Penguin359

Welcome and thanks for fixing the LFSR polynomial

Hello. Welcome to Wikipedia.

Thanks for fixing fixed the feedback polynomial that I added in the description of the GPS C/A code. If you do not mind these questions: How did you figure that the polynomial was incorrect?. Did you systematically and carefully check all of the GPS signals article, do you have the polynomial memorized and knew that the article was wrong or did you check specifically those polynomials (did they seem especially prone to being mistaken?)?. I am curious as to how you found the mistake because it seems hard to find by chance.

Regards. Mario Castelán Castro (talk) 14:11, 6 August 2015 (UTC).

I have an interest in learning more about Direct-Sequence Spread Spectrum and thought I'd give decoding GPS signals with my HackRF One a try. DSSS typically uses a sequence generated by a Linear feedback shift register which I was reading about yesterday. One key point that article made is that for Fibonacci LFSRs is, "The number of taps should be even." When I looked up the polynomials for GPS signals, I found two LFSRs that are used, but the second one used an odd number of taps and it wasn't producing the results I was expecting. After a little googling, I found the authoritative documentation for GPS is ICD-GPS-200C and searching for shift register lead me to page 29 where the correct polynomial was which had an even number of taps.

Thanks. Penguin359 (talk) 23:05, 6 August 2015 (UTC)

Sorry for causing the inconvenience and thanks for fixing the mistake. The authoritative documents describing the civilian signals are found in this page. ICD-GPS-200C is likely outdated (I just looked at the date). The latest revision is “H” from 2013; you probably want to consult it instead; it also documents L2C.

HackRF looks interesting. Thanks for the link. I will take a look at it later.

I am working on improving the article “GPS signals” to a point where it provides information relevant to implement a receiver (which seems to be your use case), and as an example of a CDMA/DSSS and RF system, in other words, so that it is useful for engineering practice (including amateur engineering and study). I invite you to comment on how it can be improved.

I have a not very detailed plan as for how to improve the article. Possibly the most important point (which needs significant work) is to improve the description of how the signals are actually used; current that is limited to a “Demodulation and decoding” section which covers only C/A. Previously, I have expanded and reorganized the article “internal combustion engine”. I did most of the work so far in October 2014, including a major reorganization of sections in a logical structure, and also significant expansion. I hope to do something like that in a much smaller scale in “GPS signals”.

Regards. Mario Castelán Castro (talk) 00:16, 7 August 2015 (UTC).