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

[Ed Marciniak]

I’ve seen some Spirent GPS boxes show up that are affordable at perhaps $100 or so last I looked.

I think they only generated a single L1C satellite signal on the one happen to have.

I had no idea what the software interface is like on them, but it had a GPIB interface.

They’re cheap enough that you could use several of them, and only 2RU height and not real deep or heavy.

Depending on what your intentions are, it ought to be possible to receive a GPS signal from an antenna, run it through an isolator or two and apply the simulated signal via a directional coupler used in an unconventional way to the live signal if it helps reduce number of signals you need to generate. As a bonus, the native antenna signal could be applied to a timing receiver to generate highly accurate local clocks to drive the rest of the chain.


________________________________
From: Jim Lux via time-nuts <time-nuts at lists.febo.com>
Sent: Tuesday, November 11, 2025 12:59:17 PM
To: time-nuts at lists.febo.com <time-nuts at lists.febo.com>
Cc: Jim Lux <jim at luxfamily.com>
Subject: [time-nuts] Re: cheap GPS sim Re: Re: Another leap second problem






Oh yeah, we have a bunch of "dumb playback" devices that basically have a 1 or 2 bit DAC for each band - you use the fancy sim to generate a signal, record it with the playback device and go to town.


On Mon, 10 Nov 2025 22:55:53 +0100, Magnus Danielson <magnus at rubidium.se> wrote:

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:
>
>
>
>
> 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.
>
> 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).
>
> 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)
>
> On Sat, 8 Nov 2025 19:04:23 -0600, Steven Sommars via time-nuts  wrote:
>
>
>
> GPS simulator: https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_osqzss_gps-2Dsdr-2Dsim&d=DwIGaQ&c=euGZstcaTDllvimEN8b7jXrwqOf-v5A_CdpgnVfiiMM&r=JsDsKeR7cZC8wbZhIlxxBQ&m=5hIWzm0hDjmFgHok2NnqLq2_Po_LNQy3cXnrhEzuFAu8Yo7ftk71UVUaBfq35Wpd&s=K8-VckBmmdzOZgDXCtqpkyd0LKFRdrN9V19POALJjTw&e=
> Thanks for the pointer. [I see used Spectracom simulators on eBay for
> $1000-$2000. This is above my hobbyist budget.]
>
>
>
>
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