Radar sensor
Encyclopedia
Radar engineering details are technical details pertaining to the components of a radar
and their ability to detect the return energy from moving scatterers — determining an object's position or obstruction in the environment. This include field of view in terms of solid angle
and maximum unambiguous range and velocity, as well as angular, range and velocity resolution. Radar sensors are classified by application, architecture, radar mode, platform, and propagation window.
Applications of radar include autonomous cruise control
, autonomous landing guidance, altimetry, air traffic management, early warning, fire control, forward warning collision sensing, ground penetrating radar, surveillance
, and weather forecasting
.
. The emitter and the receiver can be in the same place, as with the monostatic radar
s, or be separated as in the
bistatic radar
s. Finally, the radar wave emitted can be continuous or pulsed. The choice of the architecture depends on the sensors to be used.
.
An electronically scanned array (ESA), or a phased array, offers advantages over mechanically scanned antennas such as instantaneous beam scanning, the availability of multiple concurrent agile beams, and concurrently operating radar modes. Figures of merit of an ESA are the bandwidth, the effective isotropically radiated power (EIRP) and the GR/T quotient, the field of view. EIRP is the product of the transmit gain, GT, and the transmit power, PT. GR/T is the quotient of the receive gain and the antenna noise temperature. A high EIRP and GR/T are a prerequisite for long-range detection. Design choices are:
(pulse-Doppler), or through the frequency modulation
(FM) ranging and range differentiation. The range resolution is limited by the instantaneous signal bandwidth of the radar sensor in both pulse-Doppler and FMCW radars. Monostatic monopulse-Doppler radar sensors offer advantages over FMCW radars, such as:
s have a spatially-dislocated transmitter and receiver. In this case sensor in the transmitting antenna report back to the system the angular position of the scanning beam while the energy detecting ones are with the other antenna. A time synchronisation is crucial in interpreting the data as the receiver antenna is not moving.
Monostatic radar
s have a spatially co-located transmitter and receiver. It this case, the emission has to be insulated from the reception sensors as the energy emitted is far greater than the returned one.
is platform-dependent. Examples of platforms are airborne, car-borne, ship-borne, space-borne, and ground-based platforms.
considerations. The radar frequency is also chosen in order to optimize the radar cross section
(RCS) of the envisioned target, which is frequency-dependent. Examples of progation windows
are the 3 GHz (S), 10 GHz (X), 24 GHz (K), 35 GHz (Ka), 77 GHz (W), 94 GHz (W) propagation windows.
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
and their ability to detect the return energy from moving scatterers — determining an object's position or obstruction in the environment. This include field of view in terms of solid angle
Solid angle
The solid angle, Ω, is the two-dimensional angle in three-dimensional space that an object subtends at a point. It is a measure of how large that object appears to an observer looking from that point...
and maximum unambiguous range and velocity, as well as angular, range and velocity resolution. Radar sensors are classified by application, architecture, radar mode, platform, and propagation window.
Applications of radar include autonomous cruise control
Cruise control
Cruise control is a system that automatically controls the speed of a motor vehicle. The system takes over the throttle of the car to maintain a steady speed as set by the driver.-History:...
, autonomous landing guidance, altimetry, air traffic management, early warning, fire control, forward warning collision sensing, ground penetrating radar, surveillance
Surveillance
Surveillance is the monitoring of the behavior, activities, or other changing information, usually of people. It is sometimes done in a surreptitious manner...
, and weather forecasting
Weather forecasting
Weather forecasting is the application of science and technology to predict the state of the atmosphere for a given location. Human beings have attempted to predict the weather informally for millennia, and formally since the nineteenth century...
.
Architecture choice
The angle of a target is detected by scanning the field of view with a highly directive beam. This is done electronically, with a phased array antenna, or mechanically by rotating a physical antennaAntenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
. The emitter and the receiver can be in the same place, as with the monostatic radar
Monostatic radar
Monostatic radar is the term given to a radar in which the transmitter and receiver are collocated. This is the conventional configuration for a radar, but the term is used to distinguish it from a bistatic radar or multistatic radar....
s, or be separated as in the
bistatic radar
Bistatic radar
Bistatic radar is the name given to a radar system which comprises a transmitter and receiver which are separated by a distance that is comparable to the expected target distance. Conversely, a radar in which the transmitter and receiver are collocated is called a monostatic radar...
s. Finally, the radar wave emitted can be continuous or pulsed. The choice of the architecture depends on the sensors to be used.
