Introduction
Aircraft employ many techniques to avoid radar detection. A common radar deception method used by military aircraft is the re-transmission of incoming radar pulses while applying a gradual Doppler shift. This Doppler shift mimics the effect of velocity occurring at a different speed and/or direction than actual. Pulse repetition interval (PRI) is the distance in time between subsequent radar pulses. By staggering the PRI in uniform time increments, electronic countermeasures (ECM) techniques allow aircraft to potentially avoid being tracked by search radar by returning a radar signature indicating false aircraft speed and direction. The ground-based radar then gradually walks off from the actual target while continuing to track false target range information, and the evading aircraft disappears from radar due to velocity track breaking.
The ultra-high frequency content of X-, K-, V-, and W-band radar signals have traditionally required use of spectrum analyzers and frequency domain equipment due to the historically higher bandwidths of spectrum analyzers and lower bandwidths of real time oscilloscopes. A spectrum analyzer can determine frequency content of a signal, but the measurement challenge has been that these frequency domain instruments cannot accurately characterize timing information. However, with the advent of real-time analog bandwidths of 100 GHz or more, oscilloscopes can now acquire the full spectrum of radar signals up to W-band radar. Using time-domain equipment to acquire radar signals allows accurate ECM flightline tests including gated PRI, PRI stagger variation, pulse duty cycle, and other parametric measurements of a demodulated pulse, as well as parameter tracking.
Flightline Test Configuration
Figure 1 depicts an example of a hardware test configuration consisting of an F-16 coupler, a real-time oscilloscope, and a radio frequency stimulation and measurement (RFSM) unit. The RFSM unit, which is programmed to generate PRI stagger, is connected to the oscilloscope via a 2.92mm cable.
Pulse Repetition Interval Stagger
AThe blue waveform in the topmost grid of Figure 2 shows an acquired series of radar pulses, in which long idle gaps occur between bursts. The timebase setting corresponds to a 2-ms time capture window of 160 million contiguous sample points (Mpts) captured at 80 GS/s. The orange waveform in the second grid utilizes a demodulation math operator to produce an envelope of each radar pulse. The third grid shows a zoom trace (blue) with a demodulated waveform overlay of a single pulse, and the fourth grid (green trace) shows a zoom-on-zoom display of the carrier at 200 ps/div. The parametric period measurement (P3, in the green box), reports the period of function F1. The function F1 has demodulated the radar pulse and displays an envelope of each burst. We apply parametric timing measurements directly to the demodulation math operator, and the period parameter directly calculates the PRI of the series of radar pulses during walkoff. With measurement gates bracketing the first two pulses, the PRI measures 510.0024 µs, forming a gated PRI measurement.
Moving the measurement gates to each consecutive set of pulses follows the change in PRI. Bracketing the third series of pulses reveals a PRI of 520.0013 µs stagger during walkoff (Figure 3).
Parametric Measurements on PRI Stagger
Larger real-time acquisitions provide even greater insight into system behavior. By acquiring 400,000,000 contiguous sample points (with each point sampled every 12.5 ps) consecutively during a 5-ms time capture window, a larger picture emerges. In Figure 4, parameters P2 and P3 represent the pulse repetition frequency and pulse repetition interval, respectively, of the demodulated waveform. By applying a Track math operator (the orange Function F5) to parameter P3, we accurately graph the PRI stagger as a function of time. It appears with a distinct stair-step shape. We may apply further measurement and math operators onto the Track for detailed analysis of the PRI stagger, as it now exists as a real waveform trace.
In addition to the PRF and PRI calculated in parameters 2 and 3, parameter 1 reports the carrier frequency of the blue zoomed region Z3. Parameters 4 and 5 measure the rise times of the pulse envelope and the carrier respectively, and parameter 6 measures the width of the demodulated RF pulse envelope.
Randomness in the minute PRI variations of consecutive radar pulses results from refraining from using PRI stagger as an ECM technique. With PRI stagger disabled, the PRI track shown in Function F5 of Figure 5 confirms this random variation. Note that in Figure 5, the Y-axis autoscaling of Function F5 is 500 ps/div during random variation, compared with the 5 µs/div autoscaling of F5 when PRI stagger was enabled in Figure 4. Note also that if the Y-axis scaling of random PRI variation had been set to 5 µs/div to match Figure 4, then the PRI track in Figure 5 would appear as a flat line (indicating no PRI stagger was present).
With PRI re-enabled in Figure 6, we’ve added a measurement parameter to the measurement table. Parameter P7 now calculates the duty cycle of the demodulated radar burst envelope and reveals an extremely low characteristic radar duty cycle of 0.193214%. This type of measurement would not be possible without first demodulating the radar pulse. Otherwise, you would be applying the duty cycle-measurement parameter to the carrier oscillations and not the demodulated pulse.
Summary
In the recent past, the use of frequency domain equipment has prevented the ability to accurately characterize timing information such as PRI stagger during walkoff. With recent developments in ultra-high bandwidth, real time oscilloscopes, the defining timing characteristics of a radar system such as pulse duty cycle, PRF, and PRI stagger during walkoff can be accurately characterized within the fields of radar and electronic warfare now using parametric tools in the time domain.
