Lectures Radar1 Hocvien

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RADAR and GNSSAssociate Professor Vu Van Yem, Ph.D. Vice Dean Head of Department of Telecommunication Systems, School of Electronics and Telecommunications, Deputy Director of the Center for Innovation Technology,Hanoi University Of Science and Technology

Email:yemvv-fet@mail.hut.edu.vnHa Noi January 20122/27/2012 RADAR 1

PART I- RADARA- Basic radar theory

Outline1.2.

3.4. 5. 6.

Principles of radar Radar antenna Radar modes Pulsed radar Doppler radar FM-CW radar

Lecture on Radar

1. Principles of radar

Lecture on Radar

1.1 A radar operator view

Lecture on Radar

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.of the magnetron permits high power transmission at high frequency, thus making airborne radar possible.Lecture on Radar

1940 Invention

1.2.1 Brief history of radar

Currently Radar

is the primary sensor on nearly all military aircraft. Roles include airborne early warning, target acquisition, target tracking, target illumination, ground mapping, collision avoidance, weather warning. Practical frequency range 100MHz-100GHz.Lecture on Radar

1.3 Airborne radar bands

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1.3.1 Airborne radar bands

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1.3.2 Airborne radar bands

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Radar Frequency Band

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1.4 Basic principle of radar

target

range, R = ct / 2Lecture on Radar

1.4.1 Basic principle of radar

Two common transmission techniques: pulses

continuous

wave

Lecture on Radar

2. Radar antennaA 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.Lecture on Radar

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.

Lecture on Radar

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).

Lecture on Radar

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

Lecture on Radar

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

Lecture on Radar

3.1 Air-to-air

search

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3.2

Air-to-air tracking

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3.3

Air-to-air track-while-scan

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3.4

Ground mapping

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3.5

Continuous wave illumination

Lecture on Radar

3.6 Multimode

Lecture on Radar

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

Lecture on Radar

4.1 Pulsed radar architecture

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4.1.1 A lab-based pulsed radar

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4.2 Pulsed modulation

Lecture on Radar

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/Lecture on Radar

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 = _____ dBWLecture on Radar

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)Lecture on Radar

4.3.1 Pulsed radar range resolution

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4.3.2 Pulsed radar range resolution

Lecture on Radar

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 / 2Lecture on Radar

4.4 Pulsed radar range ambiguity

This figure is very confusing.

Lecture on Radar

4.4.1 Range ambiguityRamb

return time PRF

A target whose range is:R

< Ramb = c / (2 PRF) = cTp / 20 10 20 30

Lecture on Radar

4.4.2 Range ambiguityRamb

return timePRF

A target whose range is :R

> Ramb = c / (2 PRF) = cTp / 20 10 20 30

Lecture on Radar

4.4.3 Range ambiguityRamb

PRF

Which target is which?0 10

?20 30

Lecture on Radar

4.5 Angle resolution

Lecture on Radar

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 monopulseLecture on Radar

5.1 Range tracking - range gating

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5.2 Angle tracking sequential lobing

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5.3 Angle tracking sequential lobing

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5.4 Angle tracking conical scan

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5.5 Angle tracking monopulse

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5.6 Angle tracking monopulse

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In-class exercises

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?

Lecture on Radar

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%

Lecture on Radar