Passive radar uses microwave and ambient radio signals from distant transmitters in order to track targets in both the air, and sea. Although passive radar is not intended to replace stealth, it can be used as a complement tool to activate higher-fidelity active sensors.
A passive radar system is composed of two components. The reference channel and the observation channel. The observation channel receives an illuminating signal from the transmitter and detects echoes of the reflected signal bounced off a target. The algorithm can calculate the direction of arrival using the signals' returns.
The accuracy of a passive radar signal isn't the same as that of a direct radio signal. The illuminating source can be either radio, television, radio broadcast, or FM radio. Passive radar is not suitable for remote or high-density locations. It must only be used on ships or stationary ground vehicles.
The signal is also subject to ambient radiofrequency radiation, which limits passive radar's range. The range of detection depends on the radiation and reflection energy of the receiver, the deployment geometry, and other noise.
Although passive radar has 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. Digital receivers have allowed for an increase in computing power. Similarly, the use of more advanced radar absorbing coatings could significantly improve the passive's effectiveness. However, passive radar technology still has a lot of potential.
Passive radar has seen some interesting developments, including the use of signal processing for the detection and tracking moving objects. To determine the direction of arrival, several research systems have been developed which incorporate phase-differences of arrival. Cross-correlation, on the other hand, can be used to calculate bistatic Doppler Shift.
Radar-absorbing coatings are another exciting innovation in passive radar. They can be used to reduce the effects radiated and reflected energy. These systems are still in the prototype stage, but they will be available within the next few years.
Advanced infrared search-and-track (IRST), combined with passive radar, are the most practical passive radar systems. These systems can track and lock onto aircraft. They could eventually even lock onto stealthy airliners.
Passive radar has been utilized in a variety of ways, including surveillance, reconnaissance, and cueing 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.
At the moment, passive radars that are most advanced are still ground-based systems. These include the Czech TAMARA and VERA 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. These systems do not have the same power or range as passive airborne radar applications.