Christopher wrote:
Hi guys,

ok, I've got an idea but it's based on determining the distance of a
transmitter from a receiver, I originally thought about a synchronised
clock in both units, the transmitter sends the time it has out, by the
time the receiver unit gets this time a period has passed (probably a
few millionth's of a second) and a diff time is detemined, combined with
the speed those waves travel, will reveal the distance.

however, physics decided this idea wouldnt work, since all
electromagnetic radiation travels at the speed of light, apparently,
DAMN, back to the drawing board.

I'm here to see if anyone has any way they can determine the distance
from transmitter to receiver, this isnt a great distance either and it
needs to be fairly, accurate, if it's not possible, it's not possible, I
just wanted to ask people far cleverer than i.

signal strength perhaps?

literally I am talking about a transmitter within a cuboid shaped
enclosure around 10m maximum and being able to pinpoint that transmitter
within that enclosure accurately, to around 1cm, perhaps 2cm.

like I said, if it's not possible, well then hey, thanks anyway, but
perhaps it is and therefore perhaps my idea will still be workable.

thanks guys! I'll be waiting for your answers or solutions

You might want to look at how GPS works.

--
I think it likely that when the successor to IPV6 is just about to be deployed
througout the Solar System there will still be null routes and deny table
entries for 205.199.212.0/24 in an uncountable number of places.
-- Michael Rathbun

Christopher wrote:
Hi guys,

ok, I've got an idea but it's based on determining the distance of a
transmitter from a receiver, I originally thought about a synchronised
clock in both units, the transmitter sends the time it has out, by the
time the receiver unit gets this time a period has passed (probably a
few millionth's of a second) and a diff time is detemined, combined with
the speed those waves travel, will reveal the distance.

Yes, but as you've deduced, the problem is keeping the clocks synchronize.

On the other hand, there's no reason you can't have one unit send out a
pulse and begin timing. The second unit receives it, immediate sends a
'reply' pulse, and you stop your clock when the first unit receives that.
Now the accuracy is determined by the accuracy of just one clock, the amount
of uncertainty in the 'turnaround' time of the second unit... and the
variation in the speed of light as a function of athmospheric conditions.
(The last one being more of a concern if you're trying to listen to
satellites -- GPS receivers run into this problem.)

You can also use multiple transmitters with high-quality clocks and one
receiver -- this is what GPS goes. The transmitters send out a message
saying, 'I sent this at exactly this time... really I did!' You record on
your local clock when you receive those messages and hence can perform
triangulation to determine your position. Better yet, if you have yet
another transmitter, you can solve for your position as well as the
_relative error in your timebase_ and thereby synchronize one (or more)
receivers to the high-quality clock in the transmitter. In GPS receivers,
this additional satellite is what gets you a 2D fix vs. a 3D fix.

signal strength perhaps?
literally I am talking about a transmitter within a cuboid shaped
enclosure around 10m maximum and being able to pinpoint that transmitter
within that enclosure accurately, to around 1cm, perhaps 2cm.

Ah, OK, if it's enclosed field strength would be a reasonably reliable
indicator. There was an article in Circuit Cellar Ink just a year or so
back where a guy did this (and cited references -- the idea has been around
for awhile), but it's sort of the oppposite of what you suggest: You stick
large (transmitting) coils at the edges of your box and then detect the
signal strength on a small receiver. 1cm in 10m is 1 part in 1000, which I
would gander is right about in the middle of 'trivial' and 'pushing
impossible.'

There are also '3D' computer mice (mouses?) out there that use a couple of
ultrasonic receivers on the 'mouse pad' and perform triangulation after
listening for the mouse's transmission. Same idea as with RF, but easier
because sound waves are so much slower than electromagnetic waves -- times
are measured in microseconds or milliseconds rather than picosecond and
nanoseconds!

