radar part1

EEE381BAerospace Systems & Avionics

RadarPart 1 – Basic radar theoryRef: Moir & Seabridge 2006, Chapter 3

Dr Ron Smith

Winter 2009 Basic radar theory - 2EEE381B

Outline

1. Principles of radar2. Radar antenna3. Radar modes4. Pulsed radar5. Doppler radar6. FM-CW radar7. Exercises

1. Principles of radar [4]

1.1 A radar operator view [4]

1.2 Brief history of radar

Conceived as early as 1880 by Heinrich Hertz Observed that radio waves could be reflected off metal

objects. Radio Aid to Detection And Ranging 1930s

Britain built the first ground-based early warning system called Chain Home.

1940 Invention of the magnetron permits high power transmission

at high frequency, thus making airborne radar possible.

1.2.1 Brief history of radar

CurrentlyRadar is the primary sensor on nearly all

military aircraft.Roles include airborne early warning, target

acquisition, target tracking, target illumination, ground mapping, collision avoidance, altimeter, weather warning.

Practical frequency range 100MHz-100GHz.

1.3 Airborne radar bands [1]

1.3.1 Airborne radar bands [1]

1.3.2 Airborne radar bands [1]

1.4 Basic principle of radar[1]

target range, R = ct / 2

1.4.1 Basic principle of radar[1]

Two common transmission techniques:pulsescontinuous wave

2. Radar antenna

A basic principle of radar is that it directs energy (in the form of an EM wave) at its intended target(s).

Recall that the directivity of an antenna is measured as a function of its gain.

Therefore antenna types most useful for radar applications include parabolic and array antenna.

2.1 Parabolic (dish) antenna

Early airborne radars typically consisted of parabolic reflectors with horn feeds. The dish effectively directs the

transmitted energy towards a target while at the same time “gathering and concentrating” some fraction of the returned energy.

2.2 Planar (phased) array antenna

Recent radars more likely employ a planar array It is electronically steerable as

a transmit or receive antenna using phase shifters.

It has the further advantage of being capable of being integrated with the skin of the aircraft (“smart skin”).

2.3 Radar antenna beam patterns

The main lobe of the radar antenna beam is central to the performance of the system. The side lobes are not only wasteful, they provide

electronic warfare vulnerabilities.

3. Airborne radar modes

Airborne radars are designed for and used in many different modes. Common modes include: air-to-air search air-to-air tracking air-to-air track-while-scan (TWS) ground mapping continuous wave (CW) illumination multimode

3.1 Air-to-air search [1]

3.2 Air-to-air tracking [1]

3.3 Air-to-air track-while-scan [1]

3.4 Ground mapping [1]

3.5 Continuous wave illumination

3.6 Multimode [1]

4. Pulsed radar

A pulsed radar is characterized by a high power transmitter that generates an endless sequence of pulses. The rate at which the pulses are repeated is defined as the pulse repetition frequency.

Denote: pulse width, , usually expressed in sec pulse repetition frequency, PRF, usually in kHz pulse period, Tp = 1/PRF, usually in sec

4.1 Pulsed radar architecture [1]

4.1.1 A lab-based pulsed radar [4]

4.2 Pulsed modulation [1]

4.2.1 Pulsed radar bandwidth

In the frequency domain, the transmitted and received signals are composed of spectral components centered on the radar operating frequency, f0, with a sin(x)/x shape.

The practical limits of the frequency response is f0 1/,

and therefore the bandwidth of the receiver must be at least:

BWRx ≥ 2/

4.2.2 Pulsed radar average power

Since a pulsed radar only transmits for a small portion of the time, the average power of the radar is quite low:

Pav = Ppeak / Tp

For example a pulsed radar with a 1 sec pulse width and a medium PRF of 4 kHz that transmits at a peak power of 10kW transmits an average power of:

Pav = (10000 W) (0.000001 sec) (4000 /sec) = _____ W = _____ dBW

4.3 Pulsed radar range resolution

The range resolution of a radar is its ability to distinguish two closely spaced targets along the same line of sight (LOS). The range resolution is a function of the pulse length, where pulse length, Lp = c. For example, a 1 sec pulse width yields a pulse

length of 0.3 km. Two targets can be resolved in range if:

Lp < 2(R2 – R1)

4.3.1 Pulsed radar range resolution [4]

4.3.2 Pulsed radar range resolution [4]

4.4 Pulsed radar range ambiguity

The PRF is another key radar parameter and is arguably one of the most difficult design decisions.

The range of a target becomes ambiguous as a function of half the pulse period; in other words targets that are further than half the pulse period yield ambiguous range results.

Ramb = c / (2 PRF) = cTp / 2

4.4 Pulsed radar range ambiguity [1]

This figure is very confusing.

4.4.1 Range ambiguity

0 10 20 30

A target whose range is: R < Ramb = c / (2 PRF) = cTp / 2

return time

0 10 20 30

A target whose range is : R > Ramb = c / (2 PRF) = cTp / 2

return time

0 10 20 30

Which target is which?

4.5 Angle resolution[4]

5. Target tracking

A target that is tracked is said to be “locked on”; key data to maintain on locked targets is: range, azimuth and elevation angle.

A frame of reference using pitch and roll from aircraft attitude indicators is required for angle tracking. Three angle tracking techniques are: sequential lobing conical scan monopulse

5.1 Range tracking - range gating [1]

5.2 Angle tracking – sequential lobing1

5.3 Angle tracking – sequential lobing1

5.4 Angle tracking – conical scan[1]

5.5 Angle tracking – monopulse[1]

5.6 Angle tracking – monopulse[1]

6. In-class exercises

6. 6.1 Quick response exercise # 1

Explain the strange shapes on top of these two aircraft, E3 Sentry and AH-64 Longbow Apache [1]

6.2 Quick response exercise # 2

Given a 10.5 GHz intercept radar and a transmitter capable of providing a peak power of 44 dBW at a PRF of 2 kHz: What pulse width yields an average power of 50W? What is the bandwidth in MHz and in % of this

signal?

6.3 Pulsed radar calculations

Design the pulse parameters so as to achieve maximum average power for an unspecified Ku band pulsed radar given the following component specifications and system requirements: the receiver has a bandwidth of at least 0.5% across the band the required range resolution is 50m The required range ambiguity is 25 km For cooling purposes, ensure that the duty cycle of the

transmitter does not exceed 0.2%

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) Mark A. Hicks, "Clip art licensed from the Clip Art Gallery on

DiscoverySchool.com"

radar part1

radar applications

radar operator

basic radar theory ref

pulsed radar bandwidth

pulsed radar architecture

airborne radar bands

airborne radar possible

pulsed radar average

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