Satellite navigation

Lore & Background

The first satellite navigation system was Transit, deployed by the US military in the 1960s. Transit's operation was based on the Doppler effect: satellites traveled on well-known paths and broadcast signals on a well-known radio frequency.

The received frequency differed slightly from the broadcast frequency because of the satellite's movement relative to the receiver. By monitoring this frequency shift over a short time interval, the receiver could determine its location to one side or the other of the satellite. A team led by Harold L Jury of Pan Am Aerospace Division from 1970 to 1973 found solutions or corrections for many error sources, narrowing systematic and residual errors to accuracy sufficient for navigation.

Modern systems are more direct. The satellite broadcasts a signal containing orbital data and the precise time the signal was transmitted. Orbital data include a rough almanac for all satellites and a precise ephemeris for that satellite. The satellite uses an atomic clock to maintain synchronization.

The receiver compares the time of broadcast from three (at sea level) or four (allowing altitude calculation) different satellites, measuring time-of-flight to each. Each distance measurement places the receiver on a spherical shell centered on the broadcaster; by taking several such measurements and looking for a point where the shells meet, a fix is generated. The basic computation attempts to find the shortest directed line tangent to four oblate spherical shells centered on four satellites.

Reader's Guide

Satellite navigation systems are notable for providing Positioning, Navigation and Timing (PNT) services independently of any telephonic or internet reception, though those technologies can enhance the usefulness of the positioning information. The US Space Force's Global Positioning System was the first global satellite navigation system and the first to be provided as a free global service. Global coverage for each system is generally achieved by a satellite constellation of 18–30 medium Earth orbit satellites spread between several orbital planes. The systems use orbital inclinations of >50° and orbital periods of roughly twelve hours at an altitude of about 20000 km.

Satellite-based augmentation systems (SBAS) enhance the accuracy of global GNSS systems. These include Japan's QZSS, India's GAGAN, and the European EGNOS, all based on GPS.

GNSS systems that provide enhanced accuracy and integrity monitoring usable for civil navigation are classified into first-generation systems (combining existing satellite navigation systems with augmentation systems) and second-generation systems that independently provide a full civilian satellite navigation system, exemplified by the European Galileo positioning system. In recent years, GNSS systems have begun activating Lower L Band frequency sets for civilian use, featuring higher aggregate accuracy and fewer problems with signal reflection. As of late 2018, a few consumer-grade GNSS devices are being sold that use both Upper and Lower L Band frequencies, typically called 'Dual-band GNSS' or 'Dual-band GPS' devices.