GPSDO Component Selection
Hello,
I would like to design and build a microcontroller based GPSDO. And I have a couple of questions some of you may be able to answer.
-
I am going to use Trimble SMT GPS timing module but I'm curious. Does a GPS timing receiver produce a more precise 1 PPS output than a standard GPS tracking receiver in a static location? Or does it just produce a 1 PPS signal that is more accurate to GPS time? -
I have seen many DOCOXs' listed on eBay produced by Morion Inc St Petersburg Russia. This includes models MV89 and MV180 with manufacture dates in 2008. All of the Ox's' were removed from equipment. I don't know anything about Morion's reputation. The specifications are very good, but do they live up to their data sheet, are they reliable and stable?
What DOCXOs' would you recommend?
Thanks in advance
Don O'Connor
On Sun, Sep 9, 2012 at 6:18 AM, Don Oconnor eGunk@wowway.com wrote:
Hello,
I would like to design and build a microcontroller based GPSDO. And I have a couple of questions some of you may be able to answer.
-
I am going to use Trimble SMT GPS timing module but I'm curious. Does a GPS timing receiver produce a more precise 1 PPS output than a standard GPS tracking receiver in a static location? Or does it just produce a 1 PPS signal that is more accurate to GPS time?
Both of the above by several orders of magnitude. The best timing
mode GPSes today have the PPS within a few tens of nanoseconds of UTC
(one sigma) while the typical navigation mode GPS has the PPS within a
few microseconds. Also the timing mode GPS can do things like a self
survey to determine location. The finding a soltion for time within
knows it is not moving removes some uncertainty in the result. I'd
say it would be almost pointless to use a nav receiver in a GPSDO
especially when decent timming mode receivers sell for under $20.
-
I have seen many DOCOXs' listed on eBay produced by Morion Inc St Petersburg Russia. This includes models MV89 and MV180 with manufacture dates in 2008. All of the Ox's' were removed from equipment. I don't know anything about Morion's reputation. The specifications are very good, but do they live up to their data sheet, are they reliable and stable?
Morion has a good reputation but like anything used from eBay expect
unit to unit variation. Might be worth buying a few. Actually how
else could you measure your GPSDO?
I think the main thing that will determine performance is the phase
detector that will compare 10MHz to 1Hz. That is your design and is
the most critical part.
Chris Albertson
Redondo Beach, California
Chris,
On Sun, Sep 9, 2012 at 6:18 AM, Don Oconnor eGunk@wowway.com wrote:
Hello,
I would like to design and build a microcontroller based GPSDO. And I
have a couple of questions some of you may be able to answer.
-
I am going to use Trimble SMT GPS timing module but I'm
curious. Does a GPS timing receiver produce a more precise 1 PPS output
than a standard GPS tracking receiver in a static location? Or does it
just produce a 1 PPS signal that is more accurate to GPS time?
Both of the above by several orders of magnitude. The best timing
mode GPSes today have the PPS within a few tens of nanoseconds of UTC
(one sigma) while the typical navigation mode GPS has the PPS within a
few microseconds. Also the timing mode GPS can do things like a self
survey to determine location. The finding a soltion for time within
knows it is not moving removes some uncertainty in the result. I'd
say it would be almost pointless to use a nav receiver in a GPSDO
especially when decent timming mode receivers sell for under $20.
True for a cheap oem navigation receiver. Not true for a geodetic quality
receiver, who usually have some options (external frequency input, PPS_in)
to make them the best timing receivers available. However they are much
more expensive than the typical single frequency timing reciver.
Look at:
http://www.septentrio.com/products/receivers/polarx4-tr-pro
http://www.topconpositioning.com/products/networks/network-systems/net-g3a
http://www.novatel.com/solutions/timing/#whatWeOffer
http://javad.com/downloads/javadgnss/sheets/Delta-G3T_Datasheet.pdf
http://javad.com/downloads/javadgnss/how-to/hardware/Receiver_Clock_Synchronizing_Configuration_Example.pdf
--
Björn
On Sun, Sep 9, 2012 at 1:14 PM, bg@lysator.liu.se wrote:
True for a cheap oem navigation receiver. Not true for a geodetic quality
receiver, who usually have some options (external frequency input, PPS_in)
to make them the best timing receivers available. However they are much
more expensive than the typical single frequency timing reciver.
