position determination over short distance
All,
I'm planning doing some experiments in distance measurement. They don't
deal with atomic time directly but with extreme short periods of time.
I need to determine the position of a instrument with a 1mm accuracy or
less.
The instrument is not connected to a mechanical device but is separate &
independent.
The surface which the instrument is positioned on is close to the size of a
11"x11" square.
I thought of using 1 RF transmitters (not sure of freq) on bottom of the
device near the surface.
The surface would have RF receivers on 3 or 4 edges/corners to receive the
signal.
If each of the receivers positions are known and they then send a signal to
a central circuit (again known positions) how can I differentiate the time
of arrival
at the central location? Does anybody know of a circuit/chip or system
which would determine the time 'difference'.
Obviously this is used to triangulate the position of the instrument.
Light travels 1 mm in ~3.3 picoseconds so I would suspect the differentiator
would have to have that or better resolution.
It could also use some proportional method to extrapolate the position since
the surface has a fixed size.
Any ideas/thoughts?
Thanks in advance.
Rick Harold
Rick Harold wrote:
All,
I'm planning doing some experiments in distance measurement. They don't
deal with atomic time directly but with extreme short periods of time.
I need to determine the position of a instrument with a 1mm accuracy or
less.
The instrument is not connected to a mechanical device but is separate &
independent.
The surface which the instrument is positioned on is close to the size of a
11"x11" square.
I thought of using 1 RF transmitters (not sure of freq) on bottom of the
device near the surface.
The surface would have RF receivers on 3 or 4 edges/corners to receive the
signal.
If each of the receivers positions are known and they then send a signal to
a central circuit (again known positions) how can I differentiate the time
of arrival
at the central location? Does anybody know of a circuit/chip or system
which would determine the time 'difference'.
Obviously this is used to triangulate the position of the instrument.
Light travels 1 mm in ~3.3 picoseconds so I would suspect the differentiator
would have to have that or better resolution.
It could also use some proportional method to extrapolate the position since
the surface has a fixed size.
Any ideas/thoughts?
Thanks in advance.
Rick Harold
Rick
The techniques used in early geodimeters spring to mind.
Just modulate the RF transmissions with various carriers and make a
sequence of modulation phase shift measurements.
Of course one could also use the same techniques employed in the GPS system.
Are you sure that you cant just use optical techniques with corner cube
reflectors?
Can you not triangulate using 3 or more cameras?
Bruce
----- Original Message -----
From: "Rick Harold" rickharold@gmail.com
To: time-nuts@febo.com
Sent: Sunday, November 23, 2008 9:34 PM
Subject: [time-nuts] position determination over short distance
All,
I'm planning doing some experiments in distance measurement. They don't
deal with atomic time directly but with extreme short periods of time.
I need to determine the position of a instrument with a 1mm accuracy or
less.
The instrument is not connected to a mechanical device but is separate &
independent.
The surface which the instrument is positioned on is close to the size of
a
11"x11" square.
I thought of using 1 RF transmitters (not sure of freq) on bottom of the
device near the surface.
The surface would have RF receivers on 3 or 4 edges/corners to receive the
signal.
If each of the receivers positions are known and they then send a signal
to
a central circuit (again known positions) how can I differentiate the time
of arrival
at the central location? Does anybody know of a circuit/chip or system
which would determine the time 'difference'.
Obviously this is used to triangulate the position of the instrument.
Light travels 1 mm in ~3.3 picoseconds so I would suspect the
differentiator
would have to have that or better resolution.
It could also use some proportional method to extrapolate the position
since
the surface has a fixed size.
Any ideas/thoughts?
Thanks in advance.
Rick Harold
Rather vague as to what you are doing.
Have you considered ultrasonics or even laser rather than RF?
Rick Harold wrote:
All,
I'm planning doing some experiments in distance measurement. They don't
deal with atomic time directly but with extreme short periods of time.
I need to determine the position of a instrument with a 1mm accuracy or
less.
The instrument is not connected to a mechanical device but is separate &
independent.
The surface which the instrument is positioned on is close to the size of a
11"x11" square.
