[time-nuts] NMEA time off

Joseph Gray jgray at zianet.com
Thu Mar 3 09:57:41 UTC 2011


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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