Playing with acquisition code, coherent time and number of incoherent blocks

After several unsuccessfully attempts at acquisition on data obtained from the dbsrx board, I'm taking a step back to play with the acquisition code to learn more about the problem. A very discouraging post was on the gnuradio mailing list by Matt. Basically, it looks like the current oscillator is going to cause a problem, "The oscillator on the USRP is spec'ed to 50 ppm, so you could be as much
as 79 kHz off plus doppler." With doppler adding +/- 10 kHz, a search of +/- 90 kHz will not be good for a real application. I will have to look into getting a better oscillator (not really a big deal).

Here are a few example plots using the data from the previous post. The first graph shows coherent correlation on 1 ms of data. For a positive hit on acquisition, the best threshold that could be achieved was 5-sigma.



For SV 19, the carrier frequency was found to be off by 3125 kHz and the CA code offset was 3107. Which can be plainly seen in the plot. This amplitude really doesn't 'stand out' and tells us we have a week signal. Getting a better trust on whether or not we have detected the correct signal can be done by noncoherent integration. Basically, we sum together blocks of data. Below is a picture of using 6 blocks of data.



Here we can definitely see a good peak and the threshold was set to 9-sigma. Now it is time to do some variation and see how it affects our acquisition. Let's first due coherent integration over 2ms. Below is the output



Well, a new peak occurs at an offset of 7107. I think since the CA code repeats every 1 ms, if we divide by 2, we get an offset of 3553.5, close to the 3107. Let's see what more blocks show.



Which gives the same offset 7107. Interesting enough, to get 9-sigma it only took 4 blocks of data instead of the 6 blocks needed for 1ms blocks. Finally, lets try 5ms before we move on to other variables. We get more peaks, but nothing overly strong



We can now see 5 peaks, about equally spaced with the strongest at 7107.