ELINT Alchemist

There are some special types of plots that I have not seen outside ELINT analysis. It's not always obvious exactly what they represent or what they are used for, so I'll try to provide a brief description.

PRI plot

A plot of consecutive PRIs versus time.

A number of consecutive pulses can be represended by PRIs, the difference between consecutive TOAs.

PRIs

Now, plot PRIs versus TOAs on an XY-plot. Then reduce the pulses to points.

PRIs

This is the basic plot type which reveals the character of the radar in a graphic and easily understood way. See the examples below:

staggered PRF, four PRIs in four positions
constant unstable PRF
unstable PRF with scan

Raster plot

In a raster plot the PRF is compared to a reference frequency, much the same as in a Lissajous figure. But in the plot we compare the PRI to the reference frequency's period.

You can picture it as an oscilloscope trace which is triggered with a fixed frequency, and displaying the amplitude of the signal. Then you move the trace vertically so the traces won't overlap eachother.

PRIs

If the PRI is greater than the reference time, the pulse position between consecutive traces will reflect that difference. This difference may be small between consecutive traces but the effect is cumulative and will be apparent over many traces.

In the left image below the PRI is greater than the reference time which manifests itself as a slight shift of consecutive pulses to the right, when consecutive traces are below each other. In the right image the PRI is even greater than the reference time resulting in a greater shift of consecutive pulses.

PRIs