They are very economical to implement but suffer from variable propagation delays and signal fading. They are good choices in areas of strong reception and if the lower accuracy is acceptable. GOES, Geostationary Operational Environmental Satellite, is widely used as a time system. Unfortunately there is no commitment on the part of the Government to maintain these satellites as a source of UTC time and their future, especially as GPS becomes more fully operational, is in doubt. Range Telemetry Receivers are used on large test ranges.
Users then can obtain time with a telemetry receiver equipped to distribute the broadcast time code.
For the remainder of this paper it will be assumed that UTC time is being acquired with a GPS receiver. That is where time code comes in. In the early 1950's the expansion of Government test ranges and their increasing time accuracy requirements made it necessary to develop machine readable means of transmitting time and synchronization signals within ranges. Figure 2 illustrates a frame of these IRIG A, B, or G time codes. One very desirable feature of time code is its ability to be recorded on analog magnetic tape recorders along with analog test data.