Passive radar uses microwave signals from distant transmitters and ambient radio to track and detect targets in the air or sea. This technology cannot replace stealth. However, it can be used to complement higher-fidelity active sensor cues.
Two major components of a passive radar system are an observation channel as well as a reference channel. The observation channel detects echoes from a target and receives an illuminating message from the transmitter. Using the returns of these signals, the algorithm can determine the direction of arrival of the echoes.
A passive radar signal is not as precise as an RF signal. A radio, television or FM radio broadcast can serve as the illuminating signals. Passive radar can't therefore be used in remote areas or at high density. It should be installed on ships and stationary ground vehicles.
The signal is also subject to ambient radiofrequency radiation, which limits passive radar's range. As such, passive radar's detection range is limited by the amount of reflected and radiated energy received from the receiver, deployment geometry and noise.
Passive radar is not perfect, but it has its benefits. It was limited to a narrow range of frequencies in the past and required more data processing than other systems. However, with the advent of digital receivers, the computing power has skyrocketed. The passive's effectiveness can also be greatly improved by the use of advanced radar absorbing materials. However, passive radar technology still has a lot of potential.
One of the most interesting developments in passive radar is the application of signal processing to the detection and tracking of moving objects. Several research systems have been developed that incorporate phase-difference of arrival to determine the direction of arrival. Likewise, the use of cross-correlation can provide estimates of bistatic Doppler shift.
Passive radar has also seen a lot of exciting developments, including radar-absorbing coatings that reduce the effects of reflected or radiated energy. While these systems are currently in prototype stages, they are expected to become available within the next several years.
A combination of a passive and an advanced infrared tracking system (IRST), is the best type of passive radar system. These systems are capable of tracking aircraft and locking onto them. They could eventually lock onto stealthy planes.
Passive radar can be used in many ways including surveillance, reconnaissance and cueing others sensors. There are many applications for cell phone base station. Passive radar is an untested and complex technology that is not appropriate for all situations.
The most advanced passive radars currently available are ground-based stationary systems. These systems include the VERA and Czech TAMARA systems and the Ukrainian Kolchuga system. Software Defined Radio technology (SDR), which allows the processing of radio waves within the microwave spectrum, is used in some systems. However, these systems are limited in power and range making them incompatible to the wider spectrum of passive airborne radio applications.