[time-nuts] Linux PPS clues?
bruce.griffiths at xtra.co.nz
Fri Oct 21 02:45:52 EDT 2016
Another issue is that the finer the timestamp quantisation step size the larger the signal of interest (Intensity correlation). The signal doesn't vanish as the timestamp quantisation step size increases however the signal decreases requiring a longer observation time to achieve a given SNR. Handwaving without calculation to inform the resultant guesses is likely to result in a required observation greater than the age of the universe. This of course is impractically long. This fundamental error was made by several investigators in the some of the first attempts to do intensity interferometry.
On Friday, 21 October 2016 7:32 PM, Chris Albertson <albertson.chris at gmail.com> wrote:
On Wed, Oct 19, 2016 at 11:15 PM, Ilia Platone <info at iliaplatone.com> wrote:
> I need to know how much precise this system can be. How much resolution
> can I obtain with a cheap receiver (maximum quantization frequency)?
> Formulas are well accepted.
Even a cheap receiver will have error that is orders of magnitude smaller
then the resolution that the linux kernel can work in.
You should expect the system time to have error on the order of about 10
The PPS signal error comping out of events low cost GPS receiver has error
oon order of say a few tens of nano seconds, that is 100 to 1000 times less
The major source of error is not GPS , but is the interrupt latency
uncertainty. But this is not bad at all, you can expect roughly a 10
microsecond uncertainly in the time stamps more or less.
But a lot depends on how you handle the second GPIO interrupt. the GPS
interrupt is handled inside a kernel level handler. The handler snap shots
the system clock and stores it in RAM, then sets a flag that the user space
process checks. Does your GPIO interrupt have a kernel level driver to
snapshot the system clock ir is this happening in a user space process? If
the later expect worse performance. For best performance copy and paste
the Linux PPS code and then re-build the Linux kernel with your new driver.
Getting event time stamps much better then this requires some purpose built
hardware outside of the computer.
Redondo Beach, California
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