RE: [Discuss-gnuradio] GPS/DSSS question

[Deary, Bryce D. (BDEARY)]

A second option for finding the signal is to use a FFT to do the correlation
instead of doing an actual correlation.  This often lets you get very close
in both frequency and time (code phase) before turning on the FLL.  Also
remember that the PLL used in GPS is typically a costas loop instead of a
full PLL because once you stop the phase rotation with the FLL and drive all
the power into I(in-phase) by forcing Q(quadrature) to zero in your loop,
the in-phase will continually flip sign due to the 20 ms data bits. Normal
PLLs hate this.


-----Original Message-----
From: Brent Ledvina [mailto:address@hidden 
Sent: Saturday, August 16, 2003 6:39 PM
To: Krzysztof Kamieniecki
Cc: GNU-RADIO DISCUSS MAILING LIST
Subject: Re: [Discuss-gnuradio] GPS/DSSS question


Your algorithm should be modified in the following way:

1. Detect the code phase offset and carrier doppler shift using your 
acquisition algorithm (by finding the peak signal power at a particular 
Doppler offset).
2. Track the code offset using a delay-locked loop (DLL)
3.  Track the carrier frequency using a FLL or the frequency and phase 
using a PLL.

An good text on GPS receiver design using software receivers is 
Fundamentals of Global Positioning Receivers: A Software Approach by 
James Tsui, Wiley and Sons, 2000.

Regards,
Brent


On Saturday, August 16, 2003, at 08:14 PM, Krzysztof Kamieniecki wrote:

> I've been working on my GPS software, and I have a question as to the
> validity of one of my algorithms.
>
> Simplified Background:
>
>   Each GPS satellite broadcasts a DSSS(Direct Sequence Spread
> Spectrum) signal. The chipping rate is 1023M/s and the PRN code is 
> 1023 chips long.
>
>   To lock onto a satellite signal you have to:
>     1. Find (and track) the correct carrier frequency (because of
> Doppler / Relativity effects, Receiver effects, etc...)
>     2. Find (and track) the correct chip offset
>     3. Find (and track) the correct chipping rate (same problems at 
> carrier frequency, but this may not be a noticeable effect)
>
>
> My Algorithm:
> 1. Look for signal peak by trying 1023 different chipping offsets at
> frequencies separated by 200Hz
> 2. When a peak signal is found, measure the signal coming out of DSSS 
> integrators.
> 3. Adjust carrier NCO based on measured signal (frequency and phase 
> offset)
> 4. Activate carrier PLL
>
> So the question is, Is step 3 valid thing to do, or do I need a more
> complicated FLL/PLL?
>
> --
> Krzysztof Kamieniecki
> mailto:address@hidden
>
>
>
> _______________________________________________
> Discuss-gnuradio mailing list
> address@hidden 
> http://mail.gnu.org/mailman/listinfo/discuss-gnuradio
>



_______________________________________________
Discuss-gnuradio mailing list
address@hidden
http://mail.gnu.org/mailman/listinfo/discuss-gnuradio