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List:       gnuradio-discuss
Subject:    Re: [Discuss-gnuradio] Software GPS:: BPSK demodulation
From:       Krzysztof Kamieniecki <krys () kamieniecki ! com>
Date:       2003-12-08 23:22:22
Message-ID: 3FD507AE.3000703 () kamieniecki ! com
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Disclaimer: I don't have a Degree in Signal Processing, just a B.S. in Mathematics/Computer Science, 
this is mearly what I have picked from slowly writing a Software GPS Receiver.


I hope this is what you were looking for.

Short answer is
  C/A BPSK Demodulation is inherent in the down-mixing and correlator operation of the GP2015 and 
GP2021.


Longer answer is

  The GPS signal is (Carrier * Spreading_Code * C/A_signal)

where
   Carrier is the 1575.42 Mhz carrier
   Spreading_Code is the 1.023Mhz BPSK Spreading Code
   C/A_Signal is the 50 bits / second BPSK Navigation Message

The GP2015, when used in conjuction with the GP2021, mixes down the Carrier to 1.405MHz, then 
digitizes [(1.405Mhz Carrier) * Spreading_Code * C/A_signal] and sends that to the GP2021.

The GP2021 then does the final mixing down of the Carrier to Baseband.

At which point you have either (-1 * Spreading_Code * C/A_signal) or (+1 * Spreading_Code * C/A)

The correlator then "removes" the Spreading_Code and what your are left with is either (-1 * C/A) or 
(+1 * C/A)

At which point you have to use the navigation message decoding algorithms to figure out if you have 
(-1 * C/A) or (+1 * C/A) and decode C/A appropriately


P.S. Would it be possible for you to send me some of 2bit sampled signal?


Saeed Beyty wrote:
> Hi
>  
>    Some while ago we started working on software
> implementation of GPS user segment (handset).in the
> usual GPS, a GP2015 chip is used to do the RF-Down
> conversion and 2bit digitization, the sampled signal
> is then passed to a GP2021 chip for the Correlation
> and tracking procedure to be done. Here we first tried
> to process the sampled 2bit signal from GP2015 using
> the GPP (General Purpose Processors like PIV), but as
> it’s a time-wasting parallel process, it’s still
> better to be done in a FPGA.
>     We decided to use a FPGA chip to process the IF
> digital stream from GP2015.in fact we want to replace
> the GP2021 chip with a FPGA (to get closer to the SDR
> idea), and then pass its output to the PC to do the
> remaining processes. 
>     The only problem in the theorical section is:
> "Where and in which phase the BPSK Demodulation is
> taking place??” as you may know, the original message
> from the Satellite is Spread Spectrum coded and BPSK
> modulated before transmission.
>     I asked the OpenGPS mailing list, but they
> redirected me to this page as here are many peoples
> with communication expertise.
>     It is very kind of you to answer me, if you could!
>    
> Dec 8th, 2003
>  -- Saeed
> 
> 
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-- 
Krzysztof Kamieniecki
mailto:krys@kamieniecki.com




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