Tony Hoyle wrote:
David J Taylor wrote:
My GPS 18 LVC powered up in the correct mode (I think), but the
Windows control program wouldn't talk to it. HyperTerminal saw the
sentences correctly, though! I wrote a small piece of software to
switch it to 19200 baud, and back to 4800 baud, and it's OK now.
Mine was the other way around - the Windows control program could see
it but all hyperterminal (or minicom) could see was binary junk. Had
to switch it manually using $PGRMC1... you first get the windows
program to enable text mode (forcing 'enable NMEA mode' does this)
then quit it and start minicom, and issue a $PGRMC1 with 1 as the
second digit (disable binary phase).
Let me see what my system says:
# ntpq -p
*GPS_NMEA(1) .PPS. 0 l 7 64 377 0.000 0.001
0.008
Can't get PPS to work on mine... I'm recompiling the kernel even
though some places say it works without it (not happy about this -
I've been using stock kernels on that machine). Been compiling for
an hour now...
I changed the refid to "PPS". Did you tell the driver which
sentence to use? As you can see, I used the default $GPRMC.
It should be OK.. it's picking it up.
There's a strong GPS signal in this room (got a TomTomOne sat on the
desk in diagnostic mode, and it sees 3 satellites at near 100%
strength) so sync shouldn't be a problem.
It did latch on for a couple of seconds earlier, but was over 100ms
out! A long way from the claimed 1ms accuracy...
Tony
Tony,
[You sent the e-mail directly - I'm guessing that was a slip of the
finger]
Don't give up yet!
- thanks for the notes on the GPS 18 LVC. Mine wasn't set like yours, and
by chance I got the version with the longer 5m lead.
- kernel recompile took 5 hours on this 133MHz/48MB system
- without the kernel recompile (as I understand), the PPS line looks just
like another Internet source, so the jitter might be milliseconds rather
than microseconds. You /have/ to do the recompile for best accuracy.
- you /did/ wire the PPS to the DCD pin, and not somewhere else? Even a
multimeter should show the signal (well, an analog one might). IIRC, the
GPS delivers the PPS signal even if it's not position locked.
- actually, this is bringing back another memory. For a long while after
I first started the GPS it didn't report a sensible position. I suspect
it was taking a long time to read the almanac, and then to find the
correct satellites. It did eventually sort itself out, but only after
about 30 minutes of exposure to a good fraction of clear sky.
- the unit is quite a bit less sensitive than my other GPS, which has an
add-on active antenna.
- the pulse is nominally 200ms long, so if you have the "wrong" number of
inverters between the GPS and the RS-232 connector, you may observe a
200ms offset, and you may need to tell NTP to lock onto the other edge.
I've added some of this to the write-up.
I believe the claim of 1us accuracy, although I have no means of proving
it! BTW: that's one microsecond.
Cheers,
David
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