I looked at every link and can't see where they give a timing accuracy
spec on the PPS with respect to UTC. Possition accurracy is very
good and we might assume the timing is as good. But they don't say it
is. What's interesting is these GPSes will accept an accurate clock
input in order to give better location data. That is the opposite of
a timing GPS where you tell it accurate location data so that it can
get better timing. Cutting down the unknown in one lets you do
better in the other. I assume these all cost well over $50. You can
get a pretty good timing GPS for $30 and it WILL have the PPS error
specified.
To the OP. None of this matters a lot because PPS is a standard input
signal. It is easy to swap out a GPS receiver later. Same with the
OCXO. From a control point of view they are all pretty much the same.
You can swap them out later
Chris Albertson
Redondo Beach, California
Hi
Position accuracy and timing accuracy are two very different things. Firmware is optimized to improve either one. "Position" firmware is often pretty poor for timing.
Bob
On Sep 9, 2012, at 5:05 PM, Chris Albertson albertson.chris@gmail.com wrote:
On Sun, Sep 9, 2012 at 1:14 PM, bg@lysator.liu.se wrote:
True for a cheap oem navigation receiver. Not true for a geodetic quality
receiver, who usually have some options (external frequency input, PPS_in)
to make them the best timing receivers available. However they are much
more expensive than the typical single frequency timing reciver.
I looked at every link and can't see where they give a timing accuracy
spec on the PPS with respect to UTC. Possition accurracy is very
good and we might assume the timing is as good. But they don't say it
is. What's interesting is these GPSes will accept an accurate clock
input in order to give better location data. That is the opposite of
a timing GPS where you tell it accurate location data so that it can
get better timing. Cutting down the unknown in one lets you do
better in the other. I assume these all cost well over $50. You can
get a pretty good timing GPS for $30 and it WILL have the PPS error
specified.
To the OP. None of this matters a lot because PPS is a standard input
signal. It is easy to swap out a GPS receiver later. Same with the
OCXO. From a control point of view they are all pretty much the same.
You can swap them out later
Chris Albertson
Redondo Beach, California
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
Sounds like the Heisenberg Uncertainty principle at work :-)
jerry
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Bob Camp
Sent: Sunday, September 09, 2012 5:53 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] GPSDO Component Selection
Hi
Position accuracy and timing accuracy are two very different things.
Firmware is optimized to improve either one. "Position" firmware is often
pretty poor for timing.
Bob
On Sep 9, 2012, at 5:05 PM, Chris Albertson albertson.chris@gmail.com
wrote:
On Sun, Sep 9, 2012 at 1:14 PM, bg@lysator.liu.se wrote:
True for a cheap oem navigation receiver. Not true for a geodetic
quality receiver, who usually have some options (external frequency
input, PPS_in) to make them the best timing receivers available.
However they are much more expensive than the typical single frequency
timing reciver.
I looked at every link and can't see where they give a timing accuracy
spec on the PPS with respect to UTC. Possition accurracy is very
good and we might assume the timing is as good. But they don't say it
is. What's interesting is these GPSes will accept an accurate clock
input in order to give better location data. That is the opposite of
a timing GPS where you tell it accurate location data so that it can
get better timing. Cutting down the unknown in one lets you do
better in the other. I assume these all cost well over $50. You can
get a pretty good timing GPS for $30 and it WILL have the PPS error
specified.
To the OP. None of this matters a lot because PPS is a standard input
signal. It is easy to swap out a GPS receiver later. Same with the
OCXO. From a control point of view they are all pretty much the same.
You can swap them out later
Chris Albertson
Redondo Beach, California
time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
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and follow the instructions there.
On Sun, Sep 9, 2012 at 1:14 PM, bg@lysator.liu.se wrote:
True for a cheap oem navigation receiver. Not true for a geodetic
quality
receiver, who usually have some options (external frequency input,
PPS_in)
to make them the best timing receivers available. However they are much
more expensive than the typical single frequency timing reciver.
