[time-nuts] again on GPS antennas

Attila Kinali attila at kinali.ch
Wed Apr 11 20:48:14 UTC 2012


On Wed, 11 Apr 2012 16:37:42 +0200
francesco messineo <francesco.messineo at gmail.com> wrote:


> seeing this spec sheet:
> 
> http://www.saderet.co.uk/Admin/Datasheet/New%20antenna_spec.pdf
> 
>  I was wondering why these antennas have so different GPS bandwidth.
> Does the BW  affect performance of a trimble thunderbolt for example?
> Thanks in advance

Yes. There are a few considerations, the two most important ones are:

The BW of the signal:
For GPS L1 you need IIRC 2MHz, for GPS P(Y) IIRC 10MHz and for Galileo E1
8MHz of bandwidth to capture the first lobe. There are additional lobes
and the more you catch of them, the more signal energy you have to work
with and also your signal will have less distortion due to the input filter.

Nearby noise sources:
You usually have other signals or noise sources that enter your antenna.
The better you can isolte those noise sources at the antenna the easier
it is for your receiver to cope with strong noise (less intermodulation
effects). Most GPS receivers (especially the cheap positioning ones) have
a narrow BW input filter of 2-4MHz because of this.

So, there is a tradeoff between more signal and less noise. What you choose
depends on your application. Precision timing and positioning equipment
usually chooses to have more signal, because they can (to some extend)
control the antenna position and thus how much noise they get.
"Normal" positioning equipment chooses the narrow BW because it makes
them more imune to noise.

Also keep in mind that your receiver has a limited BW itself. Ie its ADC
will not catch a full 10MHz of bandwidth anyways if you use a "cheap"
receiver. So feeding it more than the reveiver BW is a wastefull and
can degrade the noise performance under certain circumstances.

			Attila Kinali

-- 
Why does it take years to find the answers to
the questions one should have asked long ago?




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