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### Transcript of Radar Part3

• EEE381BAerospace Systems & AvionicsRadarPart 3 Continuous Wave RadarsRef: Moir & Seabridge 2006, Chapter 3

Dr Ron Smith

Winter 2009Continuous wave radars - *

Winter 2009Continuous wave radars - *

1.IntroductionPulsed radar is typically used to detect targets, determining range and bearing. These radars generally require high-power, are quite complex and thus expensive.Continuous wave (CW) radars typically determine target velocity, and can achieve considerable ranges without the high peak power. These radars are typically simpler, more compact and less costly.

Winter 2009Continuous wave radars - *

2.Doppler radar [4]Recall that the Doppler effect is the change in frequency that occurs when a source and a target are in relative motion.The Doppler affect can be used in a CW radar in order to determine velocity.

Winter 2009Continuous wave radars - *

2.1.1 Doppler radar theory[4]Depicted below is a Doppler radar with transmit wavelength t and period Tt. As a closing target approaches at velocity v, the radar will observe a shift in return wavelength, r as a function of v. r = t 2vTt

Winter 2009Continuous wave radars - *

2.1.1 Doppler radar theory[5]Depicted below is a Doppler radar with transmit wavelength t and period Tt. As a closing target approaches at velocity v, the radar will observe a shift in return wavelength, r as a function of v. r = t 2vTt

Winter 2009Continuous wave radars - *

2.1.2 Why 2vTt ? [4]

Winter 2009Continuous wave radars - *

2.1.3 Doppler radar line of sight [4]

Winter 2009Continuous wave radars - *

2.1.4 Doppler radar velocity [4]Substituting frequency for wavelength and considering direction of target to line of sight, yields a general expression for Doppler velocity.

v = c(1- ft / fr ) / (2 cos( ))

Winter 2009Continuous wave radars - *

2.2 Doppler navigator radarlamda configuration

Winter 2009Continuous wave radars - *

3.FM-CW radarAn unmodulated CW radar is incapable of detecting range, as there is no reference point in the transmitted or returned signal for measuring elapsed time.By frequency modulating the CW signal, differences between the transmitted and received frequencies can be used to estimate range. The further the target, the larger the frequency difference.

Winter 2009Continuous wave radars - *

3.1.1 FM-CW radar theory [4]The modulation parameters are frequency deviation, f, and modulation period, Tm .

Winter 2009Continuous wave radars - *

3.1.2 FM-CW radar theory [4]

Winter 2009Continuous wave radars - *

3.1.3 FM-CW radar theory [4]closing target

Winter 2009Continuous wave radars - *

3.1.4 FM-CW radar theory [4]Given an FM-CW radar with triangular frequency modulation of fm and frequency deviation f, the range of a stationary target can be derived as follows:fb = tr dft/dt, where the round-trip transit time, tr = 2R/c, and the changing transmit frequency, dft/dt = 4fmf.Therefore fb =(8Rfmf/c), or R = cfb/(8fmf)

Winter 2009Continuous wave radars - *

3.1.5 FM-CW radar theory [4]Recall that the range resolution of a radar is a measure of its ability to distinguish closely spaced targets. The range resolution of a FM-CW radar is a function of its modulating bandwidth, and is c/(4f).The range ambiguity is the range beyond which the radar yields ambiguous range results. The range ambiguity of a FM-CW radar is a function of its modulating frequency, and is cTm.This is usually well beyond the signal range.

Winter 2009Continuous wave radars - *

3.2 FM-CW radar architecture [4]

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4.CW Radar applications [1]Radar altimeterSection 3.7.1 and Section 8.6.11Terrain-following radarSection 3.7.2CW illumination Section 3.7.2

Winter 2009Continuous wave radars - *

4.1 Radar altimeterTriangular FM-CW radar is commonly used in aircraft to determine the instantaneous altitude above the terrain it is flying.

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4.2 Terrain-following radar [1]

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4.3 CW illumination [1]Used in conjunction with semi-active missiles. The aircraft radar illuminates the target, while the missile uses the received return signal to track the target.What are the advantages and disadvantages?

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5.In-class exercises

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5.1Quick response exercise # 1Recalling the radar range equation, why is it possible for a CW radar to achieve much greater ranges than a pulsed radar?

Can you think of an application in sports where a simple Doppler radar may be employed?

Winter 2009Continuous wave radars - *

5.2Doppler calculationJust after take-off you realize that you are following a military CC-138 (Twin Otter) in a Cessna 152. Your air speed is 190 km/hour. You estimate the that the Twin Otter is at an approximate 15 angle above you.You have a home-made 10.6 GHz Doppler radar installed on the Cessna oriented straight ahead.If the beat frequency on your Doppler radar is 1517 Hz, what is speed of the Twin Otter?What range resolution can you get with this crude radar?

Winter 2009Continuous wave radars - *

5.3Radar altimeter calculationAn aircraft is equipped with an FM-CW radar altimeter with a modulation frequency of 1.0 kHz and a frequency deviation of 0.60 MHz.Compute the beat frequency as a function of range.If the system has a measured beat frequency of 60 kHz, what is the aircraft altitude? What is the range resolution of the altimeter?What frequency variation in MHz is required to give a range resolution of 10m?

Winter 2009Continuous wave radars - *

ReferencesMoir & Seabridge, Military Avionics Systems, American Institute of Aeronautics & Astronautics, 2006. [Sections 2.6 & 2.7]David Adamy, EW101 - A First Course in Electronic Warfare, Artech House, 2000. [Chapters 3,4 & 6]George W. Stimson, Introduction to Airborne Radar, Second Edition, SciTch Publishing, 1998.Principles of Radar Systems, student laboratory manual, 38542-00, Lab-Volt (Quebec) Ltd, 2006.Georgia State University, hyperphsyics,, http://hyperphysics.phy-astr.gsu.edu/Hbase/sound/radar.htmlMark A. Hicks, "Clip art licensed from the Clip Art Gallery on DiscoverySchool.com"