I thought of using 1 RF transmitters (not sure of freq) on bottom of the
device near the surface.
The surface would have RF receivers on 3 or 4 edges/corners to receive the
signal.
If each of the receivers positions are known and they then send a signal to
a central circuit (again known positions) how can I differentiate the time
of arrival
at the central location? Does anybody know of a circuit/chip or system
which would determine the time 'difference'.
Obviously this is used to triangulate the position of the instrument.
Light travels 1 mm in ~3.3 picoseconds so I would suspect the differentiator
would have to have that or better resolution.
It could also use some proportional method to extrapolate the position since
the surface has a fixed size.
Any ideas/thoughts?
Thanks in advance.
Rick Harold
Use a laser distance meter (or homebuild one)
Regards,
Ignacio, EB4APL
I need to determine the position of a instrument with a 1mm accuracy or less.
The instrument is not connected to a mechanical device but is separate &
independent.
The surface which the instrument is positioned on is close to the size of a
11"x11" square.
1 mm or better accuracy on a 300x300 mm surface can be
obtained with a cheap webcam mounted above the surface
and a little bit a creative software.
Obviously this is used to triangulate the position of the instrument.
Light travels 1 mm in ~3.3 picoseconds so I would suspect the differentiator
would have to have that or better resolution.
Sound travels about a million times slower than light. 343 m/s
is 1 mm in 2.9 microseconds. Perhaps this is the way to go?
I remind visitors here that a WWV tick takes the same time to
travel from Ft Collins to Seattle as it does to travel from the SW
radio speaker to your ear.
/tvb
Tom Van Baak wrote:
I need to determine the position of a instrument with a 1mm accuracy or less.
The instrument is not connected to a mechanical device but is separate &
independent.
The surface which the instrument is positioned on is close to the size of a
11"x11" square.
1 mm or better accuracy on a 300x300 mm surface can be
obtained with a cheap webcam mounted above the surface
and a little bit a creative software.
Obviously this is used to triangulate the position of the instrument.
Light travels 1 mm in ~3.3 picoseconds so I would suspect the differentiator
would have to have that or better resolution.
Sound travels about a million times slower than light. 343 m/s
is 1 mm in 2.9 microseconds. Perhaps this is the way to go?
I remind visitors here that a WWV tick takes the same time to
travel from Ft Collins to Seattle as it does to travel from the SW
radio speaker to your ear.
/tvb
If you use sound then compensation for the temperature, pressure and
humidity dependence of the speed of sound may be required.
If your accuracy requirements are higher then compensating for
variations in the C02 content will also be required.
One of the simpler ways is to use the same techniques to measure a fixed
reference path difference.
Bruce
Tom Van Baak wrote:
I need to determine the position of a instrument with a 1mm accuracy or less.
The instrument is not connected to a mechanical device but is separate &
independent.
The surface which the instrument is positioned on is close to the size of a
11"x11" square.
1 mm or better accuracy on a 300x300 mm surface can be
obtained with a cheap webcam mounted above the surface
and a little bit a creative software.
/tvb
Dont forget to calibrate the camera distortion and ensure that this
doesnt over over time.
i.e. lock the focus and ensure the camera position, tilt etc with
respect to the reference surface do not vary.
Bruce
There's a fair amount of F/OSS software from JPL available to do this sort of calibration. It's used to calibrate cameras used on Mars rovers, among other things. The target pattern for calibration is a bunch of big circular dots on a background.
On 11/23/08 7:56 PM, "Bruce Griffiths" bruce.griffiths@xtra.co.nz wrote:
Tom Van Baak wrote:
I need to determine the position of a instrument with a 1mm accuracy or less.
The instrument is not connected to a mechanical device but is separate &
independent.
The surface which the instrument is positioned on is close to the size of a
11"x11" square.
1 mm or better accuracy on a 300x300 mm surface can be
obtained with a cheap webcam mounted above the surface
and a little bit a creative software.
/tvb
Dont forget to calibrate the camera distortion and ensure that this
doesnt over over time.
i.e. lock the focus and ensure the camera position, tilt etc with
respect to the reference surface do not vary.
