GPS 1 pulse-per-second signal

[Richard Henry]

Mnay GPS receivers have a 1 PPS output which is supposedly
synchronized to UTC, given enough good satellite signlas. However,
the pulse shape and timing seems to vary from one device to another.
Does anyone have any advice on how to use this signal for
synchronizing the operation of two or more objects spread over the
Earth's surface?

 

[mpm]

Mnay GPS receivers have a 1 PPS output which is supposedly
synchronized to UTC, given enough good satellite signlas. However,
the pulse shape and timing seems to vary from one device to another.
Does anyone have any advice on how to use this signal for
synchronizing the operation of two or more objects spread over the
Earth's surface?

I would check out how paging companies (remember them?) used GPS to
simulcast their transmitters in a given area. The timing resolutions
were much, much tighter than what you're looking for. I want to say
there's a 10MHz carrier you can lock to, but I just don't recall. It
was a long time ago....

Glenayre was a major player in this technology so I would start there.
Depeneding on how many you needed, I'll bet you can get this gear very
cheaply in the used market. -mpm

 

[Vladimir Vassilevsky]

Mnay GPS receivers have a 1 PPS output which is supposedly
synchronized to UTC, given enough good satellite signlas. However,
the pulse shape and timing seems to vary from one device to another.

This is very true; the output pulse is different. Also, if a module is
not specified as a timing source, there can be the significant jerks of
phase on the 1 PPS signal.

Does anyone have any advice on how to use this signal for
synchronizing the operation of two or more objects spread over the
Earth's surface?

It depends on the accuracy. If you need a precise synchronization, you
should use modules which are specified as the timing sources.

Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

 

[James Arthur]

Mnay GPS receivers have a 1 PPS output which is supposedly
synchronized to UTC, given enough good satellite signlas. However,
the pulse shape and timing seems to vary from one device to another.
Does anyone have any advice on how to use this signal for
synchronizing the operation of two or more objects spread over the
Earth's surface?

Use an ovenized local reference and lots of averaging...

http://www.rt66.com/~shera/index_fs.htm

Certain of Rockwell's Jupiter chipsets output a 10kHz reference
signal--that allows much faster acquisition. Googling frequency
reference + jupiter + gps gives some good examples.

HTH,
James Arthur

 

[Jeff Liebermann]

Mnay GPS receivers have a 1 PPS output which is supposedly
synchronized to UTC, given enough good satellite signlas. However,
the pulse shape and timing seems to vary from one device to another.

Yep. The time between pulses is guaranteed to be exactly 1 second.
However, the time of any particular pulse is NOT guaranteed to be
synchronized with any particular time or event. If you take two GPS
receivers, and compare the 1 pps outputs, they could easily be at
quite different times. However, each succeeding pulse will be exactly
1 second later.

Does anyone have any advice on how to use this signal for
synchronizing the operation of two or more objects spread over the
Earth's surface?

Sure. Get a receiver that is suitable for running a clock, not an
interval timer. The complexity and cost will depend on how accurately
you're trying to sychronize. Lacking numbers, I can't offer any real
solutions.

Maybe something like this will work:
<http://www.gpsclock.com/specs.html>
"The GPSClock 200 has an RS-232 output that provides NMEA
time codes and a PPS output signal. About a half-second before,
it outputs the time of the next PPS pulse in either GPRMC or
GPZDA format. Within one microsecond of the beginning of the
UTC second, it brings the PPS output high for about 500 ms."

If you can live with some atmospheric and propagation drift, the
cheapest solution is a WWVB 60KHz receiver and clock.
<http://tf.nist.gov/stations/radioclocks.htm>
The problem is that WWVB only works in North America.

 

[mpm]

Yep. The time between pulses is guaranteed to be exactly 1 second.
However, the time of any particular pulse is NOT guaranteed to be
synchronized with any particular time or event. If you take two GPS
receivers, and compare the 1 pps outputs, they could easily be at
quite different times. However, each succeeding pulse will be exactly
1 second later.

Another interesting oddity about GPS is that the position error
reported by two otherwise identical units will be dependent on which
satellites have been acquired. This problem comes up a lot in
differential GPS time & distance measurements.

 

[Hal Murray]

Mnay GPS receivers have a 1 PPS output which is supposedly
synchronized to UTC, given enough good satellite signlas. However,
the pulse shape and timing seems to vary from one device to another.
Does anyone have any advice on how to use this signal for
synchronizing the operation of two or more objects spread over the
Earth's surface?

What sort of things are you trying to do? How accurate do
they need to be synchronized?

The GPS units I've worked with all specify that the leading edge
of the pulse will be very close to the second tick of UTC.

If you need (or could use) a PC to do whatever you want to do,
I'd plug the GPS unit into a PC running ntpd.

If you only need time within a few 10s or 100s of ms, you
may not need the GPS unit.

