K9YC wrote
"Now, reading that Brian had started it all, perhaps he might take it on."
TA, the terrain analysis program I wrote in the 1990s, handled vertical
as well as horizontal polarization.
I went to some effort to ensure that TA was accurate, including
comparing results against helicopter-borne radiation pattern
measurements. However, in the years since, I've become convinced that
ray tracing that considers only a single azimuth angle has serious
accuracy limitations that preclude its use in all but the simplest terrain.
Ground reflection and diffraction at any azimuth angle can wind up at
the angle of interest. Imagine what happens when radiation intersects
the slope of a hill off your target angle. Since even a directional HF
antenna has a broad forward lobe, it illuminates lots of ground away
from where it's aimed. Some of this power can come back to haunt you.
I've cautioned HFTA users with complex terrain about the limitations of
single-azimuth ray tracing. The response is invariably, "I know it's
accurate." When asked how they know, the answer is never satisfactory. I
think HFTA and TA blind users to their shortcomings by offering
fascinating and easily digestible results.
Ray tracing involves calculating power not only for direct reflection
and diffraction, but for reflection from reflection, diffraction from
reflection, reflection from diffraction, and diffraction from
diffraction. Then do it again for higher-order cascades. This must be
repeated over a dense elevation angle set to capture everything
relevant. The power of 1990s computers limited the speed of TA. I wrote
the time-consuming code in assembler to provide results in a reasonable
amount of time. Today's computers are much faster, have multiple CPUs,
and come with powerful vector instructions that can do eight
floating-point calculations simultaneously. Ray tracing over all
azimuths should be feasible today in a reasonable amount of time.
I've thought of writing a 2D (or is it 3D?) terrain analysis program.
But there's a showstopper: there's no empirical data to test it against.
Because the calculations are so complex, there's no way to ensure they
are correct without checking results against measured data for complex
terrain. As far as I know, none exists. I've searched for it and come up
empty.
I've thought about what it might take to make radiation pattern
measurements over complex terrain with a drone. But it's a complicated
problem with many hidden sources of error. When I was considering this,
each day I'd wake up with a new source of error that hadn't occurred to
me the day before. I think it would be easy to get in over your head
without ever knowing it. A computer program validated with fishy data is
not worth anyone's attention.
Brian
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