[amsat-bb] More on GPS time (was Re: Accuracy?)
Fri Aug 25 13:36:44 PDT 2006
According to the US Naval Observatory web site and gpsinformation.net, the leap second offset is contained in the NAV message sent every 12.5 minutes, along with 2 correction values (time difference and rate of system time change referenced to the USNO master clock), allowing most current generation time receivers and software to calculate UTC to within better than 40 nanoseconds of the master clock. GPS time itself is automatically "steered" to USNO UTC on a daily basis to keep system time within 1 microsecond, but has generally been "within a few hundred nanoseconds" over the last several years, since selective availability was turned off. No doubt this is how my Motorola Oncore software supplies accurate UTC time info.
>>From: Ray McKnight <firstname.lastname@example.org>
>>Sent: Aug 25, 2006 1:16 AM
>>To: George Henry <email@example.com>
>>Subject: Re: [amsat-bb] Re: Accuracy?
>>You are aware that GPS time and "earth" time are two different critters?
>>The GPS constellation is not adjusted for leap seconds, so the time your
>>GPS receiver displays is something like 13-14 seconds off actual earth time.
>>Could make for some big problems trying to track a highly eliptical orbit
>>or even LEO with that much error.
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