Re: [time-nuts] 5370B on eBay, carrier phase tracking GPS receiver
Magnus Danielson wrote:
There is another paper just before it. There is also a patent for a
specific
form of implementation.
Note they used an Oncore VP, the Superstar has a better carrier phase
tracking.performance.
What they did was that they hooked their Oncore VPs up to cesium beams
for
test purposes. Locking that clock with a not-so-perfect but sufficient
for
the task ratio.
You can do about the same without too much difficulty, but the Allstar,
Superstar and Superstar II receivers belong to those that is built out
of the
Zarlink chipsets
An interesting product originating from Mitel (Mike and Terry's
lawnmowers later became MITEL - according to their local rep about 20
odd years ago)
which all feature an interesting issue, the use of a 10 MHz
TCXO. Now, this frequency should be fairly know to us time-nuts, no? ;)
Wiring in an externally supplied 10 MHz should not be all that difficult.
The receiver is now effectively a high resolution phase comparator
between the
selected sats and the timing source used for reference. The PPS output
of the
receiver is not used for reference, but rather the carrier phase
pseudo-ranges.
Either you tap the pseudo-range data out and do the dirty stuff
outside, or
you have the receiver do all the calculations. Having a reciever do some
interesting corrections on pseudoranges may be a bit harder thought.
Anyway, if you do things properly, it will be a much better solution than
looking at that PPS.
However if all you have is a 10811 it will limit the short term
satbility of such a GPSDO.
The phase carrier measurements will have a lower noise floor than the
10811 for tau > 10s or so.
A better oscillator (FTS1200, FTS100 Oscilloquartz OSA8607 etc.) will
have a significantly lower noise floor.
However since the latter generally have 5MHz outputs a low phase noise
frequency doubler is required.
Cheers,
Magnus
Bruce
From: Dr Bruce Griffiths bruce.griffiths@xtra.co.nz
Subject: Re: [time-nuts] 5370B on eBay, carrier phase tracking GPS receiver
Date: Mon, 19 Feb 2007 17:41:52 +1300
Message-ID: 45D92A90.307@xtra.co.nz
Magnus Danielson wrote:
There is another paper just before it. There is also a patent for a
specific
form of implementation.
Note they used an Oncore VP, the Superstar has a better carrier phase
tracking.performance.
What they did was that they hooked their Oncore VPs up to cesium beams
for
test purposes. Locking that clock with a not-so-perfect but sufficient
for
the task ratio.
You can do about the same without too much difficulty, but the Allstar,
Superstar and Superstar II receivers belong to those that is built out
of the
Zarlink chipsets
An interesting product originating from Mitel (Mike and Terry's
lawnmowers later became MITEL - according to their local rep about 20
odd years ago)
Hehe... yeah, I know it was in the Mitel days they started this. I even beleive
that the GPS stuff came in with another buy, but I don't recall the details.
The Zarlink reference is more relevant for initial attempts to dig up
datasheets, where as Mitel is usefull for some of the older material.
which all feature an interesting issue, the use of a 10 MHz
TCXO. Now, this frequency should be fairly know to us time-nuts, no? ;)
Wiring in an externally supplied 10 MHz should not be all that difficult.
The receiver is now effectively a high resolution phase comparator
between the
selected sats and the timing source used for reference. The PPS output
of the
receiver is not used for reference, but rather the carrier phase
pseudo-ranges.
Either you tap the pseudo-range data out and do the dirty stuff
outside, or
you have the receiver do all the calculations. Having a reciever do some
interesting corrections on pseudoranges may be a bit harder thought.
Anyway, if you do things properly, it will be a much better solution than
looking at that PPS.
However if all you have is a 10811 it will limit the short term
satbility of such a GPSDO.
The phase carrier measurements will have a lower noise floor than the
10811 for tau > 10s or so.
A better oscillator (FTS1200, FTS100 Oscilloquartz OSA8607 etc.) will
have a significantly lower noise floor.
