Synchronisation using GPS
Hello, I am currently looking at GPS as a possible way of synchronising
transmission/reception within a radar network. I was wondering if anyone
had perhaps previously used GPS to synchronise various locations in this way
(mine will likely be a couple of km apart max) and if there were any
recommended receiver models that might be suitable for this purpose.
Ideally something approaching differential location accuracy of <1m with
synchronisation error between receiver locations of <5ns would be do-able;
has anyone ever undertaken anything similar?
Shaun
Shaun,
With a short baseline of a few km, this should be
doable. You might want to contact Rick Hambly
of CNS Systems ( http://www.cnssys.com/ ) who
has done a lot of work in this area, including
calibrating GPS receivers at USNO.
Do you need absolute synchronization against
UTC, or just relative synchronization among each
of several sites?
Depending on how quick you need the results,
I think many GPSDO such as Z3801A may not
give you this level of accuracy, either in absolute
or differential mode. But a plain GPS timing
receiver might be able to do it.
I have a similar need this summer and plan to
use sawtooth corrected M12+ GPS receivers
with identical antennas and cable feeds. By
co-locating all the antennas in a one-time trial
experiment you can determine the fixed receiver
offset of each unit, which you then subtract for
all subsequent runs.
The other time-honored approach, of course, is
carry a Cesium clock between the sites. Since
the sites are located within a few minutes of each
other a good cesium will hold 5 ns.
/tvb
http://www.LeapSecond.com
----- Original Message -----
From: "Shaun Doughty" s.doughty@ee.ucl.ac.uk
To: time-nuts@febo.com
Sent: Monday, August 08, 2005 05:24
Subject: [time-nuts] Synchronisation using GPS
Hello, I am currently looking at GPS as a possible way of synchronising
transmission/reception within a radar network. I was wondering if anyone
had perhaps previously used GPS to synchronise various locations in this
way
(mine will likely be a couple of km apart max) and if there were any
recommended receiver models that might be suitable for this purpose.
Ideally something approaching differential location accuracy of <1m with
synchronisation error between receiver locations of <5ns would be do-able;
has anyone ever undertaken anything similar?
Shaun
From: "Shaun Doughty" s.doughty@ee.ucl.ac.uk
Subject: [time-nuts] Synchronisation using GPS
Date: Mon, 8 Aug 2005 13:24:14 +0100
Message-ID: 005901c59c14$16ec96d0$61292880@willow
Shaun,
Hello, I am currently looking at GPS as a possible way of synchronising
transmission/reception within a radar network. I was wondering if anyone
had perhaps previously used GPS to synchronise various locations in this way
(mine will likely be a couple of km apart max) and if there were any
recommended receiver models that might be suitable for this purpose.
The VLBI community excells at this. Look at what they are doing.
Ideally something approaching differential location accuracy of <1m with
synchronisation error between receiver locations of <5ns would be do-able;
has anyone ever undertaken anything similar?
Relative timing like that should certainly be doable using the right equipment
and right setup. People that do this uses Geodesic receivers such as
Ashtech Z12 and a good antenna.
With the relative proximity of only a few km away, the common view aspect will
correlate quite well.
Using terms like "common view" and "Ashtech Z12" you will Google yourself to
alot of usefull information. You should harvest the archives of NIST, PTTI etc.
for good articles on the topic.
This in addition to Toms M12+ links should surely help you to get a better
understanding.
Cheers,
Magnus
Thanks, the VLBI stuff looks pretty relevant and interesting, and I notice
that CNS managed to get an accuracy of a few nanoseconds between receivers
with the M12+'s that Tom mentioned at a 21.5km baseline, which is
encouraging too.
Within the radar network itself we are only really concerned about relative
synchronisation between several sites, but this doesn't seem to be something
that is reported on the majority of receiver data sheets - generally I guess
it's just accuracy to UTC reported, so any information on relative
synchronisation like the CNS tests is pretty useful for deciding whether GPS
is a good idea to replace the current wired system being used as we move to
slightly longer baselengths, and which receiver might be most suitable.
Shaun
----- Original Message -----
From: "Magnus Danielson" cfmd@bredband.net
To: time-nuts@febo.com; s.doughty@ee.ucl.ac.uk
Sent: Monday, August 08, 2005 8:02 PM
Subject: Re: [time-nuts] Synchronisation using GPS
From: "Shaun Doughty" s.doughty@ee.ucl.ac.uk
Subject: [time-nuts] Synchronisation using GPS
Date: Mon, 8 Aug 2005 13:24:14 +0100
Message-ID: 005901c59c14$16ec96d0$61292880@willow
Shaun,
Hello, I am currently looking at GPS as a possible way of synchronising
transmission/reception within a radar network. I was wondering if
anyone
had perhaps previously used GPS to synchronise various locations in this
way
(mine will likely be a couple of km apart max) and if there were any
recommended receiver models that might be suitable for this purpose.
