[ntp:questions] how do you like the Trimble Resolution T

unruh unruh at invalid.ca
Wed Feb 29 15:50:56 UTC 2012

On 2012-02-29, Ron Frazier (NTP) <timekeepingntplist at c3energy.com> wrote:
> Hi all,
> Is anyone using the Trimble Resolution T GPS for timing?  I was looking 
> at an ad for one on ebay.  For my purposes, any accuracy under 1 ms is 
> great.  I don't have to have 15 ns.  Of course, smaller numbers are 
> always better.  Is this device hard to setup and program for PPS and use 
> with NTPD?  What accessories are needed?
> This is what the ad says is included:
> quote on ->
> The Resolution T Starter Kit provides everything you need to start 
> integrating the module into your application. The kit includes an 
> active, external 5-VDC Bullet-style antenna 
> 50 feet of RG-59 cable, and an AC/DC power adapter. The starter kit 
> enclosure includes a mother board that provides serial output, and a 
> serial interface cable. A reference manual and monitor programs are 
> provided on CD-ROM.
><- quote off
> I notice the unit only tracks 12 satellites and has 141 dbm sensitivity, 
> which seems to be less than some units.  Is that a problem?

I think both would be fine.

> Finally, what is sawtooth correction?

The timing PPS pulse output turns out to be far easier to generate if
the system is set up to send out the pulse when the local oscillator
waveform goes through 0. Since that may not be exactly on time, the
pulse will come out late by up to about one period of the local
oscillator. But the system can track how late that actually is, and send
out a message telling you how late that last pulse came out. ( in the
couple of ns that the pulse has to come out, there is not enough time to
actively correct. a few ns is an incredibly short time.) That is called
the sawtooth correction since the amount of the correction jumps
evertime a full cycle of the local oscillator has gone by. Thus if you
plot the offset of the PPS pulse with true time, you will get a sawtooth
output That correction is in the 10s of ns range. If you are not a radio
astronomer or a neutrino speed cop, you do not need to worry about it. 

Note that there is a far greater correction due to the 50 ft long
antenna cable. ( that is a correction of about 80ns).

> Sincerely,
> Ron

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