Lectures Radar3

8/2/2019 Lectures Radar3

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PART I- RADAR

C- Continuous Wave Radars



3/2011 Continuous wave radars - 2

Outline

1. Introduction

2. Doppler radar

3. Frequency-modulated CW radar

4. Terrain-following radar (TFR)

5. CW illuminationExercises




1. Introduction

Pulsed radar is typically used to detect targets,determining range and bearing. These radarsgenerally require high-power, are quite complexand thus expensive.

Continuous wave (CW) radars typicallydetermine target velocity, and can achieveconsiderable ranges without the high peakpower. These radars are typically simpler, morecompact and less costly.




2. Doppler radar

Recall that the Doppler effect is the change infrequency 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.




2.1.1 Doppler radar theory

Depicted below is a Doppler radar with transmitwavelength λt and period T t . As a closing target

approaches at velocity v , the radar will observe ashift in return wavelength, λr as a function of v .

λr = λt – 2vT t




2.1.2 Why 2vT t ?




2.1.3 Doppler radar line of sight




2.1.4 Doppler radar velocity

Substituting frequency for wavelength andconsidering direction of target to line of sight,

yields a general expression for Doppler velocity.

v = c(1- f t / f r ) / (2 cos( ))




2.2 Doppler “navigator” radar

lamda configuration




3. FM-CW radar

An unmodulated CW radar is incapable ofdetecting range, as there is no reference point inthe transmitted or returned signal for measuringelapsed time.

By frequency modulating the CW signal,differences between the transmitted and

received frequencies can be used to estimaterange.

The further the target, the larger the frequencydifference.




3.1.1 FM-CW radar theory

The modulation parameters are frequencydeviation, f, and modulation period, Tm .





closing target





Given an FM-CW radar with triangular frequencymodulation of f m and frequency deviation f , the

range of a stationary target can be derived asfollows:

f b = t r df t /dt , where the round-trip transit time, t r = 2R/c ,

and the changing transmit frequency, df t /dt = 4f m f .

Therefore

f b =(8Rf m f/c), or R = cf b /(8f m f)





Recall that the range resolution of a radar is ameasure of its ability to distinguish closelyspaced targets. The range resolution of a FM-

CW radar is a function of its modulatingbandwidth, and is c/(4f).

The range ambiguity is the range beyond which

the radar yields ambiguous range results. Therange ambiguity of a FM-CW radar is a functionof its modulating frequency, and is cTm.

This is usually well beyond the signal range.




3.2 FM-CW radar architecture




4.1 Radar altimeter

Triangular FM-CW radar iscommonly used in aircraft to

determine the instantaneousaltitude above the terrain it isflying.




4.2 Terrain-following radar




4.3 CW illumination

Used in conjunction withsemi-active missiles. The

aircraft radar “illuminates”the target, while the missileuses the received returnsignal to track the target.

What are the advantagesand disadvantages?




In-class exercises




exercise 1

Recalling the radar range equation, why is itpossible for a CW radar to achieve much greater

ranges than a pulsed radar?

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




Doppler calculation

Just after take-off you realize that you are following amilitary CC-138 (Twin Otter) in a Cessna 152. Your airspeed 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?




Radar altimeter calculation

An aircraft is equipped with an FM-CW radaraltimeter 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 60kHz, 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?

Lectures Radar3

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