---Joel Kolstad

That's basically how GPS works, except that you need more than one
transmitter, or veddy expensive clocks in the receiver. Basically with GPS
you receive accurate time signals from four satallites. Since your receiver
doesn't know the time or it's position you have four equations (from four
satellites) in four unknowns and voila! you have an answer. You can do
differential GPS down to millimeter accuracy if you sense the phase of the
carrier.

Loran uses the same basic concepts (synchronized transmitters, speed of
light, yadda yadda), but it asks you to sense the phase difference between a
master and a slave TX, and it isn't nearly so accurate.

"Christopher" wrote in message
...
Hi guys,

ok, I've got an idea but it's based on determining the distance of a
transmitter from a receiver, I originally thought about a synchronised
clock in both units, the transmitter sends the time it has out, by the
time the receiver unit gets this time a period has passed (probably a
few millionth's of a second) and a diff time is detemined, combined with
the speed those waves travel, will reveal the distance.

however, physics decided this idea wouldnt work, since all
electromagnetic radiation travels at the speed of light, apparently,
DAMN, back to the drawing board.

I'm here to see if anyone has any way they can determine the distance
from transmitter to receiver, this isnt a great distance either and it
needs to be fairly, accurate, if it's not possible, it's not possible, I
just wanted to ask people far cleverer than i.

signal strength perhaps?

literally I am talking about a transmitter within a cuboid shaped
enclosure around 10m maximum and being able to pinpoint that transmitter
within that enclosure accurately, to around 1cm, perhaps 2cm.

like I said, if it's not possible, well then hey, thanks anyway, but
perhaps it is and therefore perhaps my idea will still be workable.

thanks guys! I'll be waiting for your answers or solutions

kosh

distance of radio transmitter from receiver
 

Hi guys,

ok, I've got an idea but it's based on determining the distance of a
transmitter from a receiver, I originally thought about a synchronised
clock in both units, the transmitter sends the time it has out, by the
time the receiver unit gets this time a period has passed (probably a
few millionth's of a second) and a diff time is detemined, combined with
the speed those waves travel, will reveal the distance.

however, physics decided this idea wouldnt work, since all
electromagnetic radiation travels at the speed of light, apparently,
DAMN, back to the drawing board.

I'm here to see if anyone has any way they can determine the distance
from transmitter to receiver, this isnt a great distance either and it
needs to be fairly, accurate, if it's not possible, it's not possible, I
just wanted to ask people far cleverer than i.

signal strength perhaps?

literally I am talking about a transmitter within a cuboid shaped
enclosure around 10m maximum and being able to pinpoint that transmitter
within that enclosure accurately, to around 1cm, perhaps 2cm.

like I said, if it's not possible, well then hey, thanks anyway, but
perhaps it is and therefore perhaps my idea will still be workable.

thanks guys! I'll be waiting for your answers or solutions

kosh

literally I am talking about a transmitter within a cuboid shaped
enclosure around 10m maximum and being able to pinpoint that transmitter
within that enclosure accurately, to around 1cm, perhaps 2cm.

This is a typical application of an acoustic positioning system. With a
speed of sound of about 340 m/s, this is feasible. I have made such a system
with acoustic tags on 40 kHz using 8 receiver nodes in the room. That's more
than required, but it gives enough redundancy to be robust against shadowing
as things move around in the room. Accuracy is in the 1-2 cm range.

--
Sverre Holm, LA3ZA
---------------------------------
www.qsl.net/la3za

On Mon, 26 Jan 2004 16:55:44 -0800, "Tim Wescott"
wrote:

That's basically how GPS works, except that you need more than one
transmitter, or veddy expensive clocks in the receiver. Basically with GPS
you receive accurate time signals from four satallites. Since your receiver
doesn't know the time or it's position you have four equations (from four
satellites) in four unknowns and voila! you have an answer. You can do
differential GPS down to millimeter accuracy if you sense the phase of the
carrier.

Since the measurement should be done in a confined space, why not
switch the roles and use one transmitter on the moving object and four
receivers on known fixed locations around the perimeter of that space?

