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Abstract
It was German engineer Christian Huelsmeyer who first used the radar principle to
build a simple ship detection device intended to help avoid collisions in fog
(Reichspatent Nr. 165546).
First widely used radar technology was developed for military purpose during World
War II. Today, more than half a century later, there is a much wider radar application
area beyond the military one. Radar is needed for weather forecast, airport traffic
control and automotive applications such as car distance surveillance and pedestrian
detection. Additionally radar technology today is affordable on a mass production basis
due to highly integrated signal processing components which make it possible to detect
even low power signals in applications where at former times much more RF energy
was needed. Low power radar components automatically mean savings in costs and
size. In addition there are a lot of CAD tools available for the development of such
systems and to deal with higher frequencies up to 110 GHz and beyond.
R&S created two complementary papers, application note 1MA127 and white paper
1MA207 regarding current radar technology in order to demonstrate its contribution to
test and measurement of radar systems and components. The white paper gives an
overview on radar Systems and important measurements on them. The corresponding
application note 1MA127 goes into details in explaining radar test technology along
with the specific products needed to perform the tests. Both documents, 1MA127 and
1MA207 are addressing students who want to become familiar with radar issues as
well as radar professionals who want to solve certain test and measurement tasks.
Typical radar applications
Typical radar applications are listed here to give an idea of the huge importance of
radar in our world.
Surveillance
Military and civil air traffic control, ground-based, airborne, surface coastal, satellitebased
Searching and tracking
Military target searching and tracking
Fire control
Provides information (mainly target azimuth, elevation, range and velocity) to a firecontrol
system
Navigation
Satellite, air, maritime, terrestrial navigation
Automotive
Collision warning, adaptive cruise control (ACC), collision avoidance
Level measurements
For monitoring liquids, distances, etc.
Proximity fuses
Military use: Guided weapon systems require a proximity fuse to trigger the explosive
warhead
Altimeter
Aircraft or spacecraft altimeters for civil and military use
Terrain avoidance
Airborne military use
Secondary radar
Transponder in target responds with coded reply signal
Weather
Storm avoidance, wind shear warning, weather mapping
Space
Military earth surveillance, ground mapping, and exploration of space environment
Security
Hidden weapon detection, military earth surveillance
Common Radar Types
This section lists the most common types of radar systems with brief explanations of how they
work.
4.1 CW (Doppler) and FMCW (Doppler (Speed)/Range)
Radar)
A continuous wave (CW) radar system with a constant frequency can be used to measure speed.
However, it does not provide any range (distance) information. A signal at a certain frequency is
transmitted via an antenna. It is then reflected by the target (e.g. a car) with a certain Doppler
frequency shift. This means that the signal's reflection is received on a slightly different
frequency. By comparing the transmitted frequency with the received frequency, we can
determine the speed (but not the range). Here, a typical application is radar for monitoring traffic.
Radar motion sensors are based on the same principle, but they must also be capable of
detecting slow changes in the received field strength due to variable interference conditions that
may exist.
Radar speed traps operated by the police use this same technology. Camera systems take a
picture if a certain speed is exceeded at a specified distance from the target.
There are also military applications:
CW radars are also used for target illumination. This is a straightforward application: The radar
beam is kept on target by linking it to a target tracking radar. The reflection from the target is then
used by an antiaircraft missile to home in on the target.
CW radars are somewhat hard to detect. Accordingly, they are classified as low-probability-ofintercept
radars.
CW radars lend themselves well to detecting low-flying aircraft that attempt to overcome an
enemy's air defense by "hugging the ground". Pulsed radar has difficulties in discriminating
between ground clutter and low-flying aircraft. CW radar can close this gap because it is blind to
slow-moving ground clutter and can pinpoint the direction where something is going on. This
information is relayed to co-located pulse radar for further analysis and action. [7]
FMCW radar
The disadvantage of CW radar systems is that they cannot measure range due to the lack of a
timing reference. However, it is possible to generate a timing reference for measuring the range
of stationary objects using what is known as "frequency-modulated continuous wave" (FMCW)
radar. This method involves transmitting a signal whose frequency changes periodically. When
an echo signal is received, it will have a delay offset like in pulse radar. The range can be
determined by comparing the frequency. It is possible to transmit complicated frequency patterns
(like in noise radar) with the periodic repetition occurring at most at a time in which no ambiguous
echoes are expected. However, in the simplest case basic ramp or triangular modulation is used,
which of course will only have a relatively small unambiguous measurement range.
