At 3:24 PM +0000 2/24/12, time-nuts-requ...@febo.com wrote:
Date: Fri, 24 Feb 2012 14:42:36 +0100
From: b...@lysator.liu.se
To: "Discussion of precise time and frequency measurement"
<time-nuts@febo.com>
Subject: Re: [time-nuts] Neutrinos not so fast? (defective connector)
Bj?rn,
[snip]
>>
High precision GPS receivers use various correlator schemes that try to
minimize multipath. "Normal" GPS receivers are more vulnerable than
geodetic quality receivers.
http://webone.novatel.ca/assets/Documents/Papers/PAC.pdf
You are of course correct, but timing receivers may not go to such lengths
are are needed for geodetic receivers. A lot of the magic of geodetic
receivers is in the choke-ring antenna, which ignores signals arriving
from too low an angle above the horizon.
In the Neutrino case, the multipath is built into the cable between
antenna
and receiver, so the antenna cannot help. But I will read the article,
which looks interesting. Wonder if it would solve such a triple-transit
echo problem.
Joe
I am not discussing antenna multipath attenuation. That is a separate
topic. Look at figure 12 in the Novatel paper - noting for the y-axis -
that 1 C/A chip length is about 300 meters.
Hmm. Isn't 60 nanoseconds (18 meters at C) well within this envelope?
Here is an article from the Ashtech/JPS/JNS/Topcon-family.
http://tinyurl.com/7w9wl4p
Overview results are seen in figure 5.
Thanks. I'll read it.
Timing receivers used by time-labs to compare Cs are usually geodetic
receivers with the option to lock the internal clock to external
10MHz/1PPS signals coming from the Cs.
My point is that good receivers attenuate multipath fairly well.
If it's old, it may well have none of these improvements. The timing
receivers I have used usually say that they are within 100
nanoseconds; I assume this to be the three sigma range. I do know
that in stationary receivers, the Allan Deviation of a given such
receiver is quite good, implying that the error is a slowly drifting
offset. A cable multipath could well cause a bias error.
Is it known which GPS receiver type was used in the Neutrino experiment?
Another poster (Antonio I8IOV) answered this:
"It was a Septentrio PolaRx2e. See the paper on setup and procedures:
<http://dl.dropbox.com/u/13409775/cern-cal.pdf>"
I don't know anything about that make and model. But someone will.
I wonder when this unit was made. And if it has a Rubidium local
oscillator.
Actually, I bet there are multiple Time Nuts with more than one GPS
timing receiver. It is a simple experiment to feed a pair from a
single antenna plus splitter, and put an impedance bump or two in one
feed line, and compare the 1PPS outputs. (I don't have the
equipment.) Or, split, delay one path, recombine, using a variable
attenuatior in the delay path to adjust the overall multipath effect
with which to challenge one of the two receivers receiver.
Joe
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