Bruce
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.
Hi,
A graphic tablet (11" square is a common size) should give you about 120
positions/mm for something like a hundred bucks.
That's not very different from the "mouse" solution, but give you absolute
position rather than relative.
Regards,
Jean-Louis Oneto
----- Original Message -----
From: "Rick Harold" rickharold@gmail.com
To: time-nuts@febo.com
Sent: Monday, November 24, 2008 2:34 AM
Subject: [time-nuts] position determination over short distance
All,
I'm planning doing some experiments in distance measurement. They don't
deal with atomic time directly but with extreme short periods of time.
I need to determine the position of a instrument with a 1mm accuracy or
less.
The instrument is not connected to a mechanical device but is separate &
independent.
The surface which the instrument is positioned on is close to the size of
a
11"x11" square.
I thought of using 1 RF transmitters (not sure of freq) on bottom of the
device near the surface.
The surface would have RF receivers on 3 or 4 edges/corners to receive the
signal.
If each of the receivers positions are known and they then send a signal
to
a central circuit (again known positions) how can I differentiate the time
of arrival
at the central location? Does anybody know of a circuit/chip or system
which would determine the time 'difference'.
Obviously this is used to triangulate the position of the instrument.
Light travels 1 mm in ~3.3 picoseconds so I would suspect the
differentiator
would have to have that or better resolution.
It could also use some proportional method to extrapolate the position
since
the surface has a fixed size.
Any ideas/thoughts?
Thanks in advance.
Rick Harold
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.
Of course you can use any of the standard photogrammetric tecniques and
programs. Even you can do it automatically using image correlation to
find the paralax and hence the distance.
But I have a simpler and cheaper (and also accurate) solution:
Put a transmiting coil in the object to be measured, about 1-2 cm
diameter, 0.5 cm high (I don´t remember the frequency now, it is quite
low but it doesn't matter).
Lay a receiving antenna grid of vertical and horizontal wires, spaced
about 1 cm, in a surface below the object.
Scan the antennas noting the amplitudes an phases on the 4 or 6
horizontal and vertical wires surrounding the Tx coil in each moment .
Calculate the position in real time (you obtain a precision of about 0.1
mm if the antenna surface is calibrated)
Sound complicated? buy a digitizing tablet (used ones are really
cheap). This is how these things works.
Regards,
Ignacio Cembreros
Lux, James P wrote:
There's a fair amount of F/OSS software from JPL available to do this sort of calibration. It's used to calibrate cameras used on Mars rovers, among other things. The target pattern for calibration is a bunch of big circular dots on a background.
On 11/23/08 7:56 PM, "Bruce Griffiths" bruce.griffiths@xtra.co.nz wrote:
Tom Van Baak wrote:
I need to determine the position of a instrument with a 1mm accuracy or less.
The instrument is not connected to a mechanical device but is separate &
independent.
The surface which the instrument is positioned on is close to the size of a
11"x11" square.
1 mm or better accuracy on a 300x300 mm surface can be
obtained with a cheap webcam mounted above the surface
and a little bit a creative software.
/tvb
Dont forget to calibrate the camera distortion and ensure that this
doesnt over over time.
i.e. lock the focus and ensure the camera position, tilt etc with
respect to the reference surface do not vary.
Bruce
Hi all,
I have to carry a 10 MHz standard frequency signal inside an EMC
screened room via fiber optic cable.
Not willing to re-invent the wheel, do something like an optical
standard frequency link exist on the market?
I think it is possible to use standard 100MB LAN transceivers, and
POF. Phase noise requirements
are not very stringent, and the distance is in the order of some tens
of meters.
Marco IK1ODO / AI4YF
Why fibre optic? You'll only need to convert back to copper when you get it
in there (unless the kit that needs the 10Mhz reference has optical 10MHz
in), and if you do that you may as well just take it in there using high
quality coax (say RG-400U) and a bulkhead connector in the wall of the room.
