Jim,

Generally Navigation receivers

I think the most common interpretation is that the first standard deviation
of the error is 10ns.

But many times this applies only after you have applied a correction based
on the sawtooth function and the cable lengths  And even then only with a
clear view of the sky and a correct antenna.

On Sun, Feb 16, 2014 at 11:55 AM, Jimmy D. Burrell jimmydburr@gmail.comwrote:

--

Chris Albertson
Redondo Beach, California

On 2/16/14 11:55 AM, Jimmy D. Burrell wrote:

Hah..
It's read the specs carefully and ask questions.

And is that "always" within 1 us? or 90% or 2 sigma or ??

ANd is that spec assuming some accuracy of the underlying GPS.  At 1 us,
it probably doesn't matter, but at 10ns, you start worrying about
position uncertainty.

Jim,

Generally navigation receivers don't include survey and position hold
features so the time solution accuracy is typically about +/- 1us.
Timing receivers survey their position over a large number of samples
(typically 10,000) and go into position hold mode once the survey
completes. The fixed position allows higher accuracy in determining
the time solution, typically to +/- 1ns. However the 1PPS output is
placed on the nearest GPS clock edge, typically derived from an XO,
so the pulse placement resolution is limited by the GPS clock period.
The GPS XO clock drifts so the 1PPS placement also drifts over the
clock period, creating a "sawtooth" like displacement in time over
the GPS clock period. With a receiver like the M12+ the placement
varies roughly +/- 12ns for a 25ns 1 sigma 1PPS accuracy. For better
accuracy the M12+ also includes a message with the predicted 1PPS
placement error of the next pulse to the +/- 1ns time calculation
resolution. The combination of the 1PPS placement to the nearest
clock edge and the sawtooth correction message giving the placement
error allows resolution of the GPS time to +/- 1ns using either a
software correction of the sample data or hardware correction of
the 1PPS pulse using a variable delay.

Richard

There are two numbers that need to be specified, one is relative (back and forth wander around some fixed offset) one is absolute (how close to the real UTC pulse is it?).

Most of the time relative stability is given which is the first number, and one has no idea how much unit to unit offset and offset to UTC there is. Unless you sit at the USNO or NIST office there really is no way to tell how much offset to the real UTC your receiver has.

Please note that almost all GPS receivers I know claim to generate UTC time not GPS time. There can be up to 100ns offset between the two but lately the offset is held at around 10ns or less.

Having access to a lot of GPS receivers, I can say that the Motorola M12M is very close to the ublox receivers (better than 20ns on typical units), and a typical ublox to ublox offset would be about 5ns on the timing units with SBAS in position hold mode.

The days where GPS receivers only had 1us stability or accuracy are long gone and today even typical navigation receivers achieve better than 25ns accuracy and stability.

Can you get down to 2ns stability? Supposedly with the M2M using sawtooth correction. But who knows what the offset to UTC is on late model iLotus M2M..

Not an issue since most receivers have antenna delay correction features that lets a user fine-tune the UTC offset to some reference if that is desired so you can tune a batch of GPS to within 1ns of each other on the bench.

Bye,
Said

Sent From iPhone

On Feb 16, 2014, at 13:31, "Richard H McCorkle" mccorkle@ptialaska.net wrote:

Richard,

I confess, heretofore when I saw "sawtooth" conversations, and tried to follow the thread, my eyes would glaze over after one or two emails. I presumed they were strictly discussing some RC constant circuit-design/tradeoffs type "sawtooth".

Your explanation 'lit the light bulb' for me on this topic.

Thanks for your response.

Jim...
N5SPE

On Feb 16, 2014, at 3:31 PM, Richard H McCorkle mccorkle@ptialaska.net wrote:

The smart guys have already contributed to this thread.  What I have is an example taken when I was doing some testing.  The plot at the top is the DAC on my GDPSO.  It is moving the oscillator back and forth about 4.6E-4 Hz.  Not a lot. 

The bottom plot is the interesting one, as it is the 1PPS from my Adafruit Nav Receiver plotted with respect to the OCXO.  The labeling is unfortunate for this discussion.  Just think of "-10" as being +180 degrees of phase, and -30 as being -180 degrees, for a total phase of 360 degrees between them..  Notice the plot at about 10:40.  It points out that the 1PPS error isn't nicely spread out.  Rather, it moves around as the difference between the time signal being received from the satellite and the free-running oscillator in the GPS receiver

http://www.evoria.net/AE6RV/GPSstd_PLL/Plots/PPS%20Anomaly%202.png

Bob

Bob,

The 5ns time stamper data attached shows a 500 second hang in the
1PPS phase from an M12+. A graphic example of why including
sawtooth correction in a GPSDO is a good idea. With 500 samples
having an offset even long filter times create a source control
error unless the offset is corrected in the samples before
entering the control loop.

Richard

http://www.leapsecond.com/pages/m12/sawtooth.htm

/tvb (i5s)

Yes,  "Sawtooth" is data sent over the serial port.    Actually it is not a
sawtooth until you plot its value over time.  When you are applying it, it
is just a single number.

On Sun, Feb 16, 2014 at 4:33 PM, Jimmy D. Burrell jimmydburr@gmail.comwrote:

--

Chris Albertson
Redondo Beach, California