Principles of Modern Radar - Volume 2 1891121537

14.6 Passive Radar ATR 657radar systems. Furthermore, since the majority of the cost associated with traditional radarsystems is associated with the transmitter, passive radar systems are comparatively inexpensiveto build and maintain. Although passive radar is not inherently restricted to aparticular frequency band, the allocation of bandwidth for common illuminators of opportunity(e.g., commercial TV and FM radio) serendipitously relegates most passiveradar systems to low-frequency regimes. Consequentially, their performance is not proneto substantial degradation in inclement weather, providing a tactical advantage. Anotherfortunate consequence of this association with low-frequency sources is that the longerwavelengths are well-suited for many ATR applications, such as aircraft classification[115–116]. Recall that at X-band (e.g., 10 GHz), a typical fighter plane is roughly 500wavelengths long, while at FM-band frequencies (e.g., 100 MHz), the same fighter is onlyfive wavelengths long. Where the X-band radar cross section of the aircraft is extremelysensitive to small perturbations in aircraft orientation, the FM-band radar cross sectionof the aircraft varies slowly and smoothly with changes in aspect, providing a featurethat varies enough to allow classification but not so much as to require high precisionprediction of aircraft orientation, which can be difficult to achieve.Even so, a number of unique challenges are associated with passive radar. The followingsub-sections describe these challenges in the context of the unified framework for ATR.14.6.1 Step 1: Identify the Target SetFirst and foremost, the target sets that are candidates for passive radar ATR are largelydictated by the illuminators of opportunity in the deployment area. As a result, these systemsare most commonly associated with urban areas in developed countries that havestringent communications regulations. Deployment in under-developed countries is difficultdue to the high noise figures that result from lax emissions regulations. Deploymentin rural (but regulated) areas is challenging due to the lack of available illuminators ofopportunity. Hence, when identifying the target set, the passive radar ATR designer mustbegin by considering the available sources in the deployment area. These, in turn, dictatethe operational region (i.e., the region in which the signals from multiple transmitters canbe reliably received). The target sets are thus restricted to targets that will pass throughthis region and generate sufficient signal-to-noise ratio to be detected. The physical size ofthe targets (relative to the size of the illuminators’ wavelengths) must also be considered.14.6.2 Step 2: Select the Feature SetThe feature sets available to passive radar ATR systems are also somewhat limited. Radarcross section and target kinematics can be estimated over time by the passive radar ATRsystem, but additional features are difficult to collect since the system is unable to controltransmitted waveforms.14.6.3 Step 3: Observe the Feature SetSince direct path signals (i.e., those moving directly from the transmitters to the receiverwithout hitting the target) are orders of magnitude stronger than the desired target signals,deployment of passive radar receivers to adequately observe the feature set requires carefulplanning. The transmitters’ power and locations are typically known and the associatedspikes can be removed from the ambiguity function via signal processing, but the direct pathsignals raise the noise floor, potentially swamping the indirect target signals. Hence, passive