Dave
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Marco IK1ODO -2
Sent: 24 November 2008 15:31
To: Discussion of precise time and frequency measurement
Subject: [time-nuts] 10 MHz over optical fiber?
Hi all,
I have to carry a 10 MHz standard frequency signal inside an EMC screened
room via fiber optic cable.
At 16.41 24/11/2008, you wrote:
Why fibre optic? You'll only need to convert back to copper when you get it
in there (unless the kit that needs the 10Mhz reference has optical 10MHz
in), and if you do that you may as well just take it in there using high
quality coax (say RG-400U) and a bulkhead connector in the wall of the room.
Dave
Dave,
the customer wants no bulkheads and asks specifically for an optical
connection.
The signal will be, of course, be converted back to electrical inside the room.
Marco
In message 20081124152247.DCDB0E91529@mail.ebirds.it, Marco IK1ODO -2 writes:
Hi all,
I have to carry a 10 MHz standard frequency signal inside an EMC
screened room via fiber optic cable.
Not willing to re-invent the wheel, do something like an optical
standard frequency link exist on the market?
I think it is possible to use standard 100MB LAN transceivers, and
POF. Phase noise requirements
are not very stringent, and the distance is in the order of some tens
of meters.
HP used to have kits consisting of transmitter + receiver + plastic
fiber in various lengths.
HParchive.org has a service note that explains how to use that for
reference frequency distribution.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
Her's another approach, see:
http://www.youtube.com/watch?v=FBy_e4yvqhQ
Regards,
Bob Martinson, N1VQR
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com]On
Behalf Of EB4APL
Sent: Monday, November 24, 2008 9:20 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] position determination over short distance
Of course you can use any of the standard photogrammetric tecniques and
programs. Even you can do it automatically using image correlation to
find the paralax and hence the distance.
But I have a simpler and cheaper (and also accurate) solution:
Put a transmiting coil in the object to be measured, about 1-2 cm
diameter, 0.5 cm high (I don´t remember the frequency now, it is quite
low but it doesn't matter).
Lay a receiving antenna grid of vertical and horizontal wires, spaced
about 1 cm, in a surface below the object.
Scan the antennas noting the amplitudes an phases on the 4 or 6
horizontal and vertical wires surrounding the Tx coil in each moment .
Calculate the position in real time (you obtain a precision of about 0.1
mm if the antenna surface is calibrated)
Sound complicated? buy a digitizing tablet (used ones are really
cheap). This is how these things works.
Regards,
Ignacio Cembreros
Lux, James P wrote:
There's a fair amount of F/OSS software from JPL available to do this sort
of calibration. It's used to calibrate cameras used on Mars rovers, among
other things. The target pattern for calibration is a bunch of big circular
dots on a background.
On 11/23/08 7:56 PM, "Bruce Griffiths" bruce.griffiths@xtra.co.nz wrote:
Tom Van Baak wrote:
I need to determine the position of a instrument with a 1mm accuracy or
less.
The instrument is not connected to a mechanical device but is separate &
independent.
The surface which the instrument is positioned on is close to the size
of a
11"x11" square.
1 mm or better accuracy on a 300x300 mm surface can be
obtained with a cheap webcam mounted above the surface
and a little bit a creative software.
/tvb
Dont forget to calibrate the camera distortion and ensure that this
doesnt over over time.
i.e. lock the focus and ensure the camera position, tilt etc with
respect to the reference surface do not vary.
Bruce
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.
I'm looking for the same thing, but between buildings over singlemode
fiber. I have not evaluated any of these but I've been looking at:
http://www.terahertztechnologies.com
http://www.highlandtechnology.com/
http://www.luxlink.com/
Scott
Marco IK1ODO -2 wrote:
Hi all,
I have to carry a 10 MHz standard frequency signal inside an EMC
screened room via fiber optic cable.
Not willing to re-invent the wheel, do something like an optical
standard frequency link exist on the market?
I think it is possible to use standard 100MB LAN transceivers, and
POF. Phase noise requirements
are not very stringent, and the distance is in the order of some tens
of meters.
Marco IK1ODO / AI4YF
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.