I looked at every link and can't see where they give a timing accuracy
spec on the PPS with respect to UTC.
Here is an older quote for geodetic quality time receivers.
http://www.bipm.org/static/gpst/mail/13Jul99.1
Table 5. Inferred RMS day-boundary clock
variations attributable to individual receivers
---==============
USNJ (Javad) 216 ps
USNO (AOA TR) 630 ps
USNB (Ashtech) 574 ps
---==============
and some other links.
http://tycho.usno.navy.mil/ptti/ptti2003/paper7.pdf
http://tycho.usno.navy.mil/ptti/ptti2007/paper29.pdf
Possition accurracy is very
good and we might assume the timing is as good. But they don't say it
is. What's interesting is these GPSes will accept an accurate clock
input in order to give better location data. That is the opposite of
a timing GPS where you tell it accurate location data so that it can
get better timing.
You got it wrong, the external frequency input is primarily there to let
you compare accurate clocks. There are few applications that really
require an OCXO or better for position uses.
Cutting down the unknown in one lets you do
better in the other. I assume these all cost well over $50.
That I said already... ;-) But then you will find a gem from time to time
on the big epay river.
--
Björn
On 09/09/2012 11:05 PM, Chris Albertson wrote:
On Sun, Sep 9, 2012 at 1:14 PM,bg@lysator.liu.se wrote:
True for a cheap oem navigation receiver. Not true for a geodetic quality
receiver, who usually have some options (external frequency input, PPS_in)
to make them the best timing receivers available. However they are much
more expensive than the typical single frequency timing reciver.
I looked at every link and can't see where they give a timing accuracy
spec on the PPS with respect to UTC. Possition accurracy is very
good and we might assume the timing is as good. But they don't say it
is. What's interesting is these GPSes will accept an accurate clock
input in order to give better location data. That is the opposite of
a timing GPS where you tell it accurate location data so that it can
get better timing. Cutting down the unknown in one lets you do
better in the other. I assume these all cost well over $50. You can
get a pretty good timing GPS for $30 and it WILL have the PPS error
specified.
To the OP. None of this matters a lot because PPS is a standard input
signal. It is easy to swap out a GPS receiver later. Same with the
OCXO. From a control point of view they are all pretty much the same.
You can swap them out later
There can be uncompensated delays, that always needs to be calibrated
out. If you go looking around, you will find that they have gone through
fair amount of effort to characterize the delay and it's temperature
dependence among other things.
Also, the PPS generation and it's precision isn't a good quality measure
here, as the PPS is typically generated out of the internal clock and
stepped in that clock. For the higher precision you need to use the data
alongside it.
For most of these, you do not provide external time but a better
reference clock for a stable frequency, which will increase the quality
of both time and position results. There are receivers in which you
provide time as in 5/10 MHz and PPS, but then you get the time
difference and position out of the receivers.
So I agree with Björn here, but it may be beside the point unless you
are eager to go over the steep step and into really good receivers.
There even are rubidiums which learns of time and frequency errors out
of GPS receiver messages.
Cheers,
Magnus
Hi Bob,
Probably true for Motorola Oncores. Not very true for geodetic receivers.
Until you have a receiver clock that is on par with the satellite clocks
AND you are short on visable satellites. This might be true if you can
load up a modern cesium in your vehicle, and go for a downtown "urban
valley" type of scenario.
On a stationary site, your expensive clock will not matter to much, since
your solution is already pretty over-determined with some 60 measurements
on each epoch. (9 GPS +6 Glonass)*2(L1/L2)*2 (code + phase)
--
Björn
Hi
Position accuracy and timing accuracy are two very different things.
Firmware is optimized to improve either one. "Position" firmware is often
pretty poor for timing.
Bob
On Sep 9, 2012, at 5:05 PM, Chris Albertson albertson.chris@gmail.com
wrote:
On Sun, Sep 9, 2012 at 1:14 PM, bg@lysator.liu.se wrote:
True for a cheap oem navigation receiver. Not true for a geodetic
quality
receiver, who usually have some options (external frequency input,
PPS_in)
to make them the best timing receivers available. However they are much
more expensive than the typical single frequency timing reciver.
