[ntp:questions] almost decided which new PPS GPS to buy

unruh unruh at invalid.ca
Sun Mar 4 20:18:52 UTC 2012

On 2012-03-04, Ron Frazier (NTP) <timekeepingntplist at c3energy.com> wrote:
> 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.

There is the need to add a connecting wire to the board. You can go with
David's procedure which involves soldering two wires between legs of
surface mount devices. Altgernatively you can solder a wire rom the hole
marked PPS on the board to the dcd pin of the serial port. Both of these
are no-brainer soldering jobs-- almost impossible to get it wrong.
(soldering to the surface mount chips is delicate and requires a good
iron and steady hands). Most serial chips will be fine with the TTL
level of the PPS signal (Again, does anyone know of some that are not OK
with it?)
> D) Good timing performance.
> E) High sensitivity for potential indoor operation.
> F) David Taylor's excellent tutorial on setup: 
> http://www.satsignal.eu/ntp/Sure-GPS.htm
>  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

Sorry, what is this for? 

> 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.)

A serial line you can buy at your local dollar/euro/pound store. 

> 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).

?? not sure what you mean by this. Note that you are going to get of the
order of 10ms ( after correcting for the 500 or so ms offset of the nmea
output). Part of the problem is that the nmea sentences are of variable
length. Each character takes about 1ms (at 9600 bd) to come out. 

You need the pps to get good timing (sub ms)

> 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 

See above for this.

> 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.

You can get 2-3us if you use a real serial port with dcd interrupt.

> Sincerely,
> Ron

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