With the receivers connected by cables receiving a common clock
signal, the accuracy of the clock is not important, contrary to the
situation in GPS, in which the four satellites must have atomic clocks
to have a common time base.

Loran uses the same basic concepts (synchronized transmitters, speed of
light, yadda yadda), but it asks you to sense the phase difference between a
master and a slave TX, and it isn't nearly so accurate.

Space probe ranging is done by sending a high data rate pseudo noise
sequence from earth to the probe, in which a simple frequency
translation is done with a high accuracy local oscillator to a
different frequency and sent back to earth. From this, the total
earth-probe-earth phase difference of the PRN sequence (and possibly
also the total RF phase difference) can be determined and hence also
the distance.

However, in this case the accuracy requirement was 1 cm, so I guess
that at least 10 GHz (3 cm) radiation should be used.

Paul

You don't necessesarily need to have a carrier wavelength smaller than the
distance you want to measure, as long as you can accurately measure phase.
Assuming that you could measure phase to 10 degrees, for instance, a 1cm
accuracy would only require 900MHz (33cm).

Surveyor-quality differential GPS uses the 2.4GHz GPS carrier and measures
the differences in the carrier phase (not the phase of the pseudo-random
sequence) to get sub-cm accuracies.

"Paul Keinanen" wrote in message
...
On Mon, 26 Jan 2004 16:55:44 -0800, "Tim Wescott"
wrote:

-- snip --

However, in this case the accuracy requirement was 1 cm, so I guess
that at least 10 GHz (3 cm) radiation should be used.

Paul

Yep! acoustic is one. another is infrared

Over this short distance RF travel time is in the 30 nano second range.
This requires some pretty good timing measurements. Sound, on the other
hand, has a speed of around 30 mili seconds for 10M (30 ft).

Something I wanted to do for a long time is a model rocket altitude system.
Rocket has a one transistor 10M Rx ( have the circuit around here from some
1960's Pop Electronics for a 1 tranny FM broadcast Rx) and a one or two
transistor 2M Tx (FM) -- rocket antenna easier.. Ground station on 10M
(lots of power available for the wooden rocket Rx) transmits a TONE (not a
pulse). Rocket transponds (retransmits it on 2M). Measure the zero
crossing time delay and subtract the overhead time. With proper (and
simple) digital design, you can have a digital readout in feet, inches,
whatever. With a tone, common ham radios are just fine.
With audio, pulses are easy except how do you keep the target from hearing
it's own echo and "oscillating by itself...?


or use subcarriers - this is a little harder. Transmit an FM modulated tone
(carrier, say 10KHz, tone 1KHz) and transponding it back on another
carrier, say 15 KHz.

Who was it that used to sell the sonar modules from the cameras for
experimentation

acoustic


"Sverre Holm" wrote in message
...
literally I am talking about a transmitter within a cuboid shaped
enclosure around 10m maximum and being able to pinpoint that transmitter
within that enclosure accurately, to around 1cm, perhaps 2cm.

This is a typical application of an acoustic positioning system. With a
speed of sound of about 340 m/s, this is feasible. I have made such a
system
with acoustic tags on 40 kHz using 8 receiver nodes in the room. That's
more
than required, but it gives enough redundancy to be robust against
shadowing
as things move around in the room. Accuracy is in the 1-2 cm range.

--
Sverre Holm, LA3ZA
---------------------------------
www.qsl.net/la3za

Steve Nosko wrote:

Something I wanted to do for a long time is a model rocket altitude system.
Rocket has a one transistor 10M Rx ( have the circuit around here from some
1960's Pop Electronics for a 1 tranny FM broadcast Rx) and a one or two
transistor 2M Tx (FM) -- rocket antenna easier.. Ground station on 10M
(lots of power available for the wooden rocket Rx) transmits a TONE (not a
pulse). Rocket transponds (retransmits it on 2M). Measure the zero
crossing time delay and subtract the overhead time. With proper (and
simple) digital design, you can have a digital readout in feet, inches,
whatever. With a tone, common ham radios are just fine.
With audio, pulses are easy except how do you keep the target from hearing
it's own echo and "oscillating by itself...?