I looked at every link and can't see where they give a timing accuracy
spec on the PPS with respect to UTC. Possition accurracy is very
good and we might assume the timing is as good. But they don't say it
is. What's interesting is these GPSes will accept an accurate clock
input in order to give better location data. That is the opposite of
a timing GPS where you tell it accurate location data so that it can
get better timing. Cutting down the unknown in one lets you do
better in the other. I assume these all cost well over $50. You can
get a pretty good timing GPS for $30 and it WILL have the PPS error
specified.
To the OP. None of this matters a lot because PPS is a standard input
signal. It is easy to swap out a GPS receiver later. Same with the
OCXO. From a control point of view they are all pretty much the same.
You can swap them out later
Chris Albertson
Redondo Beach, California
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
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Hi
Indeed true for most non-geodetic gps units. Put another way - true unless you have a lot of money.
Bob
On Sep 9, 2012, at 7:25 PM, bg@lysator.liu.se wrote:
Hi Bob,
Probably true for Motorola Oncores. Not very true for geodetic receivers.
Until you have a receiver clock that is on par with the satellite clocks
AND you are short on visable satellites. This might be true if you can
load up a modern cesium in your vehicle, and go for a downtown "urban
valley" type of scenario.
On a stationary site, your expensive clock will not matter to much, since
your solution is already pretty over-determined with some 60 measurements
on each epoch. (9 GPS +6 Glonass)*2(L1/L2)*2 (code + phase)
--
Björn
Hi
Position accuracy and timing accuracy are two very different things.
Firmware is optimized to improve either one. "Position" firmware is often
pretty poor for timing.
Bob
On Sep 9, 2012, at 5:05 PM, Chris Albertson albertson.chris@gmail.com
wrote:
On Sun, Sep 9, 2012 at 1:14 PM, bg@lysator.liu.se wrote:
True for a cheap oem navigation receiver. Not true for a geodetic
quality
receiver, who usually have some options (external frequency input,
PPS_in)
to make them the best timing receivers available. However they are much
more expensive than the typical single frequency timing reciver.
I looked at every link and can't see where they give a timing accuracy
spec on the PPS with respect to UTC. Possition accurracy is very
good and we might assume the timing is as good. But they don't say it
is. What's interesting is these GPSes will accept an accurate clock
input in order to give better location data. That is the opposite of
a timing GPS where you tell it accurate location data so that it can
get better timing. Cutting down the unknown in one lets you do
better in the other. I assume these all cost well over $50. You can
get a pretty good timing GPS for $30 and it WILL have the PPS error
specified.
To the OP. None of this matters a lot because PPS is a standard input
signal. It is easy to swap out a GPS receiver later. Same with the
OCXO. From a control point of view they are all pretty much the same.
You can swap them out later
Chris Albertson
Redondo Beach, California
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
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On 09/10/2012 01:28 AM, Bob Camp wrote:
Hi
Indeed true for most non-geodetic gps units. Put another way - true unless you have a lot of money.
Well, you raise up from the normal noise just by adding carrier phase
support and external clock. Can you do double frequency it's even
better. Double freq and GPS + GLONASS is not what I can do right now...
Cheers,
Magnus
Hi Bob,
This argument has been done before on time-nuts... sorry for repeating.
There are geodetic quality GPS reveivers, like the Ashtech Z12-CORS (with
external 5-20MHz input - not the true Z12 Metronome) available for a few
hundred dollars occasionally. I got my Z12 CORS for free, from a site
where it had been replaced by modern GPS/GLONASS receivers.
I also found three Novatel Millenium OEM3 for ca $100 a piece, which in
their days were used by national time labs. Unfortunately two of them has
developed a problem with a custom IC.
I am far from having the economic freedom to purchase a new H-Maser or
Cesium. However I still have a HP5065A running in the basement. When new
in the early 1970ties, you could probably have traded the rubidium for the
house it is now running in.
