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 consists of two main components: an observation channel, and a refer channel. The observation channel detects echoes from a target and receives an illuminating message from the transmitter. The algorithm uses the returns to determine the direction in which the echoes are arriving.
Passive radar signals aren't always as accurate as direct RF signals. A radio, television or FM radio broadcast can serve as the illuminating signals. Passive radar is not suitable for remote or high-density locations. It must be mounted on both ships and stationary ground vehicles.
Passive radar is also limited by the fact that the signal is subject to ambient RF radiation. The receiver's radiated and refracted energy, the deployment geometry, as well as external noise, all affect the detection range.
Although passive radar can have many benefits it isn’t perfect. In the past, passive radar had a limited frequency range and required more data and processing to be effective. With the advent of digital receivers the computing power has increased dramatically. The passive's effectiveness can also be greatly improved by the use of advanced radar absorbing materials. But passive radar technology continues to evolve and has great potential.
One of the most fascinating developments in passive radio is the application signal processing to the detection, tracking and identification of moving objects. Numerous systems have been designed that include phase-differences in arrival to determine arrival direction. Cross-correlation, on the other hand, can be used to calculate bistatic Doppler Shift.
Another exciting development in passive radar is the use of radar-absorbing coatings to mitigate the effects of reflected and radiated energy. Although they are still in prototype, these systems will soon be available.
These systems combine a passive radar with an advanced infrared scan and track (IRST), to make them the most practical. These systems have the potential of tracking and possibly locking onto aircraft. They could eventually even lock onto stealthy airliners.
Passive radar is used in surveillance, reconnaissance, cueing and cueing of other sensors. Many applications exist for base stations on cell phones. Passive radar is complex and not yet tested so it is not suitable in all cases.
For now, the best passive radars are those that are fixed to the ground. These systems include VERA and TAMARA in Czech and VERA in Ukraine. Software Defined Radio is (SDR) technology enables processing of radiowaves in the microwave spectrum. Nevertheless, these systems have only minimal power and range, making them incompatible with the broader spectrum of passive airborne radar applications.