[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

Hi all,

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 
following reasons:

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 
by USB.

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.)

Ron Frazier
timekeepingdude AT c3energy.com

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