Lots of info in either comp.protocols.time.ntp or
http://www.ntp.org/
There is a wiki at
http://support.ntp.org/bin/view/Support/WebHome
which has a page on setting up refclocks
http://support.ntp.org/bin/view/Support/ConfiguringRefclocks

The Garmin USB 18 LVC is popular because it's the lowest cost
unit with a PPS signal. Some assembly required.

 

[Steve]

Yep. The time between pulses is guaranteed to be exactly 1 second.
However, the time of any particular pulse is NOT guaranteed to be
synchronized with any particular time or event. If you take two GPS
receivers, and compare the 1 pps outputs, they could easily be at
quite different times. However, each succeeding pulse will be exactly
1 second later.


Sure. Get a receiver that is suitable for running a clock, not an
interval timer. The complexity and cost will depend on how accurately
you're trying to sychronize. Lacking numbers, I can't offer any real
solutions.

Maybe something like this will work:
<http://www.gpsclock.com/specs.html>
"The GPSClock 200 has an RS-232 output that provides NMEA
time codes and a PPS output signal. About a half-second before,
it outputs the time of the next PPS pulse in either GPRMC or
GPZDA format. Within one microsecond of the beginning of the
UTC second, it brings the PPS output high for about 500 ms."

If you can live with some atmospheric and propagation drift, the
cheapest solution is a WWVB 60KHz receiver and clock.
<http://tf.nist.gov/stations/radioclocks.htm>
The problem is that WWVB only works in North America.

--
Jeff Liebermann [email protected]
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

GPS timing receivers, such as Trimble Resolution-T or disciplined
oscillators such as Trimble mini-Thunderbolt or Jackson Labs Fury will
output a 1PPS whose leading edge is within +/- 15 nsec of UTC at any
location on the earth (once fully locked). The pulse widths vary from one
vendor to another, but the leading edges are the only significant data in
the 1PPS outputs. Different vendor receivers have better or worse jitter
specs, but the fact that all are aligned to UTC (or GPS master time which is
an integral number of seconds offset from UTC) makes them suitable for
aligning clocks that are scattered around the world.

These timing receivers all make use of their GPS position in the world to
account for propagation delays, and provide a stable pulse that is very
accurate in frequency as well as time-aligned to a universal standard.
Accuracy is lower if your clocks are moving.

Steve

 

[Richard Henry]

Mnay GPS receivers have a 1 PPS output which is supposedly
synchronized to UTC, given enough good satellite signlas. However,
the pulse shape and timing seems to vary from one device to another.
Does anyone have any advice on how to use this signal for
synchronizing the operation of two or more objects spread over the
Earth's surface?

Thanks for the responses and links.

What I have learned so far from this thread, the links referred to,
and a little studying on my own (please correct any errors I make
here):

There is an instant every second where UTC time transitions from one
second to the next. The point of 1 PPS signal is to allow determining
that exact instant so that devices placed remotely from each other can
synchronize their operation without direct contact. An example use of
such a system would be synchronizing a radio transmitter with a
receiver.

If a device knows its exact postion on the earth it only needs a good
signal from one satellite to determine the second transition instant.
If the device does not know its position, or knows that it is moving,
it needs data from 4 satellites to determine the position and thus the
accurate timing.

Even though there is an indicator of the exact instant in the
satellite's transmitted data, the signal cannot simply be received,
decoded and used as is. The receiver must account for the estimated
rf space and atmospheric propogation delays, a local estimate of
antenna and cabling delays, snd some electronic circuitry delay. It
can then adjust the received signal back to UTC second transition
time, and compare that calculation to an estimate derived from an
internal clock, adjusting the internal estimate accordingly.
Hopefully the internal estimate will be good enough that the
adjustment will never be more than one cycle of the reference clock.
The estimate is what is used to generate the 1 PPS UTC-synch
ronized pulse.

 

[Iwo Mergler]

Mnay GPS receivers have a 1 PPS output which is supposedly
synchronized to UTC, given enough good satellite signlas. However,
the pulse shape and timing seems to vary from one device to another.
Does anyone have any advice on how to use this signal for
synchronizing the operation of two or more objects spread over the
Earth's surface?

Watch the jitter on the pulse. Some GPS implementations do it
in software - with a few microseconds of jitter. Some receivers
generate the pulse in hardware and can achieve nanosecond jitter.
The later ones are normally special designs for time keeping.

It very much depends on your application. All GPS PPS
implementations are accurate long term.

To synchronize things, you need to watch the *specified* edge
of the PPS signal (usually the 0-1 transition), which usually
marks the moment which you'll find in the NMEA messages of the
next second.

In other words, the timestamps in the NMEA will give you time
accurate to the second and the edge of the PPS signal takes it
to fractions of a second.

Kind regards,

Iwo