[Discuss-gnuradio] Troubleshooting GPS on DBSRX

[Chris Stankevitz]

Short Version:
Please help me troubleshoot why I cannot acquire GPS with my USRP+DBSRX, 
but I can using KD7LMO's sample data recorded with USRP+DBSRX.

Long Version:

Hi,

I wrote a GPS acquisition routine which operates inside a GR block fed 
by the USRP or gr.file_source.  The acquisition routine is a variation 
of the weak signal approach presented in "Fundamentals of Global 
Positioning System Receivers a Software Approach".

My acquisition routine works great when applying GR's file_source on the 
KD7LMO USRP-recorded GPS samples.  However, I cannot acquire using a 
USRP source_c.  Please share any tips you have.  Here are some things I 
tried:

1. Antenna/environment
Assuming GPS was simply not visible from my window, used the Leadtek 
9546EVK (http://tinyurl.com/yqx3nv) with the same GPS antenna I used on 
the USRP.  The Leadtek was able to acquire and track GPS with C/N0 > 40. 
 I have tried using this antenna on the USRP with and without a 30dB 
low noise amplifier.

2. Birdies
When commanded to tune to GPS L1 (1.57542GHz), the DBSRX front end tunes 
to a slightly higher freq (r=8,n=3151,Fs=4e6) and compensates with a 
software DDC.  Witht these values of r and n, a birdie can be seen at 
L1+1MHz in USRP's FFT and in an HP spectrum analyzer hooked to a GPS 
antenna near the USRP.  I eliminated it with different values of r and n 
(r=2, n=788) and a slightly higher DDC.  I accomplished this by adding a 
hack at line 289 of db_dbs_rx.py that works when Fs=4e6:
        # Avoid birdie when LO is set close to L1
        if freq==1575420000:
          best_r = 2
          best_n = 788

3. Signal generator
Thinking I was improperly tuning the USRP, I connected a signal 
generator to it and verified that L1 was being brought to baseband. 
However I was frightened to see some artifacts on the USRP FFT that do 
not appear on the HP spectrum analyzer.  They appear out of the noise 
for strong input signals (-75dBm).

4. Enclosure
I do not have an enclosure for my USRP.  This could be a problem.

5. Others
KD7LMO managed to record a beautiful signal (40+ C/N0) and I don't think 
he did anything fancy except use an amplifier.  I wonder if i am doing 
something boneheaded e.g. setting the gain to zero.


Q: Should I be worried about the birdie I saw?
Q: Does my hack do what I think it does?
Q: Should I be worried about these artifacts I see?
Q: What does set_gain() do on the USRP with a DBSRX board?  What should 
I set it to?

Thanks for your help and ideas.  I am a c++ guy new to this antenna stuff.

Thanks,

Chris