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 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. The algorithm can determine which direction the echoes are coming from by analysing the returns.
A passive radar signal is not as precise as an RF signal. The illuminating signal can be a radio, television, or FM radio broadcast. Passive radar is not suitable for remote or high-density locations. It should be installed on ships and stationary ground vehicles.
Passive radar suffers from the effects of 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 can have 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. Digital receivers have allowed for an increase in computing power. The passive's effectiveness can also be greatly improved by the use of advanced radar absorbing materials. The long-term evolution in passive radar technology is still very promising.
The application of signal processing to detect and track moving objects is one of the most exciting developments in passive radar. To determine the direction of arrival, several research systems have been developed which incorporate phase-differences of arrival. Cross-correlation also provides estimates of bistatic Doppler change.
The use of radar-absorbing coated to minimize the effects of radiation and reflected energy is another exciting development in passive radar. Although they are still in prototype, these systems will soon be available.
Passive radar systems that combine an advanced infrared track and search (IRST) system with a passive radar are the most useful. These systems could track and possibly lock onto aircraft. Eventually, they could even lock onto stealthy aircraft.
Passive radar can also be used to cue other sensors and for surveillance. Many applications exist for base stations on cell phones. However, passive radar is a complex and untested technology, and is not suitable for all situations.
For now, the best passive radars are those that are fixed to the ground. 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 do not have the same power or range as passive airborne radar applications.