Indeed. Carrier phase will cut in early as the quieter source than the PPS
method.
However since the latter generally have 5MHz outputs a low phase noise
frequency doubler is required.
Indeed. Some are however available in 10 MHz.
Also, the carrier phase method is usefull for rubidium and cesium comparisions
not to speak about hydrogen maser (not a relevant problem just yeat).
Cheers,
Magnus
Hej Magnus
Magnus Danielson wrote:
An interesting product originating from Mitel (Mike and Terry's
lawnmowers later became MITEL - according to their local rep about 20
odd years ago)
Hehe... yeah, I know it was in the Mitel days they started this. I even beleive
that the GPS stuff came in with another buy, but I don't recall the details.
The Zarlink reference is more relevant for initial attempts to dig up
datasheets, where as Mitel is usefull for some of the older material.
Fuller version is that the later Mitel founders worked for another
telecoms (Bell??) company when they left, they were restrained from
setting up in the same business area for a couple of years so they setup
a business selling lawnmowers until the restraint period expired. Then
the name of their telecoms startup, (MITEL) was derived from the name of
their interim lawnmower business.
Bruce
From: Dr Bruce Griffiths bruce.griffiths@xtra.co.nz
Subject: Re: [time-nuts] 5370B on eBay, carrier phase tracking GPS receiver
Date: Mon, 19 Feb 2007 23:39:47 +1300
Message-ID: 45D97E73.3050702@xtra.co.nz
Hej Magnus
Hej Bruce,
Fuller version is that the later Mitel founders worked for another
telecoms (Bell??) company when they left, they were restrained from
setting up in the same business area for a couple of years so they setup
a business selling lawnmowers until the restraint period expired. Then
the name of their telecoms startup, (MITEL) was derived from the name of
their interim lawnmower business.
Ah, makes perfect illogical sense! :)
It is totally natural to jump from lawnmowers to chipdesign house! :)
Cheers,
Magnus
Magnus Danielson wrote:
From: Dr Bruce Griffiths bruce.griffiths@xtra.co.nz
Subject: Re: [time-nuts] 5370B on eBay, carrier phase tracking GPS receiver
Date: Mon, 19 Feb 2007 23:39:47 +1300
Message-ID: 45D97E73.3050702@xtra.co.nz
Hej Magnus
Hej Bruce,
Fuller version is that the later Mitel founders worked for another
telecoms (Bell??) company when they left, they were restrained from
setting up in the same business area for a couple of years so they setup
a business selling lawnmowers until the restraint period expired. Then
the name of their telecoms startup, (MITEL) was derived from the name of
their interim lawnmower business.
Ah, makes perfect illogical sense! :)
It is totally natural to jump from lawnmowers to chipdesign house! :)
Cheers,
Magnus
Hej Magnus
Mitel actually used to make PABX equipment about 20 years ago. They also
did specialised ICs for such applications.
Bruce
Magnus Danielson wrote:
However if all you have is a 10811 it will limit the short term
satbility of such a GPSDO.
The phase carrier measurements will have a lower noise floor than the
10811 for tau > 10s or so.
A better oscillator (FTS1200, FTS100 Oscilloquartz OSA8607 etc.) will
have a significantly lower noise floor.
Indeed. Carrier phase will cut in early as the quieter source than the PPS
method.
That by itself seems like a significant advantage, being able to set the
loop much faster and therefore being phase locked sooner after power up,
for portable applications.
It seems that in this case, the acquisition time of the receiver will be
the most significant delay.
I guess as a result, it will become more important to have an algorithm
that effectively filters out outliers. Are there any such things as
hanging bridges with carrier phase receivers? (I hate asking that
question...)
Didier KO4BB
Dr Bruce Griffiths wrote:
Hej Magnus
Mitel actually used to make PABX equipment about 20 years ago. They also
did specialised ICs for such applications.
Bruce
That's what I knew them for, we even had a Mitel PABX for a while where
I work. Did not know they were into GPS and never seen their lawnmowers
though...
