Re: [Discuss-gnuradio] GPS for USRP ideas

[David Willmore]

> Brian Whitaker said:
> Because  of this, GPS front-ends need to offer a couple of key things to
> work well:
> -> 2-5dB NF from the antenna to the low-IF (usually about 4MHz),
> -> ultra-low phase noise, especially at 50-100Hz (as I understand, there is
> a lot of information at about 50Hz in the signal -- I need to get a book,
> though!)

The main unencoded nav information is a 50 BPS data stream spread by the
1.023 Mbps 'chirp' code, so yes, there is *a lot* of information at
50 Hz in the signal. :)

Basically, each satallite gets a unique PN (pseudo noise) code (a gold code
if anyone cares) that it uses to spread its nav message.  Each of the 50 bps
bits gets spread 20 times by the PN sequence, so 50 * 20 * 1023 = 1.023 Mbps
This is the C/A code or Coarse Aquisition code.  The P code (precision?) is
encoded on the other L band channel, the L2 in quadrature to the C/A code
and 10 times faster--I don't think the spreading sequence for it is mentioned
in any document that non security clearance people like me have access to.

So, that's GPS signals in a nutshell.  You can be a little less than 
log10(1.023M/50) under the noise floor of your receiver and still get unspread
signal back out.

Check out www.gpsinformation.net and follow some of the links from there,
that'll get you the specs for the GPS system and a bunch of nice tutorials.

Cheers,
David