Doppler Effect & RADAR Resolution

Transcription

1 oppler ffect esolution Lecture 16 r anjeev Kumar Mishra

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3 ntroduction Clutter: n real condition, radar echoes receive the echoes from the natural environment such as land, sea, and weather. Clutter echoes can be as much as 60 or 70dB of magnitude larger than aircraft echoes. he aircraft might not be detectable when an aircraft echo and clutter echo appear in the same radar resolution cell. olution: hould have clutter characteristics oppler effect [the most powerful method] is used: to detect the moving targets in the presence of large clutter. he ability of some radars to resolve the strong clutter regions into discrete areas, between which targets may be detected, is called interclutter visibility.

4 Clutter oppler pectra

5 he oppler ffect [ustrian Mathematician]

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7 ) ( ) ( ) ( ) ( 1.03 m kt v m kt v z f r r d oppler frequency as a function of radar frequency and target relative velocity oppler hift Frequency

8 When multipath propagation occurs, waves arrive with several directions, each of which then has its own associated oppler frequency. he bandwidth of the received signal is therefore spread relative to its transmitted bandwidth. his is the phenomenon of oppler spread. oppler Bandwidth

9 ign conventions ed: eceding from radar Blue: oward radar

10 Magnitude of the oppler hift ransmitted Frequency X band C band band adial velocity 9.37 z 5.62 z 3.0 z 1 m/s 62.5 z 37.5 z 20.0 z 10 m/s 625 z 375 z 200 z 50 m/s 3125 z 1876 z 1000 z hese frequency shifts are very small: for this reason, oppler radars must employ very stable transmitters and receivers

11 f the source and observer are moving toward each other, then the observer detects an increase in frequency compared to the case when the source and observer are at rest. f the source and observer are moving away from each other, then the observer detects a decrease in frequency. his phenomenon is called the oppler ffect. he detected frequency (apparent) is given by f f ' d f s c v 0 f ' f s c vs Where, f s is the frequency of the source as measured with no relative motion, c is the speed of sound, v o is the speed of the observer, and v s is the speed of the source

12 : f the observer is moving toward the source, then v 0 is positive. f the observer is moving away from the source, then v 0 is negative. Likewise, v s is positive if the source is moving toward the observer and negative is the source is moving away from the observer. s s v c v c f f 0 '

13 adar resolution of a radar is a radar metric that describes the ability to detect the target in close proximity to each other as distinct objects. adar resolution is usually divided into two categories; range resolution and angular (bearing) resolution. adar resolution

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15 adar ngular esolution ange ngular resolution is the minimum angular separation at which two equal targets at the same range can be separated..

16 he angular resolution as a distance between two targets depends on the slant-range and can be calculated with help of the following formula: 2sin 2 Where, Θ = antenna beam width = angular resolution as a distance between the two targets = slant range aims - antenna

17 esolution Cell he resolution cell

18 esolution Cell

19 eometry of L operation and resolved surface area he along-track resolution is then: a a r cos a he slant-range resolution is then: 2 c r 2sin c r

20 Coherence Coherence refers to the radar s ability to accurately measure (extract) the received signal phase.