From davem@zeppo.cs.ubc.ca Fri Jan  1 08:30:31 PST 1999
Article: 86733 of sci.geo.satellite-nav
From: davem@zeppo.cs.ubc.ca (Dave Martindale)
Newsgroups: sci.geo.satellite-nav
Subject: Leap second observations: GPS 25LP and 12XL
Date: 31 Dec 1998 19:11:59 -0800
Organization: Computer Science, University of B.C., Vancouver, B.C., Canada
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I had my GPS 25LP board receiver and GPS 12XL connected during the leap
second that happened this afternoon.

The 25LP handled the leap second exactly, as expected.  Here is the
NMEA output from that second:

$GPRMC,235960,A,4913.2303,N,12305.9333,W,000.0,169.8,311298,019.6,E*65
$GPGGA,235960,4913.2303,N,12305.9333,W,1,04,3.1,105.0,M,-17.6,M,,*76
$GPGSA,A,3,02,,07,,,,26,27,,,,,4.1,3.1,1.0*30
$GPGSV,2,1,08,02,60,078,48,04,02,161,,07,69,175,50,09,27,305,*71
$GPGSV,2,2,08,15,22,051,,21,01,344,,26,37,253,40,27,25,140,29*7C

I was setting up a second computer to capture the NMEA output of the
12XL.  Unfortunately, it was ready about 20 seconds too late!  Because
of setting it up, I wasn't watching the 12XL at midnight UTC.  However,
based on observations 18 months ago, I think it just keeps counting
without adding in the leap second.

In addition, after the leap second happened, the displayed time on
the 12XL's LCD was about half a second *early*, which I don't think
I've ever seen in normal operation.  This strongly suggests that the
display is merely half a second behind a time calculated with the
previous value of the GPS-UTC time offset.

I left the unit this way for 2 hours - plenty long enough for it to
collect a new almanac, with a new GPS-UTC offset value.  The displayed
time remained a half second fast.  Then I turned the 12XL off and
back on again.  As soon as it had started navigating (a few seconds),
the displayed time was about half a second late, as usual.  It did
this immediately, almost certainly picking the new offset out of
a saved almanac - there wasn't time for it to have obtained a new
value after turnon unless it was extremely lucky.

This says to me that the 12XL does not know how to deal with the
event of a leap second happening.  Further it uses a UTC time offset
that was obtained when the unit was first turned on, even when that
value no longer matches the current almanac which has been collected
in the unit's memory.

	Dave