Block the RX input for a few ohnoseconds[1] longer

or use subcarriers - this is a little harder. Transmit an FM modulated tone
(carrier, say 10KHz, tone 1KHz) and transponding it back on another
carrier, say 15 KHz.

Might be a useful alternative. Have to have sharp filters.

Who was it that used to sell the sonar modules from the cameras for
experimentation

Polaroid, IIRC.

[1] The shortest possible unit of time: the interval between hitting
the "do it" key and the realization that you shouldn't have.

--
I swear to god, if people treated their cars they way they treat their
computers, half the cars on the road would be covered in bumper stickers
advertising porno, and their trunks would be filled with rotting garbage.
-- Christian Wagner

Hi Joel,

Joel Kolstad wrote:
Christopher wrote:

Hi guys,

ok, I've got an idea but it's based on determining the distance of a
transmitter from a receiver, I originally thought about a synchronised
clock in both units, the transmitter sends the time it has out, by the
time the receiver unit gets this time a period has passed (probably a
few millionth's of a second) and a diff time is detemined, combined with
the speed those waves travel, will reveal the distance.


Yes, but as you've deduced, the problem is keeping the clocks synchronize.

On the other hand, there's no reason you can't have one unit send out a
pulse and begin timing. The second unit receives it, immediate sends a
'reply' pulse, and you stop your clock when the first unit receives that.
Now the accuracy is determined by the accuracy of just one clock, the amount
of uncertainty in the 'turnaround' time of the second unit... and the
variation in the speed of light as a function of athmospheric conditions.
(The last one being more of a concern if you're trying to listen to
satellites -- GPS receivers run into this problem.)

my problem with this is that radio waves are electromagnetic radiation
which travels at the speed of light, therefore can travel 1 meter in 3.3
nanoseconds, if my range is around 10 meters (I was a bit conservative
before, lets say 30meter width/depth and 10 meters high, that'd be big
enough I think for any situation I'd use this system within.

Is there a clock out there that can measure accuracy on this scale, it
seems impossible. That's of course, if radio waves travel through our
atmosphere at almost the speed of light, I'm going to look now as to how
fast they travel through our atmosphere.


You can also use multiple transmitters with high-quality clocks and one
receiver -- this is what GPS goes. The transmitters send out a message
saying, 'I sent this at exactly this time... really I did!' You record on
your local clock when you receive those messages and hence can perform
triangulation to determine your position. Better yet, if you have yet
another transmitter, you can solve for your position as well as the
_relative error in your timebase_ and thereby synchronize one (or more)
receivers to the high-quality clock in the transmitter. In GPS receivers,
this additional satellite is what gets you a 2D fix vs. a 3D fix.

well if you have one transmitter attached to say the top of your head,
you'd need multiple receivers to be able to triangulate that
transmitters position wouldnt you, you'd record the time it was sent and
diff it against the time you have in your local clock, that difference
would give you a rough distance to the transmitter, from each receiver
you'd have a distance/transmitter value, which would denote the RADIUS
of a circle from which the transmitter could be, of course, the location
of the transmitter is where all three circles overlap. More receiver
units could be used to increase the accuracy of those measurements.

the problem again is clock sync. I did originally think that you could
have one master receiver unit, whose clock all others are determined by,
this clock could send out it's time with a "reset" code that all the
other clocks would reset their time to, this would happen fairly
accurately would it? I think it would. Then you're clocks are fairly
sync'd. You'd probably not want to run them for long without resync'ing
again though


signal strength perhaps?
literally I am talking about a transmitter within a cuboid shaped
enclosure around 10m maximum and being able to pinpoint that transmitter
within that enclosure accurately, to around 1cm, perhaps 2cm.


