Re: [Discuss-gnuradio] GPS/DSSS question

[Brent Ledvina]

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
>
>
>
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