Passive radar, a technology that uses both ambient radio and microwave signals to track targets in the air and at sea, is called passive radar. This technology is not a replacement for stealth, but it is a useful complementary tool that could be used to cue higher-fidelity active sensors.
A passive radar system is composed of two components. The reference channel and the observation 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.
The precision of the passive radar signal can be less than that of an RF signal. A radio, television or FM radio broadcast can serve as the illuminating signals. Passive radar cannot therefore be used at remote and high-density places. It must be installed both on ships and ground vehicles.
Passive radar is also limited by the fact that the signal is subject to ambient RF radiation. As such, the range of detection is a function of the radiated and reflected energy from the receiver, the geometry of the deployment, and external noise.
Although passive radar has many benefits, it isn't perfect. In the past, the technology was limited to a small range of frequencies, and it required more data and processing than other systems. With the advent of digital receivers the computing power has increased dramatically. The passive's effectiveness may be enhanced by the use radar absorbing coateds with higher performance. The long-term evolution in passive radar technology is still very promising.
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. Cross-correlation can also be used to estimate bistatic Doppler shifting.
Another exciting development in passive radar is the use of radar-absorbing coatings to mitigate the effects of reflected and radiated energy. Although these systems are currently in prototyping, they are expected be commercially available within the next few decades.
The most practical passive radar systems are those that utilize an advanced infrared search and track (IRST) system, in combination with a passive radar. These systems have the potential of tracking and possibly locking onto aircraft. They could eventually even lock onto stealthy airliners.
Passive radar can be used in many ways including surveillance, reconnaissance and cueing others sensors. There are many options for mobile phone base stations. However, passive radar is a complex and untested technology, and is not suitable for all situations.
The most advanced passive radars currently available are ground-based stationary systems. These include VERA, VERA and TAMARA systems from Czech Republic and the Ukrainian Kolchuga. Some systems are based on Software Defined Radio (SDR) technology, which enables the processing of radio waves in the microwave spectrum. These systems are not compatible with passive airborne radar applications because they have limited power and range.