Trying to conger up a means to measure distance from a radio tower. Thought that maybe one could capture the time of reception of the direct wave then from a reflected wave at known distance from the antenna doing the direct capture measure the difference (shift) to know the distance from the tower. Without a unique signature, I am presuming that a computer program could be written to find where the 2 signals should have matched in order to find the difference. Is this a viable idea or have I slipped over the edge or forgotten basic physics? Dale
I know you can measure the distance to the reflector that way, and I supose if you know the distance to the reflector, and the distance from the relector to the source you can then comput the final leg of the triangle...
What exactly are you trying to do? I am assuming you (the device?) is mobile, and you want to know how far you (the device?) is from one or more particular towers. Since "towers" are almost always stationary, if you (the device) are also stationary, then some simple math will take care of the request.
For cell phones, many carriers use a technique called time difference of arrival (TDOA). Do a Google search.
You would have to do this in reverse, and it might not be possible given the information you will likely have at your disposal. You might be able to do something with Loran or GPS, etc...??
In fact, both Frank and Joseph have it wrong. Flight-time differences taken using two receivers and one transmitter (or two transmitters and one receiver) give you a solution that places the unknown unit somewhere on a hyperbolic curve. Both direction and distance are ambiguous until you add a third receiver (or transmitter), which allows you to draw two more curves. The unit you're trying to locate then lies at the intersection of those curves, giving both direction and distance.
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(Interferometry, which is what Joseph is alluding to, is the limiting case of this, where the transmitter is very far from the two receivers. All of the ambiguity is in distance, and the direction is known.)
Attempting to do this with a single tower along with some sort of multipath (passive reflections) would require that you know exactly where the reflector(s) are and that they are in fact the reflections being used for all positions of the unknown unit. In fact, you would need to place directional antennas on the tower that point at the reflectors you want to use, with each one going to its own receiver so that you know which signal is which.
If all you really want is distance, and the mobile unit can participate actively -- i.e., carry a transponder -- you might want to consider the technique used for aircraft "DME" (distance measuring equipment).
Really? How/why does the time TDOA data result in hyperbolic curves? As i said, TDOA does not give bearing, for two data values, but a line/curve of delta distance. You correctly pointed out comparing TDOA from three or more points can produce 2-d (or spherical) location.
For any doublet of two transmitters - one receiver or vice versa, it gives delta distance, not bearing.
I have no clue as to your age, but i am of quite an age. I have been conversant with the theory and practice of Loran, DME, TACAN, VOR VORTAC, IFF, 2-d and 3d radar, and sonar for decades. Please do not be too dismissive of me. I might consider it impolite.
What shape do you think the curve has? If you are indeed "conversant" with these systems, you should know this, especially given the Wikipedia reference I gave to refresh your memory.
Was I being dismissive?
I was just trying to help the OP understand his options, using your message as a springboard for my own additional comments. Age is not the issue here; accurate and useful information is.
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