AI, ASV, & AEW RADARS / RADAR DISPLAYS

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If fighters were sent up against intruders in daylight and clear weather, ground-based radars could generally get them close enough to perform an interception by eyeball. However, there was little chance of finding an intruder visually at night or in bad weather, and so night fighters carried their own radars, allowing them to target intruders after being vectored to their vicinity by ground search radars.

Night fighter or “airborne intercept (AI)” radar had limited maximum range — and early sets had long pulse widths, giving the radars a long minimum range, meaning they had to be near a target to find it with radar and then could easily lose it while trying to close in on it. Another problem with these early night fighter radars was that they operated at long wavelengths, making them difficult to focus into a narrow beam. The problem with such a wide beam was not really limited angular accuracy; night fighter radars used lobe switching and conical scanning to obtain useful targeting precision with relatively long wavelengths, obtaining enough accuracy to find a target even with a wide beam. The major difficulty with all early pulse radars was that they had no “discrimination”. If the radar pulse hit something, anything, the echo came back and showed up on the display. That meant that if a hostile aircraft was low to the ground, reflections from the terrain or “ground clutter” kept it invisible to radar. A wide beam meant that ground clutter remained a problem at relatively high altitudes, lost in the noise; the need to produce an AI with a narrow beam with an antenna that could be carried in a night fighter was one of the drivers of microwave radar.

The classic AI radar was the US “SCR-720”, a 3 GHz / 10 centimeter set used in the Northrop P-61 Black Widow night fighter. The SCR-720 remained in first-line service into the early 1950s in improved versions.

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* Work on radars that could be carried by patrol aircraft to hunt for ships and submarines in the dark and bad weather went on in parallel with the development of AI radars. Such “air to surface vessel (ASV)” radars were useful because a ship target was big enough to be picked out of the clutter returned from the surface of the water, though unsurprisingly the clutter got worse when the weather was worse and the waves were higher. Microwave radars were also useful for ASV, since their higher resolution allowed them to better pick out targets. Systems were developed that linked into the ASV radar to automatically release bombs during low-level attacks on shipping.

* Following the development of ASV radars, other radars were developed for targeting air strikes against cities and other area targets on land. These were very crude bombing aids, since they really couldn’t do much more than distinguish between dry ground and bodies of water, and only worked well when the target could be identified by lakes or the confluence of rivers. Once again, microwave radars were preferred since they gave a higher resolution image.

Such bombing radars used a PPI display to give a map of the terrain below. The radar had to compensate for the fact that radar echo returns farther away from the center of the display were fainter, distorting the radar “image”, and so the receiver sensitivity was adjusted to be greater at greater angles. This is known as “cosecant-squared” operation, since that’s the mathematical function used to determine the gain function. It was actually implemented by modifying the antenna to provide the cosecant-squared pattern, with the antenna designed with different inner and outer curvatures. The cosecant-squared configuration was also used in naval search radars, to allow the radar to pick up targets at higher altitudes while avoiding pickup of sea-surface clutter.