Re: [time-nuts] Some results of PRS10 and Trimble Resolution
Hello Tom,
I had a conversation with Sam S. from TSC the other day, and he said that
it's probably not possible to get <10ns GPS accuracy anyways due to the
multipath issues, Ionospheric issues, antenna survey issues, thermal issues etc.
I tend to agree with Sam; to get this kind of accuracy to UTC ([or better:
GPS time] a documented 2ns within 300 hours as tested on the M12+ by
Synergy/USNO) one needs to average over several hours, or even days as done by USNO.
This is documented in many long term plots of GPS versus Cesium.
Instantaneous drift in all of these plots seems to be >>+-5ns due to diurnal effects etc.
Our units typically average the GPS 1PPS over 30 minutes, so having less
than 3.33ns error on the 1PPS capture may not improve things much because while
the error stays always at +-3.33ns it get's averaged over the measurement
intervall.
Of course this would be different if we use a Cesium 1PPS output to lock to.
Also, one must ask the question: what is the correct time? Is the relative
time between two GPS disciplined stations close to each other such as are used
for CDMA base stations (some of the errors cancel out between the two
stations' frequency differences) good enough? Certainly diurnal and ionospheric
effects between the two stations will cancel if they see the same sat's.
Sam had a point in that to get 1ns accuracy from your GPS, you must have a
fantastic antenna not affected by environmental effects, and also know it's
position to within 1 foot in all three dimensions!
bye,
Said
Why are dual freq. receivers not used in timing instruments? Ionospheric
They are. But only in the high-end instruments.
errors are almost removed. Surveyors in every small town on earth are
routinely getting sub 5cm accuracy in real time. Are not time labs using
it to transfer their cesium time to each other?
True, but what's your guess on the price ratio between
a 5 cm dual frequency survey-grade timing receiver and
a M12 or Res-T? I'd guess 100x to 1000x. Anyone
know for sure?
/tvb
On Fri, June 30, 2006 4:34, SAIDJACK@aol.com said:
Hello Tom,
I had a conversation with Sam S. from TSC the other day, and he said that
it's probably not possible to get <10ns GPS accuracy anyways due to the
multipath issues, Ionospheric issues, antenna survey issues, thermal
issues etc.
Why are dual freq. receivers not used in timing instruments? Ionospheric
errors are almost removed. Surveyors in every small town on earth are
routinely getting sub 5cm accuracy in real time. Are not time labs using
it to transfer their cesium time to each other?
--
Björn
From: "Tom Van Baak" tvb@leapsecond.com
Subject: Re: [time-nuts] Some results of PRS10 and Trimble Resolution
Date: Thu, 29 Jun 2006 22:44:53 -0700
Message-ID: 001b01c69c08$50e08280$be0ff204@computer
Why are dual freq. receivers not used in timing instruments? Ionospheric
They are. But only in the high-end instruments.
The metrology labs use survey GPS receivers for this obvious reason.
errors are almost removed. Surveyors in every small town on earth are
routinely getting sub 5cm accuracy in real time. Are not time labs using
it to transfer their cesium time to each other?
True, but what's your guess on the price ratio between
a 5 cm dual frequency survey-grade timing receiver and
a M12 or Res-T? I'd guess 100x to 1000x. Anyone
know for sure?
Hmm, come to think of it, I have never really cared to figure it out. The
surveying folks have fancy antennas too. They go around the world (south pole
included) and measure all kinds of stuff, as the drift-rate of the polar ice
in the Antarctic.
The necessary components isn't directly high-volume compared to the normal
L1 C/A receivers.
Cheers,
Magnus
On Fri, June 30, 2006 7:44, Tom Van Baak said:
Why are dual freq. receivers not used in timing instruments? Ionospheric
They are. But only in the high-end instruments.
errors are almost removed. Surveyors in every small town on earth are
routinely getting sub 5cm accuracy in real time. Are not time labs using
it to transfer their cesium time to each other?
True, but what's your guess on the price ratio between
a 5 cm dual frequency survey-grade timing receiver and
a M12 or Res-T? I'd guess 100x to 1000x. Anyone
know for sure?
www.javad.com does have online pricing, not sure what discount "real"
customers buy at. Dual freq receivers are available on ebay.
Whats the price rations between TXCO, OCXO, Rb, Cs and H-maser? When is it
worth the gain do go dual freq GPS, depending on what frequency standard
you are comparing, transfering or steering...
--
Björn
Hello Tom,
I had a conversation with Sam S. from TSC the
other day, and he said that it's probably not
possible to get <10ns GPS accuracy anyways
due to the multipath issues, Ionospheric issues,
antenna survey issues, thermal issues etc.
