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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 includes 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
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, autonomous landing guidance, radar altimeter,
air traffic management 334x334px, Air traffic management (ATM) is an aviation term encompassing all systems that assist aircraft to depart from an aerodrome, transit airspace, and land at a destination aerodrome, consisting of air traffic services (ATS) including air tr ...
, early-warning radar, fire-control radar, forward warning collision sensing,
ground penetrating radar Ground-penetrating radar (GPR) is a geophysical method that uses radar pulses to image the subsurface. It is a non-intrusive method of surveying the sub-surface to investigate underground utilities such as concrete, asphalt, metals, pipes, cables ...
, surveillance, and weather forecasting.


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 In antenna theory, a phased array usually means an electronically scanned array, a computer-controlled array of antennas which creates a beam of radio waves that can be electronically steered to point in different directions without moving t ...
, or mechanically by rotating a physical antenna. The emitter and the receiver can be in the same place, as with the
monostatic radar Bistatic radar is a radar system comprising a transmitter and receiver that are separated by a distance comparable to the expected target distance. Conversely, a conventional radar in which the transmitter and receiver are co-located is called ...
s, or be separated as in the
bistatic radar Bistatic radar is a radar system comprising a transmitter and receiver that are separated by a distance comparable to the expected target distance. Conversely, a conventional radar in which the transmitter and receiver are co-located is called ...
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

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 Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...
, 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: * Active versus passive: In an
active electronically scanned array An active electronically scanned array (AESA) is a type of phased array antenna, which is a computer-controlled array antenna in which the beam of radio waves can be electronically steered to point in different directions without moving the ant ...
(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
Antenna aperture In electromagnetics and antenna theory, the aperture of an antenna is defined as "A surface, near or on an antenna, on which it is convenient to make assumptions regarding the field values for the purpose of computing fields at external points. T ...
of a radar sensor is real or synthetic. Real-beam radar sensors allow for real-time target sensing. Synthetic aperture radar (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, \Delta \tau, constant over frequency, instead of by applying a progressive phase shift, constant over frequency. Usage of true-time-delay (TTD) phase shifters avoids beam squinting with frequency. The scanning angle, \theta, is expressed as a function of the phase shift progression, \beta, which is a function of the frequency and the progressive time delay, \Delta \tau, which is invariant with frequency: Note that \theta 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 \Sigma/\Delta receive patterns depends on a feed network which combines two subarrays using a wideband hybrid coupler. * 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 (antenna): The polarization of ground-based radar sensors is vertical, in order to reduce multipath (
Brewster angle Brewster's angle (also known as the polarization angle) is an angle of incidence at which light with a particular polarization is perfectly transmitted through a transparent dielectric surface, with ''no reflection''. When ''unpolarized'' lig ...
). 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 effect The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who d ...
( 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 frequency modulated continuous wave ( 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 Monopulse radar is a radar system that uses additional encoding of the radio signal to provide accurate directional information. The name refers to its ability to extract range and direction from a single signal pulse. Monopulse radar avoids prob ...
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 compression 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 Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in de ...
(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 transform In mathematics, the discrete Fourier transform (DFT) converts a finite sequence of equally-spaced samples of a function into a same-length sequence of equally-spaced samples of the discrete-time Fourier transform (DTFT), which is a comp ...
(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.A. Ludloff: "Praxiswissen Radar und Radarsignalverarbeitung, 2. Auflage," Viewegs Fachbücher der Technik, 1998


Bistatic versus monostatic

Bistatic radar Bistatic radar is a radar system comprising a transmitter and receiver that are separated by a distance comparable to the expected target distance. Conversely, a conventional radar in which the transmitter and receiver are co-located is called ...
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 Bistatic radar is a radar system comprising a transmitter and receiver that are separated by a distance comparable to the expected target distance. Conversely, a conventional radar in which the transmitter and receiver are co-located is called ...
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 clutter 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 level 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 propagation windows 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 waveform


See also

* Amplitude monopulse for amplitude-comparison monopulse *
Phase interferometry Phase or phases may refer to: Science *State of matter, or phase, one of the distinct forms in which matter can exist * Phase (matter), a region of space throughout which all physical properties are essentially uniform *Phase space, a mathematic ...
for phase-comparison monopulse


References

{{reflist, 2 Radar Radio frequency antenna types Antennas (radio)