[time-nuts] Sub mm measurements with gps timing antennas?

Said Jackson saidjack at aol.com
Wed Apr 25 16:37:12 UTC 2012


Hi Attila,

I agree, keep the gps away from such fast and large temperature excursions. The internal tcxo is not as stable as one would expect.. Building a larger box and burying it could help slow the temp gradient..

Bye,
Said

Sent From iPhone

On Apr 25, 2012, at 8:52, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:

> Hi Attila,
> 
> On 04/25/2012 04:56 PM, Attila Kinali wrote:
>> Moin,
>> 
>> We've a customer who does sub mm measurements using GPS in alpine
>> enviroment. This is done using LEA-6T modules, logging of raw phase
>> data and offline post processing using long averaging windows.
>> 
>> Now, the customer had some problems reaching the precision requirements
>> and i'm wondering whether one of the causes might be the use of a
>> Trimble Bullet antenna [1] (3V type) and not a geodetic antenna. Can it be
>> that the phase center of the Trimble Bullet antenna isn't as well defined
>> as it should be for this application? Would the addional gain of the 5V
>> version help (35dB instead of 30dB)? Or should we evaluate a different
>> antenna all together.
>> 
>> A major restriction in this application is that there is a very harsh
>> enviroment, temperature wise. We have measured -40°C to +30°C jumps in
>> just 2h. Most of the time the devices are below freezing temperature,
>> but can go up to 50°C when in direct sunlight.
>> 
>> The next big restriction is, that this is a research project. So
>> there isn't as much money available as there should be to "do it right".
>> 
>> So, if someone could give a few tips how to improve things, this would
>> be much appreciated.
>> 
>>            Attila Kinali
>> 
>> 
>> [1] http://www.trimble.com/timing/bullet-gps-antenna.aspx?dtID=overview&
>> 
> 
> You can get both L1 choke rings and pin-wheels on Ebay for fairly reasonable money. That should help on multipath.
> 
> Another aspect is the temperature shielding of the receiver itself. If it can be kept at a fairly stable temperature would also help. Using a better reference oscillator could also be worthwhile. Consider what a FEI 5680 could do for you.
> 
> Also, the way the antenna is mounted can be relevant. Up, up and away... such that any funny reflections is below the antenna.
> 
> A simple temperature stabilisation might be to simply dig the GPS receiver down.
> 
> Cheers,
> Magnus
> 
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