Aerospace and Electronic Systems Magazine October 2017 - 25

airborne radar system relying solely on LOS coverage [2]. This
conclusion is suggested in Figure 1a, in which an airborne platform is progressively stepped by distance D from a point directly
over a roadway located between two buildings. As the platform
is moved, it is evident that coverage of the roadway provided by
the LOS path (blue line) decreases from 100% to 0%. The doublebounce multipath mode is defined as the path from the radar, to
the building, to the vehicle, back to the building, and finally back
to the radar, as indicated by the green line in this image. (The convention established during this program is that when defining the
number of bounces, or reflections, the interaction with the vehicle
is not counted.) It is seen that the portion of the roadway illuminated by the double-bounce multipath mode starts at 0%, steadily
increases, and then drops back to 0% at a distance farther than
that associated with LOS. (The quadruple bounce indicated by the
red line in Figure 1a would further increase the range coverage.)
These results motivated a statistical analysis over more realistic,
three-dimensional geometries with varying, representative building sizes and road widths [2]. The results from this study suggested
that a grazing angle of approximately 12° to 18° was appropriate.
As shown in Figure 1b, the results also indicated that exploitation
of the double-bounce multipath return extends range coverage by
a factor of about 2.5 or area coverage by a factor of 6. The impact
of the significantly increased area coverage is that the number of
airborne platforms required to provide persistent coverage of a
1,000-km2 region is reduced from 17 to just 3.
In addition to these geometric arguments, system analysis supported the feasibility of MER. For example, although the geometric analysis is based on a simple specular model for the multipath,
measurements at radio frequencies as low as 2 GHz suggested that
the amplitude of the specular component would dominate that of
the diffuse component in urban areas [3]. Furthermore, the 5 to 10
dB loss in signal power that was expected for each reflection was
deemed acceptable.
To predict and exploit the multipath, a KA approach employing three-dimensional city models was suggested. The city models
could be constructed from LIDAR data sets [4]-[6], SAR imagery,
or optical imagery. If, in a future operational deployment of a MER
system, suitable data were not already available for an urban area
of interest, it could be collected within a relatively short period
OCTOBER 2017

(several hours to a day) from a single platform. This period would
be acceptable compared to the extended surveillance (weeks to
months) associated with the MER concept. Additional analytical
and experimental work is required to examine the expected lifetime of the urban city model, i.e., to determine whether and when
it becomes stale. As a single example, the LIDAR imagery that
was used in the MER test discussed in this article was approxi-

Figure 1.

Extended range coverage provided by MER [2]. (a) Two-dimensional
view. (b) Results from three-dimensional analysis.

IEEE A&E SYSTEMS MAGAZINE

25



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