Mike Andrews wrote:

Allan Butler wrote:

[snip preconditions]


The first receiver detects the transmit pulse and it is known that the
transmitter is within 450 feet of that receiver. That starts a clock. The
second receiver detects the transmit pulse and the time since the clock
started is noted. The third receiver detects the transmit pulse and the
time since the first receiver detected the signal is noted. With this much
information the position of the transmitter can be determined on a two
dimensional plot. The fourth receiver could be used for a sanity check to
make certain that the transmitter is in the expected location and it would
allow better coverage for when only three receivers can detect the signal.


The space between clock start and second receive detect is the difference in
distance between these two receivers. The next detect is the difference in
distance between the first receiver and the third. And lastly the fourth
detect sets the distance between the fourth receiver and the first. If the
math is done right there will be four circles drawn each has the center at
the corner of your property. When the drawings are made they will all cross
in only one place. There will be other places where two or three circles
cross.


But for this to work, as I pointed out in another post, you need to
know the true distance from the transmitter to any one or more of the
receivers already, or (equivalently) you need to know the exact time
of transmission relative to the receiver clock. Otherwise all you
have is the delta-Time Of Arrival (TOA) from the receiver that gets
the pulse first to the other receivers, and that's not sufficient to
locate the transmitter.

Even where the maximum distance is known, you still need the true
distance from the transmitter to any one receiver at a minimum. If
you don't have that, you can't draw any circles.

Or I'm missing something obvious. I really don't think I am, but if
someone can point out what I'm missing I _will_ be grateful.


If you know the positions of any two receivers, and the time delta
between the reception of the pulses from the transmitters, the set of
possible positions of the transmitter lies on a hyperbola that
intersects the line between the two receivers.

With two pairs of receivers you get two hyperbolas and two intersections
-- and three receivers gives you three pairings to calculate with.

So you don't need to know the absolute time of the transmitted pulse,
just the difference.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

"Tim Wescott" wrote in message
...
Mike Andrews wrote:

Allan Butler wrote:

[snip preconditions]


Withoug the previous parts of the thread this may be duplication since I
don't know the original constraints...but

I have always wanted to build a model rocket altitude transponder using this
concept:

On-board the rocket we have a "transponder"... a one transistor superregen
Rx on, say 10 or 6 Meters. I hae a schematic around here somewhere for a
one tranny FM broadcast receiver from Pop Tronics. Have it's demodulated
signal modulate a 2 meter TX. This can be a 9 or 12 Mhz. xtal osc and 2
meter tuned circuit (gets you a few miles on the ground from a tower
receiver).
On the ground you have the "Ground Station" 10 or 6 meter mobile. Since
you can have lots of power, the crummy Rx onboard the rocket is no problem.
From the mobile, transmit a tone. Measure the time delay of a zero crossing
of the transponded tone. A simple freq counter can be made to do this if it
had (I forget the official name--triggered event counter maybe??) the
ability to start counting on the TX tone's edge and stop on the RX tone's
edge. Just a few chips for this.
You pick the frequency that the counter is counting to display feet or
whatever units you like.
What's-it, something like a nano second per foot?
You also need a delay in the proper place to account for the "zero distance"
delay of the receivers & transmitters.
Seems like one other "Ground" receiver is all that's needed to triangulate
the location horizontally...

There was an article in QST within the last year or so where some fellas
measured the delay through a, I think, 2M repeater versus distance...same
concept, 'cept they just used a dual trace scope..

--
Steve N, K,9;d, c. i My email has no u's..

"Tim Wescott" wrote in message
...
Mike Andrews wrote:

Allan Butler wrote:

[snip preconditions]


Withoug the previous parts of the thread this may be duplication since I
don't know the original constraints...but

I have always wanted to build a model rocket altitude transponder using this
concept:

On-board the rocket we have a "transponder"... a one transistor superregen
Rx on, say 10 or 6 Meters. I hae a schematic around here somewhere for a
one tranny FM broadcast receiver from Pop Tronics. Have it's demodulated
signal modulate a 2 meter TX. This can be a 9 or 12 Mhz. xtal osc and 2
meter tuned circuit (gets you a few miles on the ground from a tower
receiver).
On the ground you have the "Ground Station" 10 or 6 meter mobile. Since
you can have lots of power, the crummy Rx onboard the rocket is no problem.
From the mobile, transmit a tone. Measure the time delay of a zero crossing
of the transponded tone. A simple freq counter can be made to do this if it
had (I forget the official name--triggered event counter maybe??) the
ability to start counting on the TX tone's edge and stop on the RX tone's
edge. Just a few chips for this.
You pick the frequency that the counter is counting to display feet or
whatever units you like.
What's-it, something like a nano second per foot?
You also need a delay in the proper place to account for the "zero distance"
delay of the receivers & transmitters.
Seems like one other "Ground" receiver is all that's needed to triangulate
the location horizontally...

