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EW processing - determining values of parameters.

The first step in the analysis of threat signals is to measure the received signal parameters. To understand the measurement mechanisms, it is worthwhile to consider the way these measurements were made before computers were available to radar warning receivers (RWRs). Each parameter measurement circuit was built from discrete components and could only do a single task. The computers in modern systems do the same jobs, but with more dignity (or at least a lot more efficiently).

Pulse Width

When a pulse is passed through a high-pass filter, the result is a positive spike at the leading edge and a negative spike at the trailing edge, as shown in Figure 1. By using the positive spike to start a counter and the negative spike to stop the count, it was possible to very accurately measure the pulse width. A second approach is shown in Figure 2. The pulsed signal can be digitized at a high sample rate and analysis made to determine the pulse width. This approach also provides detailed information about the shape of the pulse. This approach is required in systems that measure rise time, overshoot, etc., in addition to the pulse width.

Frequency

In early RWRs, which used crystal video receivers, the frequency of received signals could only be determined by dividing the input into frequency ranges with filters and placing a crystal video receiver on each filter output. The frequency of pulsed or continuous wave (CW) signals could also be measured by tuning a narrow-band receiver to a signal. The frequency of the signal was the frequency to which the receiver was tuned.

With the advent of practical instantaneous frequency measurement (IFM) receivers - and computers to collect the data - the frequency of each pulse could be measured and stored.

Direction of Arrival

The direction of arrival (DOA) of each pulse is measured using one of several direction finding approaches that were described in the "EW101" columns printed from October 1994 to June 1995. Low accuracy DOA measurement was (and is) done using amplitude-comparison direction finding and high-accuracy DOA measurement was (and is) performed using an interferometric approach.

Pulse Repetition Interval

In the good (but hard) old days, the pulse repetition interval (PRI) of pulsed signals was measured using what was called a "digital filter." This was a device designed to detect the presence of a specific pulse interval. The digital filter opened an accept gate a fixed amount of time after a received pulse. If a pulse occurred when the gate was open, it would look for another pulse at the same interval. When a sufficient number of qualifying pulses had been received, the presence of a signal with the specified PRI could be determined. It was necessary to have one digital filter circuit per threat PRI, and multiples to handle staggered pulse trains. One of the charms of this approach was that the pulses from a single signal could thus be "deinterleaved" from the combined pulse trains of many signals in a wide-band receiver.

Now, of course, a computer can collect the times of arrival of the leading edges of a large number of pulses and determine multiple PRIs and staggered PRIs mathematically.

Antenna Scan

Early RWRs had to determine the beam width of a threat emitter by setting a threshold and measuring the number of sequential pulses received above that threshold as shown in Figure 3. As a threat antenna beam scans past the receiver's location, the amplitude of the received pulses varies as shown in the figure. Thus, counting pulses worked unless there were other signals present during the count. Now, because we have better tools to deinterleave signals, the pulses from a single signal can often be isolated, and the shape of the pulse amplitude history curve calculated.

A histogram of DOA versus received power can be used to determine the type of antenna scan. Figure 4 shows the (highly unlikely) situation in which three signals with different types of antenna scans are located along one DOA. The vertical axis is the number of hits (or pulses) received at that power level. If you will think about the time versus received power history for various types of scans, you will be able to see that the shapes shown differentiate among the three scan types.

Receiving Pulses in the Presence of CW

Last month, we discussed an RWR that operated in an ideal world without any continuous wave (CW) or very high duty cycle pulsed (primarily pulse Doppler) signals. The logarithmic response of a wide band receiver (for example, a crystal video receiver) will be distorted when CW signals are present along with pulses. When very high duty cycle pulse signals are present, their pulses overlap the low duty cycle pulses causing the same problem. Since accurate amplitude measurements are required in order to determine DOA, the CW signal precludes proper system operation against pulses. It should also be noted that an IFM receiver is a wide band receiver that can operate on only one signal at a time. The answer is to filter out the CW signal with a band stop filter. The wide band receiver can then "see" pulses elsewhere in its frequency range while the narrow-band receiver handles the CW (or pulse Doppler) signal.

What's Next

Next month, we'll continue the discussion of processing with a detailed discussion of the deinterleaving problem and the extension of these techniques to types of receivers other than RWRs. For your comments and suggestions, Dave Adamy is at Internet: 74674.2605@compuserve.com.
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Title Annotation:electronic warfare
Author:Adamy, Dave
Publication:Journal of Electronic Defense
Geographic Code:1USA
Date:Nov 1, 1998
Words:919
Previous Article:A sampling of EW training systems.
Next Article:Intelligence revisited.
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