Why GPS Automatically Shuts Itself Down Above 1,200 mph

Summary

• GPS’s speed and altitude limits are intentional, not technical necessities. The video says GPS could theoretically work at 1,200 mph and 60,000 feet. Many receivers instead shut down at those thresholds because of restrictions intended to prevent GPS equipment from being used to guide ballistic missiles.

• GPS began as a military navigation system.

  • In the early 1970s, the U.S. military wanted a more reliable way to locate its nuclear submarines.
  • An earlier satellite-navigation system, Transit, used only a small number of satellites. Submarines often had access to just one satellite at a time, making position estimates possible but not very reliable.
  • The military developed Navstar GPS, with more satellites and improved equipment, so receivers could usually use signals from at least four satellites.

• GPS was initially kept secret. The system was developed for military purposes, where knowing the locations of submarines, missiles, and targets had obvious security implications. For its first year, GPS was classified and unavailable to the public.

• A fatal aviation accident helped lead to civilian access.

  • In 1983, Korean Air Flight 007 strayed into Soviet airspace and was shot down.
  • Two weeks later, President Ronald Reagan opened GPS to civilian use. The video presents this as a response to the potential safety benefits of civilian navigation, while noting that the Soviet Union already had its own system, GLONASS.

• Civilian GPS initially had two security restrictions.

  • Its signal was deliberately made less accurate—about 300 feet (100 meters)—so it would be less useful for precise targeting. The video says President Bill Clinton ended this restriction in 2000.
  • Exportable GPS receivers were required to shut down above specified speed and altitude limits if they might be used on ballistic missiles. The video says limits vary by device: some trigger if either threshold is crossed, while others require both.

• The restrictions affect few people today. They generally do not interfere with ordinary users but can affect hobbyists who send high-altitude balloons. Since the limits cause little inconvenience to most users, the video suggests there has been little incentive to remove them.

• The video ends with a sponsor message promoting Brilliant, an online learning service offering interactive courses in STEM topics.

Speakers and Sources Featured

• Narrator: The video’s Half as Interesting voiceover; no individual speaker is identified in the subtitles.
• Brilliant: Featured as the sponsor in the closing advertisement.
• Wikipedia: Referenced in the narration as the source of a briefly shown article about the physics and mathematics behind Transit’s one-satellite position estimates.