[time-nuts] GPS multipath

Magnus Danielson magnus at rubidium.dyndns.org
Sat Aug 9 13:45:31 EDT 2014


Jim,

On 08/09/2014 07:02 PM, Jim Lux wrote:
> On 8/9/14, 9:33 AM, Magnus Danielson wrote:
>> 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..
>>>
>>>
>
>>> 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.
>>
>
> If you have a correlator process that gives you all the cross
> correlation bins, it's actually easier to see multipath that is greater
> than one chip away (e.g. 1 microsecond .. about 300 m, for C/A code)

Indeed. An efficient way to do that is to use the FFT correlation trick. 
If you only want narrow range, then you can have an additional 
correlator bin.

Another approach is to use the full sample-rate sample-stream of I&Q 
values after carrier frequency removal, and then to autocorrelation on 
that, and then subtract the expected autocorrelation, possibly with some 
shift. That would give you a fairly high resolution picture.

Feel tempted to try something like that.

Cheers,
Magnus



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