access memory (RAM) occupied just 40 bytes! The microprocessor clock ran at 4.096 MHz, which was frequency divided by 40960 (8 X 2 X 16 X 16 X 10) to produce 100 Hz and then phase locked to the received data clock from the satellite. The phase locked 100 Hz served as the time base for the microprocessor time-of-year (TOY) clock.

Complete plans for a GOES time code receiver, including schematics and assembly language source code, were published in NBS Technical Note 681 [35] and widely distributed. As a result of this work, a United States patent was awarded to NBS engineers Cateora, Davis, and Hanson in 1977 for their Satellite Controlled Digital Clock System [36]. Figure 8 shows the original satellite clock receiver built by NBS around 1976. Figure 9 shows a GOES controlled time display built for the United States bicentennial celebration in 1976.