There was an article in QST within the last year or so where some fellas
measured the delay through a, I think, 2M repeater versus distance...same
concept, 'cept they just used a dual trace scope..

--
Steve N, K,9;d, c. i My email has no u's..

Mike Andrews wrote:

Allan Butler wrote:

[snip preconditions]


The first receiver detects the transmit pulse and it is known that the
transmitter is within 450 feet of that receiver. That starts a clock. The
second receiver detects the transmit pulse and the time since the clock
started is noted. The third receiver detects the transmit pulse and the
time since the first receiver detected the signal is noted. With this much
information the position of the transmitter can be determined on a two
dimensional plot. The fourth receiver could be used for a sanity check to
make certain that the transmitter is in the expected location and it would
allow better coverage for when only three receivers can detect the signal.


The space between clock start and second receive detect is the difference in
distance between these two receivers. The next detect is the difference in
distance between the first receiver and the third. And lastly the fourth
detect sets the distance between the fourth receiver and the first. If the
math is done right there will be four circles drawn each has the center at
the corner of your property. When the drawings are made they will all cross
in only one place. There will be other places where two or three circles
cross.


But for this to work, as I pointed out in another post, you need to
know the true distance from the transmitter to any one or more of the
receivers already, or (equivalently) you need to know the exact time
of transmission relative to the receiver clock. Otherwise all you
have is the delta-Time Of Arrival (TOA) from the receiver that gets
the pulse first to the other receivers, and that's not sufficient to
locate the transmitter.

Even where the maximum distance is known, you still need the true
distance from the transmitter to any one receiver at a minimum. If
you don't have that, you can't draw any circles.

Or I'm missing something obvious. I really don't think I am, but if
someone can point out what I'm missing I _will_ be grateful.


If you know the positions of any two receivers, and the time delta
between the reception of the pulses from the transmitters, the set of
possible positions of the transmitter lies on a hyperbola that
intersects the line between the two receivers.

With two pairs of receivers you get two hyperbolas and two intersections
-- and three receivers gives you three pairings to calculate with.

So you don't need to know the absolute time of the transmitted pulse,
just the difference.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Allan Butler wrote:

[snip preconditions]

The first receiver detects the transmit pulse and it is known that the
transmitter is within 450 feet of that receiver. That starts a clock. The
second receiver detects the transmit pulse and the time since the clock
started is noted. The third receiver detects the transmit pulse and the
time since the first receiver detected the signal is noted. With this much
information the position of the transmitter can be determined on a two
dimensional plot. The fourth receiver could be used for a sanity check to
make certain that the transmitter is in the expected location and it would
allow better coverage for when only three receivers can detect the signal.

The space between clock start and second receive detect is the difference in
distance between these two receivers. The next detect is the difference in
distance between the first receiver and the third. And lastly the fourth
detect sets the distance between the fourth receiver and the first. If the
math is done right there will be four circles drawn each has the center at
the corner of your property. When the drawings are made they will all cross
in only one place. There will be other places where two or three circles
cross.

But for this to work, as I pointed out in another post, you need to
know the true distance from the transmitter to any one or more of the
receivers already, or (equivalently) you need to know the exact time
of transmission relative to the receiver clock. Otherwise all you
have is the delta-Time Of Arrival (TOA) from the receiver that gets
the pulse first to the other receivers, and that's not sufficient to
locate the transmitter.

Even where the maximum distance is known, you still need the true
distance from the transmitter to any one receiver at a minimum. If
you don't have that, you can't draw any circles.

Or I'm missing something obvious. I really don't think I am, but if
someone can point out what I'm missing I _will_ be grateful.

--
Mike Andrews

Tired old sysadmin

(Washed Phenom) wrote in message om...


Any advice, direction, URLs, or discussion is much appreciated.


Thanks for the replies so far. They've been helpful, and have sent me
on some interesting Google searches learning a lot about RDF in
general. Some more specifics:

I have no preferred frequency ranges, but that is more out of
ignorance than flexibility! In terms of transmitter power, in my
first post I mentioned that a continuous signal wasn't necessary if
that would help matters. It seemed plausable to me that a transmitter
which used its power to transmit a short, strong signal could be
detected at a much longer range than a weaker signal.

The reply regarding carrier phase was interesting, and sent me on a
tour of GPS basics. Half the fun of this project is going to be
learning new stuff. As I understand it, the reason you called this a
"reverse GPS" is because the multiple receivers are in known locations
instead of multiple transmitters. It would be feasible to build a
second transmitter (a time reference?) and place it somewhere in the
100 yard square. There is a house roughly in the center of the
property which would be where the "guts" of the processing equipment
would be anyway. How does the carrier phase solution compare to VHF
and shortwave in terms of power requirements, size of transmitter,
etc?

