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