5 ms·
A ground-based transmitter could be destroyed in an attack, so having the clocks on orbit meant they were protected and would still be operational for a retalia
by chiph 11y ago
A ground-based transmitter could be destroyed in an attack, so having the clocks on orbit meant they were protected and would still be operational for a retaliatory ICBM launch.
Edit: So far as latency - the on-board clocks get resynced today every now and again against a ground-based reference time as they pass overhead of a USAF transmitter (primary + backup). If they used only moderately-accurate clocks as a cost-saving measure (or were at the limit of late 1970's technology) drift would be more severe and the tail-end of each orbit would be less correct than at the start of the orbit. So additional ground stations would be required in parts of the planet not always so friendly to the US, and they would have to coordinate their time sources, allowing for the light-speed delay travelling around the earth. Much more expensive/harder solution from an operational aspect.
Edit Edit: Don't forget that the GPS system was designed around military requirements. All the satellites (even today) have nuclear weapon detonation detectors on board and are a primary input to NORAD.
> The 2nd SOPS has three missions: global navigation, time transfer and nuclear detection.
http://www.schriever.af.mil/GPS/ http://www.schriever.af.mil/GPS/
- madaxe_again 11y agoTotally get it in the military context, and why it's useful in the context of a missile lobbing holiday, but was rather more curious as to why it'd be used as a time source for synchronising packets in commercial applications as the article describes. Based on what others have said and my own inkling I think it's about synchronisation at a fine timescale without having to trouble yourself with bothersome shit like relativity, and leaving that particular pain in the ass to the US military. That's what I meant by latency, but phone keyboard, walking, lazy!
- chiph 11y agoWhat it is is outsourcing your time source to the USAF. Aka introducing an external dependency. If these people are upset over 13 microseconds, wait until the DoD turn selective availability back on during a war or start adjusting satellite positions to avoid incoming ASAT missiles. Interesting fact: GPS time doesn't take leap seconds, etc. into account. As of last summer, it's ahead of UTC by 17 seconds. http://tycho.usno.navy.mil/leapsec.html http://tycho.usno.navy.mil/leapsec.html
- kylegordon 11y agoThere is an executive order to keep SA at zero. They have other means to deny GPS availability now, and it doesn't involve futzing global availability.