GNSS Disciplined Clock
Dear members
I am a new amateur member in your group. Maybe my question has been asked.
Recently I found Trimble Mini-T GG, whose data sheet is attached, as a good
GNSS disciplined time reference. I would like to know if there exists a
more accurate one, since it does not employ dual frequencies to compensate
ionospheric delay, though it handles both GPS and GLONASS. Besides, could
anyone guide me about the presented accuracy in the datasheet? There, it is
stated that "When operating in Over Determined Timing Mode, the accuracy of
pulse per second (PPS) is within 15 nanoseconds of GNSS/UTC." Does it mean
that it includes both bias and the noise? In other words, is it true to say
that "The time-synchronization error between two of them with a long
distance is less than 2*15 ns"?
Best Regards
Hi
I don’t speak for Trimble and their interpretation of all this may be a bit unique. Normally
what the 15 ns means is the time out of a simulator compared to the 1 pps out of the module.
Put another way, it’s just a measure of the receiver. It does not include any ionosphere / troposphere
issues. It assumes a perfect estimate of the location (no bias from antenna multipath). IT also
does not take into account any delay in the antenna or coax to the antenna. Time errors between
Glonass and GPS are not included (bad broadcast offset estimate etc). Finally there is the
fairly important qualifier of “one sigma” on the 15 ns number.
All that said, two devices with the same antennas, same cables, close to each other, looking at the
same sats, using the same systems, … should track pretty well.
Bob
On May 25, 2017, at 9:27 AM, Ebrahim Roghanizad e.roghanizad@gmail.com wrote:
Dear members
I am a new amateur member in your group. Maybe my question has been asked.
Recently I found Trimble Mini-T GG, whose data sheet is attached, as a good
GNSS disciplined time reference. I would like to know if there exists a
more accurate one, since it does not employ dual frequencies to compensate
ionospheric delay, though it handles both GPS and GLONASS. Besides, could
anyone guide me about the presented accuracy in the datasheet? There, it is
stated that "When operating in Over Determined Timing Mode, the accuracy of
pulse per second (PPS) is within 15 nanoseconds of GNSS/UTC." Does it mean
that it includes both bias and the noise? In other words, is it true to say
that "The time-synchronization error between two of them with a long
distance is less than 2*15 ns"?
Best Regards
<Trimble Mini-T GG.pdf>_______________________________________________
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The long term stability of GPS is very good. Some one here will point
out exactly how one measures it. But roughly when speaking of
accuracy you always need to specify a time interval. For example
if the 1PPS is "off" by 15ns that is not bad and yes there are much
better systems if you need to measure time intervals on the order of
one second. But if the signal is "off" by 15 ns over 100,000 seconds
that is well, 100,000 time better.
This is a basic reference and for some specialized end use case you
might couple it with other equipment. Many of the concerns you had,
such as effects of the atmosphere get averaged out because the unit is
looking at satellites from all over the sky. (averaging over space)
And you do git better results with better antenna locations that are
away from multi-path and have a 360 degree view of the horizon. But
notice the unit has an temperature stabilized crystal oscillator that
is stable over many seconds. an is much more stable in the short term
then is a GPS receiver. Trimble uses this crystal to average over
time
You also have to ask where is the tine data going to be used. Are you
synchronizing a computer's internal clock or trying to measure the
frequency of a microwave transmitter
SO it falls back to the old thing about there being no "better" only
better for a specific use case.
Some of use were lucky enough to buy Trimble Thunderbolts, a previous
version of this unit when they were on eBay for $100 each. For those
without 5 digits budget they ar pretty much the Gold Standard. I have
mine installed with a good filtered DC power supply and an outdoor
antenna on mast well above the roofs of surrounding buildings. I
get long term stability of about one part in 10E13. Yes 13 digits
over long periods. (I think?) It is really hard to know because my
measurement system is a little circular referenced
On Thu, May 25, 2017 at 6:27 AM, Ebrahim Roghanizad
e.roghanizad@gmail.com wrote:
Dear members
I am a new amateur member in your group. Maybe my question has been asked.
Recently I found Trimble Mini-T GG, whose data sheet is attached, as a good
GNSS disciplined time reference. I would like to know if there exists a
more accurate one, since it does not employ dual frequencies to compensate
ionospheric delay, though it handles both GPS and GLONASS. Besides, could
anyone guide me about the presented accuracy in the datasheet? There, it is
stated that "When operating in Over Determined Timing Mode, the accuracy of
pulse per second (PPS) is within 15 nanoseconds of GNSS/UTC." Does it mean
that it includes both bias and the noise? In other words, is it true to say
that "The time-synchronization error between two of them with a long
distance is less than 2*15 ns"?
Best Regards
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To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
--
Chris Albertson
Redondo Beach, California
The long term stability of GPS is very good. Some one here will point
out exactly how one measures it. But roughly when speaking of
accuracy you always need to specify a time interval. For example
if the 1PPS is "off" by 15ns that is not bad and yes there are much
better systems if you need to measure time intervals on the order of
one second. But if the signal is "off" by 15 ns over 100,000 seconds
that is well, 100,000 time better.
Chris,
It's not 100,000 times better; it's not better at all.
If you are looking for timing accuracy then 15 ns is 15 ns. Doesn't matter if it right now, an hour from now, or tomorrow. It's an error, plain and simple. It doesn't get significantly better or worse over time. It is often quoted as an rms statistic on GPS receiver 1PPS specs. You can measure over a few minutes, or a few hours, or a few days -- you'll get approximately the same rms timing error.
What you're probably thinking of is long-term average frequency accuracy -- and then, yes, a bounded error like 15 ns rms looks less and less like a problem as you average longer and longer. But this does not mean the "GPSDO is getting better over time". All it means is the parameter you chose to measure (average frequency) happens to have elapsed time in the denominator, so of course the number gets lower as elapsed time goes on. But nothing tangible is getting "better" over time.
/tvb