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Keysight Technologies
Techniques for Precision Validation of Radar
System Performance in the Field
Using FieldFox handheld analyzers
Application Note
Introduction
This application note provides an overview ating a two-way data link between a ground When maintaining and troubleshooting
of field testing radar systems and Line station and an aircraft. Secondary radar radar systems and components in the
Replaceable Units (LRU) using high- originated from the Identification Friend or field, it is often necessary to measure
performance FieldFox combination analyzers Foe (IFF) radar system developed during both the time domain and frequency
having multiple measurement modes includ- World War II and complements the limita- domain performance over a variety of test
ing a peak power analyzer, vector network tions of the primary radar. Modern beacon conditions. While traditional methods for
analyzer, spectrum analyzer and vector systems, such as the Air Traffic Control measuring time and frequency performance
voltmeter. This application note will show Radar Beacon System (ATCRBS), separate of radar systems included 3-4 different
several measurement examples of pulsed the interrogation and reply frequencies benchtop instruments, modern "all-in-one"
and secondary radar signals and also reviews resulting in stronger received signal levels at or combination analyzers provide the most
the basics of monopulse radar. the ground station and improved weather- convenient and economical solution to
related performance. field testing. This application note provides
Modern radar systems are typically classi- an overview of field testing radar systems
fied as ground-based, airborne, ship-based For example, figure 1a shows a field and Line Replaceable Units (LRU) using
or space borne. Radars have numerous measurement of a beacon interroga- high-performance FieldFox combination
applications including civilian air-traffic tion waveform captured using a peak analyzers having multiple measurement
control, meteorology, traffic enforcement power sensor and FieldFox analyzer. The modes including a peak power analyzer,
and military air defense. Key aspects of any waveform includes coded-pulse pairs for vector network analyzer, spectrum analyzer
radar system include frequency of operation, requesting aircraft identity and altitude. This and vector voltmeter. This application note
waveform characteristics and antenna type. time domain measurement display shows will show several measurement examples
Unmodulated continuous wave (CW) radars the pulse profile as a function of time and of pulsed and secondary radar signals and
can measure target velocity and angular includes a table for peak power, average also reviews the basics of monopulse radar
position. Range information is typically power, pulse width and rise and fall times. starting in the next section.
extracted using some form of modulation Figure 1b shows the measured spectrum
such as a pulsed waveform. These types of a radar transmitter using a rectangular
of "primary" radars work by transmitting a pulsed waveform. This frequency domain
waveform that is reflected off the target's measurement can be used to determine
surface and then these echoes are mea- the center frequency of the RF carrier as
sured at the radar's receiver. There are other well as the absolute amplitude of individual
types of secondary or "beacon radars" cre- frequency components.
(a) Beacon waveform in time (b) Spectrum of a radar pulse
Figure 1. (1a) Time domain measurement of a beacon interrogation waveform and (1b) frequency domain measurement of a pulsed radar
signal
2
Monopulse radar basics
One of the most widely used radar tech- that is perpendicular to the antenna plane. difference channels otherwise errors in
niques for deriving the angular information This beam direction is often called the angle calculations will occur. A low-noise
of a target is the monopulse system. The boresight of the antenna. This transmitted stable local oscillator, or STALO, provides
monopulse technique can estimate these signal illuminates the target which returns the signal source for the downconversion.
angles with higher accuracy than compa- a reflected signal. The receive antenna
rable systems while using a single (mono) simultaneously creates two overlapping pat- One issue with this basic monopulse system
pulse measurement in time. Figure 2 shows terns referred to as the "sum" (sigma) and occurs at short ranges when the antenna
a simplified block diagram of a monopulse "difference" (delta) patterns. As shown in sidelobes may receive signals high enough
radar with capability for determining target the figure, the sum pattern maintains a peak to exceed the detection threshold and
angle in either elevation or azimuth. The in the boresight direction and the difference incorrectly report a target. The next section
transmitter creates a pulsed waveform that pattern contains a null in the boresight reviews a technique for suppressing any
is applied to a duplexing network, such direction. In this figure, the antenna pattern large amplitude signals that may enter
as a circulator or switch, which directs sidelobes are omitted for simplicity. through the sidelobes of the antenna pattern.
