RE: [LEAPSECS] RAS hits the news

From: Hornaday, Tem SPAWAR <tem.hornaday_at_NAVY.MIL>
Date: Mon, 26 Sep 2005 09:46:41 -0700

Regarding GPS receiver date determination:

1. The GPS navigation message is 12.5 minutes long. A receiver should
resolve UTC correctly within 12.5 minutes. See ICD-GPS-200 (publicly
released).

2. Virtually all receivers can correctly resolve date (and will do so
quickly), given an initialization "seed" date that is within about +/-
512 weeks of true date. The receiver will adjust date forward or
backward based on this seed date, the 10-bit GPS Week Number (WN), and
GPS time-of-week (TOW) (i.e., day of week) transmitted by the
satellites.

3. As has been pointed out, some receivers also implement a clever hack
to determine date that looks at UTC Leap Second (LS) value, and chooses
a date based on WN, TOW, and LS. That is, the receiver implements a
sliding 1024-week window whose limits are determined by the current
value of LS. Current date "will" then reside within this 1024-week
window.

4. A receiver may be at risk for needing a software update if its
authors picked too small a value to associate with each LS increment,
and if Mother Nature doesn't cooperate and slow the earth's rotation at
something close to the mean rate. I note that the interval between the
last LS increment and the next one will be 7 years.

--Tem
Received on Mon Sep 26 2005 - 09:56:53 PDT