Maybe they had GPS navigation on their lawnmowers?
Didier KO4BB
Saw an interesting article on GPS steered lawnmowers a few months ago. I
think it was in GPS World and some sort of competition....
:-)
Rob K
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Didier Juges
Sent: 19 February 2007 12:42
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] 5370B on eBay, carrier phase tracking GPS receiver
Dr Bruce Griffiths wrote:
Hej Magnus
Mitel actually used to make PABX equipment about 20 years ago. They
also did specialised ICs for such applications.
Bruce
That's what I knew them for, we even had a Mitel PABX for a while where I
work. Did not know they were into GPS and never seen their lawnmowers
though...
Maybe they had GPS navigation on their lawnmowers?
Didier KO4BB
time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
From: Dr Bruce Griffiths bruce.griffiths@xtra.co.nz
Subject: Re: [time-nuts] 5370B on eBay, carrier phase tracking GPS receiver
Date: Tue, 20 Feb 2007 01:14:37 +1300
Message-ID: 45D994AD.10000@xtra.co.nz
Magnus Danielson wrote:
From: Dr Bruce Griffiths bruce.griffiths@xtra.co.nz
Subject: Re: [time-nuts] 5370B on eBay, carrier phase tracking GPS receiver
Date: Mon, 19 Feb 2007 23:39:47 +1300
Message-ID: 45D97E73.3050702@xtra.co.nz
Hej Magnus
Hej Bruce,
Fuller version is that the later Mitel founders worked for another
telecoms (Bell??) company when they left, they were restrained from
setting up in the same business area for a couple of years so they setup
a business selling lawnmowers until the restraint period expired. Then
the name of their telecoms startup, (MITEL) was derived from the name of
their interim lawnmower business.
Ah, makes perfect illogical sense! :)
It is totally natural to jump from lawnmowers to chipdesign house! :)
Cheers,
Magnus
Hej Magnus
Hej Bruce,
Mitel actually used to make PABX equipment about 20 years ago. They also
did specialised ICs for such applications.
I especially enjoy their 16x8 analog mux chips. Wonder if I can still get hold
of them. They would be immensly usefull in another hobby of mine. :)
Cheers,
Magnus
From: Didier Juges didier@cox.net
Subject: Re: [time-nuts] carrier phase tracking GPS receiver
Date: Mon, 19 Feb 2007 06:37:35 -0600
Message-ID: 45D99A0F.1020103@cox.net
Magnus Danielson wrote:
However if all you have is a 10811 it will limit the short term
satbility of such a GPSDO.
The phase carrier measurements will have a lower noise floor than the
10811 for tau > 10s or so.
A better oscillator (FTS1200, FTS100 Oscilloquartz OSA8607 etc.) will
have a significantly lower noise floor.
Indeed. Carrier phase will cut in early as the quieter source than the PPS
method.
That by itself seems like a significant advantage, being able to set the
loop much faster and therefore being phase locked sooner after power up,
for portable applications.
You can acheive much greater speedup by a combined frequency/phase approach.
You will get a very accurate frequency error estimate, so you will very
quickly be close enought to go into phase lock. At least if your clock isn't
too noisy. So, the lock-in time should not be the major concern, but rather
the behaviour of the full setup when running. Also, another classic trick is
to vary the bandwidth, so you have a much wider bandwidth in the beginning and
then step down towards your target bandwidth as some suitable conditions have
been met prior to the step.
It seems that in this case, the acquisition time of the receiver will be
the most significant delay.
Your OCXO needs to heat up anyway. You can usually acheive a good GPS lock in
that time. A full-fledge correction would probably require data collected over
some time anyway, so don't fool yourself here.
I guess as a result, it will become more important to have an algorithm
that effectively filters out outliers. Are there any such things as
hanging bridges with carrier phase receivers? (I hate asking that
question...)
