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

[Bob kb8tq]

Hi

The lowest cost “commercial” simulator that I know of is the Jackson 
Labs Claw:

http://www.jackson-labs.com/index.php/products/claw_gps_simulator <http://www.jackson-labs.com/index.php/products/claw_gps_simulator>

Taking a look at the manual:

http://www.jackson-labs.com/assets/uploads/main/CLAW-Simulator.pdf <http://www.jackson-labs.com/assets/uploads/main/CLAW-Simulator.pdf>

It appears to be a fully documented interface. ( unlike the device that started
this thread off). The support software is freeware so there is no lockdown 
there either. 

Time transfer is claimed to be in the 3 to 5 ns range. My guess is that you
can trust that number :) :). As far as I can see, it will do pretty much everything
you would want to do to check out a GPS device or to drive one off of some
other source ( like your basement ensemble of Cs fountains …. )

It looks like a nice little box. 

Bob

> On Apr 24, 2021, at 1:56 PM, 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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