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Author Topic: New Method of Direction Finding  (Read 14649 times)

KB7WVJ

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New Method of Direction Finding
« on: October 18, 2006, 10:42:15 PM »

Here is a project I have been working on professionally for about 5 years - a method of tracking electromagnetic sources that finds the coordinates (x,y,z) of the source and the power level if a calibration is performed beforehand. I have tested with low frequency (200 KHz), visible and IR optical, and with gamma rays from a 1 microcurie source that is fairly harmless but a good test of the theory at all frequency ranges.

A detailed description is at http://www.signaldisplay.com/omni.html and this is fairly mathematical (had to be to get passed peer-reviewed publishers), but there is a software program on this link that implements the algorithm for radioactive materials. I need to rewrite this for radio, but in the meantime if you want to test you can just plug in values from field strength meters at each antenna and substitute this in where it says "curies" in the program. The calibration constants of the antenna have to be determined - just input the field strength values into the program (again in place of curies) and the known distance from the source to the antenna. The antenna is hopefully non-directional - omni is preferred but the equations work over any angle where the directionality is valid, it just limits the range of tracking to have directional gains.

Everything is relative here so the units are irrelevant as long as the calibration constants are found with true measured values. There is also some C source code for those who want to plug into their own program. The greater the antenna offset distance (xoff or e in the formulas), the greater the accuracy.

Have fun with this and let me know what results you get. I have not tested with high-power (just fox hunting inside of buildings) so I am interested in how long ranges work out. Happy hunting!

Mike
KB7WVJ

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WB6BYU

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New Method of Direction Finding
« Reply #1 on: December 20, 2006, 06:24:27 PM »

I would be very interested how well this works on radio signals in the
real world, especially on VHF or UHF where shadows and reflections
are a major problem.  When hunting on 80m the signal strengths
are relatively constant even in hilly terrain, but the actual signal
strength will depend on the ground conductivity where you are
standing and the height above ground.  For long distance calculations
the power difference between two receiving sites will be very small and
these differences could be significant.  (Perhaps this approach is more
appropriate to fixed receiving sites than to mobile or on-foot hunting?)
On VHF the real-world differences are much more significant - in fact,
when mobile hunting the signal strength differences due to terrain
shielding become a significant source of information in addition to the
antenna direction.  Often drivers will swear that a stretch of road is
flat, but driving it with a receiver will show differences of 10 to 20dB in
signal strength between the crest of a rise and the dip between them.
We often estimate the location of a transmitter based on what hill is
now between it and us when the signal strength drops (and I've seen
aircraft doing the same thing by flying around a mountian.)

Several cell phone subscribers have tried comparative amplitude to find
the location of subscribers, but with little success.  Most have now
changed to the time difference of arrival (TDOA) method, where each
receiving site measures the exact time of some characteristic edge of
the transmitted waveform.  This is less susceptable to amplitude errors
due to shielding and reflections, though major reflections will still
throw the calculations off.

So I can see that it may work at the very low frequencies, or with
sources such as gamma rays that aren't easily blocked or reflected, but
I wouldn't invest heavily in it for terrestial VHF/UHF DF work.  Even for
an unobstructed path such as an aircraft, the variation in radiation
pattern due to aircraft attitude and bearing may be considerably more
than the expected signal strength difference at the receiving sites due
to the distance to the source.

KB7WVJ

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New Method of Direction Finding
« Reply #2 on: December 20, 2006, 07:47:27 PM »

Thanks for the reply. Which signal of the same strength gets through mountains better - AM or FM radio? 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.

With strong transmitter power the errors from multipath reflection can be somewhat overcome, whereas time and phase distortions from a reflection are pretty hard to control.

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. Also, frequency-dependent algorithms suffer more from interfering sources at the same or near frequency. 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.

KB7WVJ
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WB6BYU

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New Method of Direction Finding
« Reply #3 on: December 21, 2006, 12:57:38 PM »

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

KB7WVJ

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New Method of Direction Finding
« Reply #4 on: December 22, 2006, 12:33:32 PM »

Thanks for the info. I beg to differ that phase shifts due to reflections are so small. My experience with an unmodulated carrier at 2.4 GHz has shown that multipath delays can be a full phase shift or more, easily cancelling out back at the receiver. Also, ferrite materials like ferrite-titanate have a relative permittivity of 60*(2-j), a complex value that when converted to a reflection coefficent produces a phase angle as part of the coefficient that can actually be used as a quarter wave plate (when the right thickness is used it can cancel incoming waves). A smaller thickness will just reflect at a large phase shift. Depending upon the material, phase shifts of all sort are possible.

My example of the AM vs. FM signal was not to compare propagation, although this is interesting, but more about the problems of reflections and the effect on demodulation. It's easy to see that two signals reaching a receiver at quarter phase shift will cause some problems for the phase locking of the demodulator.

This being said, I agree to that there are problems with a straight amplitude approach. I like the idea of using both in conjunction to compare results and get a better result.

Thanks for your thoughts!
KB7WVJ
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K8MHZ

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New Method of Direction Finding
« Reply #5 on: March 12, 2007, 02:46:42 PM »

Have you tried your methods out at a fox hunt?

I have found that the more complex systems usually finish last.

Just curious to see how all this works in the real world.

Fox hunting is a blast!
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