Hi The thing that I find useful about a GPS simulator is it’s ability to calibrate the time delay through a GPS based system. In the case of a GPSDO, there may be things beyond the simple receiver delay that get into the mix. Getting the entire offset “picture” all at once is nice thing. Yes, that’s a Time Nutty way to look at it…..
So far, I have not seen anybody extending this sort of calibration to the low cost SDR based devices. Without digging into the specific device, I’m not sure how well a “generic” calibration would do. Indeed, it might work quite well. Without actually doing it … no way to tell. So if anybody knows of the results of such an effort, I suspect it would be of interest to folks here on the list. Bob > On Apr 24, 2021, at 8:43 PM, Forrest Christian (List Account) > <[email protected]> wrote: > > I have used GPS-SDR-SIM with good results. > > It's an open source tool that will generate the right files to be able to > generate simulated GPS signals using many of the open source SDR platforms > including HackRF. It uses the publically available ephermis files along > whith desired receiver position data to generate the "RF" output files. > > My experience has been that clocking the sdr with a output from a > disciplined source results in the 1pps from a typical GPS receiver > remaining at the same relative phase during the entire playback, for a > definition of same which was good enough for my purposes. > > Two notes: > > This is gps only, no other constellations. Would love someone to write a > similar tool for other constellations. > > Several platforms are supported, some are dirt cheap. I used HackRF > because I already had one. Not sure about any of the others. > > On Sat, Apr 24, 2021, 11:56 AM Lux, Jim <[email protected]> wrote: > >> On 4/24/21 10:31 AM, Andrew Kalman wrote: >>> Hi Paul. >>> >>> Yes, I've been on this same journey. After I learned (somewhat unrelated) >>> that one is supposed to have an FCC license to rebroadcast GNSS signals >>> (e.g. via a repeater inside a lab, makes eminent sense), I started >> thinking >>> more about GNSS simulators and how they might be added to my company's >>> workflow. So I bid on a couple of units, got them for pennies on the >>> dollar, and started messing with them in the hope of ending up with an >>> ATE/rack-type setup that I can build into a nearly automatic test & >>> validation suite. >>> >>> Let's say I was much more successful with the Spectracom/Orolia GSG-5 >> than >>> with the Spirent GSS4200 ... In the case of the GSG-5, it's really just a >>> question of how many options you can afford -- the rest is all there, you >>> don't need a support contract, it's all easily accessible in the unit >>> itself, and as long as the Internet exists the GSG-5 will probably keep >>> working (it gets time, ephemeris and almanac data from servers -- it can >>> simulate stuff NOW (wth the right options), not just in the past and >>> future). The GSS4200 is about 10-15 years older, and it shows (in terms >> of >>> ease-of-use), along with how Spirent chose to monetize their users / >>> subscribers. Also, the GSG-5 adds things like interference to the signals >>> (all for a price, of course). IOW, the newer units (at least, from >>> Spectracom was XL Microwave is now Orolia) are a whole lot easier to use >>> ... but they come at a price. It's an interesting business. >>> >>> I will say that the build quality of the Spirent is very good. I have not >>> opened up the GSG-5, just did a calibration and it was very close. >>> >>> I'm a little bit surprised that there is not an open-source, SDR-based >> GNSS >>> simulator (at least, one I could find). >> >> >> >> Not much demand, I suspect. I seem to recall a GNSS generator that was >> open source about 5-10 years ago, but I can't find it now. >> >> The record/playback boxes are actually pretty simple - just a single bit >> in many cases. After all, a lot of the receivers use a single bit input, >> because the signal of interest is below the thermal noise floor. >> >> The real challenge isn't the SDR part (a USRP would work just fine as >> long as you get a daughter card that supports L-band) - it's the >> "scenario building" which requires simulating the orbits of the GNSS >> satellites, simulating the track of the receiver, calculating the time >> delays (including iono and tropo effects), and generating the PN codes >> appropriately. >> >> Each of those isn't too tough, but putting it all together is quite >> challenging, and, apparently, it's not "dissertation topic" suitable >> (which is where a lot of niche SDR stuff comes from). >> >> A *real* challenge is that to do it right, you need very good orbit >> propagators - if you're looking to simulate nanosecond scale >> phenomenology, you need to be able to generate orbit behavior on a few >> cm or better sort of uncertainty. For some applications (differential >> GPS, RTK surveying) you could probably get away with something that's >> not perfect, but doesn't have problems for YOUR specific application. >> But it wouldn't be a generalized box. >> >> One strategy we've used at JPL is to have the fancy expensive box >> generate the signals for a scenario, and record them with a much cheaper >> record/playback box, then use the playback for testing. >> >> Right now, my project (SunRISE mission) is working on how to generate >> realistic test signals for a space interferometer - Where we need to >> generate signals that can be received, and the output of the receiver >> fed into GIPSY-X for post processed precision orbit determination. >> _______________________________________________ >> time-nuts mailing list -- [email protected] -- To unsubscribe send >> an email to [email protected] >> To unsubscribe, go to and follow the instructions there. > _______________________________________________ > time-nuts mailing list -- [email protected] -- To unsubscribe send an > email to [email protected] > To unsubscribe, go to and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] -- To unsubscribe send an email to [email protected] To unsubscribe, go to and follow the instructions there.
