[time-nuts] Re: cheap GPS sim Re: Re: Another leap second problem

[Magnus Danielson]

Hi Jim,

Many of the GNSS issues that was suggested to be tested doesn't really 
need a very clean RF chain, and a HackRF One with some decent source and 
some open source code will have you generate signals. Testing all the 
nitty bits of the signals needs more code to fill in and fiddle with 
those bits, but it is not stellar development really.

However, the bitrate of the data on the satellite carrier is 50 bps as 
modulated onto the ranging code, and that in a very strict structure, so 
many of these changes does not happen quick. That is why I proposed to 
inject the trouble after the actual GPS/GNSS receiver as an alternative 
to have that part tested quicker.

The good RF chain becomes important when you want to calibrate receivers 
and work way down in the nitty gritty. Some of us do, but these simpler 
devices can clean the pipe.

One GNSS-simulator vendor had a simpler device for L1 only signals 
originally, and didn't think it would sell to the heavier design houses. 
How wrong they where, they where very popular if you only dared to ask, 
as they could have a bunch of them spread out in the lab and let the big 
expensive simulator do more of the hard work while trivial things could 
cheaply be tested separately by each designer.

Anyway, do not dismiss the simpler solutions, they may be just as 
efficient for many problems, even if they currently may not have all the 
tools in place to do that. I've intended to develop something like that 
myself, but you know, a few lines of code, how hard can it be, and spare 
time did not suffice.

Cheers,
Magnus

Den 2025-11-10 kl. 19:26, skrev Jim Lux via time-nuts:
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> Interesting - one would need an SDR with very good frequency accuracy and stability, if you're doing anything other than just checking GPS software.
> That's part of what makes the commercial simulators expensive - in theory, they have good quality synthesis chains.
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> I'm not sure that the AD936x (used in most inexpensive SDRs as up/down converter) is of "time-nuts" quality in terms of the LO synthesis. Certainly one can feed in a good quality reference clock (derived from your H maser ensemble, disciplined by your set of Cs fountains, of course).
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> At work, I have been fooling with a variety of cheap SDRs (real cheap - as in RTL-SDRs for $30-50) and they have all sorts of interesting internal compromises or settings in the synthesis chains which make no difference for their design purpose of receiving Over the Air TV broadcasts, but DO make a difference for other uses.
> (I note as an aside, that there are people who claim to make multichannel receiver setups for direction finding, who seem unaware of the details of achieving phase coherence, even with common 28.8 MHz clocks)
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> On Sat, 8 Nov 2025 19:04:23 -0600, Steven Sommars via time-nuts <time-nuts at lists.febo.com> wrote:
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> GPS simulator: https://github.com/osqzss/gps-sdr-sim
> Thanks for the pointer. [I see used Spectracom simulators on eBay for
> $1000-$2000. This is above my hobbyist budget.]
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