Pulse Doppler radar
A pulse Doppler radar also provides radial speed information about the target in addition to range
information (and direction information). In case of coherent operation of the radar transmitter and
receiver, speed information can be derived from the pulse-to-pulse phase variations. I/Q
demodulators are normally used. The latest pulse Doppler radar systems normally use different
pulse repetition frequencies (PRF) ranging from several hundred Hz up to 500 kHz in order to
clarify any possible range and Doppler ambiguities.
More advanced pulse Doppler radar systems also " use "staggered PRF, i.e. the PRF changes
on an ongoing basis to get rid of range ambiguity and reduce clutter as well.
Important criteria for achieving good performance in pulse Doppler radar systems include very
low phase noise in the LO, low receiver noise and low I/Q gain phase mismatch (to avoid "false
target indication") in addition to the measurement parameters listed above.
When measuring the pulse-to-pulse performance of a radar transmitter, it is important to
understand the variables that can impact the uncertainty of the measurement system for accurate
Doppler measurements:
Signal-to-noise ratio of the signal - the better the signal to noise ratio of the signal, the lower
the uncertainty due to noise contribution.
Bandwidth of the signal - the bandwidth of the IF acquisition system must be sufficient to
accurately represent the risetime of the pulsed signal, however too much bandwidth can
result in added noise contribution uncertainty.
Reference (or timebase) clock stability.
Jitter or uncertainty due to the measurement point of the rising edge of the signal ñ rising
edge interpolation or signals that have changing edges impact this uncertainty.
Overshoot and preshoot of the rising and falling edges ñ any ringing on the rising and falling
edges can impact the measurement points adversely on a pulse to pulse basis. It is important
that the measurement point, or the average set of measurement points, are sufficiently far
away in time from the leading and falling edges of a pulse. Applying a Gaussien filter to
smooth the impact of the rising and falling edges can reduce this phenomena and is often
implemented in the Doppler measurement system of a radar receiver.
Time between measured signals ñ due to the PRI of the measured signal, the close-in phase
noise of the measurement system needs to be considered due to the integration time at
lower offset frequencies.
The same variables can also contribute to the uncertainty in the signal generator when
testing the receiver circuit and Doppler measurement accuracy
Pulse Compression Radar (FM Chirp and Phase Coded)
Classic pulse and pulse Doppler radar transmits extremely short pulses. Increasing the pulse
power allows the radar system to achieve greater range results. Decreasing the duration of the
transmit pulses also decreases the pulse volume and provides better range resolution for the
radar system, i.e. closely spaced targets can be distinguished with smaller distances between
them.
Pulse compression combines the power-related benefits of very long transmit pulses (good
range) with the benefits of very short transmit pulses (high distance resolution). Lower peak
power can then be used.
By modulating the transmit pulses, a timing reference is produced within the transmit pulse,
similar to frequency-modulated continuous wave (FMCW) radar systems. Several different
modulation techniques can be used. The most common are:
Linear frequency modulation (FM chirp)
Non-linear frequency modulation
Encoded pulse phase modulation (e.g. Barker code)
Polyphase modulation and time-frequency coded modulation
Although pulse compression technique has various benefits such as low pulse power with good
range and distance resolution, there is a significant disadvantage: The minimum measurement
range is degraded depending on the pulse length, since the radar receiver is blocked during the
transmit pulse. As this is a major disadvantage for radar systems used for air traffic control, they
typically use both techniques: Between the frequency-modulated pulses for the larger range,
small (very short) pulses are transmitted which only have to cover the nearby area and do not
require very high pulse power.
Linear FM is most common in older radar systems. An example is the air-defense radar RRP-
117 [4][4].
Non-linear FM (NLFM) is becoming more practical use because of its various benefits such
as inherently low range sidelobes which yields an advantage in SNR compared to Linear FM.
[16]
Encoded pulse phase modulation is very common, particularly Barker codes with lengths of
11 and 13 [15].
In advanced military radar systems, polyphase pulse compression is also used increasingly
with special codes [14].
Pulse compression radar signal require baseband IQ collection of the signal covering the BW of
the pulse risetime, wideband analog FM demodulation or vector demodulation and new displays
of the information for analysis (amplitude, frequency, and phase vs. time), and digital
demodulation/EVM measurement for BPSK/QPSK modulations.