THE TIME AND FREQUENCY CONTROL SUBSYSTEM ONBOARD NAVSTAR/GPS

Abstract

The commitment of the Global Positioning System (GPS) to use atomic clocks dates back more than ten years ago when the first major contractual commitment was made with the award of the first space vehicle procurement contract to Rockwell International in 1973. The primary purpose of atomic clocks was, and still is, to provide autonomy of time and frequency to the individual spacecraft. The objective of the atomic clock was, and still is, the assurance of time accuracy to the nanosecond level. This implies a positioning accuracy of the order of feet. We note that timing accuracy and positioning accuracy are related via the speed of light; thus, 1 ns time accuracy translates to 1 foot positioning accuracy. The role of the atomic clock is to preserve this accuracy autonomously out to many days in case uploading should be impossible or undesirable. Thus, because 10 ns per day equals 10¯¹³ fractional frequency stability and 10 ns per week represents parts in 10¹⁴, atomic clock stabilities of 10¯¹³ and better for periods of days are needed for GPS. Because of the state-of-the-art prevalent in the early seventies, rubidium devices were chosen for the first GPS satellites (Navstar 1 thru Navstar 4). At the same time, FTS went under contract to develop a space-qualified cesium device. Navstar 5 thru Navstar 12 will carry a complement of three rubidium and one cesium clock. Starting with Navstar 13, the Block II GPS Satellite Time/Frequency Subsystem will consist of two rubidium and two cesium clocks. The rubidium devices are produced by the Autonetics Division of Rockwell International using an Efratom physics package. The cesium clocks are produced by Frequency and Time Systems, Inc. The need for a total of four clocks per satellite results from the systems planner’s risk assessment focussing on the assurance of in excess of 7 1/2 years of “in-spec” life for this basic subsystem of the Navstar satellite.