[time-nuts] antennas was Re: Common-View GPS Network

bg at lysator.liu.se bg at lysator.liu.se
Thu Apr 18 20:40:01 UTC 2013


Magnus, Jim,

> On 04/18/2013 04:00 PM, Jim Lux wrote:
>> On 4/18/13 12:01 AM, Chris Albertson wrote:
>>> If I read the paper correctly you can skip the choke ring if you mount
>>> the
>>> antenna on top of a 2 meter or longer mast. Iron pipe comes on 10 foot
>>> lengths. The choke ring is for portable survey antenna that can't be
>>> placed on tall rooftop masts. I think a 2 meter pole on a roof pretty
>>> much meets their criteria of multi path difference being over 10
>>> meters.
>>
>>
>>
>> yes.. multipath that is more than a chip away is generally filtered out
>> by the PN tracking loop, so all you really worry about is multipath
>> signals within a chip. For C/A code at 1 Mchip/sec, the chips are 300
>> meters long. If you're doing P/Y code, it's a tenth of that.
>>
>> In reality, if the multipath signal is lower, and it's "far" away (a
>> good fraction of a chip) it doesn't contribute much to the output of the
>> correlator. So their 10 meter thing is probably a good number for a
>> "typical" receiver they make.
>
> It's an interesting mix of sample-rate/bandwidth, code you track and
> distance between early-late detectors (normal distance is one chip)
> comes in when analyzing and combat the multi-path. I recall there is
> some subtle points with some of the C/A codes.

All of the "high quality" GNSS receiver manufacturers have their own
version  of correlator that try to mitigate multipath. See for example
this Ashtech-document (for a ca 10 year old L1 only receiver (DG14/16)).

 ftp://ftp.ashtech.com/OEM,%20Sensor%20&%20ADU/DG16%20&%20DG14/Reference%20Material/Correlator.doc


/Björn




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