What Is a Passive Radar?

Passive radar is a type of radar that does not use the transmitter to transmit energy and detects targets by receiving microwave energy reflected from warm objects or other sources. It has an antenna and a highly sensitive receiving device. The ability of a passive radar to identify a target mainly depends on the surface temperature difference between the targets and the reflection coefficient of the target, the incident angle between the antenna beam and the target, the wireless polarization and beam width, and the minimum detectable electron of the receiver.

Passive radar

Passive radar itself does not emit energy, but passively receives the electromagnetic signals of non-cooperative radiation sources reflected by the target to track and locate the target. The so-called non-cooperative external radiation source refers to the fact that the radiation source and the radar "do not make a boundary" and there is no direct cooperative combat relationship. This prevents detection equipment and anti-radiation missiles from using electromagnetic signals to capture, track, and attack passive radars.

Passive radar systems are simple, small in size, can be installed on mobile platforms, are easy to deploy, and have low ordering and maintenance costs. Passive radar does not emit signals that illuminate the target, so it is not easy to be perceived by the other party, and there is generally no problem of interference. It can work around the clock and around the clock: it can continuously detect targets, usually once every second, and the signal source is low-frequency electromagnetic waves of 40-400 MHz, which is conducive to the detection of stealth targets and low-altitude targets: no frequency allocation is required, so it can be deployed Radar area.

Passive radars do not emit signals themselves, which brings both advantages and disadvantages. Due to the reliance on a third-party transmitter, the operator cannot actively control the illuminator, and remains at the detected target.

Passive radar systems can be classified based on the detection object or configuration. According to the configuration, passive radar is divided into two types: fixed (ground-based) and mobile (installed on platforms such as submarines, ships, aircraft, and ground vehicles).

In fact, passive radar is not a new concept. Its history is almost as long as the radar technology itself. In 1935, Robert Watson Watt used it in a single base passive system

Lockheed Martin of the United States began researching non-cooperative bistatic passive radars in 1983, and in 1998 developed a new type of "silent sentry" passive detection system. This passive radar uses 50-80 MHz continuous wave signals transmitted by commercial FM radio and television stations to detect, track, and monitor moving targets in the area. The system consists of a large dynamic range digital receiver, a phased array receiving antenna, a high-performance parallel processor with gigabit floating point operations per second and

The British "cellular" radar system can detect, track and identify moving targets on land, at sea and in the air, including vehicles moving in trees, and it can theoretically detect ground targets in the wild environment of 10 to 15 kilometers and 100 thousand Large aircraft. When the target enters the detection area, it causes

The "Vera" series of passive radars was developed by the Czech Republic. "Vera-E" is the latest model of this series, which can detect, locate, identify and track air, ground and maritime targets. The maximum distance for air detection is 450 kilometers, and it can identify targets and generate aerial target images.

The "Vera-E" system consists of 4 parts: the analysis and processing center is centered, and 3 signal receiving stations are distributed in a circular arc shape around the station, and the distance between the stations is more than 50 kilometers. The analysis and processing center is deployed in the square cabin vehicle.

Passive radar systems (especially those using external non-cooperative radiation sources) may be an important development direction in the next 10-20 years. With the implementation of several major international communications satellite plans, more than 1,000 communications satellites will be in orbit in the future. Many of them will be able to emit high enough RF energy, and most locations on the ground will be illuminated by several spaceborne radiation sources at the same time. Passive radar systems can make full use of these radiation sources for target detection and tracking. In general, passive radar will be developed in the following areas:

(1) Expand the types of available external radiation sources. The external non-cooperative radiation sources range from the earliest television signals, FM signals, to current mobile communication signals, global positioning system satellite signals, and a variety of satellite signals and other possible radiation sources in the future. The number of sources will increase.

(2) Fourier imaging of radar targets. Researchers at the University of Illinois have confirmed that passive multistatic radar can be used to generate synthetic aperture images of aircraft targets. By using multiple transmitters with different frequencies and different positions, a dense Fourier domain data set can be established for an object, and the image of the object can be reconstructed by inverse Fourier transform.

(3) Networking of passive radars on different platforms. Due to the variety of available external radiation source signals, different radiation source signals occupy different frequency bands, and the same target will have different radar characteristics in different frequency bands. Therefore, in order to improve the detection capability of the target as much as possible, the passive radars of different platforms can be networked.

(4) The combination of passive radar and active radar. When the external electromagnetic radiation equipment is turned off or cannot be used, the passive radar cannot detect and locate the target. Therefore, the combination of passive radar and active radar can be considered. For example, the passive and active radars are reasonably arranged in a bi- / multi-base manner. When external electromagnetic radiation does not exist or cannot be used, the passive radar is used to receive the direct signal of the active radar and the reflected signal of the target to detect the target. This not only improves the utilization rate of passive radar, but also enhances the concealment and survivability of active radar.