Text preview for : Radar_252C EW & ELINT Testing_ Identifying Common Test Challenges - Application Note 5990-7036EN c20 part of
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Radar, EW & ELINT Testing:
Identifying Common Test Challenges
Application Note
Introduction
The roots of modern radar systems stretch back to 1940 when the U.S. Navy developed what was
then called radio detection and ranging or RADAR. Today, this technology has been adapted to
applications that range from the ubiquitous supermarket door opener, which is a simple moving-
target indicator (MTI), to highly complex shipboard phased-array fire-control radars.
In military applications, two new fields followed close on the heels of radar: electronic intelligence
(ELINT) and electronic warfare (EW). ELINT is used to extract information from enemy radar sys-
tems and provide insights into coping with potential threats attached to those radar signals: ships,
aircraft, missiles, and so on. The associated development of EW technologies provides active and
passive responses to those potential threats.
From the simplest to the most complex, all radar, EW and ELINT systems pose a variety of chal-
lenges when it comes time to test components, assemblies and systems. To complicate matters,
all radars operate in an increasingly cluttered spectral environment. For example, the airwaves
in an urban setting may include countless wideband RF and microwave emitters--and therefore
potential interferers--such as wireless communications infrastructure, wireless networking systems
and civilian radars.
This application note focuses on test equipment that will help you address the challenges you're
most likely to face during system development. To provide context, the note starts with a review
of radar, EW and ELINT basics. After providing an overview of key test challenges, the remainder
of this note covers three main topics: the generation of test signals, an example of a synthetic test
range, and the validation and analysis of radar signals.
03 | Keysight | Radar, EW & ELINT Testing: Identifying Common Test Challenge - Application Note
Beam
scan p
Long pulses attern
overlap Pulse-on mean power
Echoes
Pulse
width
I)
al (PR
terv
n in
itio
)
PRF
et cy (
rep uen
1
e eq
puls Pul
se itio
n fr
ort ution
Sh sol = ls e re
pet
PRI Pu
re
oes
Ech
Figure 1. Radar pulse terminology and tradeoffs
Primary radars suffer significant sig- one large target indistinguishable chirp is, in concept, a simple modula-
nal losses from the transmitted pulse from several smaller ones in tion to create and to decompress.
to the received echo. The transmit- close formation. Frequency modulating (FM) the radar
ted signal must bounce off and travel pulse with a linear voltage ramp
back from the target to the receiver If a radar's pulse width is long, creates a frequency-chirped pulse.
without amplification. One way to echoes from adjacent targets can The chirped pulse is then transmit-
overcome these large signal losses bounce back together, overlapping ted, as an uncompressed pulse would
is to transmit longer pulses and in- in time. To the radar, this appears as normally be.
tegrate the larger total energy in the one large target instead of adjacent
received echo. A longer pulse width smaller targets. Thus, to get the best The radar receiver uses a special filter
thus provides longer operating range radar resolution, a narrower pulse with a significant linear group delay
for a given antenna and transmit width is desirable. opposite that of the chirped pulse.
power amplifier. The filter's group delay slows the
One can see that optimal range and lower-frequency portion of the chirp
Radar "resolution" is also an impor- resolution involves conflicting criteria. and allows the higher-frequency part
tant characteristic related to pulse Best range implies a long pulse of the chirp to emerge from the filter
width. The ability to resolve small whereas best resolution implies a earlier. This has the effect of taking
objects allows a radar to provide a short pulse. a long pulse, easily integrated for
more detailed picture of the target. A greater total power, and compress-
radar that can resolve details down to To solve the range-versus-resolution ing it to a short pulse easily identified
1 meter will provide much more infor- optimization problem, many radar among other pulses.
mation about approaching targets. A systems use pulse compression or
resolution of 100 meters might render modulation. The linear frequency
04 | Keysight | Radar, EW & ELINT Testing: Identifying Common Test Challenge - Application Note
Chirped pulse De Compressed pulse
lay
ed
lo w
Delay
-fr
eq
ue
nc
yc
om
po
ne
nts
ies
equenc Frequency
d high fr
ll y delaye
Minima
Receiver pulse filter
Time Time
Figure 2. Frequency chirped pulse compression
Pulse compression or modulation There are many cases, however, in As mentioned earlier, pulse
offers other advantages in which a slower PRF degrades overall compression can be used to eliminate
unambiguous range. To see these radar performance. For example, it ambiguity between successive pulses.