We do lots of this sort of thing at JPL. But the high precision does come at a cost..
Here's a paper from Bob Tjoelker and colleagues..
http://ipnpr.jpl.nasa.gov/progress_report/42-167/167C.pdf
There are various off the shelf products too, (you could buy a receiver and transmitter module from Ortel, for instance)
James Lux, P.E.
Task Manager, SOMD Software Defined Radios
Flight Communications Systems Section
Jet Propulsion Laboratory
4800 Oak Grove Drive, Mail Stop 161-213
Pasadena, CA, 91109
+1(818)354-2075 phone
+1(818)393-6875 fax
-----Original Message-----
From: time-nuts-bounces@febo.com
[mailto:time-nuts-bounces@febo.com] On Behalf Of Scott Mace
Sent: Monday, November 24, 2008 8:37 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] 10 MHz over optical fiber?
I'm looking for the same thing, but between buildings over
singlemode fiber. I have not evaluated any of these but I've
been looking at:
http://www.terahertztechnologies.com
http://www.highlandtechnology.com/
http://www.luxlink.com/
Scott
Marco IK1ODO -2 wrote:
I've used the ortel/emcore 5100 series for L-band IF links in places where the
distances and costs of running coax were too high. Some of the products
launch hot as they were designed for passive/splitter based distribution
so you have to pad the optical rx.
Scott
Lux, James P wrote:
We do lots of this sort of thing at JPL. But the high precision does come at a cost..
Here's a paper from Bob Tjoelker and colleagues..
http://ipnpr.jpl.nasa.gov/progress_report/42-167/167C.pdf
There are various off the shelf products too, (you could buy a receiver and transmitter module from Ortel, for instance)
James Lux, P.E.
Task Manager, SOMD Software Defined Radios
Flight Communications Systems Section
Jet Propulsion Laboratory
4800 Oak Grove Drive, Mail Stop 161-213
Pasadena, CA, 91109
+1(818)354-2075 phone
+1(818)393-6875 fax
-----Original Message-----
From: time-nuts-bounces@febo.com
[mailto:time-nuts-bounces@febo.com] On Behalf Of Scott Mace
Sent: Monday, November 24, 2008 8:37 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] 10 MHz over optical fiber?
I'm looking for the same thing, but between buildings over
singlemode fiber. I have not evaluated any of these but I've
been looking at:
http://www.terahertztechnologies.com
http://www.highlandtechnology.com/
http://www.luxlink.com/
Scott
Marco IK1ODO -2 wrote:
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.
Thanks to all for the quick answers and all the info. As usual,
time-nuts is a great resource :-)
Marco IK1ODO
You might look at video fiber optic transmitter/receiver. I used one
once to carry a certain telemetry signal for the same reasons your are
looking for. The fiber optics were analog in this case and rated from
30 hz to 10 Mhz at 1 volt P-P. We had to attenuate the signal going
into the transmitter and we had to amplify the receiver. Our run was
around 400 feet. The signal to noise ratio was somewhere around 55 db.
Hi Marco ,
You might try firecoms if it's not too hot in there. (plastic)
Steve
--- On Mon, 11/24/08, Marco IK1ODO -2 ik1odo@spin-it.com wrote:
From: Marco IK1ODO -2 ik1odo@spin-it.com
Subject: [time-nuts] 10 MHz over optical fiber?
To: "Discussion of precise time and frequency measurement" time-nuts@febo.com
Date: Monday, November 24, 2008, 7:31 AM
Hi all,
I have to carry a 10 MHz standard frequency signal inside an EMC
screened room via fiber optic cable.
Not willing to re-invent the wheel, do something like an optical
standard frequency link exist on the market?
I think it is possible to use standard 100MB LAN transceivers, and
POF. Phase noise requirements
are not very stringent, and the distance is in the order of some tens
of meters.
Marco IK1ODO / AI4YF
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.
Marco IK1ODO -2 skrev:
Hi all,
I have to carry a 10 MHz standard frequency signal inside an EMC
screened room via fiber optic cable.
