[time-nuts] Re: question about GPS time

[Bob Camp]

Hi

There are GPS modules out there that report GPS time rather than UTC. Typically there is a setting buried deep in the docs that controls this. The uBlox folks usually include this on their modules. Furuno does as well. 

If you want a packaged device that will do this and chat via ethernet, the Trimble NetRS is not a bad candidate. There are other Trimble devices that also will "do GPS time”. 

Bob

> On Mar 28, 2024, at 2:40 PM, David Bridgham via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> On 3/28/24 2:03 PM, Tom Van Baak via time-nuts wrote:
> 
>> If a second needs to be added to UTC it is named 23:59:60
> 
> 
> This brings to mind a question I've been meaning to ask one of these days.  I guess today is a good day.
> 
> I was contemplating building a system that would keep its internal time in TAI.  That makes more sense to me than jumping the internal time around to keep up with leap seconds; that conversion, if needed, can be done in a UI library.
> 
> In support of this, I was looking over GPS receiver data-sheets to see what I had to work with.  The GPSes I found all liked to report times in UTC, rather than TAI or GPS time.  Hmmm.  But one thing I noticed was that the seconds field of that UTC time report was defined to be [00..59].  Uhhh, so what do they do when a leap second comes along?  Does the GPS receiver double up with :59 or maybe it rolls over to :00 and doubles that one up?  Neither of those are very good answers.  Or maybe the documentation is just wrong and the receiver actually does the right thing and report :60.  I suppose documenting it wrong is better than doing it wrong.
> 
> So while I'm curious about that, my real question is whether there's a way to get GPS time out of a GPS receiver.  Or, lacking that, is there a way to reliably get the information out of a GPS receiver as to what leap-second offset it's currently using to calculate the UTC that it's reporting.  Yeah, I *should* be able to figure that out with my own leap-second database but how can I be sure that the GPS is really using the same list of leap-seconds that I have?  Better if it just tells me.  Better still if I could get GPS time without the leap-second offset applied.
> 
> Thanks,
> Dave
> 
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