Yes, with a standard GPS receiver or GPSDO, I
very much agree with this. See also the links to
GPS papers I posted earlier today which will give
you a feel for what level of accuracy or stability
you get from various GPS time transfer techniques.
Note accuracy and stability are two different goals.
Related to that, GPS-based frequency reference
products are plentiful, cheap, and in widespread
use while GPS-based time transfer products are
few, very expensive, and have a small use base.
I think most of us time-nuts use GPS as a source
of precise time interval (GPS as a ~ 1e-13 stable
frequency reference) rather than a source of
absolute time (~ ns accurate UTC). This is why
uncalibrated GPS receivers work for all of us.
See Rick's papers on M12 calibration at USNO.
http://www.gpstime.com/
http://tycho.usno.navy.mil/ptti/ptti2002/paper9.pdf
Note NIST's use of regular GPS receivers (Oncore?)
for frequency stability (not time accuracy):
http://www.tf.nist.gov/service/fms.htm
So 10 ns stability with GPS over a day is quite
doable on the cheap. < 10 ns accuracy with GPS
is quite another matter and requires a huge amount
of work. On this list, DougH is probably the only
one who's pulled it off:
http://www.leapsecond.com/time-nuts.htm
Our units typically average the GPS 1PPS over
30 minutes, so having less than 3.33ns error on
the 1PPS capture may not improve things much
because while the error stays always at +-3.33ns
it get's averaged over the measurement intervall.
3.3ns / 30 m = 2e-12 and sets your lower bound.
But it would depend on your choice of LO and PLL
if this is a limiting factor or not.
Here's something to try: deliberately degrade your
1PPS TIC resolution in software and see what effect
it makes on the stability of your RF or 1PPS output.
/tvb
Hi Björn:
I don't think that's that case. I live in a small town and hired a
local surveyor chosen because he's the guy that uses GPS.
He was able to use two GPS receivers separated by say 100 feet and
connected with a cable to determine an accurate relative bearing between
his total station and the remote GPS. I had placed wood stakes at a few
locations that were very close to Longitude and Latitude points ending
exactly on the seconds, for example 123:09:50.0W by 39:11:24.0N. The
idea was for him to locate the exact points and to locate the house GPS
antenna. To do this he needed to go back to his office and post process
the data.
Since the points I wanted to know about were separated by exactly 1
second of Lat and 1 second of Lon he just placed his total station a few
feet from the corner one and shot the other stakes.
It's my understanding the the military has the crypto key needed to use
the L2 channel to full effect and that's not available to surveyors.
What surveyors do is record the carrier phase of both the L1 and L2
signals and resolve the ambiguity in post processing. There are a
couple of flavors of post processing. In one only the data recorded in
the field is used, in the other the filed data is combined with actual
data taken at a nearby reference station from the satellites used in the
field data. The second method corrects for errors in the ephemeris data
broadcast by the satellites.
I think what you may be referring to are the differential correction
methods, either Low Frequency or satellite broadcasts that improve the
position accuracy of GPS receivers. But I don't think these can be used
to improve timing accuracy.
Have Fun,
Brooke Clarke
--
w/Java http://www.PRC68.com
w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml
http://www.precisionclock.com
bg@lysator.liu.se wrote:
. . . .
Why are dual freq. receivers not used in timing instruments? Ionospheric
errors are almost removed. Surveyors in every small town on earth are
routinely getting sub 5cm accuracy in real time. Are not time labs using
it to transfer their cesium time to each other?
--
Björn
time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
On Fri, June 30, 2006 20:49, Brooke Clarke said:
Hi Björn:
I don't think that's that case. I live in a small town and hired a
local surveyor chosen because he's the guy that uses GPS.
Maybe I was wrong with extrapolating the local situation to a global level.
With the present dense network of "CORS"-like stations [1]. Surveyors are
moving away from running their own RTK-base stations. With close to full
coverage RTK-corrections is distributed via the cellphone network, which
means the surveyor only need the rover side of the traditional equipment.
antenna. To do this he needed to go back to his office and post process
the data.
That makes his GPS recievers cheaper, than the ones where the software
options for RTK is enabled. He also does not need a real time radio
connection. Precision is about the same or better, since he can download
better ephemeris, and can tinker more with parameters in the software.
It's my understanding the the military has the crypto key needed to use
the L2 channel to full effect and that's not available to surveyors.
That is true in theory. In reality Ashtech (and others) early found that
the crypto code rate is much lower then the chipping rate. Details was
posted here in the last 6 months? IIRC the known P-code will flip sign
every 511 bits depending on the crypto code. This makes the highend
receivers do much more than intended on L2.
