PSN-L Email List Message
Subject: Re: from Angel in Panama
From: "kpayea" kpayea@...........
Date: Sat, 3 May 2003 18:44:23 -0700
NTP offsets of 100's of milliseconds are not unusual. It depends on how far
away the server is. Can you tell me more about how you have Tardis
configured? Just telling it to set the time every second may not do what
you want. Most NTP client software tries to keep the PC clock within a
second of UTC.
GPS by itself can deliver time well below a microsecond without any trouble,
better if you have a good receiver. I used to work for TrueTime (now a
division of Symmetricom) and we and our competitors claim accuracy in the
area of 25 nanoseconds RMS, 100 ns peak. Many of them use Motorola OnCore
receivers to get there.
The real trick is getting the time from the receiver to the PC. If you just
use the serial message, all of the precision from the GPS has gone out the
window. Some people use the 1 Pulse Per Second (PPS) output from the
receiver into one of the RS-232 control lines on the PC serial port to
improve the timing. Then they dig deep into the PC to use the interrupts
for the best response. Even then, you can't get the PC's clock set closer
than a few milliseconds. The people who wrote TAC32 claim 25 milliseconds,
which is pretty reasonable.
Based on your setup, I would question Tardis and NTP before TAC32 and GPS.
At these levels, you can ignore cable delay from the antenna. Even 300
meters of cable will only shift the time about a microsecond or so.
----- Original Message -----
Sent: Saturday, May 03, 2003 4:31 AM
Subject: from Angel in Panama
> Hello Everyone,
> I have a general question about GPS timing.
> I have several systems that have GPS timing systems and have always
> assumed that they were "right on", but now I have my doubts.
> I have one system on that is using a Motorola Oncore with and
> interface from Larry. I then use TAC32 to read the GPS time and set the
> PC time.
> Based on some conversations with Mauro and some of the work on the
> array I decided to check the GPS clocks.
> I set TAC32 to read the GPS but not set the PC time, ever. Then I set
> Tardis to get the time via NTP and set the PC time every second. Then
> I set TAC32 to show the difference between GPS time and the PC time.
> I get a consistent difference of .825 seconds. I have calculated the
> cable losses from the antenna and they don't seem to be able to
> account for all of that.
> Anyone have any ideas??
> On a difference note, just last night I was able to locate two
> events using nothing but the small array!! I will try to update the web
> page this weekend.
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