SE880 GPSDO
On Wed, 27 Apr 2016 08:25:55 +1200
Bruce Griffiths bruce.griffiths@xtra.co.nz wrote:
- Relative position of any pair of clocks located up to 2km apart has to be
known to within 3cm or so. Post processing is OK, however differential Earth
tides between the clock locations may need to be considered.
That's doable. People at ETHZ got sub cm accuracy from LEA-6T modules
with post-processing of the recorded phase data with an integration time
of several hours. Using phase data of multiple timing modules should give
relative positions with better than 1cm accuracy on these short baselines.
I don't know how much post-processing is necessary though. Haven't looked
into the the field of RTK[1] and PPP[2] yet. Probably data from IGS[3] is
needed as well.
- The difference in the time offset between any pair of clocks located up to
2km apart shall not vary by more than 200ps (1ns time stamp quantisation) or
2ns (10ns timestamp quantisation) over an 8 hour period (at night).
Post processing of data to fit wander etc is not practical as the SNR is too
low to support this.
Now this is quite a bit more challenging. While i'd say 1ns should be doable
(using receivers that are calibrated against each other and using common in
view mode during post-processing of the data), i'm not so sure whether 200ps
is possible. What might work is using an LEA-M8F with it's external frequency
input, to record the phase of an stable external reference (e.g. Rb).
Averaging that over a dozen minutes or so should make it possible to
measure the phase of the reference oscillator with 200ps precision, relative
to the other stations.
Another way would be to use L1/L2 receivers with calibrated antennas.
I know that BIPM has a GPS station that can deliver time transfer
accuracy <2ns over a distance of several 100km. It could be possible
to use such receivers with the <3km distances to deliver 10 times better,
if they are frequently calibrated (eg. every couple of days).
But of course, this makes things much more expensive.
But all this is a wild guess. I haven't seen anything like this done.
If you want a more precise answer i would need to think about the design
of the system for some time.
I guess using some cable/fibre between the telescopes is out of question?
Attila Kinali
[1] http://www.navipedia.net/index.php/Real_Time_Kinematics
[2] http://www.navipedia.net/index.php/Precise_Point_Positioning
[3] http://www.igs.org/
--
Reading can seriously damage your ignorance.
-- unknown
On Tue, 26 Apr 2016 22:24:08 +0200
Ilia Platone info@iliaplatone.com wrote:
I must print each photon event captured by an APD at the focus of more
telescopes, these events must be timestamped with an accuracy of 2.5ns,
As i wrote before, 1ns should be doable with standard timing GPS receiver
and calibration at those short baselines. Going below that will be a
challenge though.
and the expected rate is lower than 10MHz.
10MHz is quite a high rate of events that need to be timestamped.
How do you intend to measure events at such a high rate? And how
do you intend to store so much data? Each measurement will need
at least 40bit resolution, at 10MHz that's 50MByte/s. Even if you
say that the average rate is only a fraction of that and you use
lots of buffers, that's still a high data rate for a measurement
instrument.
Attila Kinali
--
Reading can seriously damage your ignorance.
-- unknown
I will use a dedicated FPGA design, and the data will be stored into an
SDXC card (UHS), or an IDE drive (maybe not), in RAW mode (no filesystem).
Each timestamp will be stored into a word of 48bits, the clock will be
400MHz, so the quantization will be 2.5ns, but the maximum error/clock
offset should be 1/10 of this, so 500ps , as said by Bruce.
Regards,
Ilia.
Il 27/04/2016 00:29, Attila Kinali ha scritto:
On Tue, 26 Apr 2016 22:24:08 +0200
Ilia Platone info@iliaplatone.com wrote:
I must print each photon event captured by an APD at the focus of more
telescopes, these events must be timestamped with an accuracy of 2.5ns,
As i wrote before, 1ns should be doable with standard timing GPS receiver
and calibration at those short baselines. Going below that will be a
challenge though.
and the expected rate is lower than 10MHz.
10MHz is quite a high rate of events that need to be timestamped.
How do you intend to measure events at such a high rate? And how
do you intend to store so much data? Each measurement will need
at least 40bit resolution, at 10MHz that's 50MByte/s. Even if you
say that the average rate is only a fraction of that and you use
lots of buffers, that's still a high data rate for a measurement
instrument.
