KB7WVJ wrote:
> Thanks for the reply. Which signal of the same strength gets through mountains better - AM or FM radio?
But here you have to differentiate between the modulation type and the
transmitter frequencies. A 1 MHz AM broadcast signal will probably do
better than a 100 MHz FM signal more due to the difference between
MF and VHF propagation rather than because of the modulation type.
Coverage at 1 MHz is primarily ground wave, where the attenuation per
km varies somewhat with the soil type, but which travels over hills
relatively unimpeded. At 100 MHz the propagation is more commonly
space (direct) wave aided by reflections and diffraction, with severe
blocking by solid objects such as hills.
> The signal is more intelligible to the receiver if the phase and frequency aren't shifted all around. Generally, any signal that experiences multiple reflections will have different amplitudes as signals each reflect off of a different object, but both will also be phase and somewhat frequency shifted, causing major havoc with ToA or doppler implementations. As such, multipath reflections are a problem in any triangulation method, but reflection coefficents off of rocks or trees are usually nowhere near unity, whereas phase changes off of these same objects can be much larger in a reflected signal because of the wide variety of materials that reflect the signal.
Except that the TDoA implementations in use for cell phone tracking
are based on the modulation waveform (perhaps resolved to 10’s of
microseconds) rather than the phase of the RF wave (1 nanosecond or
less), so phase shifts due to reflections are of little importance. On the
other hand, amplitude differences of +/- 20dB between sites due to
shielding or antenna orientation don’t affect the result, either.
> With strong transmitter power the errors from multipath reflection can be somewhat overcome...
Actually, no. The limitation on night-time range of an AM broadcast
station is the vertical radiation pattern of the antenna and the
ionosphere height, as these determine the minimum range at which
the sky wave signal can have the same amplitude as the ground wave
signal. At that point they will either reinforce or cancel each other out,
depending on the phase difference - a classic multipath problem. This
distance is independent of transmitter power.
> If one is lucky enough to have just one dominant reflection source, the reduced amplitude from reflection effects the signals equally for each antenna in the array and the ratio of the signals is all that is required in amplitude triangulation ... A broad-band amplitude method can be used as a sanity check on a doppler or TOA to make sure it's not getting some false interference.
Yes, in some cases all antennas might pick up the signal via the same
reflection source, and if all the receiving sites were in a straight line
with it you might triangulate the location of the reflector rather than
the initial source. But in a typical case (especially to get good
accuracy) the receiving sites would be more dispersed, and the
amplitude of the reflection in each direction would be different. More
common in my experience is the case where the path to each receiver
has a different combination of terrain and building shielding and/or
reflections, re-radiation from power lines, etc., which can cause far
more difference in amplitude than the simple difference in distance to
each receiver. (You can test this at optical frequencies with some sort
of reflector surface illuminated by signal source that can't be seen
directly by the receivers.)
Take, for example, a VHF omnidirectional antenna mounted on a mast
at one end of a building: for simplicity we will assume one receiver in
line with the building and one at 90 degrees (which should be the
geometry for maximum accuracy.) When the antenna is high enough
above the building both sites should see the same signal strength.
As the antenna is lowered, at some point the metal roof of the building
acts like a good ground and we may see 6dB gain in that direction. As
the antenna is lowered more it drops behind the building and we might
see 20dB or more of loss due to shielding at one site, with relatively no
change at the other (or perhaps up to 6dB increase if the walls of the
metal building are acting as a reflector.) These are the sorts of real-
world effects that make it very difficult to get good results with
amplitude comparison systems in a practical installation, especially at
VHF/UHF.
But I don’t want to be a naysayer and say it cannot work, and there is
no reason that it wouldn’t in a uniform medium. You could try it on
local AM broadcast stations and it should work fairly well (though
probably not on more distant stations at night.) If you were closer to
Portland I'd set out some of my 80m or 2m hidden transmitters and we
could actually try it out - we could calibrate it using a single
transmitter at a known location then try to determine the locations of
several other transmitters and see how close we get. Sounds like a
fun and informative way to spend an afternoon! You can do the same
thing using any local ham with a mobile rig. I'd be very interested to
hear about the results!
Good luck,
- Dale WB6BYU