[time-nuts] Re: 20210423: Introduction and Request for Spirent GSS4200 User Manual / Help

[Bob kb8tq]

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) <lists at packetflux.com> 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 <jim at luxfamily.com> 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.
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