Attila Kinali
--
Ilia Platone
via Ferrara 54
47841
Cattolica (RN), Italy
Cell +39 349 1075999
On Tue, 26 Apr 2016 23:39:40 +0200
Ilia Platone info@iliaplatone.com wrote:
BTW, the u-Blox NEO-M8N doesn't fit RTK specs (patched it returns RAW
data): it works on band L1 (20cm range), what I need is a GPS module
capable on wiorking on L1+L2 to get 1cm range precision (if it's correct).
L1/L2 receivers are quite a bit more expensive than simple L1 receivers.
An receiver+antenna costs easily 1k€ and more.
On these short baselines you can assume that the same satellite will
see the same ionospheric delay, hence you can compensate for that
when doing common-view with only using L1 data. Or alternatively,
you can work with the IGS data and compensate not only ionospheric
delay but also clock and orbit errors.
Attila Kinlai
--
Reading can seriously damage your ignorance.
-- unknown
Hi All,
I read from an article about this receiver: C-Max CMMR-6P-60
Can it be useful?
One of the places where I'll setup the telescopes will be in mount
Carpegna, near where I live. There are the repetitors of television and
radio over there. Can the carrier wave of such repetitors be used as
clock? they will be distant 5 Km or less from the observation location.
Regards,
Ilia.
Il 26/04/2016 23:51, Attila Kinali ha scritto:
On Wed, 27 Apr 2016 08:25:55 +1200
Bruce Griffiths bruce.griffiths@xtra.co.nz wrote:
- Relative position of any pair of clocks located up to 2km apart has to be
known to within 3cm or so. Post processing is OK, however differential Earth
tides between the clock locations may need to be considered.
That's doable. People at ETHZ got sub cm accuracy from LEA-6T modules
with post-processing of the recorded phase data with an integration time
of several hours. Using phase data of multiple timing modules should give
relative positions with better than 1cm accuracy on these short baselines.
I don't know how much post-processing is necessary though. Haven't looked
into the the field of RTK[1] and PPP[2] yet. Probably data from IGS[3] is
needed as well.
- The difference in the time offset between any pair of clocks located up to
2km apart shall not vary by more than 200ps (1ns time stamp quantisation) or
2ns (10ns timestamp quantisation) over an 8 hour period (at night).
Post processing of data to fit wander etc is not practical as the SNR is too
low to support this.
Now this is quite a bit more challenging. While i'd say 1ns should be doable
(using receivers that are calibrated against each other and using common in
view mode during post-processing of the data), i'm not so sure whether 200ps
is possible. What might work is using an LEA-M8F with it's external frequency
input, to record the phase of an stable external reference (e.g. Rb).
Averaging that over a dozen minutes or so should make it possible to
measure the phase of the reference oscillator with 200ps precision, relative
to the other stations.
Another way would be to use L1/L2 receivers with calibrated antennas.
I know that BIPM has a GPS station that can deliver time transfer
accuracy <2ns over a distance of several 100km. It could be possible
to use such receivers with the <3km distances to deliver 10 times better,
if they are frequently calibrated (eg. every couple of days).
But of course, this makes things much more expensive.
But all this is a wild guess. I haven't seen anything like this done.
If you want a more precise answer i would need to think about the design
of the system for some time.
I guess using some cable/fibre between the telescopes is out of question?
Attila Kinali
[1] http://www.navipedia.net/index.php/Real_Time_Kinematics
[2] http://www.navipedia.net/index.php/Precise_Point_Positioning
[3] http://www.igs.org/
--
Ilia Platone
via Ferrara 54
47841
Cattolica (RN), Italy
Cell +39 349 1075999
On Wed, 27 Apr 2016 01:30:49 +0200
Ilia Platone info@iliaplatone.com wrote:
I will use a dedicated FPGA design, and the data will be stored into an
SDXC card (UHS), or an IDE drive (maybe not), in RAW mode (no filesystem).
Please be aware that SDCards performance spec is peak and best case.