Conclusion: used geodetic gps equipment are no more expensive, than many
of the oscillators we play with. They might actually once have been used
in the same national time lab... ;-)
--
Björn
Hi
Indeed true for most non-geodetic gps units. Put another way - true unless
you have a lot of money.
Bob
On Sep 9, 2012, at 7:25 PM, bg@lysator.liu.se wrote:
Hi Bob,
Probably true for Motorola Oncores. Not very true for geodetic
receivers.
Until you have a receiver clock that is on par with the satellite clocks
AND you are short on visable satellites. This might be true if you can
load up a modern cesium in your vehicle, and go for a downtown "urban
valley" type of scenario.
On a stationary site, your expensive clock will not matter to much,
since
your solution is already pretty over-determined with some 60
measurements
on each epoch. (9 GPS +6 Glonass)*2(L1/L2)*2 (code + phase)
--
Björn
Hi
Position accuracy and timing accuracy are two very different things.
Firmware is optimized to improve either one. "Position" firmware is
often
pretty poor for timing.
Bob
On Sep 9, 2012, at 5:05 PM, Chris Albertson albertson.chris@gmail.com
wrote:
On Sun, Sep 9, 2012 at 1:14 PM, bg@lysator.liu.se wrote:
True for a cheap oem navigation receiver. Not true for a geodetic
quality
receiver, who usually have some options (external frequency input,
PPS_in)
to make them the best timing receivers available. However they are
much
more expensive than the typical single frequency timing reciver.
I looked at every link and can't see where they give a timing accuracy
spec on the PPS with respect to UTC. Possition accurracy is very
good and we might assume the timing is as good. But they don't say it
is. What's interesting is these GPSes will accept an accurate clock
input in order to give better location data. That is the opposite of
a timing GPS where you tell it accurate location data so that it can
get better timing. Cutting down the unknown in one lets you do
better in the other. I assume these all cost well over $50. You can
get a pretty good timing GPS for $30 and it WILL have the PPS error
specified.
To the OP. None of this matters a lot because PPS is a standard input
signal. It is easy to swap out a GPS receiver later. Same with the
OCXO. From a control point of view they are all pretty much the same.
You can swap them out later
Chris Albertson
Redondo Beach, California
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
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To unsubscribe, go to
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Hi
Except that on the same basis, the non-timing GPS gear is in the $10 to $20 range…
Bob
On Sep 9, 2012, at 7:55 PM, bg@lysator.liu.se wrote:
Hi Bob,
This argument has been done before on time-nuts... sorry for repeating.
There are geodetic quality GPS reveivers, like the Ashtech Z12-CORS (with
external 5-20MHz input - not the true Z12 Metronome) available for a few
hundred dollars occasionally. I got my Z12 CORS for free, from a site
where it had been replaced by modern GPS/GLONASS receivers.
I also found three Novatel Millenium OEM3 for ca $100 a piece, which in
their days were used by national time labs. Unfortunately two of them has
developed a problem with a custom IC.
I am far from having the economic freedom to purchase a new H-Maser or
Cesium. However I still have a HP5065A running in the basement. When new
in the early 1970ties, you could probably have traded the rubidium for the
house it is now running in.
Conclusion: used geodetic gps equipment are no more expensive, than many
of the oscillators we play with. They might actually once have been used
in the same national time lab... ;-)
--
Björn
Hi
Indeed true for most non-geodetic gps units. Put another way - true unless
you have a lot of money.
Bob
On Sep 9, 2012, at 7:25 PM, bg@lysator.liu.se wrote:
Hi Bob,
Probably true for Motorola Oncores. Not very true for geodetic
receivers.
Until you have a receiver clock that is on par with the satellite clocks
AND you are short on visable satellites. This might be true if you can
load up a modern cesium in your vehicle, and go for a downtown "urban
valley" type of scenario.
On a stationary site, your expensive clock will not matter to much,
since
your solution is already pretty over-determined with some 60
measurements
on each epoch. (9 GPS +6 Glonass)*2(L1/L2)*2 (code + phase)
--
Björn
Hi
Position accuracy and timing accuracy are two very different things.
