GPS Acquisition

After a break of almost a year to finish my thesis, I am back to explore GPS using the USRP. The good news is that I was able to 'acquire' (coarse) satellites with the dbsrx daughter board. Unsure of the best gain to use, I collected data for four different gains: 76 dB, 86 dB, 96 dB and 104 dB. Also the active antenna I am using is Gilsson part # SMA-M, which adds 28 dB of gain and can be found here. The gnuRadio commands used to collect the data were

./usrp_rx_cfile.py -g 76 -d 8 -f 1.57542G -N 500M gps76.usrp
./usrp_rx_cfile.py -g 86 -d 8 -f 1.57542G -N 500M gps86.usrp
./usrp_rx_cfile.py -g 96 -d 8 -f 1.57542G -N 500M gps96.usrp
./usrp_rx_cfile.py -g 104 -d 8 -f 1.57542G -N 500M gps104.usrp

Since the decimation is 8, then 500M samples/ 8 Ms/s = 62.5 s, so there is just over 1 minute of data for each gain value. The time the data was taken is around 10:02 AM, May 22, 2007 in Colorado Springs, Lat = N38 52.160', Lon = W104 49.589'. The satellites in view at this time according to my Garmin receiver (which did not track all of these) were

Sats PRN: 03, 06, 07, 09, 14, 16, 18, 21, 22, 24, 26, 29

Of course a bunch were at low elevations and with all the trees in my yard, I was only able to track about half of them. On to the data.

Once again acquisition is achieved using the python code develeped here. Note that you must change some values to correspond to how the data was taken so set these values to

SAMPRATE = 8e6
CENTERFREQ = 0e3
NSIDEFREQ = 50

The number of frequencies to search is increased because of the oscillator on the USRP has a higher error than GPS receivers. One could expect to add at most around +/- 10kHz freqs. to be searched because of doppler and I was getting frequency offsets as high as 24kHz. Results with an 8 sigma acquisition threshold are shown below for the 86 dB gain case

Sat. , Freq. , Code offset, Num. of chunks
-------------------------------------------------------------------------
07 , -17000 , 2648, 6
18 , -2000 , 1988 , 7
21, 20000, 1420, 3
22, 22000, 5464 , 2
24, -18000, 3655 , 5

Plots of the data: