Introduction

Abstract

The Global Positioning System was designed and built, and is operated and maintained by the U.S. Department of Defence (c.f., e.g., Parkinson and Spilker 1996). The first GPS satellite was launched in 1978, and the system was fully operational in the mid-1990s. The GPS constellation consists of 24 satellites in six orbital planes with four satellites in each plane. The ascending nodes of the orbital planes are equally spaced by 60 degrees. The orbital planes are inclined 55 degrees. Each GPS satellite is in a nearly circular orbit with a semi-major axis of 26 578 km and a period of about twelve hours. The satellites continuously orient themselves to ensure that their solar panels stay pointed towards the Sun, and their antennas point toward the Earth. Each satellite carries four atomic clocks, is the size of a car and weighs about 1 000 kg. The longterm frequency stability of the clocks reaches better than a few parts of 10^-13 over a day (cf. Scherrer 1985). The atomic clocks aboard the satellite produce the fundamental L-band frequency, 10.23 MHz.