Additionally, you need to prepare for writes taking several ms when
the SDCard firmware remaps blocks. Ie you will need to be able to
buffer several MB of data before you write to the SDCards. It would
probably be a good idea to push the data into a CPU (ARM9, or Cortex-A)
and handle the buffering/writing in software instead of an FPGA.
Each timestamp will be stored into a word of 48bits, the clock will be
400MHz, so the quantization will be 2.5ns, but the maximum error/clock
offset should be 1/10 of this, so 500ps , as said by Bruce.
I thought a little bit more about this problem this morning and came up
with something that should be rather simple to implement:
Assuming that you have an amateur radio license, you could use a
well located central station to transmit a CW signal in the 70cm or
23cm band. There should be some effort put into this station
to make it stable (eg by using a good rubidium as frequency source,
or even an ensemble) and low noise.
Use this CW signal on all the telescope stations to phase lock a local
OCXO. Using a good OCXO, it should be possible to use loop bandwidths
in the 0.1-10Hz range. My guess is, that this frequency transfer system
would yield stabilities in the order of 10^-12 @ 1s (or even better).
For additional performance, one could modulate the CW with a PRN sequence
to get a better SNR and probably get another order of magnitude out of it.
For the simple CW case, the circuitry should be fairly simple and easy
to do. The PRN case would require at least some processing in an FPGA.
Now that all stations have the "same" frequency, one can use the GPS
module to get the time information using long integration times.
Under the assumption that the (sawtooth corrected) PPS is good to +/-10ns
an has a nice, time-invariant distribution, it should be possible to get
below 1ns in precision within 100s. Using common view phase data it
should be possible to get even better than that.
Of course, the GPS modules would still need to be calibrated against
eachother to get the accuracy below the 10ns level. I am still not
sure how to go below 1ns, though I think this approach should make
that easier. Most likely you will need some temperature stabilization
of the electronics.
It's also quite easy to verify the frequency transfer by letting
each telescope station transmit a CW signal back to the central
station on a different frequency. Then use some SDR system to extract
the relative phase/frequency of each station.
Attila Kinali
--
Reading can seriously damage your ignorance.
-- unknown
On Wed, Apr 27, 2016 at 7:51 AM, Attila Kinali attila@kinali.ch wrote:
Another way would be to use L1/L2 receivers with calibrated antennas.
I know that BIPM has a GPS station that can deliver time transfer
accuracy <2ns over a distance of several 100km. It could be possible
to use such receivers with the <3km distances to deliver 10 times better,
if they are frequently calibrated (eg. every couple of days).
But of course, this makes things much more expensive.
But all this is a wild guess. I haven't seen anything like this done.
There was a discussion in 2005 that's pertinent to this:
https://www.febo.com/pipermail/time-nuts/2005-August/019158.html
The references are interesting, with some real data about what is
achievable on short baselines.
eg Fig 7 in http://gpstime.com/files/PTTI/PTTI_2002_CNS_Testbed.pdf
shows that two receivers keep within about 5 ns of each other on a
21.5 km baseline.
As I think I said in a similar discussion a few weeks ago, two
identical geodetic receivers that I have on a 400 m baseline keep
within about 100 ps of each other, using a Precise Point Positioning
solution for the (common) clock.
If you were brave, then 2 km would scale to 500 ps synchronization
(works for 5 ns/20 km too ..)
Cheers
Michael
A 60kHz receiver is unlikely to be useful for nanosecond timing applications.
Bruce
On Wednesday, 27 April 2016 6:36 PM, Ilia Platone <info@iliaplatone.com> wrote:
Hi All,
I read from an article about this receiver: C-Max CMMR-6P-60
Can it be useful?
One of the places where I'll setup the telescopes will be in mount
Carpegna, near where I live. There are the repetitors of television and
radio over there. Can the carrier wave of such repetitors be used as
clock? they will be distant 5 Km or less from the observation location.
Regards,
Ilia.
Il 26/04/2016 23:51, Attila Kinali ha scritto:
On Wed, 27 Apr 2016 08:25:55 +1200
Bruce Griffiths bruce.griffiths@xtra.co.nz wrote:
- Relative position of any pair of clocks located up to 2km apart has to be
known to within 3cm or so. Post processing is OK, however differential Earth
tides between the clock locations may need to be considered.
