[time-nuts] Best GPS 1PPS Accuracy

[paul@wireless]

Mit freundlichen Grüssen
 
Paul Klöckler
Coral links Gmbh
Postfach 6955
CH-3001 Bern
Schweiz

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Randy Warner
Sent: Mittwoch, 13. Dezember 2006 19:27
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Best GPS 1PPS Accuracy

Jack,

Jason is right. I think the M12M is currently at the top of the heap.
Rick Hambly and Tom Clark have developed some circuitry that knocks the
jitter down quite a lot.

Basically, for a "normal" timing receiver (please don't flame me guys, I
know there is other stuff out there) the jitter is dependent on the receiver
micro-controller's clock frequency. For instance, the old UT+ timing
receiver had a jitter spec of +/- 45ns. If you were to compare the short
term 1PPS output from a UT+ to a rubidium, OCXO, or any "stable" reference
oscillator with a 1PPS output you would see that one pulse might show up
about 45ns ahead of the reference 1PPS output, the next one would be about
45ns behind the reference, etc., etc......

The reason for this particular "jitter" of 90ns is because the UT+ used a
crystal running at about 11MHz, giving a period of about 90ns per clock
cycle. Since the receiver can only place 1PPS pulses with a 90ns granularity
it places the pulse as close to the what it thinks is the actual UTC time
tick. One time it will be on one side of the UTC tick, the next time it may
occur after the tick. This is where the term "sawtooth" comes from. In one
of the timing messages the receiver puts out its best guess of how far off
the NEXT pulse is going to be. If you plot this data it looks roughly like a
sawtooth as on average 50% of the pulses are ahead of the time tick and the
other 50% are behind it. I have attached a screenshot from Winoncore12
showing this waveform.

Note that in this screenshot the jitter is about +/-15ns. That's because the
receiver I used for this plot is an M12+ timer. The reason for the increased
resolution is that the M12+T uses a 16.384MHz clock, and that it can place
the pulse on both the rising and falling edges of the clock. This means that
the M12+T has a 30ns granularity, resulting in a
+/-15ns amplitude of the sawtooth (jitter).

Note that these are all "perfect world" numbers, which is why they are
expressed as 1 Sigma values. Actual realizable performance ultimately
depends on how good the underlying code running the GPS receiver is.
This is why we all ultimately rely on the experts in the timing community
(many of whom are on this list) to test and publish "real world" numbers,
instead of some marketeer's spin on the performance of the receiver he
happens to hawking at the time..... 

Sorry for the long post....

Randy Warner
Senior Applications Engineer
Synergy Systems, LLC
randy at synergy-gps.com
________________________________________________________________________
____________________

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Jason Rabel
Sent: Wednesday, December 13, 2006 9:39 AM
To: 'Discussion of precise time and frequency measurement'
Subject: Re: [time-nuts] Best GPS 1PPS Accuracy

One problem is that older GPS receiver spec sheets give numbers with SA on,
even though SA has been off for quite some time, so their numbers will be
inherently higher.

Probably the most accurate GPS receiver today would be the Motorola M12M
Timing. I would go into more detail but I'm about to head out the door.
I'm sure others will chime in on this soon.

There were some posts a few days ago about some new papers that were
published, there is some good info on the various recent versions of the
Motorola receivers... Might want to check that out.

Jason 

> Can someone explain the accuracy numbers that are represented in specs

> for
GPS
> receivers?

> I find that Trimble says the Resolution T
> (http://www.trimble.com/resolutiont.shtml) has 15 ns (1 Sigma) like 
> the
M12+.

> So I guess the real question is; are you comparing apples to apples 
> when
the
> standard deviation (Sigma) is specified?

> Another question is; what's the most accurate GPS receiver module
available? I
> realize this is only as accurate at the antenna system, but I'll save 
> a multipath discussion for later.


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