Scanning antenna
- Active versus passive (see Fig. 1): In an active electronically scanned array (AESA), each antenna is connected to a T/R module featuring solid state power amplification (SSPA). An AESA has distributed power amplification and offers high performance and reliability, but is expensive. In a passive electronically scanned array, the array is connected to a single T/R module featuring vacuum electronics devices (VED). A PESA has centralized power amplification and offers cost savings, but requires low-loss phase shifters
- Aperture: The aperture of a radar sensor is real or synthetic. Real-beam radar sensors allow for real-time target sensing. Synthetic aperture radarSynthetic aperture radarSynthetic-aperture radar is a form of radar whose defining characteristic is its use of relative motion between an antenna and its target region to provide distinctive long-term coherent-signal variations that are exploited to obtain finer spatial resolution than is possible with conventional...
(SAR) allow for an angular resolution beyond real beamwidth by moving the aperture over the target, and adding the echoes coherently. - Architecture: The field of view is scanned with a highly directive frequency-orthogonal (slotted waveguide), spatially-orthogonal (switched beamforming networks), or time-orthogonal beams . In case of time-orthogonal scanning, the beam of an ESA is scanned preferably by applying a progressive time delay,
, constant over frequency, instead of by applying a progressive phase shift, constant over frequency. Usage of true-time-delay (TTDTTDTTD may refer to:* Telecommunications device for the deaf, a teleprinter specifically designed for text communication over public switched telephone network...
) phase shifters avoids beam squinting with frequency. The scanning angle,
, is expressed as a function of the phase shift progression, 
, which is a function of the frequency and the progressive time delay, 
, which is invariant with frequency:
Note that
is not a function of frequency. A constant phase shift over frequency has important applications as well, albeit in wideband pattern synthesis. For example, the generation of wideband monopulse 
receive patterns depends on a feed network which combines two subarrays using a wideband hybrid couplerHybrid couplerA hybrid coupler is a passive device used in radio and telecommunications. It is a type of directional coupler where the input power is equally divided between two output ports. Since it is a special case of directional coupler, it is discussed in Power dividers and directional...
. - Beam forming: The beam is formed in the digital (digital beamforming (DBF)), intermediate frequency (IF), optical, or radio frequency (RF) domain.
- Construction: An electronically scanned array is a brick, stick, tile, or tray construction. Brick and tray refers to a construction approach in which the RF circuitry is integrated perpendicular to the array plane. Tile, on the other hand, refers to a construction approach in which the RF circuitry is integrated on substrates parallel to the array plane. Stick refers to a construction approach in which the RF circuitry is connected to a line array in the array plane.
- Feed Network: The feed network is constrained (corporate, series) or space-fed.
- Grid: The grid is periodic (rectangular, triangular) or aperiodic (thinned).
- Polarization: The polarization of ground-based radar sensors is vertical, in order to reduce multipath (Brewster angle). Radar sensors can also be polarimetric for all-weather applications.
FMCW versus pulse-Doppler
The range and velocity of a target are detected through pulse delay ranging and the Doppler effectDoppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
(pulse-Doppler), or through the frequency modulation
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...
(FM) ranging and range differentiation. The range resolution is limited by the instantaneous signal bandwidth of the radar sensor in both pulse-Doppler and FMCW radars. Monostatic monopulse-Doppler radar sensors offer advantages over FMCW radars, such as:
- Half-duplex: Pulse-Doppler radar sensors are half-duplex, while FMCW radar sensors are full-duplex. Hence, pulse-Doppler provide higher isolation between transmitter and receiver, increasing the receiver's dynamic range (DR) and the range detection considerably. In addition, an antenna or an array can be time-shared between transmitter and receiver of the T/R module, whereas FMCW radars require two antennas or arrays, one for transmit and one for receive. A drawback of half-duplex operation is the existence of a blind zone in the immediate vicinity of the radar sensor. Pulse-Doppler radar sensors are therefore more suited for long-range detection, while FMCW radar sensors are more suited for short-range detection.
