[teqc] MultiPath with L1 GPS receiver

[James Johnson]

Dear Hannan,

Teqc requires dual frequency data in order to a multipath estimate.  
This is because teqc looks at a long term difference between the phase 
and code measurements while removing the ionosphere.  In principle, you 
can do this with single frequency data over a short period of time, but 
after some time the L1 phase and C/A measurements will diverge due to 
the ionosphere (phase advance on L1 and group delay on C/A).  What you 
want to look at is a running average of C/A - L1*lambda where C/A is 
the pseudo range measurement, L1 is the phase measurement in cycles, 
and lambda is the wavelength of L1.  If you use a running average of a 
10 min, you should be able to see the multipath signal, an you could 
look at the RMS of this value (with steps removed caused by possible 
cycle slips).  Unfortunately teqc does not do this.

I think you will find quite a bit of multipath with long wavelengths 
(2-5 min), especially if you are using a cheap patch antenna.

Hope this helps,
-Jim

On Feb 3, 2005, at 5:53 PM, Hannan Sadar wrote:

> Hi,
>
> I have what is consider low cost GPS receiver and i would like to find
> the multipath i was having on my measurements. I have psuedorange,
> phase and Doppler of L1. I would to know if teqc could find the
> multipath on of the measurements of teqc needs P1 data to do some
> averaging if I remember correctly.
>
> Thanks
>
> Hannan
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