That's doable. People at ETHZ got sub cm accuracy from LEA-6T modules
with post-processing of the recorded phase data with an integration time
of several hours. Using phase data of multiple timing modules should give
relative positions with better than 1cm accuracy on these short baselines..
I don't know how much post-processing is necessary though. Haven't looked
into the the field of RTK[1] and PPP[2] yet. Probably data from IGS[3] is
needed as well.
- The difference in the time offset between any pair of clocks located up to
2km apart shall not vary by more than 200ps (1ns time stamp quantisation) or
2ns (10ns timestamp quantisation) over an 8 hour period (at night).
Post processing of data to fit wander etc is not practical as the SNR is too
low to support this.
Now this is quite a bit more challenging. While i'd say 1ns should be doable
(using receivers that are calibrated against each other and using common in
view mode during post-processing of the data), i'm not so sure whether 200ps
is possible. What might work is using an LEA-M8F with it's external frequency
input, to record the phase of an stable external reference (e.g. Rb).
Averaging that over a dozen minutes or so should make it possible to
measure the phase of the reference oscillator with 200ps precision, relative
to the other stations.
Another way would be to use L1/L2 receivers with calibrated antennas.
I know that BIPM has a GPS station that can deliver time transfer
accuracy <2ns over a distance of several 100km. It could be possible
to use such receivers with the <3km distances to deliver 10 times better,
if they are frequently calibrated (eg. every couple of days).
But of course, this makes things much more expensive.
But all this is a wild guess. I haven't seen anything like this done.
If you want a more precise answer i would need to think about the design
of the system for some time.
I guess using some cable/fibre between the telescopes is out of question?
Attila Kinali
[1] http://www.navipedia.net/index.php/Real_Time_Kinematics
[2] http://www.navipedia.net/index.php/Precise_Point_Positioning
[3] http://www.igs.org/
--
Ilia Platone
via Ferrara 54
47841
Cattolica (RN), Italy
Cell +39 349 1075999
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.
Stabilising the GPS receiver antenna temperature is probably a good idea particularly if it has bandpass filter(s).
Bruce
On Wednesday, 27 April 2016 9:01 PM, Attila Kinali <attila@kinali.ch> wrote:
On Wed, 27 Apr 2016 01:30:49 +0200
Ilia Platone info@iliaplatone.com wrote:
I will use a dedicated FPGA design, and the data will be stored into an
SDXC card (UHS), or an IDE drive (maybe not), in RAW mode (no filesystem)..
Please be aware that SDCards performance spec is peak and best case.
Additionally, you need to prepare for writes taking several ms when
the SDCard firmware remaps blocks. Ie you will need to be able to
buffer several MB of data before you write to the SDCards. It would
probably be a good idea to push the data into a CPU (ARM9, or Cortex-A)
and handle the buffering/writing in software instead of an FPGA.
Each timestamp will be stored into a word of 48bits, the clock will be
400MHz, so the quantization will be 2.5ns, but the maximum error/clock
offset should be 1/10 of this, so 500ps , as said by Bruce.
I thought a little bit more about this problem this morning and came up
with something that should be rather simple to implement:
Assuming that you have an amateur radio license, you could use a
well located central station to transmit a CW signal in the 70cm or
23cm band. There should be some effort put into this station
to make it stable (eg by using a good rubidium as frequency source,
or even an ensemble) and low noise.
Use this CW signal on all the telescope stations to phase lock a local
OCXO. Using a good OCXO, it should be possible to use loop bandwidths
in the 0.1-10Hz range. My guess is, that this frequency transfer system
would yield stabilities in the order of 10^-12 @ 1s (or even better).
For additional performance, one could modulate the CW with a PRN sequence
to get a better SNR and probably get another order of magnitude out of it.
For the simple CW case, the circuitry should be fairly simple and easy
to do. The PRN case would require at least some processing in an FPGA.
Now that all stations have the "same" frequency, one can use the GPS
module to get the time information using long integration times.
Under the assumption that the (sawtooth corrected) PPS is good to +/-10ns
an has a nice, time-invariant distribution, it should be possible to get
below 1ns in precision within 100s. Using common view phase data it
should be possible to get even better than that.
