[time-nuts] GPS simulator,

jimlux jimlux at earthlink.net
Fri Nov 26 15:34:38 UTC 2010


Luis Cupido wrote:
> Hi,
> 
> The fact they refer that atmospheric s/n degradation and dropouts etc 
> being replayed precisely lead me to think this is just a spectrum 
> rec/play machine (no mod/demod of any kind)
> like the ham-oriented "time machine" but for GPS.
> http://www.expandedspectrumsystems.com/prod2.html
> 
> Luis Cupido.
> ct1dmk.
> 


But they mention software to generate sequences.  It's actually a fairly 
clever idea.  It's easy to stream 16.3xx Mbps from a modern computer, so 
you could generate an arbitarily complex signal off line..


For a lot of applications this would be great.  A nice test source that 
you could replay on command.

  The only question I'd have is whether the 1 bit DAC can adequately 
represent the real-life complex signal.  I suppose, intuitively, since 
your receiver is digitizing it with a 1 bit ADC...

And, I have a question whether the 1 bit DAC can produce a high enough 
fidelity waveform for a higher performance receiver that uses a multibit 
digitizer.

Since the clock you are recording and playing back with isn't 
necessarily as good as the one on the satellite, you probably couldn't 
use this for testing your precision receiver.  It would be like 
introducing a random propagation variation or coax length variation in 
the system.

It is intriguing though.. a homebrew version could be quite simple.. you 
can get the USB interface and a small FPGA pretty easily.  One of those 
10.23 MHz crystals from Rick might be handy...



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