Ah, OK, if it's enclosed field strength would be a reasonably reliable
indicator. There was an article in Circuit Cellar Ink just a year or so
back where a guy did this (and cited references -- the idea has been around
for awhile), but it's sort of the oppposite of what you suggest: You stick
large (transmitting) coils at the edges of your box and then detect the
signal strength on a small receiver. 1cm in 10m is 1 part in 1000, which I
would gander is right about in the middle of 'trivial' and 'pushing
impossible.'

hmmm, thats interesting idea. although I can't find much about it,
could you remember the article name?



There are also '3D' computer mice (mouses?) out there that use a couple of
ultrasonic receivers on the 'mouse pad' and perform triangulation after
listening for the mouse's transmission. Same idea as with RF, but easier
because sound waves are so much slower than electromagnetic waves -- times
are measured in microseconds or milliseconds rather than picosecond and
nanoseconds!

---Joel Kolstad

Yea. Easy.Steve N.

"Mike Andrews" wrote in message
...
Steve Nosko wrote:

...With audio, pulses are easy except how do you keep the target from
hearing
it's own echo and "oscillating by itself...?

Block the RX input for a few ohnoseconds[1] longer

or use subcarriers - this is a little harder. Transmit an FM modulated
tone
(carrier, say 10KHz, tone 1KHz) and transponding it back on another
carrier, say 15 KHz.

Might be a useful alternative. Have to have sharp filters.

Who was it that used to sell the sonar modules from the cameras for
experimentation

Polaroid, IIRC.

[1] The shortest possible unit of time: the interval between hitting
the "do it" key and the realization that you shouldn't have.

--
I swear to god, if people treated their cars they way they treat their
computers, half the cars on the road would be covered in bumper stickers
advertising porno, and their trunks would be filled with rotting garbage.
-- Christian Wagner

Steve Nosko wrote:
. . .
Who was it that used to sell the sonar modules from the cameras for
experimentation


It was Polaroid.

You can get sonar measuring devices at home handyman stores for a few
bucks. They might be useful with a little hacking.

Roy Lewallen, W7EL

In article , Christopher
writes:

Joel Kolstad wrote:
Christopher wrote:

Hi guys,

ok, I've got an idea but it's based on determining the distance of a
transmitter from a receiver, I originally thought about a synchronised
clock in both units, the transmitter sends the time it has out, by the
time the receiver unit gets this time a period has passed (probably a
few millionth's of a second) and a diff time is detemined, combined with
the speed those waves travel, will reveal the distance.

Yes, but as you've deduced, the problem is keeping the clocks synchronize.

On the other hand, there's no reason you can't have one unit send out a
pulse and begin timing. The second unit receives it, immediate sends a
'reply' pulse, and you stop your clock when the first unit receives that.
Now the accuracy is determined by the accuracy of just one clock, the
amount
of uncertainty in the 'turnaround' time of the second unit... and the
variation in the speed of light as a function of athmospheric conditions.
(The last one being more of a concern if you're trying to listen to
satellites -- GPS receivers run into this problem.)

my problem with this is that radio waves are electromagnetic radiation
which travels at the speed of light, therefore can travel 1 meter in 3.3
nanoseconds, if my range is around 10 meters (I was a bit conservative
before, lets say 30meter width/depth and 10 meters high, that'd be big
enough I think for any situation I'd use this system within.

Is there a clock out there that can measure accuracy on this scale, it
seems impossible. That's of course, if radio waves travel through our
atmosphere at almost the speed of light, I'm going to look now as to how
fast they travel through our atmosphere.

For some background on an established system, research aviation
navigational aids DME (Distance Measuring Equipment, civilian use)
and TACAN (TACtical Area Navigation, military use) for an aircraft
to get their slant range to a ground station.

