GPS — The Constellation That Geolocates the World

On 17 July 1995, GPS reached full operational capability with 24 satellites. The civil specification was 100 m or less of horizontal error for 95% of the time, under its stated conditions. A 2019 RTI/NIST study puts the cumulative benefits to the US private sector at $1.4 trillion since GPS was opened to civilian use in the 1980s.

Discovery

Parameter	Value
Program	NAVSTAR GPS, US Department of Defense
Program start	1973 (merger of Navy TIMATION and Air Force 621B projects)
First satellite	Navstar 1 (Block I), February 22, 1978 — preceded by the US Navy NTS-2 demonstrator, June 23, 1977
Full operational capability (FOC)	July 17, 1995
Nominal constellation	24 satellites (6 orbital planes × 4 satellites)
Orbital altitude	20,200 km (MEO orbit, semi-synchronous)
Orbital period	11 h 58 min (2 orbits/sidereal day)
Civilian frequencies	L1: 1,575.42 MHz · L2: 1,227.60 MHz · L5: 1,176.45 MHz
Onboard clock	Cesium (precision ~10⁻¹³) and rubidium (10⁻¹²)

Technical Explanation

1. Signal timing — The receiver compares the incoming pseudorandom code with a local copy. A 1 ns timing difference corresponds to about 30 cm in pseudorange.

2. Position solution — With suitable geometry, four satellite measurements normally provide the four equations needed to estimate three coordinates and the receiver clock bias.

3. Relativistic corrections — Special relativity slows the satellite clocks by −7.2 μs/day (orbital velocity of 3.87 km/s). General relativity speeds them up by +45.8 μs/day (weaker gravitational field at 20,200 km).

4. Control segment — The ground network includes a master control station (Schriever Space Force Base, Colorado), 11 command antennas, and 16 monitoring stations that continuously track the satellites, calculate precise ephemerides, and upload clock corrections every 2 hours.

Why It Worked

GPS solves a problem that seemed intractable: nanosecond-level time synchronization between clocks separated by 20,200 km, moving at 3.87 km/s, in a different gravitational field. The solution rests on two pillars: onboard atomic clocks (cesium and rubidium) with stability exceeding 10⁻¹³, and the operational application of Einstein's general relativity — the first civilian technology requiring Einstein's physics to work correctly.

Ending Selective Availability in May 2000 improved the expected accuracy of the basic civil service and helped broaden its uses.

Causal Chain

Transit (first navigation satellite, US Navy, 1960) → miniaturized atomic clocks (1960s) → NAVSTAR program initiated by DoD (1973) → US Navy NTS-2 demonstrator with cesium clock (1977) → first Block I satellite, Navstar 1 (February 22, 1978) → Selective Availability removed (May 1, 2000) → smartphone integration (iPhone 3G, 2008) → GPS III with L1C signal interoperable with Galileo (2018) → centimeter-level RTK positioning in real time (2020s)

Anecdote

On September 1, 1983, Korean Air Lines Flight 007 is shot down by the USSR after straying into Soviet airspace, killing all 269 passengers. President Reagan then announces that GPS, until then exclusively military, will be made freely available to civilians upon completion, to prevent such tragedies from recurring. This political decision transformed a military tool into a global public good.

Pseudorange and position

A pseudorange is the apparent satellite-to-receiver distance obtained by multiplying a signal timing measurement by the speed of light. It includes satellite and receiver clock biases and propagation delays. In a nominal GPS orbit the net relativistic clock rate difference is about +38.6 μs per day relative to the ground reference. After one day without that rate correction, c × 38.6 μs is about 11.6 km of equivalent pseudorange bias. It does not give a daily position error. The receiver estimates its own clock bias together with its three position coordinates. In the ideal model a bias common to all measurements can be absorbed by the clock term; the position effect of unequal errors depends on the satellite geometry.

Civil GPS in 1995

In 1995 the US civil Standard Positioning Service (SPS) specified horizontal position error of 100 m or less for 95% of the time over a 24-hour interval at any point on Earth, subject to the specified coverage, availability and reliability conditions. Selective Availability intentionally degraded the civil signal and was discontinued in May 2000.

Sources

References verified during the August 2026 fact-checking audit: these are the pages
against which this bulletin's claims were checked.

References

Editorial note

Correction of 9 November 2026: clarified the 1995 civil accuracy specification and separated pseudorange bias from position error. This correction and its translations were prepared with AI assistance. The cover is a conceptual illustration generated with AI; it does not depict an identified spacecraft.