[time-nuts] Best Chance GPS module

MLewis mlewis000 at rogers.com
Sun Dec 4 06:57:50 EST 2016

On 03/12/2016 12:33 PM, Bob Camp wrote:
> For an “antenna challenged” location, the T is the better choice. It is simply an update (as the M8Q) of the earlier uBlox parts. The function is very similar to the earlier parts. You nail down the antenna location (like with duct tape) and put the module in survey mode. Eventually it completes the survey and you save that location. That location is then used as part of the fixed location setup. This eliminates any need to ever do a survey again. The reason you want the fixed location operating mode is that it will work with a single satellite. You don’t need the accuracy, but you do want to eliminate dropouts.
Thank you!

That is exactly what I "thought" I was getting from reading various 
sources, but I wasn't confident I was getting it right.

I think:
- There's GPS Time and with the offset (currently 17 seconds?) in the 
GPS location message we get UTC Time.
- The GPS module receives the satellite's GPST, offset, and hence UTC 
Time, and its location fix allows for correcting for the message delay.
- A better fix, and that correction is more accurate.
- With a number of satellites reporting, that correction is more accurate.

If the module has a good fix and a good solution from several satellites 
and is providing me with a local UTC Time that is close to true UTC, but 
then conditions degrade to a single satellite:
- will the module be able to maintain a quality correction to true UTC 
- or perhaps I should be asking: what is the precision from a GPS 
module's best time solution vs. that from a single satellite.
- or is that irrelevant given my rather trivial goal of 1 ms precision?

> The module should be set to only put out a PPS when it has a valid timing solution. You very much do*not*  want it to simply put one out that is based on the internal oscillator on the module.
> Bob

And here I was thinking that internal oscillator was going to save me by 
continuing to provide PPS from a recently disciplined TCXO.

If I have a good sat solution and am getting PPS for some hours, then it 
loses all satellites,
     - is the internal TCXO sufficient to trust for a number of minutes?
     - if so, how would I determine how much error for how many minutes?
     - or do I just have to set an alarm, let NTP fall back to the 
internet hosts, and monitor my data clusters for drift?

I'd guess this is related to the Best Practices I saw that said that for 
time critical applications, as a minimum: use a local GPS time source, 
but have NTP polling four or more NTP hosts (non GPS based) through the 
internet as backup if the GPS system goes out.
Or in my case, simply if I can't track a sat.



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