Radar Part3

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

    Dr Ron Smith

    Winter 2009Continuous wave radars - *

    OutlineIntroductionDoppler radarFrequency-modulated CW radarTerrain-following radar (TFR)CW illuminationExercises

    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]

    Winter 2009Continuous wave radars - *

    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.

    Winter 2009Continuous wave radars - *

    4.2 Terrain-following radar [1]

    Winter 2009Continuous wave radars - *

    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?

    Winter 2009Continuous wave radars - *

    5.In-class exercises

    Winter 2009Continuous wave radars - *

    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"

    EEE381B Aerospace Systems & AvionicsRadar - Continuous Wave RadarsEEE381B Aerospace Systems & AvionicsRadar - Continuous Wave RadarsEEE381B Aerospace Systems & AvionicsRadar - Continuous Wave RadarsEEE381B Aerospace Systems & AvionicsRadar - Continuous Wave RadarsEEE381B Aerospace Systems & AvionicsRadar - Continuous Wave RadarsEEE381B Aerospace Systems & AvionicsRadar - Continuous Wave RadarsEEE381B Aerospace Systems & AvionicsNote that the distance vTt is greatly exaggerated for illustration.Radar - Continuous Wave RadarsEEE381B Aerospace Systems & AvionicsRadar - Continuous Wave RadarsEEE381B Aerospace Systems & AvionicsRadar - Continuous Wave RadarsEEE381B Aerospace Systems & AvionicsPrior to the prevalence of INS (30 years ago, this was a very common means of navigation. A disadvantage of this system is that level terrain with a low reflectivity coefficient (ice covered lake, or very calm waters) may provide insufficient returns to yield suitable Doppler results. Radar - Continuous Wave RadarsEEE381B Aerospace Systems & AvionicsRadar - Continuous Wave RadarsEEE381B Aerospace Systems & AvionicsRadar - Continuous Wave RadarsEEE381B Aerospace Systems & AvionicsRadar - Continuous Wave RadarsEEE381B Aerospace Systems & AvionicsRadar - Continuous Wave RadarsEEE381B Aerospace Systems & AvionicsThe round-trip transit time, tr = 2R/c, see figure on slide 14.The changing transmit frequency, dft/dt = 4fmf, from the slope of the transmitting waveform = f/(Tm)Radar - Continuous Wave RadarsEEE381B Aerospace Systems & AvionicsFor a FM-CW radar with a modulating frequency of 1 kHz, and a maximum frequency deviation of +/- 0.6 MHzThe range resolution is c/(4f) = 3x108 / (4(0.6x106) = 125 mThe range ambiguity is cTm = (3x108)(1/1x103) = 300,000 m = 300 km (well beyond the range of the radar!!)Radar - Continuous Wave RadarsEEE381B Aerospace Systems & AvionicsRadar - Continuous Wave RadarsEEE381B Aerospace Systems & AvionicsRadar - Continuous Wave RadarsEEE381B Aerospace Systems & AvionicsRadar - Continuous Wave RadarsEEE381B Aerospace Systems & AvionicsThe TFR scans the terrain ahead of the aircraft and receives ground returns that are used for guidance. Normally, a simple box scan is used where the active sweeps are those in the vertical direction (sections 1 and 3). In some circumstances a figure-of-eight scan is usedwhich provides broader lateral coverage than the simple box scan. The TFR therefore builds up a range/elevation picture of the terrain ahead of the aircraft and calculates an imaginary ski-toe profile that reaches out ahead of the aircraft. This profile is calculated taking into account such factors as aircraft speed, manoeuvrability, etc., and provides an envelope within which the aircraft will not be able to avoid the terrain ahead. The system is configured so that, whenever the terrain ahead broaches the ski-toe envelope, the aircraft pitches up to rectify the situation. Similarly, if the terrain drops away in front of the aircraft, the aircraft pitches down until just operating outside the profile. The system operates just like the toe of a ski, moving up or down to follow the terrain ahead of the aircraft but always ensuring the aircraft can safely manoeuvre.[1]Radar - Continuous Wave RadarsEEE381B Aerospace Systems & AvionicsThe main advantage of this