Hacker News Reader

Comment by friedgil | Hacker News Reader

> Small digression: did you know that, until May 2000, GPS satellites (which are owned and operated by the United States Space Force) provided the general public a signal with intentional error built into it?

What the hell? Why?

View on HN

They wanted to keep accurate global positioning as a US military exclusive capability. It's definitely useful for guided munitions, & alternative satellite positioning systems didn't exist or were less mature at the time, so US GPS was the only system one could realistically use for that. A missile able to hit a target within a 3 meter radius is vastly more effective than one that can only hit within 100m, for instance.

There are still some restrictions around this sort of thing: IIRC a GPS receiver for sale to the public isn't allowed to give accurate data if it's too high up &/or moving too fast, to prevent unauthorized usage in ICBMs & other similar weapons. I think there would be a lot of red tape involved if you wanted to buy an unrestricted GPS device without this limitation.

Which is idiotic, because nowadays it is trivial to build your own GPS received compared to the complexity of building a reliable ICBM.

But they said 2000.

[] There are still some restrictions around this sort of thing..

Error was built in to reduce precision and make it harder to use them for targeting. It stopped being useful once there were other constellations available.

GPS receivers sold to public also required to not operate at certain altitude/speed to prevent it from being used in ballistic (and probably other kinds?) missiles.

GPS started as a U.S. Department of Defense project, and they had qualms about freely giving the high accuracy positioning information they found so very useful for e.g. targeting bombs and missiles, to every unverified third party in the world. Depending on your preferred flavor of jadedness, one could say it was because of security concerns... or one could say it was because said third parties hadn't paid off the military industrial complex enough!

It was to degrade accuracy. Military (and presumably other gov't and allied gov't owned systems) were able to get more accurate signals. The degraded signals meant that someone couldn't use commercial GPS as a guidance system or for similar applications.

https://www.gps.gov/selective-availability https://en.wikipedia.org/wiki/Error_analysis_for_the_Global_...

Something something Yujio Hanma

Reply on news.ycombinator.com