For many years people have realized that an earth satellite would be the nearly ideal way to broadcast radio signals for time and frequency calibrations. At this writing, two systems are operational and another is being planned.

NBS provides a continuous time code via the GOES (Geostationary Operational Environmental Satellite) satellites. The U. S. Navy operates the TRANSIT series of navigation satellites with major participation by the Applied Physics Laboratory of the Johns Hopkins University and the U. S. Naval Observatory. The operation of these two systems will be explained in this chapter.

In addition to GOES and TRANSIT, the proposed Global Positioning Satellite System (GPS) is in the experimental stage. When it becomes operational, it will offer users several advantages. First, its frequency band, signal strength, and the use of stateof-the-art atomic oscillators on board each of the planned 24 satellites will allow highaccuracy calibrations. Second, its predicted popularity should make available a line of equipment that will be versatile and easy to use. However, since no firm operational date can be specified, GPS will not be covered in this chapter.

10.1 THE GOES SATELLITE TIME CODE

10.1.1 BACKGROUND

A satellite signal is no different than any other radio signal so far as the user is concerned. Table 10.1 gives the main specifications for GOES. Note that it broadcasts in the UHF band (which means small antennas are used), the system supports two satellites at this time, and the satellites are nearly stationary. That is, if you point the antenna to one part of the sky, you don't have to move it to keep a satellite in view.

The user should not be discouraged by the difference in frequency or the operating techniques needed for frequency and time calibrations using GOES. In fact, tests have shown that GOES is one of the easiest services to use and that it can be used in areas where other signals are nearly impossible to receive. The equipment provided by manufacturers is usually quite sophisticated and nearly automatic in its operation.

GOES is an operational descendant of NASA's Applications Technology Satellites.

that the user must have such a clock and maintain it with batteries, etc., in order to use the GOES signals.

There is nothing to prevent manufacturers from using frequency measurements directly and from building equipment to do this. However, GOES has a time-code output (perhaps its major application is for its time code) and since the manufacturers felt the largest number of users wanted the time code, they developed their equipment for time recovery. Frequency calibrations obtained using GOES time data fall into the accuracy range from 1 part per million to 1 part in 109 (for a 1-day measurement period) with respect to NBS, depending on the sophistication of the equipment used.

The path from the NBS master clock to the user via a geostationary satellite is, in effect, a line-of-sight path allowing the use of high carrier frequencies that are largely unaffected by the ionosphere and troposphere. This means negligible fading and path length variations which are so characteristic of terrestrial HF signals. The line-of-sight path also means that free space transit time computations will work for most timing applications.

The NBS master clock is located on the ground rather than in the satellite itself. The satellite is only a "bent pipe" or "transponder" used to relay signals. This allows for easy control and maintenance of the system, thus guaranteeing better performance and reliability.

Because of these advantages, the GOES satellite can provide a time message, repeated continuously, to clocks in its view. It can control the frequency rate of the slaved ground clock to eliminate the need for highquality oscillators and can also provide position data to correct for propagation delays.

10.1.2 COVERAGE

There are three GOES satellites in orbit, two in operational status with a third serving as an in-orbit spare. The two operational satellites are located at 135° and 75° West Longitude and the spare is at 105° West Longitude. The western GOES operates on 468.825 MHz; the eastern on 468.8375 MHz. The earth coverages of the two operational satellites 60 30

60

30

60

90

FIGURE 10.1.