Aerospace and Electronic Systems Magazine December 2017 - 12

OTH Radar Phenomenology
The resulting radar echoes, presented in beam-range-Doppler
format, are highly informative, as can be seen in Figure 9. Not
only can we instantly obtain the full velocity vector of the target,
without having to rely on a tracker to establish the cross-range velocity component by following the target for many dwells, we can
also estimate target altitude and even provide target classification
information, as reported in [9].
Data from an early experiment conducted to test this theory
confirmed the feasibility of the concept. Radar operators measured
the azimuthal spread of one-way transmissions from two emitters
at the same geographical location, one at sea level, the other airborne at 7000 m altitude. The widths were recorded at two power
levels below the peak. Later, a very detailed model of propagation
and sea scatter was used to predict the width of the emitter signal
as a function of emitter altitude, from sea level to 10,000 m. The
measured widths were then overlaid on the model predictions. As
evident from Figure 10, the agreement is excellent.

Figure 9.

Azimuth-range-Doppler display showing the diffuse scatter contributions as well as the single peak (yellow spot, circled, in Beam 5) that is
the only echo predicted by the 4-ray model in this scenario.

improve the phenomenology we need to add terms whose propagators take the general form
 
r − rtarg 
 
  
 

M diff (rx , rtarg ) = ∬M sky ( rx , r )σ  rˆ sky ,    M los r , rtarg dr


r
r
|
−
|
targ 


(

)

(2)

where σ ( rˆscat , rˆinc ) denotes the bistatic scattering coefficient, X
stands for either T (transmitter) or R (receiver), and the superscripts signify diffuse, skywave, and line-of-sight.

SCATTERING
The other principal class of models that are candidates for refinement are those concerned with scattering. It transpires that the phenomenology of scattering is just as rich in prospects for enhancement as propagation.
As a first example, we know that the assumption of perfect
electrical conductivity works exceedingly well for many radar targets, but if we look beyond that, not just to imperfect conductors
and dielectrics, but to materials whose constitutive relations are
functionally dependent on the incident field, some very interesting
possibilities emerge. The first step in exploring these is to general-

Figure 10.

Measured and predicted azimuth widths of signals received from ground-based and elevated emitters. The match correctly registers the altitude of the
elevated emitter as 7000 m.

10

IEEE A&E SYSTEMS MAGAZINE

DECEMBER 2017



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