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Re: [Discuss-gnuradio] Baseband GPS OTA USRP capture that anyone can sha

From: Chris Beaumont
Subject: Re: [Discuss-gnuradio] Baseband GPS OTA USRP capture that anyone can share?
Date: Fri, 25 May 2012 23:53:13 -0400
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.5; rv:10.0.4) Gecko/20120421 Thunderbird/10.0.4

This is the web page I was looking for.


I myself have never used RTKlib with an SDR.

Try to use a circularly polarized antenna.

GPS signals are circularly polarized to allow the antenna to have some degree of rejection of signals reflected from the ground. The simplest to make rhcp antenna for a single frequency, say 1575 MHz, is going to be a helical antenna. If your right hand is the coil windings and your thumb is facing up, the winding will go in the direction your fingers point. There are calculators on the net.

(Note that the measurements they will give you are for an air core.)

If you want a *broadband* circularly polarized antenna, you might want to look at spiral antennas like the "equiangular spiral" (logarithmic spiral) antenna or Archimedes spiral. The two arm design works best with a balun. There is also a very innovative design *that uses coax as the balun*. Its cheap and could work over a 8:1 frequency range!

IMO, with that antenna, performance could be quite good plus it works over a very wide frequency range. Google "infinite balun" or look at this page and scroll down. The antenna I am talking about is the one made out of a single piece of coax. That design does not look like it would be a good antenna but actually it is an extremely good antenna for satellite reception if you use the appropriate size for your frequency and the appropriate polarization. And of course, you can make a single antenna and simply flip it over to get the other polarization when you need it.

You could also make a similar design using a PCB and a stripline instead of coax.

Most of the low cost GPS antennas you see are not very good. To really take advantage of RTKlib you need a good antenna - good being defined as one with minimal phase center variation and high rejection of opposite polarized (reflection) signals. A vertical whip is going to be an absolutely terrible GPS antenna.

I've mostly used RTKlib with a local non-SDR receiver and gotten correction data from (as nearby as possible) RTCM data server, usually post processing. I think that is a typical use pattern.

There are lots of servers for correction data. Some of them are free.

I don't know how to use RTKlib with an SDR but its definitely being done. Look at the URL above to start. If you can't figure it out email Tomji Takasu its author. He is into software defied radio, he would probably be very helpful. Also, maybe there have been previous postings on this here on this list?

On 5/25/12 11:35 AM, Andrew Davis wrote:
I didn't see that may 9th post. I could get some samples ( USRP with
rubber duck with clear sky, would that work? ).

Does anyone know how to feed RTKNAVI? What format does the file need to be?

On Fri, May 25, 2012 at 12:06 AM, Chris Beaumont<address@hidden>  wrote:

A while ago (May 9) there was a post to the list about raw GPS data as
captured by a USRP.

These web pages may or may not be useful, I don't know if the data files on
the first one are the right format for you (they were not captured by a
USRP) but they may still be helpful.


If not, Michele might be able to help point you to where you could find data
that is.

Also, you should check out this web page:


Tomoji Takasu has done quite a bit of amazing stuff with GPS including SDR
development. (you may have to dig a bit for it but its there)

Between one or the other you should be able to find what you are looking

On Mon, 2011-05-09 at 10:27 -0500, John Andrews wrote:

Is there is anyone here who has GPS OTA capture using USRP that they
are willing to share? I don't have a USRP and I am interested in
demodulating the GPS signal.

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