[time-nuts] GPS week rollover

[jimlux]

On 3/29/19 12:22 PM, John C. Westmoreland, P.E. wrote:
> To All:
> 
> I have a related question; hope it's OK:
> 
> What are these companies using for their "GPS Simulation/Simulators?".
> 
> Is it actually simulating the RF downlink or is a direct data stream
> injection into a tap into the receiver??
> 

RF.
GPS simulators, recorders, players are a standard thing.

They range in complexity - some generate the entire sequence "offline" 
and store it in a big memory, then just play it back. Others can run it 
in real time and take dynamic inputs (e.g. from a simulated airplane 
flight).  Some have variable levels.

The tricky parts are:
you have to do Doppler too, so it's not just hooking up 1 Mchip/sec PN 
codes to a mixer and a 1575.42 MHz oscillator.   That said, because many 
receivers use 1 or 2 bit digitizers, you can make some shortcuts.

The recorder/playback ones are the simplest - they are literally a 1 bit 
thresholder running at a suitable rate hooked up to a LNA and the 
playback plays back the 1 bit through the same filter, and trusts in the 
harmonics coming through.

The fancy ones (Spirent is one mfr) are very nice - they're basically a 
real time waveform generator with a bunch of software, implemented in an 
FPGA - they can do hundreds of satellites on multiple frequencies, with 
multipath, relativistic effects, etc.  (Like many software defined 
widgets, you pay to "unlock" various features).