- Monopulse: A monopulse feed network, as shown in Fig. 2, increases the angular accuracy to a fraction of the beamwidth by comparing echoes, which originate from a single radiated pulse and which are received in two or more concurrent and spatially-orthogonal beams.
- Pulse compression: Pulse compressionPulse compressionPulse compression is a signal processing technique mainly used in radar, sonar and echography to increase the range resolution as well as the signal to noise ratio...
derelates the pulse width and the instantaneous signal bandwidth, which are otherwise inversely related. The pulse width is related to the time-on-target, the signal to noise ratio (SNR) and the maximum range. The instantaneous signal bandwidth is related to the range resolution. - Pulse-Doppler processing: Echoes originating from a radiated burst are transformed to the spectral domain using a discrete fourier transformDiscrete Fourier transformIn mathematics, the discrete Fourier transform is a specific kind of discrete transform, used in Fourier analysis. It transforms one function into another, which is called the frequency domain representation, or simply the DFT, of the original function...
(DFT). In the spectral domain, stationary clutter can be removed because it has a Doppler frequency shift which is different from the Doppler frequency shift of the moving target. The range and velocity of a target can be estimated with increased SNR due to coherent integration of echoes .
Bistatic versus monostatic
Bistatic radarBistatic radar
Bistatic radar is the name given to a radar system which comprises a transmitter and receiver which are separated by a distance that is comparable to the expected target distance. Conversely, a radar in which the transmitter and receiver are collocated is called a monostatic radar...
s have a spatially-dislocated transmitter and receiver. In this case sensor in the transmitting antenna report back to the system the angular position of the scanning beam while the energy detecting ones are with the other antenna. A time synchronisation is crucial in interpreting the data as the receiver antenna is not moving.
Monostatic radar
Monostatic radar
Monostatic radar is the term given to a radar in which the transmitter and receiver are collocated. This is the conventional configuration for a radar, but the term is used to distinguish it from a bistatic radar or multistatic radar....
s have a spatially co-located transmitter and receiver. It this case, the emission has to be insulated from the reception sensors as the energy emitted is far greater than the returned one.
Platform
Radar clutterClutter (radar)
Clutter is a term used for unwanted echoes in electronic systems, particularly in reference to radars. Such echoes are typically returned from ground, sea, rain, animals/insects, chaff and atmospheric turbulences, and can cause serious performance issues with radar systems.- Backscatter coefficient...
is platform-dependent. Examples of platforms are airborne, car-borne, ship-borne, space-borne, and ground-based platforms.
Propagation window
The radar frequency is selected based on size and technology readiness levelTechnology Readiness Level
Technology Readiness Level is a measure used by some United States government agencies and many of the world's major companies to assess the maturity of evolving technologies prior to incorporating that technology into a system or subsystem...
considerations. The radar frequency is also chosen in order to optimize the radar cross section
Radar cross section
Radar cross section is a measure of how detectable an object is with a radar. A larger RCS indicates that an object is more easily detected.An object reflects a limited amount of radar energy...
(RCS) of the envisioned target, which is frequency-dependent. Examples of progation windows
Radio propagation
Radio propagation is the behavior of radio waves when they are transmitted, or propagated from one point on the Earth to another, or into various parts of the atmosphere...
are the 3 GHz (S), 10 GHz (X), 24 GHz (K), 35 GHz (Ka), 77 GHz (W), 94 GHz (W) propagation windows.
Radar Mode
Radar modes for point targets include search and track. Radar modes for distributed targets include ground mapping and imaging. The radar mode sets the radar waveformWaveform
Waveform means the shape and form of a signal such as a wave moving in a physical medium or an abstract representation.In many cases the medium in which the wave is being propagated does not permit a direct visual image of the form. In these cases, the term 'waveform' refers to the shape of a graph...
See also
- Amplitude monopulse for amplitude-comparison monopulse
- Phase interferometry for phase-comparison monopulse