[time-nuts] GPS timing recievers and INTERFERENCE

[Randy Warner]

James,

It really comes down to clock speed on the receiver. We haven't noted
much of a change in accuracy since SA was turned off because other
factors cause much more error that SA ever did so long as you are
tracking enough satellites to average everything out.

Everything else being equal (meaning no firmware anomalies...), an M12+T
will be about 3 times better jitter-wise than the UT+ simply because the
M12+T's clock runs at a higher frequency and the fact that the M12+T is
able to place the 1PPS on BOTH the rising and falling edges of the
clock. This is where the sawtooth correction comes in. Basically, all
the sawtooth is telling you is that if the receiver had a clock rate of
infinity it would place the 1PPS as close to the top of the UTC second
as firmware and other system variables allow. Since it is limited to
clock edges you end up with the "granularity" that the sawtooth
displays. Rick ran three of the newest M12+T's for Motorola and us at
USNO and Motorola modified the code as necessary to place the 1PPS from
the M12+T right on top of the UTC second. As I recall, the UT+'s had
about a 100ns offset.

The +/-10ns jitter you are seeing is typical for the M12+. Again, it is
related to clock frequency. There really is no magic at work here. Rick
has come up with some tricky stuff that allows him to increase the
resolution (decrease the granularity) and thus shave some nanoseconds
from the jitter.

Randy

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-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of James Maynard
Sent: Wednesday, May 03, 2006 10:56 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] GPS timing recievers and INTERFERENCE

Glenn wrote:

> 
> BTW, Timing accuracy of the UT+ is listed as "In position hold mode, 
> <50ns (1 sigma) with SA on." Does anyone have claimed or measured 
> values for timing accuracry WITHOUT SA?
> 
I'm using an M12+ timing receiver, included in my new "CNS Clock II)
from Rick Hambly of CNS Systems. In the manual for the CNS Clock II,
Rick claims 20 ns or less one-sigma jitter on the 1 Hz pulses. On his
Web page describing this product,

     http://www.cnssys.com/cnsclock/CNSClockII.html,

Rick claims typical jitter of plus or minus 27 ns (15 ns one-sigma)
without the "High Performance PPS Option."  With that option, which
provides a hardware correction for sawtooth error on the 1 PPS output,
he claims plus or minus 11 ns jitter on the 1 Hz output pulses (3 ns
one-sigma).

I am seeing a one-sigma jitter of about 9 or 10 nanoseconds.  This isn't
as good as Rick claims, but then my antenna location, although up on the
roof, is shadowed by some rather large trees and may be experiencing
some multipath effects.

As to its absolute accuracy, I don't know, because I have not completely
allowed for cable delay from the antenna to the M12+, nor for cable
delay from the M12+ to my time interval counter. Also, I'm still waiting
for final settling of the long-time-constant phase-locked loop by which
my Rb frequency standard disciplines its frequency to agree with the 1
Hz pulses from the GPS receiver.

If the M12+ timing receiver had an optimum antenna location, with
multipath suppressed, I think the 3-nanosecond one-sigma accuracy on the
timing pulses is probably acheivable. That is, I have no reason to doubt
Rick's measurements. With my installation, however, I see more like
10-nanosecond one-sigma uncertainty.

-- 
James Maynard, K7KK
Salem, Oregon, USA


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