[time-nuts] GPS multipath

[Tom Miller]

Don't forget that the polarity of the reflection will reverse. RCP > LCP. 
and a >20 dB loss will occur.

Tom



----- Original Message ----- 
From: "Jim Lux" <jimlux at earthlink.net>
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
Sent: Saturday, August 09, 2014 11:31 AM
Subject: [time-nuts] GPS multipath


> 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.
>
> 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.
>
> 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.
>
> 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)
>
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