[time-nuts] NMEA time off

[Joseph Gray]

Months ago a bought an old Delorme Tripmate for $5 (for that price,
why not?). Tonight I finally got around to fooling with it. After
finding out that it was designed to talk only to Delorme software
without some finagling, I got it to spit out NMEA data to Teraterm and
other programs.

I discovered two things about the Tripmate. First, the sensitivity
sucks indoors. It is currently showing 13 satellites visible, but only
tracking three. Second, the time data from the GPRMC sentence is about
two seconds behind UTC (using my calibrated eyeball for measuring).

Is the time delay simply due to a firmware error, a low priority
processing and outputting the NMEA data, or something else?

I was going to attach the Tripmate to a modified Linksys WRT54G WiFi
router and make it an el-cheapo NTP server. No reason, other than just
for fun. Even without the two second delay, I know that the WRT54G
will not be a very accurate NTP server. I was assuming that I'd be
able to at least get better than one second accuracy, though. I guess
I can just put in a large fudge factor for ntpd to compensate.

Unrelated to the above, I also have a Timex Ironman runner's watch
with mating GPS unit (another $5 purchase). The GPS is worn on the arm
and transmits data to the watch for displaying distance, speed, etc. I
need to get a battery for the watch, but I believe everything works.
Taking the GPS unit apart, I find that it is a Garmin. I see some
interesting test points on the circuit board, so I'm going to see if I
can get some serial data directly from the GPS, without using the
watch.

Joe Gray
W5JG