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).
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