The Radar Series
As derived in Part 1 of this series, the radar range equation captures the essential variables that define the maximum distance at which a given radar system can detect objects of interest. Because the variables relate directly to the major sections of the system block diagram, they provide a powerful framework for the essential process of understanding, characterizing, and verifying the actual performance of any radar.
Parts 2 and 3 defined the pulsed radar signal, described ways to measure the power in those signals and presented readily available ways to measure the frequency, timing, power, and spectrum of pulsed signals. Part 4 examined the use of vector signal analysis (VSA) with wideband signal analyzers and oscilloscopes to measure frequency, phase, and more, in today’s increasingly complex radar signals.
Part 5 provided a closer look at the testing, analysis, and optimization of radar components and subassemblies. In Part 6 the focus was on the best ways to test antennas and antenna arrays, surveying the narrowband and wideband measurements that can be made with vector network analyzers and multi-channel digitizer-based systems. Part 7 continued our discussion of practical test methods, focusing on three ways to assess and improve radar system performance: noise figure, time sidelobe level, and phase noise.
To conclude the series, we turn to multi-emitter testing of radar and electronic warfare (EW) systems. Realistic testing of these systems depends on the generation of signals that accurately simulate multi-emitter environments consisting of thousands of emitters and millions of pulses per second, all arriving from multiple directions. Traditionally, this has required the use of large, complex systems not readily available to R&D engineers. New technology built into commercial, off-the-shelf (COTS) “agile signal generators” provides an integrated, lower-cost solution that fits on an engineer’s test bench. This technology also enables developers to generate increasingly complex simulations that get closer to reality and, ultimately, provide deeper confidence in EW system performance.
The radar series
This application note is the eighth and final installment in a series that delves into radar systems and the associated measurement challenges and solutions. Across the series, our goal is to provide a mix of timeless fundamentals and emerging ideas.
In each note, many of the sidebars highlight solutions—hardware and software—that include future-ready capabilities that can track along with the continuing evolution of radar systems.
Whether you read one, some, or all of the notes in the series, we hope you find material—timeless or timely—that is useful in your day-to-day work, be it new designs or system upgrades.
Outlining the Most Common Direction-Finding Methods
An EW receiver parameterizes every incoming RF pulse from all inbound threats. The result is a set of pulse descriptor words (PDWs) that contain essential information such as time of arrival, frequency, angle of arrival (AoA), pulse width, and modulation on pulse (MOP).
EW receivers use direction-finding (DF) methods to sort radar threats into different categories. AoA and frequency are primary parameters because they change more slowly than other attributes. The rate of change is on the order of hundreds of milliseconds to several seconds depending on the velocities and ranges of the platforms carrying the threat radars and EW receivers. Once the threats are sorted and tracked, they are presented to the pilot on a display according to type and relative bearing.
Three DF techniques are most common: amplitude comparison, time difference of arrival (TDOA), and interferometry (i.e., phase difference). We’ll take a brief look at each; however, interferometry is noteworthy because it lends itself to an effective simulation of AoA scenarios in a lab setting.
This is the most common DF method, often used in radar-warning receivers designed many years ago. The amplitude-comparison monopulse technique relies on the signal ratio P2/P1 from two displaced antenna patterns originating from one phase center and overlapping in the far field.
As shown in Figure 1, the antenna boresights are oriented 90 degrees apart to ensure there will be a measurable power difference in each channel when the same pulse is incident on both patterns. The power difference gives a meaningful result in the arctangent of P2/P1, which yields the AoA relative to the boresight of antenna 1.
This method provides DF accuracy of 10 to 15 degrees because the measured cross-channel power levels will vary due to aircraft motion and amplitude attenuation by the aircraft (i.e., “shadowing”). This level of inaccuracy is deemed acceptable because resolution is generally more important than accuracy when using AoA. Resolution is the ability to distinguish co-located threats such as different radars within a single surface-to-air missile (SAM) site.
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