EEE381BAerospace Systems & Avionics

RadarPart 3 – Continuous Wave RadarsRef: Moir & Seabridge 2006, Chapter 3

Dr Ron Smith

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Outline

1. Introduction

2. Doppler radar

3. Frequency-modulated CW radar

4. Terrain-following radar (TFR)

5. CW illumination

6. Exercises

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1. Introduction

Pulsed 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.

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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.

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2.1.1 Doppler radar theory[4]

Depicted below is a Doppler radar with transmit wavelength λt and period Tt. As a closing target approaches at velocity v, the radar will observe a shift in return wavelength, λr as a function of v.

λr = λt – 2vTt

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2.1.1 Doppler radar theory[5]

Depicted below is a Doppler radar with transmit wavelength λt and period Tt. As a closing target approaches at velocity v, the radar will observe a shift in return wavelength, λr as a function of v.

λr = λt – 2vTt

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2.1.2 Why 2vTt ? [4]

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2.1.3 Doppler radar line of sight [4]

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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( ))

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2.2 Doppler “navigator” radar

lamda configuration

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3. FM-CW radar

An 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.

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3.1.1 FM-CW radar theory [4]

The modulation parameters are frequency deviation, f, and modulation period, Tm .

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3.1.2 FM-CW radar theory [4]

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3.1.3 FM-CW radar theory [4]

closing target

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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)

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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.

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3.2 FM-CW radar architecture [4]

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4. CW Radar applications [1]

Radar altimeterSection 3.7.1 and Section 8.6.11

Terrain-following radarSection 3.7.2

CW illumination Section 3.7.2

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4.1 Radar altimeter

Triangular 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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5.1 Quick response exercise # 1

Recalling 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?

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5.2 Doppler calculation

Just 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.a. If the beat frequency on your Doppler radar is 1517 Hz, what is

speed of the Twin Otter?b. What range resolution can you get with this crude radar?

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5.3 Radar altimeter calculation

An 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.

a. Compute the beat frequency as a function of range.

b. If the system has a measured beat frequency of 60 kHz, what is the aircraft altitude?

c. What is the range resolution of the altimeter?

d. What frequency variation in MHz is required to give a range resolution of 10m?

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References1) Moir & Seabridge, Military Avionics Systems, American Institute of

Aeronautics & Astronautics, 2006. [Sections 2.6 & 2.7]2) David Adamy, EW101 - A First Course in Electronic Warfare, Artech

House, 2000. [Chapters 3,4 & 6]3) George W. Stimson, Introduction to Airborne Radar, Second Edition,

SciTch Publishing, 1998.4) Principles of Radar Systems, student laboratory manual, 38542-00, Lab-

Volt (Quebec) Ltd, 2006.5) Georgia State University, hyperphsyics,,

http://hyperphysics.phy-astr.gsu.edu/Hbase/sound/radar.html6) Mark A. Hicks, "Clip art licensed from the Clip Art Gallery on

DiscoverySchool.com"