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

[Magnus Danielson]

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