Of course, the GPS modules would still need to be calibrated against
eachother to get the accuracy below the 10ns level. I am still not
sure how to go below 1ns, though I think this approach should make
that easier. Most likely you will need some temperature stabilization
of the electronics.
It's also quite easy to verify the frequency transfer by letting
each telescope station transmit a CW signal back to the central
station on a different frequency. Then use some SDR system to extract
the relative phase/frequency of each station.
Attila Kinali
--
Reading can seriously damage your ignorance.
-- unknown
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.
It was a joke :)
Interesting the idea of Attila, it can be the less expensive solution:
"Assuming that you have an amateur radio license, you could use a
well located central station to transmit a CW signal in the 70cm or
23cm band. There should be some effort put into this station
to make it stable (eg by using a good rubidium as frequency source,
or even an ensemble) and low noise."
...
"Now that all stations have the "same" frequency, one can use the GPS
module to get the time information using long integration times.
Under the assumption that the (sawtooth corrected) PPS is good to ±10ns
an has a nice, time-invariant distribution, it should be possible to get
below 1ns in precision within 100s. Using common view phase data it
should be possible to get even better than that."
Regards,
Ilia.
Il 27/04/2016 10:12, Bruce Griffiths ha scritto:
A 60kHz receiver is unlikely to be useful for nanosecond timing applications.
Bruce
On Wednesday, 27 April 2016 6:36 PM, Ilia Platone <info@iliaplatone.com> wrote:
Hi All,
I read from an article about this receiver: C-Max CMMR-6P-60
Can it be useful?
One of the places where I'll setup the telescopes will be in mount
Carpegna, near where I live. There are the repetitors of television and
radio over there. Can the carrier wave of such repetitors be used as
clock? they will be distant 5 Km or less from the observation location.
Regards,
Ilia.
Il 26/04/2016 23:51, Attila Kinali ha scritto:
On Wed, 27 Apr 2016 08:25:55 +1200
Bruce Griffiths bruce.griffiths@xtra.co.nz wrote:
- Relative position of any pair of clocks located up to 2km apart has to be
known to within 3cm or so. Post processing is OK, however differential Earth
tides between the clock locations may need to be considered.
That's doable. People at ETHZ got sub cm accuracy from LEA-6T modules
with post-processing of the recorded phase data with an integration time
of several hours. Using phase data of multiple timing modules should give
relative positions with better than 1cm accuracy on these short baselines..
I don't know how much post-processing is necessary though. Haven't looked
into the the field of RTK[1] and PPP[2] yet. Probably data from IGS[3] is
needed as well.
- The difference in the time offset between any pair of clocks located up to
2km apart shall not vary by more than 200ps (1ns time stamp quantisation) or
2ns (10ns timestamp quantisation) over an 8 hour period (at night).
Post processing of data to fit wander etc is not practical as the SNR is too
low to support this.
Now this is quite a bit more challenging. While i'd say 1ns should be doable
(using receivers that are calibrated against each other and using common in
view mode during post-processing of the data), i'm not so sure whether 200ps
is possible. What might work is using an LEA-M8F with it's external frequency
input, to record the phase of an stable external reference (e.g. Rb).
Averaging that over a dozen minutes or so should make it possible to
measure the phase of the reference oscillator with 200ps precision, relative
to the other stations.
Another way would be to use L1/L2 receivers with calibrated antennas.
I know that BIPM has a GPS station that can deliver time transfer
accuracy <2ns over a distance of several 100km. It could be possible
to use such receivers with the <3km distances to deliver 10 times better,
if they are frequently calibrated (eg. every couple of days).
But of course, this makes things much more expensive.
But all this is a wild guess. I haven't seen anything like this done.
If you want a more precise answer i would need to think about the design
of the system for some time.
I guess using some cable/fibre between the telescopes is out of question?
Attila Kinali
[1] http://www.navipedia.net/index.php/Real_Time_Kinematics
[2] http://www.navipedia.net/index.php/Precise_Point_Positioning
[3] http://www.igs.org/
--
Ilia Platone
via Ferrara 54
47841
Cattolica (RN), Italy
Cell +39 349 1075999