[time-nuts] NMEA time off

[Hal Murray]

> I suspected it might have something to do with the almanac. The Tripmate has
> been on now for 100 minutes, but has not been able to track four satellites
> during that entire time. Does the GPS need to track at least four satellites
> continuously during the  12.5 minutes it takes to download the almanac? If
> so, then I'll probably need to place this thing outside tomorrow. 

I'm pretty sure it only needs one satellite to get the almanac.  The whole 
idea is that the almanac contains all the details about where the satellites 
are and you need that to compute the Doppler so you know what frequency to 
listen to.  It's sort of a chicken and egg problem.  If you start from 
scratch, you just hunt (in frequency) until you get lucky and find a 
satellite, then you get the almanac from it, then you can find the others.


> I discovered two things about the Tripmate. First, the sensitivity sucks
> indoors. It is currently showing 13 satellites visible, but only tracking
> three.

If you are indoors, I'm not surprised that an older unit is having troubles.


> Second, the time data from the GPRMC sentence is about two seconds behind
> UTC (using my calibrated eyeball for measuring).

Some units are off-by-one.  As long as it is consistent, you can fix it up in 
software.  I don't think I've seen any that are off by two, but it wouldn't 
surprise me.

Here is a hack I wrote when I was chasing that sort of glitch:
  http://www.megapathdsl.net/~hmurray/ntp/ntpd/timing.c
The first column is HHMMSS.sss (UTC from your local system) so it should be 
easy to see if your system time matches the time from a NMEA device.


-- 
These are my opinions, not necessarily my employer's.  I hate spam.