[time-nuts] GPS multipath

[Magnus Danielson]

Jim,

On 08/09/2014 05:31 PM, Jim Lux wrote:
> Clarifying my previous question..
> There's no doubt that multipath exists, and how to test is fairly
> straightforward, whether with multiple antennas, cables, or waving
> cookie sheets around..
>
> What I was really asking is if anyone had observed this in the output of
> their GPS receiver.

Yes. Just look at the deviations when making position averaging.
After position lock, monitor the residue errors of the T-RAIM.

> That is, say you were watching the 1pps output and comparing its to your
> ensemble of active hydrogen masers.. As you place and remove the cookie
> sheet, do you see any (fraction of a nanosecond) change in 1pps?
> (unlikely, since I assume the 1pps has a fairly long time constant).
>
> Or more interesting, if you happened to have a GPS receiver that puts
> out raw observables of carrier or code phase, would you see a bump? Or
> if you were experimenting with your KF implementation, where you were
> comparing filter output (i.e. estimate of where it "should be") and
> tracking loop output (i.e. "where it is") would you see any discontinuity.

Watch the T-RAIM output if you can't get the carrier or code phase. The 
carrier phase will me much less sensitive in absolute time to multipath.

I've looked at the residue errors to some degree. The worst out-lines 
will be dropped out of the average building, but yes, it will creep 
through if you look at it. Also the residues from a T-bolt will 
illustrate this.

> Ultimately, the way to find out is just to get a GPS sampler, record
> some raw bits, and then run the correlator and look for the second peak
> from the reflection.

Quite possible, but... outside of a chip period, the Gold-code will 
relatively effectively de-correlate multi-path, so it is only within the 
chip period that multi-path will affect things.

> There's been a lot of discussion over the years about good and bad
> locations for the antenna, and how multipath is a big issue with getting
> very good timing performance.  I was wondering if someone had a
> practical anecdote of better or worse performance that could be
> attributed to something on the order of a square meter.  (position
> inaccuracies in urban canyons are a good example of multipath from
> hundreds of square meters)

Well, in urban canyons the problem isn't as much multi-path, but lack of 
visibility of birds that gives good geometry. You will typically see 
that the position moves widely along some corridors, which is indication 
of geometry problems, but multi-path can give similar issues, but it is 
about moving the apparent phase of the chip.

So, real life positioning or timing errors is really a mixture of the 
two effects. With good logging on good receivers, you can separate the 
effects.

Cheers,
Magnus

Cheers,
Magnus