Well, the resolution of carrier phase measures is below the noise level, so
whatever errors there would be will be noised out. Also, it is not static as
the satellite orbit and speed of change will practically ensure that there is
no hanging bridge, or at least not for very long. So, effectively no.
I was kind of expecting that question, it was just a matter of time before it
came up. :)
Cheers,
Magnus
On 2/19/07, Dr Bruce Griffiths bruce.griffiths@xtra.co.nz wrote:
Hej Magnus
Magnus Danielson wrote:
An interesting product originating from Mitel (Mike and Terry's
lawnmowers later became MITEL - according to their local rep about 20
odd years ago)
Michael Copland and Terence "Terry" Matthews
These guys are/were also involved with Nortel, Newbridge Networks, and Corel.
Fuller version is that the later Mitel founders worked for another
telecoms (Bell??) company when they left, they were restrained from
setting up in the same business area for a couple of years so they setup
a business selling lawnmowers until the restraint period expired. Then
the name of their telecoms startup, (MITEL) was derived from the name of
their interim lawnmower business.
AFAIK they met at Nortel (aka Northern Telecom, Nortel Networks) and
left. I don't know about any lawnmower business though.
Magnus Danielson wrote:
You can acheive much greater speedup by a combined frequency/phase approach.
You will get a very accurate frequency error estimate, so you will very
quickly be close enought to go into phase lock. At least if your clock isn't
too noisy. So, the lock-in time should not be the major concern, but rather
the behaviour of the full setup when running. Also, another classic trick is
to vary the bandwidth, so you have a much wider bandwidth in the beginning and
then step down towards your target bandwidth as some suitable conditions have
been met prior to the step.
My concern is that closing the loop faster by itself does nothing, if it
is not stable enough. What I meant by being able to close the loop
sooner was that the carrier phase data is actually better than the OCXO
sooner after power up. I think you or Bruce said the Allan variance of
the carrier phase signal could be as good as e-10 in 1 second, or
something like that.
It seems that in this case, the acquisition time of the receiver will be
the most significant delay.
Your OCXO needs to heat up anyway. You can usually acheive a good GPS lock in
that time. A full-fledge correction would probably require data collected over
some time anyway, so don't fool yourself here.
I was assuming the OCXO remains powered, which is fairly easy to do, but
in a transportable application, you cannot guaranty continued visibility
of the GPS satellites, or even continued connection of the GPS antenna,
when the system has to be transported, and in a hobby application, I am
not sure I can design a better set of algorithms to compensate for the
OCXO variations than what the Trimble or others have come up with in
their GPSDOs (in fact, I am sure I can't), so that holdover will
probably not be optimized (read: suck).
Also, I am not sure what happens when you move a timing receiver, I
guess the Thunderbolt for instance would have to do a new survey, here
goes an hour at best...
I guess as a result, it will become more important to have an algorithm
that effectively filters out outliers. Are there any such things as
hanging bridges with carrier phase receivers? (I hate asking that
question...)
Well, the resolution of carrier phase measures is below the noise level, so
whatever errors there would be will be noised out. Also, it is not static as
the satellite orbit and speed of change will practically ensure that there is
no hanging bridge, or at least not for very long. So, effectively no.
Good
I was kind of expecting that question, it was just a matter of time before it
came up. :)
Cheers,
Magnus
Well, hopefully we are done with this one :-)
Didier
Didier Juges wrote:
Magnus Danielson wrote:
You can acheive much greater speedup by a combined frequency/phase approach.
You will get a very accurate frequency error estimate, so you will very
quickly be close enought to go into phase lock. At least if your clock isn't
too noisy. So, the lock-in time should not be the major concern, but rather
the behaviour of the full setup when running. Also, another classic trick is
to vary the bandwidth, so you have a much wider bandwidth in the beginning and
then step down towards your target bandwidth as some suitable conditions have
been met prior to the step.
