RADAR and GNSS Associate Associa te Professor Profe ssor Vu Van Van Yem, Yem, Ph.D. 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
PART I- RADA RADAR R A- Basic radar theory
Outline 1. 2. 3. 4. 5. 6.
Principles of radar Radar antenna Radar modes Pulsed radar Doppler radar FM-CW radar
1. Principles of radar
1.1 A radar operator view
1.2 Brief history of radar
Conceived as early as 1880 by Heinrich Hertz
Radio Aid to Detection And Ranging 1930s
Observed that radio waves could be reflected off metal objects.
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
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.
1.3 Airborne radar bands
1.3.1 Airborne radar bands
1.3.2 Airborne radar bands
Radar Frequency Band
1.4 Basic principle of radar
target range, R = c t / 2
1.4.1 Basic principle of radar
Two common transmission techniques: pulses continuous 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 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” 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
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
3.2
Air-to-air tracking
3.3
Air-to-air track-while-scan
3.4
Ground mapping
3.5
Continuous wave illumination
3.6 Multimode
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.. frequency Denote: pulse width, , usually expressed in sec PRF,, usually in kHz pulse repetition frequency, PRF pulse period, Tp = 1/PRF, usually in sec
4.1 Pulsed radar architecture
4.1.1 A lab-based pulsed radar
4.2 Pulsed modulation
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 range resolution 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 r ange if: Lp < 2(R2 – R1)
4.3.1 Pulsed radar range resolution
4.3.2 Pulsed radar range resolution
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) = cT p / 2
4.4 Pulsed radar range ambiguity
This figure is very confusing.
4.4.1 Range ambiguity Ramb
return time PRF
A target whose range is:
R < Ramb = c / (2 PRF) = cTp / 2 0
10
20
30
4.4.2 Range ambiguity Ramb
return time PRF
A target whose range is :
R > Ramb = c / (2 PRF) = cTp / 2 0
10
20
30
4.4.3 Range ambiguity Ramb
PRF
?
Which target is which? 0
10
20
30
4.5 Angle resolution
5. Target tracking
A target that is tracked tracked is said to be “locked on”; key data to maintain on locked targets is: range, 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
5.2 Angle tracking – sequential lobing
5.3 Angle tracking – sequential lobing
5.4 Angle tracking – conical scan
5.5 Angle tracking – monopulse
5.6 Angle tracking – monopulse
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?
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%