What surveyors do is record the carrier phase of both the L1 and L2
signals and resolve the ambiguity in post processing. There are a
couple of flavors of post processing.
And there is real time flavors as well, with the best going down below 5cm
errors.
I think what you may be referring to are the differential correction
methods, either Low Frequency or satellite broadcasts that improve the
position accuracy of GPS receivers. But I don't think these can be used
to improve timing accuracy.
These are the SBAS (WAAS, EGNOS, MSAT), commercial satellite based, "coast
guard" DGPS, etc. Where the good do sub 1m and SBAS a little worse.
Why would these not make absolute accuracy better? They do improve
positioning.
--
Björn
[1] http://swepos.lmv.lm.se/natverksrtk/nat_postj_031012.gif
From: bg@lysator.liu.se
Subject: Re: [time-nuts] Some results of PRS10 and Trimble Resolution
Date: Fri, 30 Jun 2006 22:10:42 +0200 (CEST)
Message-ID: 65357.212.181.149.145.1151698242.squirrel@webmail.lysator.liu.se
On Fri, June 30, 2006 20:49, Brooke Clarke said:
Hi Björn:
I don't think that's that case. I live in a small town and hired a
local surveyor chosen because he's the guy that uses GPS.
Maybe I was wrong with extrapolating the local situation to a global level.
With the present dense network of "CORS"-like stations [1]. Surveyors are
moving away from running their own RTK-base stations. With close to full
coverage RTK-corrections is distributed via the cellphone network, which
means the surveyor only need the rover side of the traditional equipment.
antenna. To do this he needed to go back to his office and post process
the data.
That makes his GPS recievers cheaper, than the ones where the software
options for RTK is enabled. He also does not need a real time radio
connection. Precision is about the same or better, since he can download
better ephemeris, and can tinker more with parameters in the software.
It's my understanding the the military has the crypto key needed to use
the L2 channel to full effect and that's not available to surveyors.
That is true in theory. In reality Ashtech (and others) early found that
the crypto code rate is much lower then the chipping rate. Details was
posted here in the last 6 months?
I might have been the guilty party for that one. I think I posted something on
that here.
IIRC the known P-code will flip sign every 511 bits depending on the crypto
code. This makes the highend receivers do much more than intended on L2.
The P-code is being encrypted into Y-code by XOR-ing it with a W-code (also
known as A-code in some papers) which has a chipping rate of 511 kchips/s.
There is several different strategies around this encryption. One of the first
was by squaring the received signal and lock onto that. The W-code will cancel
and P-code tracking is possible. It has however several limits. The Z-tracking
algorithm used by Aztech geodesic receivers is a bit more advanced. By doing
the C/A to P code handover on L1 (where C/A and P correlate closely) a rough
W-code estimade is made by mixing up the received L1 channel with the locally
procduced P-code. The L2 P-channel is then decoded by the W-code estimate and
P-code tracking is acheived by using a delayed variant of the P-code. The time
difference between L1 and L2 will contain the delay-difference due to mainly
ionospheric delay and with that you are enabled to come up with a more correct
model of the delays than normal C/A code ever gives you, and quite similar to
that of what the coded receivers have. What you don't get is the fancy direct
breakin into Y-code which the modern munition receiver do for instance.
The codeless L1&L2 receivers does not provide a much higher threat to the US
military than they anticipate from C/A receivers, so to the best of my
knowledge no real issue is being made of it. They still depend on the C/A code
to operate over time and there are means to deal with that situation anyway,
which the US military isn't particular afraid of applying.
L1&L2 receivers not only have added baseband complexity, it goes from the top
down with antenna etc. If you are a bit fancy, you do GLONASS too. Now we
should start considering L4 and soon GALILEO. Only a few bothers with L3, which
occassionally transmitt C/A, but for quite dedicated uses (erhm!).
What surveyors do is record the carrier phase of both the L1 and L2
signals and resolve the ambiguity in post processing. There are a
couple of flavors of post processing.
And there is real time flavors as well, with the best going down below 5cm
errors.
Indeed.
I think what you may be referring to are the differential correction
methods, either Low Frequency or satellite broadcasts that improve the
position accuracy of GPS receivers. But I don't think these can be used
to improve timing accuracy.
These are the SBAS (WAAS, EGNOS, MSAT), commercial satellite based, "coast
guard" DGPS, etc. Where the good do sub 1m and SBAS a little worse.
Why would these not make absolute accuracy better? They do improve
positioning.
Doesn't care, it's summertime so we don't know what time it really is! :P
Discussion has been going on to spread that as a freer service, but I don't
know where it ended.
Cheers,
Magnus