IEEE - Aerospace and Electronic Systems - August 2022 - 46

Integrating Multiband Active and Passive Radar for Enhanced Situational Awareness
quantifying the actual vertical accuracy of the active data for
the given target, two different levels of that accuracy have
been assumed to compute the fused altitude estimates. When
putting less trust in that accuracy, the estimated altitude shows
a visible fluctuation, and the quality degrades in the two gap
phases where no active data are available. With an assumed
higher accuracy of the altitude data, the fused estimates show
much more stable behavior.
Finding an optimized fusion strategy with respect to the
Figure 19.
Improved track continuity and accuracy due to contributions from
multiple sensors and sensor types. Fused tracks (green) versus
tracks based on active data only (magenta). Zoomed region additionally
shows active PSR (blue) and SSR (purple) plots, including
their uncertainty ellipses. The weaving target is an EADS
CASA C-295M flying at approximately 2000-m altitude with an
average speed ofabout 430 km/h.
passive radar data. A couple of tracks are continued or
maintained in regions where there are no active data anymore.
It should be noted that passive radar tracks may be
maintained even in cases of bistatic plot data that is too
sparse to produce Cartesian plots (which is done for output
purposes only, as mentioned previously).
Figure 19 not only confirms the aforementioned gapfilling
capability of passive radar but also demonstrates
further enhancements in the target situation picture gained
by the use of multiple sensors and sensor types. Due to its
inherently much higher update rate and multiple target-tosensor-and-transmitter
geometries, passive radar provides
a better accuracy during the maneuver phases where the
target performs coordinated turn movements. Note the
slower reaction to those maneuvers exhibited by the active
radar tracks at various points in the figure. Of course, the
overall horizontal position and velocity accuracy could
also be improved by using a larger number of active
radars, but that would cause additional electromagnetic
emission and come with much higher costs.
On the other hand, it is known that the vertical accuracy of
passive radar tracks strongly depends on the available sensor/
transmitter setup plus the target location relative to that [37].
In particular, for flat terrain in combination with a low target
altitude, the altitude estimation quality achieved with passive
radar alone may be degraded. With a lower WGS84 altitude
ofaround 3 kilometers, the considered weaving target poses a
challenge for altitude estimation with passive radar only.
Here, the fusion with active 3D data can significantly improve
and stabilize the altitude estimates. The achievable accuracy
in such situations is mostly driven by the accuracy ofthe corresponding
data provided by the active system. Figure 20
clearly shows this. Without having an independent source for
46
altitude remains a still somewhat open topic and would
require a more detailed analysis of the active radar data. A
simple linear regression has been used to perform an altitude
compensation of the barometric altitude provided by
SSR versus the geometric altitude reported by the 3D PSR.
More sophisticated approaches to determine systematic
effects depending on region, altitude, or target were considered
out of scope for the APART-GAS, although they
might have helped to further improve altitude estimates.
Figure 20.
Comparison of active track altitudes (top) and fused track altitudes
(bottom) for the weaving target in Figure 19 using tracker
parameters corresponding to higher (green) versus lower (orange)
assumed altitude accuracy of active radar.
Figure 21.
Comparison of estimated altitudes versus ADS-B for the two airliners
in Figure 16 using active data (top) and passive data (bottom)
only. Measured PSR (blue) and SSR (purple) plots, radar/
IFF tracks (magenta), passive radar tracks (green), created Cartesian
plots (orange and cyan), and ADS-B (yellow).
IEEE A&E SYSTEMS MAGAZINE
AUGUST 2022
IEEE - Aerospace and Electronic Systems - August 2022

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