advantages,let us consider the pulse might be preferable to have a higher Adding digital modulation to each
repetition frequency. PRF for a faster radar screen update pulse allows the adjacent pulses to
rate if the radar is tracking a fast be uniquely encoded. Using digital
The pulse-repetition frequency (PRF) moving aircraft. In this case, the PRF modulation techniques, such as bi-
is dependent on the range capability might allow an ambiguous return in phase keying, encodes pulses so the
of the radar. Sending new pulses out favor of a faster update rate. round trip delay of each pulse is easily
before previously sent pulses can measured unambiguously using each
echo back can cause an ambiguity One approach to eliminating the pulse's unique coding as a separating
in the echo response. Generally, it is clutter of echoing returns that are tool.
easiest to send a pulse out and wait not from a range of interest is to use
until all possible echo responses have time or range gating. This approach Another important feature of many
been received before sending the next blanks on or off the radar's receiver, radars is the ability to measure
pulse. Providing an unambiguous ignoring echoes from objects either Doppler shift from moving targets.
range response determines the PRF or too close or beyond the range of Measuring the change in frequency
pulse-repetition interval (PRI) between interest. An example might be a time of the RF carrier or phase shift with
successive pulses. gate that ignores echoes from the time allows some radars to accurately
bow of the ship the radar is mounted determine the target's speed. MTIs
on. Similarly, a missile might use time use Doppler shift in the return echo to
gating to ignore echo returns beyond sense movement.
the missile's maximum range.
05 | Keysight | Radar, EW & ELINT Testing: Identifying Common Test Challenge - Application Note
Echo Echoes
Early gate
Late gate
Ignore
late echoes
0 Ignore Time or range
early echoes
Figure 3. Time gating or range gating
ELINT/EW basics -- What's out there?
The various design criteria that Similarly, the scan pattern of the Beyond simply gathering ELINT
influence the chosen radar pulse radar can also convey valuable information about the radar and its
pattern also convey a great deal of information about threats in the local attached platform, knowledge about
information about the nature of the environment. For example, observing the radar can enhance and guide
platform attached to the radar. A the signal amplitude as a function of electronic warfare techniques. For
slow PRF with a long pulse might time can reveal the type of antenna example, echo patterns can be syn-
indicate a weather radar scanning the radar is scanning with and the thesized and broadcasted to an early
across hundreds of miles, where a pattern the antenna is scanning out. warning radar receiver to display
fast PRF and a short pulse width This type of intelligence is helpful for assets that are physically not there.
might indicate a missile's terminal understanding what the radar is il- Missiles can track false radar returns
homing radar scanning across a luminating and how it is being used. that alter their range gating to ignore
mile or two. The ELINT gained from their intended targets. Doppler infor-
these signals conveys vastly different mation can also be used to confuse
information. targeting equipment.
Raster scan
Sector scan
Figure 4. Antenna scan patterns
06 | Keysight | Radar, EW & ELINT Testing: Identifying Common Test Challenge - Application Note
Modern radar & EW test challenges
The above review of some of the As we have seen thus far, the many Finally, many radars use phased-ar-
design issues with radar, ELINT and advantages of using compressed ray antenna systems. These systems
EW equipment highlights the level of pulses for better resolution and use wavefront time-of-arrival among
circuit complexity required. Testing unambiguous range frequently give many antenna ports to steer the an-
these modern radar systems places rise to the need for complex test tenna beam. This calls for test signals
unique demands on test and mea- waveform synthesis. This can be and measurements that provide mul-
surement equipment. Let us briefly further compounded by the need for tiple channels of phase-coherent and
consider some common challenges added Doppler shifts for radars that phase-adjustable sources or analyz-
encountered in testing. determine velocity. ers. The so-called multi-channel ar-
ray test system poses some very real
Wide bandwidths are essential for Another challenge facing radar sys- challenges to the radar test engineer.
many radar signals. Chirped or modu- tem designers is the ubiquitous use
lated pulses can require gigahertz of of software-defined radar systems. Having examined some of the basics
bandwidth, demanding broadband Many modern types of radar not only of radar systems and the test chal-
test equipment resources. require test signals and measure- lenges they pose, next we will look at
ments in the traditional analog RF the unique features of the Keysight
Very low phase noise is another com- fashion, but also in digital formats. Technologies, Inc. test equipment
mon requirement of radar test equip- This multi-format testing can present that make some of the radar engi-
ment. Radars that use Doppler shift a real problem trying to get good neer's difficult test challenges much
information often measure the rate of agreement between digital signal easier to solve. We begin with the
phase shift over time, as radar pulses measurements and analog measure- generation of radar test signals.