My concern is that closing the loop faster by itself does nothing, if it
is not stable enough. What I meant by being able to close the loop
sooner was that the carrier phase data is actually better than the OCXO
sooner after power up. I think you or Bruce said the Allan variance of
the carrier phase signal could be as good as e-10 in 1 second, or
something like that.
Didier
Typically with a good local oscillator you can do even better than that,
around 1E-11 in 1 sec is achievable and has been achieved.
Even with the on board TCXO typically 3E-11 or so in 1s is achieved.
Bruce
Dr Bruce Griffiths wrote:
Didier
Typically with a good local oscillator you can do even better than that,
around 1E-11 in 1 sec is achievable and has been achieved.
Even with the on board TCXO typically 3E-11 or so in 1s is achieved.
Bruce
I don't understand how such good variance can be achieved without some
method of electronic frequency control on the TCXO (unless the TCXO does
have EFC)?
Otherwise, we would be back to the case where the receiver would
approximate the position of the PPS by skipping pulses, and we would
have the typical 40nS jitter (assuming a 25 MHz clock)
I am more puzzled now :-)
Didier
From: Didier Juges didier@cox.net
Subject: Re: [time-nuts] carrier phase tracking GPS receiver
Date: Mon, 19 Feb 2007 17:50:04 -0600
Message-ID: 45DA37AC.2090502@cox.net
Bon soir Didier,
Magnus Danielson wrote:
You can acheive much greater speedup by a combined frequency/phase approach.
You will get a very accurate frequency error estimate, so you will very
quickly be close enought to go into phase lock. At least if your clock isn't
too noisy. So, the lock-in time should not be the major concern, but rather
the behaviour of the full setup when running. Also, another classic trick is
to vary the bandwidth, so you have a much wider bandwidth in the beginning and
then step down towards your target bandwidth as some suitable conditions have
been met prior to the step.
My concern is that closing the loop faster by itself does nothing, if it
is not stable enough. What I meant by being able to close the loop
sooner was that the carrier phase data is actually better than the OCXO
sooner after power up. I think you or Bruce said the Allan variance of
the carrier phase signal could be as good as e-10 in 1 second, or
something like that.
Actually, it works both ways in the proposed setup. The lower (unknown)
frequency offset and lower noise of the OCXO kickstarts the GPS tracking
(which is FLL/PLL loops). The low noise of carrier phase measurement is
there, but there is an initial loss of time in order to accumulate enought of
them, so it takes time before you can actually get a more fullblown precission.
But even without those benefits you would be able to crank out much better
values even from code phase tracking by this method. Carrier tracking and
fancy corrections is just down the line.
It seems that in this case, the acquisition time of the receiver will be
the most significant delay.
Your OCXO needs to heat up anyway. You can usually acheive a good GPS lock in
that time. A full-fledge correction would probably require data collected over
some time anyway, so don't fool yourself here.
I was assuming the OCXO remains powered,
Yes, but you failed to mention this, so this is a whole different ballgame.
which is fairly easy to do, but
in a transportable application, you cannot guaranty continued visibility
of the GPS satellites, or even continued connection of the GPS antenna,
when the system has to be transported, and in a hobby application, I am
not sure I can design a better set of algorithms to compensate for the
OCXO variations than what the Trimble or others have come up with in
their GPSDOs (in fact, I am sure I can't), so that holdover will
probably not be optimized (read: suck).
:)
Come on, where are your sense of adventure? Don't you feel like wanting to try
the wings a little bit? :)
Also, I am not sure what happens when you move a timing receiver, I
guess the Thunderbolt for instance would have to do a new survey, here
goes an hour at best...
You better be running it with an antenna, so you can get a positioning
estimate. You probably want either a Kalman or particle filter approach here.
When you are talking about moving it around you again have shifted into a
different field and different set of solutions.
I was kind of expecting that question, it was just a matter of time before it
came up. :)
Well, hopefully we are done with this one :-)
Indeed. Especially since I had cached up on a good answer (or so I hope).