Not willing to re-invent the wheel, do something like an optical
standard frequency link exist on the market?
Yes. I know of several commecial systems. If you only need to do a short
jump, then using fairly basic E/O-O/E equipment should work well
enought. It all depends if you want/can to roll your own or need to buy
a finished product (aka "buy this, and you will be fine!").
Stay of plastic "fiber" if you can. Go multimode at least.
I think it is possible to use standard 100MB LAN transceivers, and
POF. Phase noise requirements
are not very stringent, and the distance is in the order of some tens
of meters.
Cheers,
Magnus
Yes. I know of several commecial systems. If you only need to do a short
jump, then using fairly basic E/O-O/E equipment should work well
enought. It all depends if you want/can to roll your own or need to buy
a finished product (aka "buy this, and you will be fine!").
Magnus, what's the typical noise floor, tempco or drift of cheap
(i.e., non JPL-level) fiber distribution systems like this? Is it less
than regular coax, or phase stabilized heliax? At 100 m lengths?
/tvb
Hi;
This may fit your needs, Wenzel made a 10MHz reference that was
disciplined thru fiber. I am currently using one. It could be easily
modified to use battery power. It had very low phase noise.
Thomas Knox
NIST
4475 Whitney Place
Boulder Colorado 80305
1-303-554-0307
tomknox@nist.gov
Quoting "Magnus Danielson" magnus@rubidium.dyndns.org:
Marco IK1ODO -2 skrev:
Hi all,
I have to carry a 10 MHz standard frequency signal inside an EMC
screened room via fiber optic cable.
Not willing to re-invent the wheel, do something like an optical
standard frequency link exist on the market?
Yes. I know of several commecial systems. If you only need to do a short
jump, then using fairly basic E/O-O/E equipment should work well
enought. It all depends if you want/can to roll your own or need to buy
a finished product (aka "buy this, and you will be fine!").
Stay of plastic "fiber" if you can. Go multimode at least.
I think it is possible to use standard 100MB LAN transceivers, and
POF. Phase noise requirements
are not very stringent, and the distance is in the order of some tens
of meters.
Cheers,
Magnus
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.
Tom Van Baak wrote:
Yes. I know of several commecial systems. If you only need to do a short
jump, then using fairly basic E/O-O/E equipment should work well
enought. It all depends if you want/can to roll your own or need to buy
a finished product (aka "buy this, and you will be fine!").
Magnus, what's the typical noise floor, tempco or drift of cheap
(i.e., non JPL-level) fiber distribution systems like this? Is it less
than regular coax, or phase stabilized heliax? At 100 m lengths?
/tvb
Tom
At 1550nm:
Tight buffered fiber has a delay tempco of around 57 ppm/K
Bare SMF28 has a delay tempco of about 7.9ppm/K
Loose fibers in gel filled tube have a delay tempco of about 6.6ppm/K
Bare Spectran fiber has a delay temcpco of about 5.1ppm/K
Sumitomo phase stabilised fiber has a delay tempco of about 0.4ppm/K at
293K.
Bruce
Tom Van Baak skrev:
Yes. I know of several commecial systems. If you only need to do a short
jump, then using fairly basic E/O-O/E equipment should work well
enought. It all depends if you want/can to roll your own or need to buy
a finished product (aka "buy this, and you will be fine!").
Magnus, what's the typical noise floor, tempco or drift of cheap
(i.e., non JPL-level) fiber distribution systems like this? Is it less
than regular coax, or phase stabilized heliax? At 100 m lengths?
The temperature stability is fairly good, but what Bruce posting didn't
expose is the temperature stability of the laser. The problem is that
the laser changes frequency (not wavelength as the optics people say, as
that depends on the refraction index of the material) with temperature.
This is a fairly dramatic effect over distances. The fiber delay changes
by three factors: thermal expansion of fiber (lengthwise), change of
wave-solution as dielectric and physical expansion occurs, and
wavelength of light. The lasers for WDM networks typically has a peltier
cooler for stabilizing temperature.