Firmware is optimized to improve either one. "Position" firmware is
often
pretty poor for timing.
Bob
On Sep 9, 2012, at 5:05 PM, Chris Albertson albertson.chris@gmail.com
wrote:
On Sun, Sep 9, 2012 at 1:14 PM, bg@lysator.liu.se wrote:
True for a cheap oem navigation receiver. Not true for a geodetic
quality
receiver, who usually have some options (external frequency input,
PPS_in)
to make them the best timing receivers available. However they are
much
more expensive than the typical single frequency timing reciver.
I looked at every link and can't see where they give a timing accuracy
spec on the PPS with respect to UTC. Possition accurracy is very
good and we might assume the timing is as good. But they don't say it
is. What's interesting is these GPSes will accept an accurate clock
input in order to give better location data. That is the opposite of
a timing GPS where you tell it accurate location data so that it can
get better timing. Cutting down the unknown in one lets you do
better in the other. I assume these all cost well over $50. You can
get a pretty good timing GPS for $30 and it WILL have the PPS error
specified.
To the OP. None of this matters a lot because PPS is a standard input
signal. It is easy to swap out a GPS receiver later. Same with the
OCXO. From a control point of view they are all pretty much the same.
You can swap them out later
Chris Albertson
Redondo Beach, California
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
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On 09/10/2012 02:14 AM, Bob Camp wrote:
Hi
Except that on the same basis, the non-timing GPS gear is in the $10 to $20 range…
True, but better performance is reachable on a private budget.
Cheers,
Magnus
First - I realize 95% of the folks reading this are well aware of what I am
going to say.
No matter how good your equipment (receiver/antenna) is, the short term
accuracy of GPS time is defined in GLOBAL POSITIONING SYSTEM STANDARD
POSITIONING SERVICE PERFORMANCE STANDARD (2008). This document can be
found at http://www.gps.gov/technical/ps/#spsps.
The guaranteed timing accuracy (short term) is found in Table 3.8-3. SPS
Position/Time Accuracy Standards. The time domain transfer accuracy is
defined as "≤ 40 nsec time transfer error 95% of time
(SIS only)". In order to achieve this the "(SIS only)" comment means you
have a perfect receiver that introduces no errors (good luck with that
one). Most commercial users set their probability at around 30 nS, and
experience virtually no estimates out of that boundary.
The 30 nS error can be reduced to a better number if position is accurately
known and the receiver "knows" that it is stationary - but still 5% of the
time you can get noisy / degraded time and still be "in spec.". I am not
sure over what time span the 95% number is used.
Now: The real answer is to take the relatively noisy GPS timing information
and use it to discipline a device that is (extremely) stable over a short
time such as a OCXO or Rubidium standard. The integration period determines
how much of the very accurate long term GPS information with the short
term, highly stable, clock information. Hence a GPSDO.
Michael / K7HIL
On Sun, Sep 9, 2012 at 3:10 PM, Jerry jsternmd@att.net wrote:
Sounds like the Heisenberg Uncertainty principle at work :-)
jerry
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Bob Camp
Sent: Sunday, September 09, 2012 5:53 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] GPSDO Component Selection
Hi
Position accuracy and timing accuracy are two very different things.
Firmware is optimized to improve either one. "Position" firmware is often
pretty poor for timing.
Bob
On Sep 9, 2012, at 5:05 PM, Chris Albertson albertson.chris@gmail.com
wrote:
On Sun, Sep 9, 2012 at 1:14 PM, bg@lysator.liu.se wrote:
True for a cheap oem navigation receiver. Not true for a geodetic
quality receiver, who usually have some options (external frequency
input, PPS_in) to make them the best timing receivers available.
However they are much more expensive than the typical single frequency
timing reciver.