Cheers,
Magnus
From: Didier Juges didier@cox.net
Subject: Re: [time-nuts] carrier phase tracking GPS receiver
Date: Mon, 19 Feb 2007 18:12:57 -0600
Message-ID: 45DA3D09.4080003@cox.net
Dr Bruce Griffiths wrote:
Didier
Typically with a good local oscillator you can do even better than that,
around 1E-11 in 1 sec is achievable and has been achieved.
Even with the on board TCXO typically 3E-11 or so in 1s is achieved.
Bruce
I don't understand how such good variance can be achieved without some
method of electronic frequency control on the TCXO (unless the TCXO does
have EFC)?
http://www.oscilloquartz.ch/file/pdf/8607.pdf
See bottom table of Page 2. OK, this is an expensive rock, but looking at
those numbers you can realize that the numbers which Dr Griffiths is giving
can be acheived for much less.
Also, we are talking about their out-of-the-box properties, not in a streered
application. You gotta read some more datasheets to see what it "typical".
It is always good to calibrate onces senses.
The TCXO can have fairly good performance in the short term, but not that
great on longer term since heat waves will have a highly noticeable effect on
them. It is also not really worth it to spend alot on a very good crystal and
oscillator if you cheating out by TCing it. You end up in the OCXO single-oven
range first, and there is plenty of variation of performance right there!
I have 3 cut-open oscillators on my office desk (OK, the ESD pad area) right
now, since the performance difference between them needed a bit of explanation,
and I did get my hunches confirmed.
Otherwise, we would be back to the case where the receiver would
approximate the position of the PPS by skipping pulses, and we would
have the typical 40nS jitter (assuming a 25 MHz clock)
Sigh!
I am more puzzled now :-)
Well, that's how we learn, right? ;)
Cheers,
Magnus
Didier Juges wrote:
Dr Bruce Griffiths wrote:
Didier
Typically with a good local oscillator you can do even better than that,
around 1E-11 in 1 sec is achievable and has been achieved.
Even with the on board TCXO typically 3E-11 or so in 1s is achieved.
Bruce
I don't understand how such good variance can be achieved without some
method of electronic frequency control on the TCXO (unless the TCXO does
have EFC)?
Otherwise, we would be back to the case where the receiver would
approximate the position of the PPS by skipping pulses, and we would
have the typical 40nS jitter (assuming a 25 MHz clock)
I am more puzzled now :-)
Didier
time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Bon soir Didier
You are confusing the disciplining precision of the oscillator with
measurement of its short term stability via the GPS receiver in this case.
Bruce
Hi Bruce,
Dr Bruce Griffiths wrote:
Bon soir Didier
You are confusing the disciplining precision of the oscillator with
measurement of its short term stability via the GPS receiver in this case.
Bruce
Bon après midi to you, I believe, and if my Clock program is any good,
you just had lunch :-) Do you have Daylight Savings Time?
Yes, I realize that I am very confused, but at the end of the day, the
two have to come together. It does me little good to know how much off
my oscillator was, some time in the past. That information has to be
available in time to steer the oscillator, in real time, because what I
am looking for is a precise oscillator, not a history lesson (like the
BIPM view of UTC, where it takes a month for the paperwork to be
processed so that everyone knowns by how much they were off last month :-)
You said:
Typically with a good local oscillator you can do even better than that,
around 1E-11 in 1 sec is achievable and has been achieved.
Even with the on board TCXO typically 3E-11 or so in 1s is achieved.
Is this 3E-11 1) measured on the 1 PPS output, 2) measured on the TCXO,
or 3) the interpretation, by external computing of the carrier phase
data, of the quality of the timing prediction from the GPS receiver?
If it is 3), how can I use that information to make a 3E-11 @ 1sec (or
anything near that) stable GPSDO?
I think I am beginning to understand, but it's slow coming...
Didier
Didier
Didier Juges wrote:
Hi Bruce,
Dr Bruce Griffiths wrote:
Bon soir Didier
You are confusing the disciplining precision of the oscillator with
measurement of its short term stability via the GPS receiver in this case.
