[time-nuts] GPSCreations SDR (software defined receiver) GPS1A

jimlux jimlux at earthlink.net
Sun Jan 16 20:43:30 UTC 2011

On 1/16/11 12:12 PM, Brooke Clarke wrote:
> Hi Pete:
> The SiGe receiver IC is not much use by itself since you need to
> literally be a rocket scientist in order to process its output data.
> The GPS1A, with open source software is interesting, but again you need
> to be a rocket scientist to modify the code.

Not necessarily...

There are a fair number of 1 bit sampler example codes for GPS receivers 
out there.

There's sort of two parts to the problem.. the first one is acquiring 
and tracking the PN codes and recovering the NAV message.  That's fairly 

You can do a FFT type cross correlation, for instance, or just a simple 
standard PN correlator.  The trick is that you have to allow for the 
doppler (that is, it's not just acquiring code phase.. you also need to 
get the doppler/clock offset).  I'm sure there's sample code out there 
(perhaps in the textbook referenced in the first link?)

  The next part is actually getting a NAV solution from the raw observables.

That's a bit trickier, but, really, if you're not "computational 
performance limited" (e.g. you're not trying for nanojoules/fix on a 
cellphone, and can burn a bunch of performance from your PC), the 
equations and Kalman filter are doable in Matlab/Octave pretty easily 
(and published).

After all, your filter update rate is probably something like 10 Hz or 
1 Hz.

BTW, it is possible to get all your observables with 1 second's worth of 
data samples.  You might even be able to get a nav solution,  with some 
side information (e.g. ephemeris data... at the 50bps, you're not going 
to get the whole almanac in 1 second)

More information about the time-nuts mailing list