RADAR

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Doppler radar

Doppler effect.

A Doppler radar is a specialized radar that uses the Doppler effect to produce velocity data about objects at a distance. It does this by bouncing a microwave signal off a desired target and analyzing how the object's motion has altered the frequency of the returned signal. This variation gives direct and highly accurate measurements of the radialcomponent of a target's velocity relative to the radar. Doppler radars are used in aviation, sounding satellites, meteorology, radar guns,^[1] radiology and healthcare (fall detection^[2] and risk assessment, nursing or clinic purpose^[3]), and bistatic radar (surface-to-air missiles).

Partly because of its common use by television meteorologists in on-air weather reporting, the specific term "Doppler Radar" has erroneously become popularly synonymous with the type of radar used in meteorology. Most modern weather radarsuse the pulse-Doppler technique to examine the motion of precipitation, but it is only a part of the processing of their data. So, while these radars use a highly specialized form ofDoppler radar, the term is much broader in its meaning and its applications.

ConceptEdit

Doppler effectEdit

The emitted signal toward the car is reflected back with a variation of frequency that depend on the speed away/toward the radar (160 km/h). This is only a component of the real speed (170 km/h).

The Doppler effect (or Doppler shift), named after Austrian physicist Christian Doppler who proposed it in 1842, is the difference between the observed frequency and the emitted frequency of a wave for an observer moving relative to the source of the waves. It is commonly heard when a vehicle sounding a siren approaches, passes and recedes from an observer. The received frequency is higher (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is lower during the recession. This variation of frequency also depends on the direction the wave source is moving with respect to the observer; it is maximum when the source is moving directly toward or away from the observer and diminishes with increasing angle between the direction of motion and the direction of the waves, until when the source is moving at right angles to the observer, there is no shift.

Imagine a baseball pitcher throwing one ball every second to a catcher (a frequency of 1 ball per second). Assuming the balls travel at a constant velocity and the pitcher is stationary, the catcher catches one ball every second. However, if the pitcher is jogging towards the catcher, the catcher catches balls more frequently because the balls are less spaced out (the frequency increases). The inverse is true if the pitcher is moving away from the man. He catches balls less frequently because of the pitcher's backward motion (the frequency decreases). If the pitcher moves at an angle, but at the same speed, the frequency variation at which the receiver catches balls is less, as the distance between the two changes more slowly.

From the point of view of the pitcher, the frequency remains constant (whether he's throwing balls or transmitting microwaves). Since with electromagnetic radiation like microwaves frequency is inversely proportional to wavelength, the wavelength of the waves is also affected. Thus, the relative difference in velocity between a source and an observer is what gives rise to the doppler effect.^[4]

Frequency variationEdit

The formula for radar Doppler shift is the same as that for reflection of light by a moving mirror.^[5] There is no need to invokeEinstein's theory of special relativity, because all observations are made in the same frame of reference.^[6] The result derived with c as thespeed of light and v as the target velocity gives the shifted frequency (${\displaystyle f_{r}}$) as a function of the original frequency (${\displaystyle f_{t}}$) :

${\displaystyle f_{r}=f_{t}\left({\frac {1+v/c}{1-v/c}}\right)}$

The "beat frequency", (Doppler frequency) (${\displaystyle f_{d}}$), is thus:^[7]

${\displaystyle f_{d}=f_{r}-f_{t}=2v{\frac {f_{t}}{(c-v)}}}$

Since for most practical applications of radar, ${\displaystyle v\ll c}$, so ${\displaystyle \left(c-v\right)\rightarrow c}$. We can then write:

${\displaystyle f_{d}\approx 2v{\frac {f_{t}}{c}}}$

TechnologyEdit

U.S. Army soldier using a radar gun, an application of Doppler radar, to catch speeding violators.

There are four ways of producing the Doppler effect. Radars may be:

• Coherent pulsed (CP),
• Pulse-Doppler radar,
• Continuous wave (CW), or
• Frequency modulation (FM).

Doppler allows the use of narrow band receiver filters that reduce or eliminate signals from slow moving and stationary objects. This effectively eliminates false signals produced by trees, clouds, insects, birds, wind, and other environmental influences. Cheap hand held Doppler radar may produce erroneous measurements.

CW Doppler radar only provides a velocity output as the received signal from the target is compared in frequency with the original signal. Early Doppler radars included CW, but these quickly led to the development of frequency modulated continuous wave (FMCW) radar, which sweeps the transmitter frequency to encode and determine range.

With the advent of digital techniques, Pulse-Doppler radars (PD) became light enough for aircraft use, and Doppler processors for coherent pulse radars became more common. That provides Look-down/shoot-down capability. The advantage of combining Doppler processing with pulse radars is to provide accurate velocity information. This velocity is called range-rate. It describes the rate that a target moves toward or away from the radar. A target with no range-rate reflects a frequency near the transmitter frequency and cannot be detected. The classic zero doppler target is one which is on a heading that is tangential to the radar antenna beam. Basically, any target that is heading 90 degrees in relation to the antenna beam cannot be detected by its velocity (only by its conventional reflectivity).

In military airborne applications, the Doppler effect has 2 main advantages. Firstly, the radar is more robust against counter-measure. Return signals from weather, terrain, and countermeasures like chaff are filtered out before detection, which reduces computer and operator loading in hostile environments. Secondly, against a low altitude target, filtering on the radial speed is a very effective way to eliminate the ground clutter that always has a null speed. Low-flying military plane with countermeasure alert for hostile radar track acquisition can turn perpendicular to the hostile radar to nullify its Doppler frequency, which usually breaks the lock and drives the radar off by hiding against the ground return which is much larger.