the high power signal to the antenna. If
the antenna is mechanically rotated, the The received signals from the sum and dif-
connection between the transmitter and the ference antenna ports are downconverted
antenna is managed through a rotary joint. and measured by the radar's signal process-
The transmit signal is applied to the "sum" ing subsystem for target detection. It is very
(sigma) port of the antenna assembly which important that amplitude and phase tracking
ideally creates a thin beamwidth pattern is tightly controlled between the sum and
Transmitter
D
Duplexer
IF RF Rotary Monopulse
Receiver Receiver Joint Antenna S
Antenna
STALO Patterns
Figure 2. Simplified block diagram of a monopulse radar system
3
Monopulse radar with sidelobe suppression
When there is a possibility that false detec- Figure 3 includes a representation of the is called Sidelobe Suppression (SLS). The
tions can result from energy entering the sidelobes in the sum pattern. In the signal secondary channel in figure 3 also shows
sidelobes in the monopulse antenna pattern, processor, the outputs from the sum, differ- a second transmitter connected to the
a secondary "omnidirectional" antenna may ence and omega channels are compared omnidirectional antenna through a separate
be added to the system to improve the over- and those signals having higher power duplexer. This auxiliary transmitter is impor-
all detection performance. Figure 3 shows level in the omega channel relative to the tant to beacon systems when attempting to
the addition of a secondary receiver which sum channel are assumed to be signals identify the location of an aircraft relative
includes the omnidirectional antenna with coming from the antenna's sidelobe. The to the ground station. The next section
pattern labeled with an "omega" symbol. total receiver gain of the omega channel shows an application for using this auxiliary
The antenna gain of the omnidirectional can also be adjusted and also used to transmitter in air traffic control radar.
antenna is lower than the peak gain of cancel the undesired energy received from
the sum pattern and this gain difference the sidelobe. The technique of adding the
will be useful when determining if a target secondary (omega) channel to improve the
is within the boresight of the antenna. performance of a monopulse radar system
Transmitter
Duplexer
IF RF Rotary Monopulse
Receiver Receiver Joint Antenna
STALO
IF RF Duplexer Rotary Omni
Receiver Receiver Joint Antenna Antenna
Patterns
Transmitter
Figure 3. Simplified block diagram of a monopulse radar system with sidelobe suppression (SLS)
4
Application of sidelobe suppression to Radar Beacon System
A typical Air Traffic Control Radar Beacon a 21 microsecond spacing (Mode C). The at the boresight of the antenna system
System (ATCRBS) is based on the similar P1/P3 pulse sequence is transmitted by the (position #1). When the aircraft is located
block diagram to the monopulse system high-gain sum (sigma) antenna. off boresight, shown as position #2 in figure
previously discussed in figure 3. The beacon 4, the received P1 amplitude no longer
system is a two-way "data link" between To avoid undesired replies from aircraft tran- exceeds P2 and the aircraft transponder will
a ground station and a transponder that is sponders receiving energy from a sidelobe not reply to any ground station requests. As
installed onboard the aircraft. The data link of the sum antenna, the ground station the antenna system is mechanically rotated
begins when the ground station transmits transmits a secondary pulse, shown in figure in azimuth, the aircraft at location #2 will
an interrogation signal requesting the air- 4 as the P2 pulse. This secondary pulse is eventually enter the main beam and prop-
craft's identification or altitude. The aircraft transmitted through the omnidirectional erly reply to ground station interrogations.
transponder replies with the requested (omega) antenna. The timing relationship
data. The ground station-to-transponder places the P2 pulse between P1 and P3. As During installation, periodic maintenance
transmissions occur at a carrier frequency all the pulses are transmitted on the same and troubleshooting of this or any radar
of 1030 MHz. The transponder-to-ground 1030 MHz carrier, the aircraft transponder system, it is often required to field test and
station replies are transmitted at a carrier will receive these waveforms as a single tune the numerous functional blocks, also
known as Line Replaceable Units (LRU),
frequency of 1090 MHz. Data is encoded time sequence which can be used to
that make up the radar. Because of the
onto the RF carriers in the form of pulsed compare the relative pulse amplitudes. If
unique amplitude and phase relationships
sequences 1. Figure 4 shows the transmitted the aircraft is located near the boresight between the various channels in a mono-
and received data sequences for the ground of the antenna system, the received P1 pulse system, testing LRUs often requires
station-to-transponder link. The pulse pairs, amplitude will exceed the P2 amplitude, as coordinating and comparing waveforms in
P1 and P3, are transmitted at specific time the antenna gain of the sum beam is much the time and frequency domains. The next
intervals denoting whether aircraft identifi- higher than the gain of the omnidirectional section of this application note will review
cation or altitude information is requested. antenna. Under these conditions, the air- the various domains and measurements
For example, identification requests (Mode craft transponder will reply to the ground required to test the operation of the LRUs in
A) use a relative spacing between P1 and station. Figure 4 shows the received pulse the field.