DME, compatible with TACAN for range, has an aircraft interrogator-
receiver that sends a pulse pair at low L Band. A ground station
within range will receive that pair, wait 50 microseconds, then
respond with another pulse pair. The aircraft determines range from
the round-trip to get a response minus the fixed 50 uSec delay using
the common radar round-trip time (which is approximately 500 uSec
per mile for coarse estimation).

The reason for the pulse pairs is to avoid false responses and
returns from noise spikes or stray single pulses or whatever. That
is easy to implement by a delay line and the equivalent of an AND
gate action. Single pulses won't get passed but a pair with the
right spacing will.

To further cut down on false replies in a crowded airspace, the
aircraft interrogator repetition rate is "dithered" slightly. Since the
aircraft uses that interrogation as the start of its timing to a return,
the dithered rep rate doesn't matter to it...but to the rest of the air-
space, those interrogations and replies seem like so much noise
from others ("fruit" in the civil avionics jargon).

TACAN has been in the U.S. military since the early 1950s (a USN
development) and DME came shortly thereafter. Used daily by
aircraft on civil airways. Accuracy depends on the calibration of
the aircraft interrogator-receiver. Old ground stations used a
supersonic delay line filled with mercury to achieve the fixed 50 uSec
delay of replies to an interrogation from aircraft. Maximum duty cycle
is rather higher than radar for ground stations but not a real problem.
That fixed delay includes the turn-around time of receiving a pulse
pair, determining that it is a pair, etc. A fixed delay allows checking
out the system prior to takeoff, using a local VORTAC station at
close range.

===========

Trying to use one-way signal strength for range isn't going to be
good. If within 5 to 10 wavelengths for the path, the path is fraught
with all kinds of variations just as a parasitic antenna works with
field phasing. Unlike parasitic antennas, the path would not be
straight line (such as a Yagi-Uda).

Beyond the "near field" effects, one would get Multipath reflections,
some of which would be in-phase, some out-phase, all resulting in
many decibels of strength variation at short relative positions.

Signal strength as a coarse distance indication is only good at very
long ranges relative to wavelength. Even then it is uncertain. It's very
difficult to get a true isotropic source with a spherical radiation
pattern in a practical antenna, any RF wavelength.

Round-trip time is the only practical way to achieve ranging whether
by RF or by acoustic means.

Len Anderson
retired (from regular hours) electronic engineer person

Signal strength as a coarse distance indication is only good at very
long ranges relative to wavelength. Even then it is uncertain.

New location systems based on WLAN (2.4 GHz) use field strength combined
with what seems to be some sort of adaptive algorithm that learns the field
strength vs environment through repeated tests. See www.ekahau.com and
www.radionor.com. These companies promise location for 'free', as they can
locate laptops by using the existing hardware infrastructure of the wireless
network. This is a very hot topic these days, and large business
opportunities are expected.

But accuracies are in the 1 m range, to get to 1 cm accuracy there does not
seem to be any alternative to ultrasound systems, see demo on
www.sonitor.com.


--
Sverre Holm, LA3ZA
---------------------------------
www.qsl.net/la3za

We did a unit at my last job called 'velocity of light'.

It was pretty simple and you cud accurately measure distance down to 1cm.

Infrared led emitter switched at 50MHz, focused using a lens to a 2 inch beam,
reflected back by a mirror to an infrared detector (5 inches away from the
emitter).

The tx 50MHz was generated using a 50MHz xtal which drove the tx led.

Their was a 50.025MHz second xtal osc which was used to mix down both the tx
signal and the rx signal to 25KHz IF (we now have 2 25KHz waveforms), these two
25KHz carriers were then phase compared - so easy to see the smallest of
movements in the mirror on a basic scope.

So all the hard work is done at 25KHz (phase measuring) - one of todays little
mcpu's will do this easily (ATmega16 for example).

You could just as easily use laser or maybe rf in place of the IR led's, though
directing RF at such low freq's would be somewhat difficult.