The VLBI community excells at this. Look at what they are doing.
Ideally something approaching differential location accuracy of <1m with
synchronisation error between receiver locations of <5ns would be
do-able;
has anyone ever undertaken anything similar?
Relative timing like that should certainly be doable using the right
equipment
and right setup. People that do this uses Geodesic receivers such as
Ashtech Z12 and a good antenna.
With the relative proximity of only a few km away, the common view aspect
will
correlate quite well.
Using terms like "common view" and "Ashtech Z12" you will Google yourself
to
alot of usefull information. You should harvest the archives of NIST, PTTI
etc.
for good articles on the topic.
This in addition to Toms M12+ links should surely help you to get a better
understanding.
Cheers,
Magnus
If the two positions are fixed, say a permanent VLBI or bi-static radar
installation, You can do pretty well (say < 5 ns delta between sites) with
GPS-steered cesium. The long-term stability of the delta can be even
better (say < 1 ns). Even with civilian one-way GPS, the cesium allows you
to average the position/time with averaging times of days. Once locked, you
have the short-term stability and phase noise performance of a good cesium,
with long-term phase and frequency accuracy driven by the GPS constellation.
Historically, there have been several off-the-shelf GPS-steered-cesium
solutions, there was an option board for the TrueTime XL-DC, which steered
the 5071A, and the Datum 6801, which steered the FTS 4040/CsIII series.
There was even a short-lived product, the Datum Cs+, which crammed it all
into one box. Alas, all of these products are out of production, but you
may find them on the used market.
-RL
Robert Lutwak, Senior Scientist
Symmetricom - Technology Realization Center
34 Tozer Rd.
Beverly, MA 01915
(978) 232-1461 Voice RLutwak@Symmetricom.com (Business)
(978) 927-4099 FAX Lutwak@Alum.MIT.edu (Personal)
(339) 927-7896 Mobile
Dear Shaun,
If you use Geodesic receivers or some special GPS receivers and requires
few ns, please use a phase stable cable and two DC bias passing attenuators
(10dB etc) to antenna side and receiver side, if possible please use
temperature stabilized antenna.
I am using a phase stable cable and two 10 dB DC bias passing attenuators
to 50dB GPS antenna and GPS receiver.
Best regards,
Masamichi Tsuchiya
From: "Shaun Doughty" s.doughty@ee.ucl.ac.uk
Subject: Re: [time-nuts] Synchronisation using GPS
Date: Tue, 9 Aug 2005 11:03:20 +0100
Message-ID: 002f01c59cc9$9233b930$61292880@willow
Thanks, the VLBI stuff looks pretty relevant and interesting, and I notice
that CNS managed to get an accuracy of a few nanoseconds between receivers
with the M12+'s that Tom mentioned at a 21.5km baseline, which is
encouraging too.
Within the radar network itself we are only really concerned about relative
synchronisation between several sites, but this doesn't seem to be something
that is reported on the majority of receiver data sheets - generally I guess
it's just accuracy to UTC reported, so any information on relative
synchronisation like the CNS tests is pretty useful for deciding whether GPS
is a good idea to replace the current wired system being used as we move to
slightly longer baselengths, and which receiver might be most suitable.
Just to give you the short story with minimal of technicalities:
Consider that you have two local sites and they have their own timescales
UTC(A) and UTC(B) (these names are chosen "arbitrarilly"), they both receive
UTC(GPS) (which is a bit incorrect, but lets just say it is what pops out of
the receiver) then you can locally measure the time-differences t1 and t2 which
we define as
t1 = UTC(A) - UTC(GPS)
t2 = UTC(B) - UTC(GPS)
if you now transmitt either one of these to the other site you can then
caluculate t12:
t = t - t = [UTC(A) - UTC(GPS)] - [UTC(B) - UTC(GPS)] = UTC(A) - UTC(B)
12 1 2
Notice how the time given from the GPS system cancels out. Most, but not all,
of the estimation error for UTC(GPS) will also cancel given equalent receivers.
This is the simple mathematical background for GPS common view and it will with
a very minimal transfer of information give you relative timing information.
A pair of M12+ and some minor post-processing should make you happy. Just some
additional processing and you can have your slave site track your master sites
phase using this common view comparision. A pair of time-interval counters and
means to control frequency should do it.
Cheers,
Magnus