[time-nuts] NMEA time off

[Said Jackson]

Hi Joe,

One way to prevent this issue is to make sure you have a good battery in the unit to keep an almanac copy in SRAM..

Try putting the unit outside with a clear sky view.

Btw: the error usually depends on when the gps firmware was compiled, as the manufacturer hard-codes the number of leap seconds at that time, and any additional leap seconds will cause the error you are seeing. So a "good" gps may go "bad" when the next leap second happens.. Units with good backup battery usually shouldn't have this problem.
Said

Sent from my iPad

On Mar 3, 2011, at 1:57, Joseph Gray <jgray at zianet.com> wrote:

> Said,
> 
> 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.
> 
> It's well past my bedtime, so I'll say goodnight.
> 
> Joe Gray
> W5JG
> 
> On Thu, Mar 3, 2011 at 2:24 AM, Said Jackson <saidjack at aol.com> wrote:
>> Hello Joe,
>> 
>> The gps probably has not finished receiving the entire almanac, and is "guessing" the number of leapseconds. The offset may go away after the almanac has been received.
>> 
>> This can take up to 12.5 minutes.
>> 
>> It is a design flaw in the gps spec in my opinion, they could have transmitted the number of leapseconds alongside the gps-time/date.
>> 
>> Bye, Said
>> 
>> Sent From iPhone
>> 
>> On Mar 3, 2011, at 1:06, Joseph Gray <jgray at zianet.com> wrote:
>> 
>>> 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
>>> 
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