Global Positioning System

A programme made from three

The Global Positioning System came from several 1960s navigation programmes, merged into one under Colonel Bradford Parkinson, who was chosen in 1972 to direct the Department of Defense programme. Ivan Getting, who had argued for space-based navigation since the 1950s, shared the Charles Stark Draper Prize with Parkinson in 2003. The first Block I Navstar satellite launched on 22 February 1978.

Initial operational capability was declared on 8 December 1993, and full operational capability in 1995.

How it works

Each satellite, orbiting at about 20,200 kilometres, broadcasts its position and the time from onboard atomic clocks. A receiver measures the time delay from four or more satellites and solves for its position and its clock error. The system is therefore a clock before it is a map: its accuracy rests on the stability of the satellite clocks and on the ground segment that tracks and updates them.

The civil signal was degraded deliberately, by selective availability, until May 2000, when the US government ended the practice and civilians gained positions good to a few metres.

An observatory in disguise

GPS signals, opened to science, turned out to be a tool for geodesy, meteorology and space physics. Networks of receivers measure the motion of the ground to millimetres a year, the water vapour in the atmosphere from signal delays, and the electron content of the ionosphere. Timing from GPS keeps telephone networks, power grids and financial markets in step. The US Space Force operates the constellation, and Europe, Russia and China operate their own, which together form a family of global navigation satellite systems.

Opening a signal to the world

The decision to give the civil signal to everyone was made for reasons that included a tragedy in 1983, when a civilian airliner strayed into Soviet airspace, and for economic ones. The step changed what a navigation signal is. Disabling selective availability in 2000 improved civilian accuracy tenfold overnight, and an industry grew around it. At the same time, dependence on a signal that is weak at the Earth's surface raised questions about jamming and spoofing, and about the need for alternatives. A single point of dependence is a risk, and the continuing investment in other constellations and in terrestrial backups is the response.

Instruments and technologies

• Atomic clocks Rubidium and caesium clocks on each satellite, steered by the control segment to GPS time.
• Navigation signals Coded signals on L1, L2 and L5 frequencies, with civil and military codes, carrying orbit and clock data.
• Ground control and monitor stations Stations around the world track the satellites, so that the control centre can update their orbits and clocks.

The people

• Bradford W. Parkinson Director of the programme from 1972; Draper Prize 2003.
• Ivan A. Getting Early advocate of satellite navigation; Draper Prize 2003.

Timeline

• 1972 Bradford Parkinson is chosen to direct the DoD GPS programme.
• 1978 First Block I Navstar satellite launched on 22 February.
• 1993 Initial operational capability announced on 8 December.
• 1995 Full operational capability declared.
• 2003 Parkinson and Ivan Getting receive the Charles Stark Draper Prize for GPS.

Key lessons for observation today

• A signal designed for one use serves many GPS was a military system and became a civil utility and a science instrument. Open signal definitions allow uses the designers did not plan.
• Time is the product Positioning is derived from time. A good timing reference underlies sensing, networks and security.
• Redundancy through diversity Several independent constellations make navigation more robust. Observation networks should not depend on a single provider of time or position.

Sources

• GPS.gov, Space Segment
• Stanford News, Draper Prize 2003 to Parkinson and Getting
• ESA Navipedia, GPS
• HISTORY, Navstar 1 launches (22 February 1978)