Choice of fiber and laser is also important. Multi-mode fiber is not
good at all since the dispersion provided by the many different
wavepaths in the fiber will shift in a not so forgiving fashion. For
shorter runs like 100 m and with not too high requirements it will be
fine, but tune up your requirements and you want to go single-mode.
The laser-type is also important. Cheap lasers exhibit a huge number of
frequencies and when looking at an optical spectrum analyzer it can be
hard to decide which has most energy... there is typcially a shape but
the spikes are so many and close that you can't say point out with
certaintly which will be the strongest peak... it will shift. Also, the
dispersion that is gained from the aggregate of those peaks is not nice.
WDM lasers like those for 1550 nm has a much cleaner spectrum which
will gain you alot in the dispersion field.
Long distance communication is very dispersion-sensitive and both
chromatic and non-chromatic dispersion is being considered. While
inter-symbol interference as such is not that important to a pure sine
distribution, the stability aspect of dispersion is.
If you want optimum performance for longer taus, there is really no
option but to do two-way time transfer compensation. Typically the
process is fairly slow on properly laid fiber. For a pure frequency
transport, just looping the signal back to the source and do phase
measure and phase-adjust so that the sent phase compensate the phase
error will work. This way you can continuously compensate the variations
out. Such systems is used for instance in DSN and similar systems with
multiple arrays of antennas at microwave frequencies.
Anyway, I beleive I had the sum of errors become about 85 ps/nm/K/km.
Could be wrong, it's just a number stuck in my head.
Does this effect occur in real life? Well, naturally I have an anecdote
to share on that. In Sweden the national power company had put fibers on
their high voltage cables running through the country. They needed it
for their internal use, but could of course provide service to others,
which they do. They have a SDH network on top of that and can then
provide SDH based services, alongside WDM channels. However, about 7 in
the Monday mornings in February their SDH network failed as the sync
went totally bananas. They called in a synchronisation expert which I
happend to know. He measured the wander and picked up a very nice
deviation with a huge swing right there in the morning hours. What
happend? Well, what happens 7 in the Monday mornings which is
exceptional? That's when they start up all the heavy machinery in the
industry, they increase heating since it is cold, people wake up and
turn TVs on etc. etc. etc. Many of the things which have been off or
turned down during the weekend is starter up. This makes the current go
up, this makes the power cables go warmer (they will expand quite
noticably!) and the fiber being pulled on the cable will change
temperature. As the heater and fiber run over a considerable length
together, many 100s of km, and the shift in temperature is several C
then it is not strange that they experienced a huge shift in delay.
Having heard my comments on this, the synchronisation expert pointed
this out. They ended up to move the fibers to the relatively unloaded
ground wire at the top, and have a much more quieter system.
This shows that uncompensated delay even in a good fiber system can eat
you for lunch.
The depth by which you have buried your fibers is another issue. Deeper
means higher stability in temperature, as mother wander (the sun) will
contribute alot at tau = 43,2 ks.
For shorter runs, one way transfer can work well. The requirements on
stability may be an issue if you have high requirements. If you do a 10
m run, few would have a problem on an uncompensated system. Off the
shelf components could probably satisfy most needs even for 100 m or so.
Interestingly enought, there is a Siemens patent for a fiber based
oscillator. The short term stability of silica fiber is actually quite
good. It is the long term aspects which can eat you out if you do not
deal with it.
Consider also things like laser gyroscopes. The stability of the fiber
is sufficient to detect even small shifts frequency due to Sagnac effect.
Cheers,
Magnus
On Fri, 2008-11-28 at 02:50 +0100, Magnus Danielson wrote:
Consider also things like laser gyroscopes. The stability of the fiber
is sufficient to detect even small shifts frequency due to Sagnac effect.
Usually called FOGs (Fiber Optic Gyroscope), where the other "laser"
type, no fibers here, is called RLG (Ring Laser Gyroscope).
--
Björn
It's actually [1]http://www.bb-elec.com for those looking.
Bill Ezell
They said 'Windows or better'
so I used Linux.
