Purpose and classification of radar systems, and key factors affecting radar performance

Latest update time：2021-09-06 20:33

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Source: Radar Communications Electronic Warfare

introduction:

Measuring the distance to a target remains the fundamental purpose of most radar systems. However, radar systems have evolved significantly in how they are composed, the signals they use, the information they can capture, and how this information can be used in different applications.

Radar is widely used in military and civilian fields, including: surveillance (threat identification, motion detection or proximity fuze), detection and tracking (target identification and tracking or maritime rescue), navigation (avoiding car collisions or air traffic control), high-resolution imaging (terrain mapping or landing guidance), weather tracking (storm warning or wind profile), etc. This article introduces the purpose and classification of radar systems in detail, and the key factors that affect radar performance.

Key factors affecting radar performance

Classification by signal type

Some common radar systems with various signal types are listed below:

Continuous Wave (Doppler) Radar: A continuous wave radar system transmits a continuous wave signal at a constant frequency. The received signal has a Doppler shift that can be used to determine the speed of the target. This radar system is often used for traffic monitoring.

FMCW radar: FMCW radar systems frequency modulate the CW signal to produce a timing reference. With this information, in addition to measuring speed, it is also possible to measure range. A significant advantage of continuous wave radars is that they provide continuous results (compared to pulse radar systems). This type of radar system is often used to accurately measure the altitude of an aircraft during landing.

Pulse radar: A basic (non-coherent) pulse radar system that determines the range and direction of a target by measuring the time difference between the transmitted and received pulses. Since the phase is random between pulses, the system is non-coherent. Long-range air surveillance is a common application scenario for these radar systems.

Doppler pulse radar: This is a coherent radar system in which information other than target range and direction, target velocity, is obtained based on the phase variation between received pulses. High pulse repetition rates (PRRs) are usually used, which makes radial velocity measurements more precise, but with lower range accuracy. The use of Doppler pulse radar systems to detect moving targets while suppressing static clutter is of great significance for meteorological monitoring applications.

Moving Target Indication (MTI) Radar: MTI radar also uses Doppler frequency to distinguish moving targets from stationary targets and clutter. Its waveform is a series of low PRR pulses, thus avoiding range ambiguity, but sacrificing velocity accuracy. These types of radar systems are often used in ground-based aircraft search and surveillance applications.

Pulse Compression Radar: Short pulse width signals provide better range resolution, but have limited range. Long pulse width signals contain more energy, providing longer detection range, but at the expense of range resolution. Pulse compression combines the power-related advantages of long pulse widths with the resolution advantages of short pulse widths. By modulating the frequency (e.g., linear frequency modulation) or phase (e.g., using Barker codes) of the transmitted signal, the long pulses can be compressed in the receiver by an amount equal to the inverse of the modulating signal bandwidth; many weather monitoring systems have moved toward using pulse compression radars.

Classification by antenna configuration

Monostatic Radar: In monostatic radar, the transmitter and receiver share the same antenna by time domain multiplexing.

Bistatic Radar: A radar system in which the transmitting and receiving antennas are separated (usually by a large distance or offset angle) is called a bistatic radar system. Bistatic radar systems are often used to detect stealth targets, where stealth technology intentionally avoids reflecting radar signals in the direction of the transmitter.

Electronically Scanned Array (ESA): Radar systems can use antenna arrays that can contain 1,000 or 10,000 antennas. By precisely controlling the phase and amplitude of each antenna element, the overall beam pattern of the array can be formed. These phased array antennas are an alternative to mechanically scanned antennas, which are generally heavier and more prone to failure.

In addition, a single point failure of a motor will cause the mechanical system to fail, whereas a failure of one or more elements of a phased array antenna will not cause the entire radar system to fail. There are two basic types of electronically scanned array (ESA) radar systems: passive ESA (PESA) and active ESA (AESA).

PESA: Passive Electronically Scanned Array. Typically, a PESA radar system takes a signal from a single source and splits it into hundreds of paths, delaying and/or attenuating some of them until each path reaches a single antenna element.

AESA: Active Electronically Scanned Array. Each antenna element of the AESA radar system array is an independent transmit/receive module (TRM). This provides great flexibility, enabling the AESA radar system to operate on multiple frequencies simultaneously, producing multiple beam patterns to achieve different radar functions. AESA radars are now the baseline for state-of-the-art fighter aircraft.

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