[time-nuts] tracking position & orientation

[Tim Lister]

On Fri, Nov 22, 2019 at 9:01 AM Bob kb8tq <kb8tq at n1k.org> wrote:
>
> Hi
>
> At least right now, it is the “king of the hill” in terms of low cost modules that
> will do L1 / L2.
>
> If the target application is precision survey work, you do want (at least) dual
> band reception. All of the post processing outfits are looking for that sort of data.
>
> I do not know of any free sites that will post process “L1 only” data. Without
> post processing, you will not get mm level results.

The NRC Precise Point Positioning service
(https://webapp.geod.nrcan.gc.ca/geod/tools-outils/ppp.php?locale=en)
will do L1 only but it seems to be the only one. Have used it
successfully with ublox6 L1-only data and after waiting the 12-18 days
for the Final GPS orbits to be available and resubmitting, it did
improve the residuals considerably. It still won't be to the mm-level
that you would get from dual frequency L1/L2 data of course.

You probably don't need to wait that many days for the Final GPS
satellite orbits; the IGS products page
(https://www.igs.org/products)shows that even the real-time
ultra-rapid products are a factor of 20x better in the orbit (but
interestingly not the satellite clocks) than the broadcast ephemeris.
But if you are trying to find small drifts over a year, rather than
say doing real-time RTK corrections, you have the luxury of time to
wait for the absolute best orbits before doing the processing.

>
> Indeed with any of these numbers you can wonder what the errors might really
> be. If the claim is 5 mm, could it really be 8 mm? sure it could. If I average L1
> data for a few days and think I’m at 500 mm could I really be over 800 mm off?
> Yes, and I do have data on that.
>
> If you are after things like continental drift or wander of a post in the ground,
> you are after mm level numbers.
>
> Bob
>

Cheers,
Tim