[LEAPSECS] Crunching Bulletin B numbers (POSIX time)

[Keith Winstein]

On Tue, 22 Feb 2011, Tim Shepard wrote:


>> Having a known list of leap seconds let one recover TAI time from a cold

>> GPS receiver in a few seconds to a minute, rather than waiting ~20

>> minutes for the almanac to download.  Many systems have their internal

>

> I thought GPS time had a constant offset from TAI of 19 seconds.  So

> as soon as you have a GPS solution, you can find TAI.  No table of

> leap seconds needed to accomplish that, as far as I know.


Hi Tim,

If you just need GPS-CC time, you're right. Adding 19 seconds gets you 
within 1 microsecond of "TAI(USNO,MC)".

But if you need UTC(USNO) or "TAI(USNO)" to better than that, you need the 
UTC steering parameters A0 and A1. Those only come around as often as the 
leap second delta, meaning you still have to wait up to 12.5 minutes on a 
cold start, after which you can spit out UTC as easily as you can TAI.

-Keith