Neon John wrote:
On Mon, 24 Nov 2008 17:33:25 -0800, "Tom Van Baak" [2]tvb@LeapSecond.com wrot
e:
Yes. I know of several commecial systems. If you only need to do a short
jump, then using fairly basic E/O-O/E equipment should work well
enought. It all depends if you want/can to roll your own or need to buy
a finished product (aka "buy this, and you will be fine!").
Magnus, what's the typical noise floor, tempco or drift of cheap
(i.e., non JPL-level) fiber distribution systems like this? Is it less
than regular coax, or phase stabilized heliax? At 100 m lengths?
OK, tom, you got me with another one. WTF is phase stabilized heliax? Is
that a hunk of ordinary heliax that has been characterized or is it made
special in some way?
re: original problem
check out B&B Electronics
[3]http://www.bandbelectronics.com/
sorry, I'm off-line right now and can't browse but they specialize in really
inexpensive implementations of stuff like this.
John
John De Armond
See my website for my current email address
[4]http://www.neon-john.com
[5]http://www.johndearmond.com <-- best little blog on the net!
Tellico Plains, Occupied TN
Save the whales, collect the whole set!
time-nuts mailing list -- [6]time-nuts@febo.com
To unsubscribe, go to [7]https://www.febo.com/cgi-bin/mailman/listinfo/time-nut
s
and follow the instructions there.
References
It's regular Heliax that's been temperature-conditioned by repeated
temp cycling to minimize dielectric size change vs temperature. This
minimizes phase shift over length. The cycling affects the foam used
as the dielectric, which apparently eventually stops expanding and
contracting as much as non-conditioned cables.
Bill Ezell
They said 'Windows or better'
so I used Linux.
Neon John wrote:
On Mon, 24 Nov 2008 17:33:25 -0800, "Tom Van Baak" [1]tvb@LeapSecond.com wrot
e:
Yes. I know of several commecial systems. If you only need to do a short
jump, then using fairly basic E/O-O/E equipment should work well
enought. It all depends if you want/can to roll your own or need to buy
a finished product (aka "buy this, and you will be fine!").
Magnus, what's the typical noise floor, tempco or drift of cheap
(i.e., non JPL-level) fiber distribution systems like this? Is it less
than regular coax, or phase stabilized heliax? At 100 m lengths?
OK, tom, you got me with another one. WTF is phase stabilized heliax? Is
that a hunk of ordinary heliax that has been characterized or is it made
special in some way?
re: original problem
check out B&B Electronics
[2]http://www.bandbelectronics.com/
sorry, I'm off-line right now and can't browse but they specialize in really
inexpensive implementations of stuff like this.
John
John De Armond
See my website for my current email address
[3]http://www.neon-john.com
[4]http://www.johndearmond.com <-- best little blog on the net!
Tellico Plains, Occupied TN
Save the whales, collect the whole set!
time-nuts mailing list -- [5]time-nuts@febo.com
To unsubscribe, go to [6]https://www.febo.com/cgi-bin/mailman/listinfo/time-nut
s
and follow the instructions there.
References
On Sun, 23 Nov 2008 20:26:52 -0800, "Lux, James P" james.p.lux@jpl.nasa.gov
wrote:
There's a fair amount of F/OSS software from JPL available to do this sort of calibration. It's used to calibrate cameras used on Mars rovers, among other things. The target pattern for calibration is a bunch of big circular dots on a background.
URLs?
I recently found this freeware open source on the net.
http://tim-jackson.co.uk/area/index.html
Caution: I've encountered this guy on Usenet where he has a bad tendency to
substitute abject BS in place of fact when it's too much trouble to dig up the
facts. I'd inspect the sources closely before using, looking for shortcuts
that don't work and pure old logic errors.
It would do the job in this instance but I'm interested in a more generalized
solution (without having to write it myself or buy anything) to pulling
measurements from photos.
I'd like to be able to take a photograph in which an object of known
dimensions is included and pull other dimensions from the photo, including
areas.
Any suggestions?
John
John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com <-- best little blog on the net!
Tellico Plains, Occupied TN
What do you call a blonde's cranial cavity? Vacuum chamber?