On 27/09/2018 16:46, brian wrote:
I use aircraft scatter at VHF/UHF and microwaves to make long-distance
non LOS contacts. It is a bit of hit and miss and I'm trying to make a
more scientific approach to the problem.

Programs like Airscout tell when an aircraft lies in a zone on the
direct path between myself and the distant station in real time. So
that removes some of the guesswork.

The system is a form of bistatic radar .Equations exist to estimate the
received signal level and hence it is possible to calculate the SNR from
knowledge of both stations equipment e.g. antenna gain, power and
effective noise figure. I'm including ground gain in antenna gain.

Now the unknown parameter is the reflection characteristics of the
aircraft. Some generic info exists on the monostatic cross-section e.g.
A 747 is about 100m^2

The bistatic equivalent does not, because it is highly dependent on
relative angles.

The case I'm interested in is however a small subset i.e the forward
scattering x-section over relatively well defined angles.The unknown is
the orientation of the aircraft. However this can be derived from
Airscout ,since the direction of travel is known.

To try to get the range of figures, I've been looking at 4NEC2, which
for a starting point has 747 wire model. To simulate the bistatic
radar case, it is possible to replace the voltage excitation , by a
plane wave at a specific angle.

When the model is run, you get a set of complex polar plots with
strongly peaked lobes in the forward direction. 4NEC2 gives the relative
gain dBi as a function of angle.

What I have done is to turn the gain figures into an equivalent bistatic
radar cross-section by multiplying them by lamda^2/4*pi , since this is
the RCS of an isotropic radiator.

What I want to know is if this approach is valid ? Also has anyone else
tried something similar ?

I'm not looking for precision, but only an indication that a particular
QSO is possible or a no-hoper.

A couple of points...

The azimuthal RCS of an aircraft results in what's sometimes known as a
'fuzzball' which can have high peaks and deep nulls. I suspect the
stated RCS is some average of this.

If you're targeting large transport aircraft having two or four large
turbofans, then perhaps your best best is to choose those aircraft that
are approaching along a vector that results in roughly equal angles
between your station, the aircraft itself, and the distant station. and
bounce your signal from the fans, which in themselves will have high
RCS. Masking the fans from radar is a critical requirement for stealth
or low RCS aircraft.You'll probably be able to recover the Doppler from
the fan rotation, if you wanted to.

It is said that during WWII Wurzburg radar operators could hear the
Doppler caused by the rotation of the target aircraft's propellers.


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Spike

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