Bruce
Bon après midi to you, I believe, and if my Clock program is any good,
you just had lunch :-) Do you have Daylight Savings Time?
We are currently observing 1 hour daylight saving so I had lunch about 2
hours ago if I remember correctly.
Yes, I realize that I am very confused, but at the end of the day, the
two have to come together. It does me little good to know how much off
my oscillator was, some time in the past. That information has to be
available in time to steer the oscillator, in real time, because what I
am looking for is a precise oscillator, not a history lesson (like the
BIPM view of UTC, where it takes a month for the paperwork to be
processed so that everyone knowns by how much they were off last month :-)
You said:
Typically with a good local oscillator you can do even better than that,
around 1E-11 in 1 sec is achievable and has been achieved.
Even with the on board TCXO typically 3E-11 or so in 1s is achieved.
Is this 3E-11 1) measured on the 1 PPS output, 2) measured on the TCXO,
or 3) the interpretation, by external computing of the carrier phase
data, of the quality of the timing prediction from the GPS receiver?
If it is 3), how can I use that information to make a 3E-11 @ 1sec (or
anything near that) stable GPSDO?
I think I am beginning to understand, but it's slow coming...
Didier
time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
The PPS output doesn't exhibit this stability as its position is
quantised to the nearest timing clock transition.
The 3E-11 is inferred from the SV carrier phase tracking data from a set
of receivers used as part of a geodetic survey network.
This is mainly due to the TCXO short term instability.
Note that it is significantly better than the spec for the Rakon TCXO used.
You need to make your own ionospheric delay corrections using the the
code and carrier phase pseudo range variations or from an equivalent
real time source
You also need to ensure that the effect of the SV motion is accurately
corrected.
Once you have corrected for the SV motion, the ionospheric delay and a
host of other small but significant effects, then the changes in
measured carrier phase are due to receiver local oscillator offset,
which is why the TCXO must be replaced by a 10MHz signal derived from
the OCXO you wish to discipline.
There are a host of other things to consider such as cycle slipping in
the carrier phase tracking loops - the receiver will detect these.
The carrier phase (actually carrier beat phase) measurements are just
like the output of an extremely high resolution (< 1ps) phase detector
and are used in the same way to control the OCXO EFC DAC or offset DDS
after suitable statistical filtering to keep the OCXO on track.
In fact it is possible to discipline the OCXO frequency so that it has a
known offset from its nominal frequency. This is useful with older OCXOs
which have drifted out of their adjustment range. A frequency standard
with a fixed known offset is just as useful as a frequency standard with
zero offset. A DDS based narrow range offset generator can always be
used to produce a low phase noise signal with an offset of 1E-14 or less
even though the OCXO may have an offset as much as 1E-5.
The first step is to log carrier phase and code range data from the
receiver using its standard TCXO, then analyse the data so that you can
see/understand what is going on.
Then use the 10MHz signal derived from your TCXO as the receiver local
oscillator, again log the carrier phase and code range data to check
that the performance is indeed improved.
Once you understand what's happening you can experiment with satellite
orbit modelling, ionospheric corrections etc using the logged data to
get your algorithms correct.
Then try closing the loop in real time. Remember you will need a high
resolution (24bit optically isolated) DAC with good short term stability
(low noise and low tempco) to adjust the OCXO EFC voltage. You will also
need a processor with adequate performance, not a PIC, to execute the
algorithms.
The final result is that you will then know your antenna phase centre
location to within a few cm and your OCXO will have an Allan deviation
of 1E-11 or less for Tau from 1 sec to 1 day. Even better performance is
possible with a really good OCXO (FTS1200, FTS1000, OSA8607 etc.). See
Quartzlock A8-MX GPS-BVA datasheet for some idea of what's possible if
you have a good enough OCXO and GPS receiver coupled with good algorithms.
Bruce