may not be long enough to integrate ments.
cycles of frequency difference. When
making these precise phase-change Full-scale system test is often a major
measurements, phase noise must issue for radar, ELINT and EW equip-
be kept very low, placing stringent ment. The primary issue is usually the
requirements on the phase-noise cost of the test assets.
performance of the test instrumenta- For example, simulating Doppler
tion. shifts, clutter and other signal ele-
ments to test a shipboard
Similarly, dynamic range require- fire-control radar may require a
ments can challenge radar test sys- ship and multiple test aircraft. Such
tems. Generally, this stems from the test platforms can quickly run into
large path losses encountered from a cost of many tens of thousands of
the transmitter through the return dollars per hour to accurately test
echo. targeting performance.
07 | Keysight | Radar, EW & ELINT Testing: Identifying Common Test Challenge - Application Note
Generating Test Signals
In the design and manufacture of In the past, bandwidth was a crucial Theoretically, each bit of resolution
radar systems, many situations limitation of most AWGs. Today, the should yield a maximum of 6.02 dB
require wideband microwave signal latest models have largely solved this of SFDR. In practice, DACs are often
generators. Test signal sources are problem for many applications. For described in terms of the effective
commonly used for applications such example, the M8190A AWG provides number of bits (ENOB) or an equiva-
as stable local oscillator (STALO) sub- 14-bit resolution up to 8 GSa/s and lent number of bits. After accounting
stitution, coherent oscillator (COHO) 12-bit resolution up to 12 GSa/s. for linearity issues, the actual SFDR
testing and threat-emitter simulation. This makes it possible to generate per bit is less than the theoretical
signals with alias-free bandwidths of 6.02 dB.
up to 5 GHz. Even greater alias-free
Creating an accurate simulation of bandwidths can be created through Broadband DACs also suffer from a
received signals can be quite difficult. the use of combining and converting phenomenon called passband tilt,
Fortunately, today's DSP-based sig- technologies. which further lessens dynamic range
nal generators and arbitrary wave- at the higher end of the frequency
form generators (AWGs) are capable When selecting an AWG, perhaps the band. Also, due to the (sin x)/x rolloff
of producing simulated emitter most important consideration is the of the sampling function, passbands
signals and electromagnetic environ- spurious-free dynamic range (SFDR) from the AWG roll off as frequency
ments with realistic impairments and of the source. This is affected by the increases; however, because this tilt
path distortions that accurately por- bits of resolution provided by the dig- is inherent in the sampling function,
tray distant targets. One important ital-to-analog converter (DAC) within it is not considered when specifying
note: With COTS signal generators the AWG. It also depends on the SFDR. Thus, an SFDR of 75 dB gener-
and AWGs, the simulated signals are quality of the frequency-conversion ally applies to the lowest frequency
typically not coherent with the radar circuitry that translates the arbitrary in the band. Dynamic range will typi-
receiver. However, non-coherent signal into the microwave range. cally be 5 to 7 dB lower at the upper
signals are an effective way to test end of the band.
passive radar, multi-static radar and
electronic countermeasure (ECM)
systems.
Keysight signal sources
and AWGs
The true power of an AWG is in its
ability to generate virtually any wave-
form downloaded into its memory.
For example, an AWG that can
provide both high resolution and wide
bandwidth--simultaneously--makes
it easy to create radar emitters and
targets scattered across a synthetic
test range that simulates hundreds of
cubic miles of airspace.
Figure 5. Keysight arbitrary waveform instruments
08 | Keysight | Radar, EW & ELINT Testing: Identifying Common Test Challenge - Application Note
Generating Test Signals (continued)
In addition to the number of bits and The E8267D PSG microwave vector Memory configuration is another im-
the SFDR loss related to the sampling signal generator offers I/Q modula- portant consideration when selecting
function, upconversion to microwave tion inputs and frequency coverage an AWG or a vector signal generator
frequencies poses another set of up to 44 GHz (and higher with exter- with AWG capabilities. Either type
problems in the creation of useful sig- nal mixers). The modulation inputs of instrument creates waveforms by
nals. Upconversion can be performed are compatible with the M8190A playing back digital information from
within the signal source or externally AWG. Working together, these two memory. The addition of standard or
with a separate device. This may high-performance instruments can optional capabilities for sequencing
seem easy to do using just a mixer, deliver 2 GHz of signal bandwidth up and playback can further enhance the
two filters and a fixed LO. In practice, to 44 GHz with excellent SFDR and utility of the signal generator.
