Tony Hoyle wrote:
sleek daelite wrote:
Could anyone help me with Garmin GPS 18lvc device that comes with PPS
output. I've been trying to configure the device to achieve about 1ms
accuracy. The specification of this device states it could achieve
about 1ns accuracy. Im quite puzzled as to why i can't achieve such
accuracy.
1ns is just not going to happen on PC hardware.
Whether you can get sub-ms speeds depends a lot on the hardware - a
fast processor helps. System load is critical (best to dedicate a
machine to ntp serviing really).
With the shm driver which requires a daemon, you're at the mercy of
the scheduler that it even runs that daemon at the right moments..
you're likely to get better results with the kernel PPS driver because
it doesn't have that issue. If you can't do that try putting gpsd at
realtime priority.
This is true in theory, but in practice it is not much of a problem. I
get similar performance from gpsd as other people report from kernel PPS
patches.
(of course, when you have a constant high load on the system it may be
different. my load usually is somewhere around 0.25)
Note that gpsd runs at an elevated "nice" level by default. This is
done during startup in gpsd.c
I have also used it at realtime priority, but be careful! When a
process running at realtime priority somehow ends up in a loop, you kill
the system.
About a year ago, gpsd sometimes went into a loop during device
detection and I removed the realtime priority after hanging my system twice.
That bug is fixed, so if you are daring you could add it again.
However, in practice it does not seem really needed.
Rob
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