Moin, On Fri, 03 Apr 2015 22:51:34 -0500 Robert Watzlavick <roc...@watzlavick.com> wrote: > On 04/03/2015 10:12 PM, Robert Watzlavick wrote: > > I have an amateur radio license (mostly CW/HF and some VHF/UHF > > experience) and I've written some driver software for an IQ > > demodulation board but I have to admit, I would have no idea how to > > begin setting up that system as initially described by Attila and > > expanded by you and others. I have a rudimentary understanding of the > > modulation schemes involved but I don't fully understand how the > > various codes mentioned fit in. I've poked around a bit at some > > articles on PN codes and I can see how data would be transmitted but I > > think I'm missing something key that allows you to extract positions, > > velocities, etc. out of the various links. I think I have some more > > reading to do :)
The basic system is that of an DSSS modulator/demodulator. The best text on spread spectrum systems I have found sofar is [1]. I explains modulation and demodulation in a hands on fashion. But, due to the age of the book, it does not contain any of the advanced stuff done today. But I think you don't need anything more fancy than an early-prompt-late correlator architecture for tracking. For the way how GPS works and how correlation and everything is done, I would suggest [2,3,4]. [2] is a good overview of how GPS is done and contains 99% of everything you need to know (special thanks to Magnus for mentioning it). It lacks some details on how to actually implement the system though. There [3] helps a lot, as it's a book specifically on building a GPS/Galileo receiver. I only skimmed trough a digital copy of [4] yet, so I cannot say too much about it, but that it's probably the most complete book on radio and inertial navigation I have seen sofar. The level of detail seems to vary from topic to topic quite a bit, but it is a treasure trove of references for everything the book covers (which is a damn lot!) If you are tight on time I would probably recommend to start with [3] and have a look at [1] and [2] when things don't make sense. > To head off a bunch of replies - I think I stumbled upon what is being > suggested. To extract the pseudorange, you have to figure out the > offset of the locally generated PN code against the one that is > received. In this reverse GPS case, I assume each ground station would > have to start their local PN codes at the same time? Then you would be > able to get the pseudoranges at each ground station and use those values > for the multilateration equations. You still would have an uncertainty > of one clock cycle since the phases of the local clocks at the stations > wouldn't be aligned but several folks have suggested ways around that. There are multiple things here: * PRN generation: The locally generated PRN has to be time synchronous with the one received from the rocket transmitter. If you are more than one clock period off, you will only get noise out of the demodulator. What you measure is the time difference of the locally generated PRN to your ground station system time. * Uncertainty: The autocorrelation function of a PRN sequence has a quite steep peak at \tau=0 with width of the clock period. Yes, this does mean that you get a one clock period uncertainty, if you do a hit/miss correlation. But as the correlation function is actually triangle shaped, you can get quite a bit better than that. The limit is afaik around your sampling clock period for naive approaches, which you can further improve with some statistics (you have multiple edges to work with, ie can average over those). * Synchronisation of ground stations: There are easy and diffuclt ways to do that. Probably the easiest is to use to use an additional transmitter at the launch point on the same frequency, but with a different PRN than the rocket. This way you can do a difference of the two PRN codes in your receiver, which gets away with a lot of nasty effects that you would need to account for otherwise. Another approach would be to use a GPSDO on each ground station and run all the receivers already synchronized. This also enables you to get the position of all stations very accurately, especially if you let the GPSDO average its position for some time. But for ultimate accuracy, you'd need to calibrate the GPSDO's (including antennas) against each other, to know what the systematic offsets are (ie set them up all together at the same location and measure the time difference of the PPS). Of course, it's possible to use a combination of multiple approaches. Eg a nice one would be to GPSDO's to provide position and a precise frequency reference, but then use a central transmitter for the synchronization. HTH Attila Kinali [1] "Spread Spectrum Systems with Commercial Applications", 3rd edition, by Robert C. Dixon, 1994 [2] "Global positioning system signals, measurements, and performance", 2nd edition, by Partap Misra and Per Enge, 2012. [3] "A Software-Defined GPS and Galileo Receiver", by Bore, Akos, Bertelsen, Rinder, Jensen, 2007 [4] "Principles of GNSS, Inertial, and Multisensor Integrated Navigation Systems", 2nd edition, by Paul D. Groves, 2013 -- < _av500_> phd is easy < _av500_> getting dsl is hard _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.