Obviously at 50MHz, the phase difference would cycle every 6 meters (total
reflected path), but if your a bit cleverer (though not hard to do) you could
get the freq to sweep from a low freq (say 5MHz) upto wot ever you like - using
a pair of single xtal referenced PLL's to generate the two oscillator freq's
(whilst maintaining the 25KHz difference) and then calculate an exact distance
in the cpu.

This method is simple and relatively cheap to judge distances very accurately
with no need for very short pulses (high bandwidths) and very fast logic.

Clive

Forgot to say.

A 90deg phase change in the reflected beam (at 50MHz) also results in a 90deg
phase change in the rx'ed 25KHz IF) - this is why no fast logic is required.

I these kind of system it is the field strength that is the most important
not the distance. A radio system needs signal strength and that is the
primary reason to measure it. There is a desire to get distance from it,
but this is more of a dream rather tham reality. In cellular systems, it
can be used and give pretty goos location in SOME cases.

I say, for the described situation, RF is NOT the way due to the timing
requirements.
Steve N.



"Sverre Holm" wrote in message
...
Signal strength as a coarse distance indication is only good at very
long ranges relative to wavelength. Even then it is uncertain.

New location systems based on WLAN (2.4 GHz) use field strength combined
with what seems to be some sort of adaptive algorithm that learns the
field
strength vs environment through repeated tests. See www.ekahau.com and
www.radionor.com. These companies promise location for 'free', as they can
locate laptops by using the existing hardware infrastructure of the
wireless
network. This is a very hot topic these days, and large business
opportunities are expected.

But accuracies are in the 1 m range, to get to 1 cm accuracy there does
not
seem to be any alternative to ultrasound systems, see demo on
www.sonitor.com.


--
Sverre Holm, LA3ZA
---------------------------------
www.qsl.net/la3za

I think this will be easier than using RF. However, even though the IF is
25 kHz, you are still measuring small time differences. I just think this
hardware will be easier than RF. Mostly because you can control where it
goes better. This, of course, assumes you can do that in the target system
The OP has to decide.

That being said.... oneof those 10 GHz door opener or motion detector units
(does the GunnPlexer have a DC output from its detector?) would be another
interesting method. You can easily see small motions in the reflected
signal phase difference and this is at the 10GHz freq. so the resolution is
high. You do need to insure that the desired reflection is the only
one...similar to the LED type of system.

An aiming problem.

Steve N.

wrote in message
...

We did a unit at my last job called 'velocity of light'.

It was pretty simple and you cud accurately measure distance down to 1cm.

Infrared led emitter switched at 50MHz, focused using a lens to a 2 inch
beam,
reflected back by a mirror to an infrared detector (5 inches away from the
emitter).

The tx 50MHz was generated using a 50MHz xtal which drove the tx led.

Their was a 50.025MHz second xtal osc which was used to mix down both the
tx
signal and the rx signal to 25KHz IF (we now have 2 25KHz waveforms),
these two
25KHz carriers were then phase compared - so easy to see the smallest of
movements in the mirror on a basic scope.

So all the hard work is done at 25KHz (phase measuring) - one of todays
little
mcpu's will do this easily (ATmega16 for example).

You could just as easily use laser or maybe rf in place of the IR led's,
though
directing RF at such low freq's would be somewhat difficult.

Obviously at 50MHz, the phase difference would cycle every 6 meters (total
reflected path), but if your a bit cleverer (though not hard to do) you
could
get the freq to sweep from a low freq (say 5MHz) upto wot ever you like -
using
a pair of single xtal referenced PLL's to generate the two oscillator
freq's
(whilst maintaining the 25KHz difference) and then calculate an exact
distance
in the cpu.

This method is simple and relatively cheap to judge distances very
accurately
with no need for very short pulses (high bandwidths) and very fast logic.

Clive


Forgot to say.

A 90deg phase change in the reflected beam (at 50MHz) also results in a
90deg
phase change in the rx'ed 25KHz IF) - this is why no fast logic is required.