Just to get a better sense of whether the range and resolutions I'm
looking at are feasible, suppose you wanted to locate your local
college radio station's transmitter. I just googled a few college
stations and found two FM stations that transmit at 300 and 400 watts.
The 300 watt station claims to cover 700 square miles. Of course I
realize that: 1) "covering an area" may not be equivalent to "area
within which their location can be pinpointed", and 2) Even if I could
generate such power in a portable transmitter, I would need to choose
bands so as not to run afoul of the FCC. (Just how much transmitting
power can be generated from a garage door sized unit is another issue,
and another reason the brief "burst" signal sounded preferable).
Anyway, I was sidetracked there. To get back to the main question, if
you had antennas at 4 corners of a 300 x 300 ft square lot, are there
any good thumbnail estimates of how accurate you could be at locating
the 300 watt college station, and how this accuracy varied as a
function of the distance to the station?

In addition to being a fun way to learn new things and tinker with
electronics, this project is also motivated by someone I care a great
deal about who often works in isolated outdoor locations and doesn't
own a cell phone or GPS. I know when she is at these sites, but worry
she will be unable to call for help if something happens. Can she be
located by her pushing a button and at what range?

She's already suggested building a Bat-Signal, but that is beyond by
technical expertise. Also, she said that buying her a cell phone and
GPS would be cheaper than building this monstrosity, but I like
intellectual challenges and need to keep my mad scientist reputation
well-exercised.

Thank you again, and please continue the excellent discussion.

-wp

If you want to build something that will locate the roving transmitter on
your plot of land that is 300 foot by 300 foot it might not be too hard to
do if you can get your four receivers to do a little bit of timing
computations for you.

Set the roving unit up to send a pulse on a regular basis. It doesn't have
to carry any data or anything like that and it wouldn't have to be too
powerful either.

There are a few assumptions that can be made that are pretty definate.

1. The distance from two diagonal corners of the square is the maximum
distance the transmitter can be from the receiver and still be in the area
that is designated as home.

2. The transmitter should be home. If the calculations are coming out wrong
then the transmitter has violated the bondary of home.

3. All four of the receivers must be set to a very accurate clock so that
they are all using the same reference.


I don't have a calculator with me that will let me do the calculation to
find the diagonal distance across the square so I will use 450 feet as the
rough number for the maximum distance from any receiver.

The first receiver detects the transmit pulse and it is known that the
transmitter is within 450 feet of that receiver. That starts a clock. The
second receiver detects the transmit pulse and the time since the clock
started is noted. The third receiver detects the transmit pulse and the
time since the first receiver detected the signal is noted. With this much
information the position of the transmitter can be determined on a two
dimensional plot. The fourth receiver could be used for a sanity check to
make certain that the transmitter is in the expected location and it would
allow better coverage for when only three receivers can detect the signal.

The space between clock start and second receive detect is the difference in
distance between these two receivers. The next detect is the difference in
distance between the first receiver and the third. And lastly the fourth
detect sets the distance between the fourth receiver and the first. If the
math is done right there will be four circles drawn each has the center at
the corner of your property. When the drawings are made they will all cross
in only one place. There will be other places where two or three circles
cross.

The nice thing about doing it this way is that there is nothing mechanical
and with todays computing power that is available a solution can be had
within milliseconds of the transmitter putting out a pulse.

An idea for the accurate clock could be to use a receiver at each receiver
in the square to receive a local TV station and use the synch pulses as a
reference. Just don't forget the propagation delay is from one side of the
square to the other and figure that in.

Another option would be to put a GPS receiver at each corner and use the
clock from these as your reference.

Or you could use a common receiver site and one clock feeds all four
detectors. Just remember that there will need to be four receiver
antennas and corresponding feed lines to take care of.

(Washed Phenom) wrote in message om...


Any advice, direction, URLs, or discussion is much appreciated.



I'm still following this thread with interest and looking up things
that I don't understand. The recent exchanges of the past day have
gone quite over my head, but I'm reaching.

In my travels, I found this link. It seems this setup is using two
antennas, but I'm wondering if a similar 4-antenna solution could be
used for what I am trying to do. It would seem that the ability to
use a common PC audio card and readily available software would be a
big bonus. Comments welcome, particularly if this idea is way-off
what I'd need! The URL:

http://www.vlf.it/rdfsoftware/rdf.html

(Washed Phenom) wrote in message om...


Any advice, direction, URLs, or discussion is much appreciated.



I'm still following this thread with interest and looking up things
that I don't understand. The recent exchanges of the past day have
gone quite over my head, but I'm reaching.

In my travels, I found this link. It seems this setup is using two
antennas, but I'm wondering if a similar 4-antenna solution could be
used for what I am trying to do. It would seem that the ability to
use a common PC audio card and readily available software would be a
big bonus. Comments welcome, particularly if this idea is way-off
what I'd need! The URL:

http://www.vlf.it/rdfsoftware/rdf.html