RADAR and GNSS

Associate 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.vn

Ha Noi – January 2012

2/27/2012 1 RADAR

PART I- RADAR

A- Basic radar theory

Lecture on Radar

Outline

1. Principles of radar

2. Radar antenna

3. Radar modes

4. Pulsed radar

5. Doppler radar

6. 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.

1940 Invention of the magnetron permits high power

transmission at high frequency, thus making airborne radar possible.

Lecture on Radar

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

Lecture on Radar

1.3.1 Airborne radar bands

Lecture on Radar

1.3.2 Airborne radar bands

Lecture on Radar

Radar Frequency Band

Lecture on Radar

1.4 Basic principle of radar

target range, R = ct / 2

Lecture on Radar

1.4.1 Basic principle of radar

Two common transmission techniques:

pulses

continuous wave

Lecture on Radar

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.

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

Lecture on Radar

3.2 Air-to-air tracking

Lecture on Radar

3.3 Air-to-air track-while-scan

Lecture on Radar

3.4 Ground mapping

Lecture on Radar

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: pulse width, , usually expressed in sec

pulse repetition frequency, PRF, usually in kHz

pulse period, Tp = 1/PRF, usually in sec

Lecture on Radar

4.1 Pulsed radar architecture

Lecture on Radar

4.1.1 A lab-based pulsed radar

Lecture on Radar

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 = _____ dBW

Lecture 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

Lecture on Radar

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 / 2

Lecture on Radar

4.4 Pulsed radar range ambiguity

This figure is very confusing.

Lecture on Radar

4.4.1 Range ambiguity

0 10 20 30

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

PRF

Ramb

return time

Lecture on Radar

4.4.2 Range ambiguity

0 10 20 30

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

PRF

Ramb

return time

Lecture on Radar

4.4.3 Range ambiguity

0 10 20 30

Which target is which?

PRF

Ramb

?

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

monopulse

Lecture on Radar

5.1 Range tracking - range gating

Lecture on Radar

5.2 Angle tracking – sequential lobing

Lecture on Radar

5.3 Angle tracking – sequential lobing

Lecture on Radar

5.4 Angle tracking – conical scan

Lecture on Radar

5.5 Angle tracking – monopulse

Lecture on Radar

5.6 Angle tracking – monopulse

Lecture on Radar

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?

In-class exercises

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%