Time and Frequency Measurements Using the Global Positioning System

GPS, well known as a versatile, global tool for positioning, has also become the primary system for distributing time and frequency. GPS receivers are fixtures in telecommunication networks, and calibration and testing laboratories. They make it possible to synchronize clocks and calibrate and control oscillators in any facility that can place an antenna outdoors for lineof-sight reception of the GPS satellites. The GPS satellites are controlled and operated by the United States Department of Defense (USDOD). The constellation includes at least 24 satellites that orbit the earth at a height of 20,200 km in six fixed planes inclined 55° from the equator. The orbital period is 11 h 58 min, which means that a satellite will orbit the earth twice per day. By processing signals received from the satellites, a GPS receiver can determine its position with an uncertainty of < 10 m. The GPS satellites broadcast on two carrier frequencies: L1 at 1575.42 MHz, and L2 at 1227.6 MHz. Each satellite broadcasts a spread-spectrum waveform, called a pseudorandom noise (PRN) code on L1 and L2, and each satellite is identified by the PRN code it transmits. There are two types of PRN codes. The first type is a coarse acquisition (C/A) code with a chip rate of 1023 chips per millisecond. The second type is a precision (P) code with a chip rate of 10230 chips per millisecond. The C/A code is broadcast on L1, and the P code is broadcast on both L1 and L2. GPS reception is line-of-sight, which means that the antenna must have a clear view of the sky. If a clear sky view is available, the signals can be received nearly anywhere on earth. Each satellite carries either rubidium or cesium oscillators, or a combination of both. The on-board oscillators provide the reference for both the carrier and Time and Frequency Measurements Using the Global Positioning System