however, LO harmonics and spurs phase noise.
often combine with the desired signal The simplest way to organize play-
to create in-band spurious signals Another way to mitigate many of back memory is to use a single large
that can severely limit SFDR. these issues is digital upconver- block of fast RAM and play the
sion, which is offered in the best of waveform directly from memory. This
Many radars measure pulse-to-pulse today's AWGs. When available in a works well for single pulses or very
phase shifts as a way to derive values wide-bandwidth AWG, this technique short RF events; however, at the data
for Doppler shift or target velocity. makes it possible to directly gener- rates required to support 12 GSa/s at
To combat the addition of unwanted ate the IF signal. In the two-channel 12-bit resolution, the signal must be
phase noise into the upconversion M8190A, each channel has a separate very short. Some manufacturers have
process, a signal generator must also digital upconversion engine and the extended this approach to work with
have low phase noise. channels can be used in "coupled large RAID arrays, thereby enabling
mode" to achieve full phase-coherent longer playback times. 1
Keysight offers a full line of signal output. Parameters such as carrier
sources and AWGs that offer excel- frequency, amplitude and waveform The single-block approach is of
lent SFDR and phase noise perfor- can be set independently and the somewhat limited usefulness because
mance. For example, the E8257D PSG complex-valued I and Q data will be most RF signals are repetitive. Even
analog signal generator offers indus- upconverted digitally to the desired with terabytes of memory or RAID
try-leading phase noise performance frequency range while providing ex- capacity, sequential playback times
as good as -143 dBc/Hz (typical) cellent signal quality with SFDR of up will be limited to a few seconds
for a 1 GHz signal at a 10 kHz offset to 80 dBc and harmonic distortion of of signal.
(option UNY). For upconversion, the less than 72 dBc (both values
analog PSG can also be configured are typical).
with an internal mixer or an internal
mixer and frequency doubler.
1. RAID: redundant array of inexpensive discs
09 | Keysight | Radar, EW & ELINT Testing: Identifying Common Test Challenge - Application Note
Generating Test Signals (continued)
Desired pulse pattern
Digital pulse pattern
Samples
Memory sequences
Segment Segment Segment Segment Segment
#1 #2 #3 #2 #1
Time
Figure 6. Waveform segmenting, sequencing and scenarios
The solution is a more efficient Once you've chosen a signal source
memory-access capability for re- that provides adequate bandwidth,
petitive signals such as radar pulse SFDR, phase noise and sequenc-
sequences. To support a repetitive ing capabilities, the next challenge
signal, fast playback memory can be is the digital creation of the desired
organized to play signal segments waveform using software tools such
as loops or an infinite sequence. as Signal Studio or SystemVue from
Advanced sequencing capabilities Keysight or MATLAB from The Math-
such as conditional branching make Works.
it possible to create highly complex
segments and scenarios. In addi-
tion, some Keysight sources offer
dynamic sequencing that supports
real-time modification of waveform
segments. When combined with
waveform memory large enough to
hold 2 GSa per AWG output chan-
nel (M8190A), highly complex and
realistic signal scenarios of long
duration are possible.
10 | Keysight | Radar, EW & ELINT Testing: Identifying Common Test Challenge - Application Note
Generating Test Signals (continued)
Key features:
Signal Studio Easy pulse building for specify parameters such as PRI, the
number of pulse repetitions, repeti-
for Pulse Building Keysight sources
tion interval jitter and PRI wobbula-
This specialized version of Signal Stu- Depending on the application, pulsed tion. Available PRI patterns include
dio (N7620B) supports a wide array of radar signals utilize a wide range of bursted, linear ramp, staggered and
imported or software-defined pulse characteristics: pulse width; PRI or its stepped, and PRI jitter can be defined
shapes and antenna patterns. inverse, PRF; modulation; and more. as Gaussian, uniform or U-shaped.
The creation of suitable test signals PRI wobbulation can be selected as
Pulse parameters is challenging, and the synthesis of sawtooth, sinusoidal and triangular.