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Re: [Discuss-gnuradio] Software GPS:: BPSK demodulation

From: Krzysztof Kamieniecki
Subject: Re: [Discuss-gnuradio] Software GPS:: BPSK demodulation
Date: Mon, 08 Dec 2003 18:22:22 -0500
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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)

  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:
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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