GPS — The Day a Military Navigation System Opened Up Its Accuracy

GPS (the Global Positioning System) is a satellite navigation system owned by the United States and operated by the US Space Force. A receiver picks up signals from satellites overhead and calculates its own position and the time. Civilian use began on a limited basis with the first satellites in 1978, but the civil signal carried Selective Availability (SA), which deliberately reduced its accuracy. On 1 May 2000 President Bill Clinton announced that SA would stop at midnight that night, and civilian GPS became able to pinpoint locations up to ten times more accurately.

Three segments

According to GPS.gov, GPS has three segments: space, control and user. The space segment is the constellation of satellites broadcasting one-way signals that give their current position and time. The United States commits to keeping at least 24 operational satellites available 95% of the time, and the satellites circle the Earth twice a day in medium Earth orbit at about 20,200 km. They are arranged in six orbital planes of four slots each, which lets users see at least four satellites from virtually anywhere. GPS.gov reported 31 operational satellites as of 3 July 2023.

The control segment is a worldwide network of monitor and control stations that keep the satellites in their orbits and adjust their clocks. The user segment is the receiver, which turns the signals into three-dimensional position and time. The civilian service is freely available to all users, continuously and worldwide.

From the 1973 plan to full operation in 1995

According to an article by the GPS Directorate at Los Angeles Air Force Base, in 1973 the Navstar GPS Joint Program Office, headed by then-Colonel Bradford Parkinson, developed the GPS architecture and set out to field a prototype proving that space-based global navigation would work. The Air Force Historical Foundation records that the first Navstar satellite was launched on 22 February 1978 from Vandenberg Air Force Base in California on an Atlas F booster and became operational on 31 March.

The same Air Force article says that once the first four developmental Block I satellites were up in 1978, GPS began providing three-dimensional positioning, navigation and timing to military and civilian users on a limited but worldwide basis. The remaining seven Block I satellites were launched by 1985. The first operational Block II satellite went up on 14 February 1989, and on 8 December 1993, with 24 Block I and Block II/IIA satellites in orbit, Air Force Space Command declared initial operational capability (IOC). On 27 April 1995 it declared full operational capability (FOC), with 24 operational Block II/IIA satellites in their assigned slots.

1983: after the Korean airliner

The State Department's historical record notes that on 1 September 1983 a Soviet fighter shot down Korean Air Lines Flight 007, which had strayed into Soviet airspace near Sakhalin, killing all 269 people on board. On 16 September Deputy Press Secretary Larry Speakes read a statement saying that, as a contribution to preventing such a tragedy, the President had determined that the United States was prepared to make the facilities of its Global Positioning System available to civilian aircraft when it became operational in 1988, giving airliners three-dimensional position information.

The episode is often retold as Reagan making GPS free for civilian use, and English Wikipedia summarises it that way. The statement itself speaks of civilian aircraft and of 1988; the word "free" does not appear.

What Selective Availability was

GPS.gov describes SA as an intentional degradation of public GPS signals implemented for national security reasons. The White House fact sheet of 2000 explains that with SA in place, car navigation could locate a vehicle only to within about a hundred metres, which made it hard to tell which of several parallel highways a car was on.

In a Presidential Decision Directive of March 1996, Clinton had committed to ending the use of SA by 2006, with an annual assessment of its continued use starting in 2000. According to his statement, the decision to stop was based on a recommendation by the Secretary of Defense, coordinated with the Departments of State, Transportation and Commerce, the Director of Central Intelligence and others. The military had demonstrated the ability to deny GPS signals regionally when national security was threatened, so global degradation was no longer needed.

The announcement of 1 May 2000

In his statement of 1 May 2000 Clinton announced that the United States would stop the intentional degradation of the GPS signals available to the public beginning at midnight that night. Civilian users, he said, would be able to pinpoint locations up to ten times more accurately than before.

Times are from the GPS.gov FAQ and its first-week data.

The fact sheet expected the GPS market to double in three years, from $8 billion to over $16 billion. Beyond car navigation, it named Enhanced 911 location for emergency calls from mobile phones, hiking, camping and fishing. According to the GPS.gov FAQ, existing receivers got the higher accuracy without modification, and because SA could not be applied region by region, switching it off improved accuracy for the whole world at once.

The US government says it has no intent to use SA again. According to GPS.gov, in September 2007 it announced that the next generation, GPS III, would be procured without the SA feature, making the 2000 decision permanent.

When location became a given

More accurate GPS moved into devices. The self-driving cars of the 2005 DARPA Grand Challenge carried GPS antennas on their roofs, and the 2008 iPhone 3G built in GPS so the Maps app could follow the user's position as a dot. In 2016 Pokémon GO turned real-world location into the playing field.

Japan's Michibiki

Satellite positioning is not only American. Japan's Cabinet Office describes Michibiki, the Quasi-Zenith Satellite System (QZSS), as Japan's satellite positioning system built mainly around satellites in quasi-zenith orbits that pass almost directly over Japan, and notes that it is sometimes called the Japanese GPS. In cities and mountains, buildings and trees cut down the number of GPS satellites in view, and Michibiki fills that gap.

According to Mitsubishi Electric, the first satellite was launched on an H-IIA rocket on 11 September 2010 (it was retired on 15 September 2023). Units 2 to 4 joined in 2017 to form a four-satellite system, and from 1 November 2018 users could receive uninterrupted, round-the-clock service complementing and augmenting GPS. Unit 6 was launched on 2 February 2025, but unit 5, launched on 22 December 2025, failed to reach orbit when the second-stage engine of the eighth H3 rocket malfunctioned. No launch date has been set for unit 7.

For hardware generally, see the hardware timeline.