[time-nuts] L1/L2 GPS Receiver

[Magnus Danielson]

On 18/01/14 04:09, Jim Lux wrote:
> On 1/17/14 11:35 AM, Magnus Danielson wrote:
>> On 2014-01-16 20:29, Hal Murray wrote:
>>>
>>> anders.e.e.wallin at gmail.com said:
>>>> The real benefit of dual-frequency is you can do post-processing with
>>>> PPP.
>>>> Javad has some modules but they start at 3 kUSD - if anyone knows of
>>>> hobby
>>>> level priced L1/L2 receivers that can produce rinex-files for PPP
>>>> processing
>>>> that would be interesting!
>>>
>>> Has anybody considered doing it in software?
>>>
>>> If I wanted to play with that sort of stuff, is there any particular SDR
>>> hardware package/project that would good to start with?
>>
>> Well, considering that you will need to get the P(Y) signal at 10,23
>> Mchip/s on both L1 and L2, requiring say 40 Msamples/s for both
>> frequencies, and that you will need to do it for say 12 channeles and a
>> bit of interesting processing beyond doing the same amount of channels
>> for the C/A code, it will be an interesting challenge to do that in CPU
>> code, rather than doing the "baseband-processing" in some form of
>> hardware/FPGA.
>>
>
> ALmost certainly in an FPGA.  But unless you want fast acquisition,
> implementing the tracking loop and despreading in FPGA isn't
> mindbendingly difficult.  I'll bet there's open source out there.

Doing it in FPGA isn't all that hard. You build a C/A receiver and then 
extend it, as you do hand-over from C/A to L1 P(Y) and then lock up on 
the L2 P(Y). FFT accelerated C/A lockup will benefit also for P(Y) as 
you get a head-start with C/A and then it is relatively trivial to take 
the next step. It's this dependence of C/A which makes it OKish from a 
military standpoint, as the C/A is fairly trivial to jam.

> You could also record raw bits and decode off line in software in
> non-real time, as long as your clock that you timestamp with isn't too bad.

Indeed. Been there and done that for L1 C/A. If your time-stamps is off, 
your solution will tell you how of they are.

Cheers,
Magnus