P3 of 8 microseconds. Altitude requests use sequence when the aircraft is positioned
P1 P3
time
Monopulse
Antenna
Antenna #1 P1 P3
Patterns
P2 P2
Omni
Antenna
time #2
P2
P1 P3
Figure 4. Block diagram of an Air Traffic Control Radar Beacon System (ATCRBS) showing transmit pulsed waveforms and the respective
received waveforms at the aircraft transponder
5
Time and frequency domain measurements
When field testing LRUs of any radar system, (a) Absolute
time domain
there is typically a set of basic measurements
measurement using
that must be made in both the time and peak power meter
frequency domains. Time and frequency
measurements result in absolute and relative
type measurements. For example, figure 5a
shows a time domain measurement of the
peak power of a pulsed radar signal. This
measurement was made using a peak power
sensor connected to a peak power meter. A
marker is used to measure the peak amplitude
at a specific point within the pulse. Absolute
measurements may also be made in the fre-
quency domain using a variety of instrument
types including a spectrum analyzer, vector
network analyzer (VNA) and vector voltmeter
(VVM). For example, figure 5b shows the
measured spectrum, the pulsed radar signal
and a marker is used to measure the ampli-
tude at a specific frequency. LRUs that contain
their own signal source are typically measured (b) Absolute
using a spectrum analyzer. VNAs and VVMs frequency domain
measurement using
are typically used to measure the amplitude
spectrum analyzer
and phase of transmission paths which may
include cables, filters and amplifiers. (c) Relative
time domain
Relative time domain measurements are also measurement using
peak power meter
made using a peak power sensor and peak
power meter. Figure 5c shows the relative
measurements between two points in time.
This type of measurement is useful for char-
acterizing timing features such as pulse width,
rise time, fall time and pulse repetition interval
(PRI) to name a few. Relative frequency
domain measurements can be performed
using a spectrum analyzer, VNA and VVM.
Figure 5d shows the relative amplitude
(insertion loss) between two different coaxial
cables. Along with relative amplitude, the
relative phase between multiple channels,
is an important measurement in monopulse
radar systems and will be discussed later in
this application note. It is worth noting that
(d) Relative
all the measurements shown in figure 5 were
frequency domain
captured using a single FieldFox analyzer measurement using
with multi-function capability. When making vector network
measurements in the field and/or challenging analyzer
test environments, selecting the appropriate
instrument types is critical to successful and Figure 5. Time and frequency domain measurements of radar signals and radar components
accurate results.
6
Instrumentation for ield testing
With the numerous measurement combina- Another option for field testing would be to When using FieldFox as a substitution for
tions required to fully characterize LRUs in replace the multiple benchtop instruments benchtop instruments, it is important to
a radar system, it is important to compare with a single "all-in-one" FieldFox analyzer. note that technology breakthroughs have
the choices between benchtop and modern FieldFox was specifically designed for field enabled high-performance measurement
handheld analyzers when installing, main- testing having a fully sealed enclosure that capabilities in the handheld analyzer that
taining and troubleshooting radar systems is compliant with US MIL-PRF-28800F Class are comparable to benchtop instruments.
in the field. For example, to characterize a 2 requirements to ensure durability in harsh It has been shown that measurements
commercial aviation radar system, the instru- environments. FieldFox includes a peak using FieldFox correlate well to benchtop
ment list includes a peak power sensor and power meter, spectrum analyzer, VNA and instruments often within hundredths of a dB.
meter, spectrum analyzer, VNA and VVM. As VVM all in a six pound instrument. At the Keysight Technologies, Inc. provides a very
most benchtop equipment was designed for test site, FieldFox includes a unique feature, informative application note that details the
indoor laboratory environments, the test site named InstAlign, that allows the spectrum correlation between handheld and benchtop
must have the adequate weather protection analyzer mode to make accurate measure- instruments 2.
to guarantee the safety of the equipment ments immediately at turn on and also
against harsh weather conditions. For the automatically corrects the measurements for
highest measurement accuracy, the equip- any temperature changes over a range of
ment typically requires a minimum of 30 -10