Aerospace and Electronic Systems Magazine September 2017 - 41

The goal of this measurements-driven and phenomena-oriented research was clear and simple-to explore the impact of very
low threshold detection processing on track formation in a fully
coherent TWS radar with an embedded four-channel sidelobe canceller. The test targets of greatest interest were small, slow moving
aircraft flying 100 m above ground level. All experiments were
conducted in close proximity (40 km) to the radar test bed. Research focused on exploiting coherent clutter maps, autoregressive
superresolution spectral estimators, and Kalman filter-based noisy
area TWS processing. Postmission analysis of data demonstrated
that slow moving targets often "broke track" due to residual interference in one or more Doppler filters. This was caused by strong
backscatter signals from natural terrain and close-in manmade
structures in addition to electromagnetic interference. The early
conjecture was that internal clutter motion and scanning modulation would also contribute to degraded track performance, but it
was later determined that these effects were negligibly small by
comparison.
The weak returns from low, slow test aircraft most interested
the analysis team. We initially implemented an adaptive Doppler
processor followed by a three-dimensional cell-averaging constant
false alarm rate (CA-CFAR) detector, with a crude form of extreme
value excision. This "modified" multidimensional CA-CFAR, a
detector that spanned range, angle, and Doppler, was permanently incorporated into the L-Band radar signal and data processing
chain (in software on the AP-120B array processor). The team decided that a symmetric training data window was adequate only
for initial analysis, and that detector performance would indicate
the direction for future CFAR research. Analysis employing multiple detectors immediately followed. Both the greatest-of (GO)
and trimmed mean (TM) CFAR were investigated. Topographical
maps and a coherent clutter map (using only several Doppler filters) were used as information sources (an early, albeit self-generated knowledge source) for suppressing outliers and to compute
clutter-plus-noise power estimates in regions of heterogeneous terrain. It was soon verified that the effects of outliers were greatly
reduced using multiscan training data in range, angle, and Doppler.
The track processor was also utilized to excise persistent returns
from "stationary movers" spatially localized in the training data.
SEPTEMBER 2017

Furthermore, asymmetric training data windows were employed
due to the close proximity (of the slow movers) to the zero Doppler
filter and the effects of numerous dominant structures surrounding
the radar test bed.
In late 1981, the LAD experiments proved successful and Mr.
Silfer transitioned results to the sponsor. Gains in wide area surveillance radar performance would be enabled by improvements
in track processing, with appropriate changes to Doppler filtering,
sidelobe cancellation, and especially false alarm control. This research concluded with little fanfare, as the topic of ground-based
wide area surveillance radar was no longer of great interest to the
basic research community. However, this experience influenced
the careers of many radar engineers at RADC, as weak target detection became the focus of signal processing experiments in the
Surveillance Laboratory for the next three decades.

EXPERT SYSTEM CFAR CONCEPT DEVELOPMENT
In 1984, as technical leader of the seedling in-house Air Defense
Initiative (ADI) at RADC, Dr. Richard Schneible requested support from the Surveillance Laboratory staff to develop advanced
radar technology in order to improve weak signal detection in
wide area surveillance radars via advanced airborne moving target indication (AMTI), Doppler processing, and track formation.
This futuristic ADI radar was to be a "concept car" 1 designed to
foster discussion and provide a baseline for developing a postAWACS (airborne warning and control system) radar capability.
Similar to the EDEW project, the mission was to develop a wide
area surveillance capability for detecting and tracking the modern
air-breathing threat under all weather conditions, albeit now from
an airborne sensor platform.
At RADC, we had examined the link budget for the airborne
radar in operation onboard AWACS. Significant performance
gains were theoretically possible through signal and data processing. However, large gains were not practically achievable through
1

Concept cars were popular in the automotive industry during the
mid-20th century as a means of illustrating planned or potential
futuristic body styles.

IEEE A&E SYSTEMS MAGAZINE

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