What is GPS?
The Global positioning System (GPS), operated by the USDoD, is comprised of a total of 24 satellites orbiting round Earth in 6 different planes. There are 4 satellites in each plane, each at 20,200 km altitude and 55 degree inclinations (see Figure 1).
A GPS receiver receives at least 4 signals from 4 satellites. The receiver can then resolve it’s exact position as well as its exact time from the time of arrival of the 4 signals (see Figure 2). A GPS receiver can provide global, all-weather, 24-hour, real-time, accurate navigation and time reference to an unlimited number of users. However, GPS receivers are sensitive to external effects, such as extreme weather, intentional or unintentional jamming. In addition the U.S. government has the capability of blocking the signal in designated areas.
A GPS receiver outputs four-dimensional data, which are comprised of location (position) and time. Part of the time signal is a 1 pulse per second (1 PPS) signal that may be considered to be an accurate 1 Hz frequency source. The short-term stability of the 1 PPS is not very good due to its relatively high jitter. For example, a jitter of 400 nanoseconds translates into a short-term stability of 4E-7 at an averaging time of 1 second, which is rather poor. However, in the long term, the stability of the 1 PPS tracks the excellent stability of the GPS system (which is comprised of atomic clocks located aboard satellites and in ground stations). For more information on GPS see http://tycho.usno.navy.mil/.
GPS Synchronized Rubidium Clocks
In order to solve the problems of interruption in the GPS reception and the jittering of the GPS signal, one may use the following technique. A Rubidium Clock is locked via a phase lock loop (PLL) to the 1 PPS signal coming from the GPS receiver. See Figure 3 for a diagram of the PLL. GPS Synchronized Rubidium Clocks combine the superb medium and short-term stability of the RFS with the excellent long term stability of the GPS receiver. In addition, if the GPS reception is interrupted for a short or long period of time, the Rubidium Standard continues to maintain accurate frequency and time.