I looked at every link and can't see where they give a timing accuracy
spec on the PPS with respect to UTC. Possition accurracy is very
good and we might assume the timing is as good. But they don't say it
is. What's interesting is these GPSes will accept an accurate clock
input in order to give better location data. That is the opposite of
a timing GPS where you tell it accurate location data so that it can
get better timing. Cutting down the unknown in one lets you do
better in the other. I assume these all cost well over $50. You can
get a pretty good timing GPS for $30 and it WILL have the PPS error
specified.
To the OP. None of this matters a lot because PPS is a standard input
signal. It is easy to swap out a GPS receiver later. Same with the
OCXO. From a control point of view they are all pretty much the same.
You can swap them out later
Chris Albertson
Redondo Beach, California
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There is something missing in this sentence:
The integration period determines
how much of the very accurate long term GPS information <missing verb?>
with the short
term, highly stable, clock information. Hence a GPSDO.
That is: how much of the very accurate GPS information leak? are
transferred to? Interact? with the short term...
I expect that the integration time is related to the end of the short term
oscillator variation and the start of the long term GPS steering. This
should help in the proper selection of this time: maybe the time should be
adapted to the temperature variation too.
On Mon, Sep 10, 2012 at 11:56 AM, Michael Perrett mkperrett@gmail.comwrote:
First - I realize 95% of the folks reading this are well aware of what I am
going to say.
No matter how good your equipment (receiver/antenna) is, the short term
accuracy of GPS time is defined in GLOBAL POSITIONING SYSTEM STANDARD
POSITIONING SERVICE PERFORMANCE STANDARD (2008). This document can be
found at http://www.gps.gov/technical/ps/#spsps.
The guaranteed timing accuracy (short term) is found in Table 3.8-3. SPS
Position/Time Accuracy Standards. The time domain transfer accuracy is
defined as "≤ 40 nsec time transfer error 95% of time
(SIS only)". In order to achieve this the "(SIS only)" comment means you
have a perfect receiver that introduces no errors (good luck with that
one). Most commercial users set their probability at around 30 nS, and
experience virtually no estimates out of that boundary.
The 30 nS error can be reduced to a better number if position is accurately
known and the receiver "knows" that it is stationary - but still 5% of the
time you can get noisy / degraded time and still be "in spec.". I am not
sure over what time span the 95% number is used.
Now: The real answer is to take the relatively noisy GPS timing information
and use it to discipline a device that is (extremely) stable over a short
time such as a OCXO or Rubidium standard. The integration period determines
how much of the very accurate long term GPS information with the short
term, highly stable, clock information. Hence a GPSDO.
Michael / K7HIL
On Sun, Sep 9, 2012 at 3:10 PM, Jerry jsternmd@att.net wrote:
Sounds like the Heisenberg Uncertainty principle at work :-)
jerry
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Bob Camp
Sent: Sunday, September 09, 2012 5:53 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] GPSDO Component Selection
Hi
Position accuracy and timing accuracy are two very different things.
Firmware is optimized to improve either one. "Position" firmware is often
pretty poor for timing.
Bob
On Sep 9, 2012, at 5:05 PM, Chris Albertson albertson.chris@gmail.com
wrote:
On Sun, Sep 9, 2012 at 1:14 PM, bg@lysator.liu.se wrote:
True for a cheap oem navigation receiver. Not true for a geodetic
quality receiver, who usually have some options (external frequency
input, PPS_in) to make them the best timing receivers available.
However they are much more expensive than the typical single frequency
timing reciver.
I looked at every link and can't see where they give a timing accuracy
spec on the PPS with respect to UTC. Possition accurracy is very
good and we might assume the timing is as good. But they don't say it
is. What's interesting is these GPSes will accept an accurate clock
input in order to give better location data. That is the opposite of
a timing GPS where you tell it accurate location data so that it can
get better timing. Cutting down the unknown in one lets you do
better in the other. I assume these all cost well over $50. You can
get a pretty good timing GPS for $30 and it WILL have the PPS error
specified.
To the OP. None of this matters a lot because PPS is a standard input
signal. It is easy to swap out a GPS receiver later. Same with the
OCXO. From a control point of view they are all pretty much the same.
You can swap them out later
Chris Albertson
Redondo Beach, California
time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
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To unsubscribe, go to
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and follow the instructions there.