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:[email protected]
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%