[ntp:questions] almost decided which new PPS GPS to buy
Ron Frazier (NTP)
timekeepingntplist at c3energy.com
Sun Mar 4 16:30:05 UTC 2012
Some of you know from my prior threads that I've been debating what GPS
to buy to upgrade from my USB based BU-353. I think I've just about
nailed it down, based on a number of conflicting factors.
I'm getting + / - 6 ms accuracy now with the BU-353. My goals for the
new device are: 1 ms if using USB-serial interface, and 100 us if using
a real serial interface. One problem with the BU-353, is that it's
average offset from the internet servers seems to wander around up to 60
ms or so. When that happens, NTPD sometimes marks my GPS as a
falseticker. ESR, the guy mentioned in another thread who wants to
connect GPS to 100 routers, called this "wobble", and said it's related
to the SIRF chipset. I haven't done any other research to confirm that.
I'm pretty sure I'm going with the Sure board (pun intended) for the
A) Low cost for a NEW product.
B) Antenna and USB cable included.
C) No need to add connectors to the board, or a power supply if powered
D) Good timing performance.
E) High sensitivity for potential indoor operation.
F) David Taylor's excellent tutorial on setup:
From my reading, it appears that I need to add the following to the
Sure Electronics package of parts:
1) DB-9 Male - DB-9 Female serial cable
2) Motherboard header - DB-9 port adapter
3) Plastic box for mounting. Misc hardware like screws, standoffs, etc.
(David T. or others, I don't suppose you can suggest any specific part
numbers for those hardware items from Radio Shack [do you have those in
the UK], Mouser, or DigiKey. Just thought I'd ask.)
My plan of attack is as follows, although the sequence may vary:
A) Get the unit working "at all", using USB NMEA data only. I would
hope to see performance equal to my BU-353.
B) Get the unit working with NMEA data from the RS-232 port, going
through a Trendnet TU-S9 USB - Serial converter, based on the Prolific
chipset. Performance should, again, be similar to the BU-353 (which
also uses the Prolific chipset internally).
C) Get the unit working with NMEA data from the RS-232 port, going to a
real serial port. I have to tinker with the motherboard on another
computer to do that.
D) Get the unit working with NMEA + PPS, with data from the RS-232 port,
going through the TU-S9, which claims to pass all the handshaking
signals. I'll have to do the mods that David recommends on the board
for this. Hopefully, the same procedure will still work on the recent
boards. This is where I'm hoping to get + / - 1 ms performance.
E) Get the unit working with NMEA + PPS, with data from the RS-232 port,
going to a real serial port on the other computer.
This should be an interesting adventure, hopefully ending with a time
server running at 100 us accuracy or better.
(PS - If you email me and don't get a quick response, don't be concerned.
I get about 300 emails per day from alternate energy mailing lists and
such. I don't always see new messages very quickly. If you need a
reply and have not heard from me in 1 - 2 weeks, send your message again.)
timekeepingdude AT c3energy.com
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