SEARCH RADARS PPI and HEIGHT FINDERS

0
707

The sort of simple pulse radar system described in the previous chapter was more or less available at the beginning of World War II, and was used on ground sites and on ships. The war led to improved radar technologies and an explosion of radar applications and types.

One of the early improvements was to build a radar that could automatically sweep around the sky to search for intruders. The early floodlight systems could cover a wide sector of the sky, but as mentioned they were inefficient. A simple steerable radar with an A-scope display was more efficient, but it had to be manually steered to find a target. Building an improved radar that could be swept around 360 degrees was a bit tricky, since it implied that the electrical connection between the antenna system and its associated electronics had to freely rotate, and designing reliable “rotary couplers” was troublesome.

It also implied a different type of display, the “plan position indicator (PPI)”, also known as the “polar plot indicator”. The PPI is a circular display, with a sweep rotating around the center in sync with the transmitter antenna, and the return for a particular angle displayed along the display sweep. As the sweep rotates around the center of the display, it paints an image of what the radar “sees” all around it. The display uses has a “long-persistence phosphor” that allows the image to linger after the sweep has passed, fading away just before the sweep comes around to refresh the image. A PPI display can be thought of as something like an A-scope being spun around in a circle, with a single A-scope trace on each radius of the circle.

Advertisement

PPI display

The PPI is the popular concept of a radar display, commonly seen in TV shows in which a mysterious or dangerous intruder is moving closer to the center of the display, where the heroes are, with every sweep. (In some shows, they use a PPI even when the radar doesn’t have a rotating antenna.) In the early days, the radar did little processing on the return echoes, and so it was up to the operator staring at the PPI to figure out what the display actually said. It wasn’t necessarily the case that there was a simple bright blip where the intruder was; there would often be sources of “clutter” in the radar sweep, such as flights of birds, swarms of insects, and other obstructions to the radar beam.

Incidentally, the time it takes for the antenna to rotate 360 degrees and for the sweep to correspondingly move all the way around the display is referred to as the “update rate”. One of the classic examples of such a radar was the US Navy “SG” shipboard radar, which was a 3 GHz / 10 centimeter system with a horizontal parabolic antenna. It could provide a “map” of threats and obstacles around a vessel on its PPI display.

* One of the issues, if not necessarily a problem, with the “search radar” scheme described above is that it gives the range and azimuth to the target, but not its altitude — it is a “two-dimensional” or “2D” radar. That was okay if the search radar was being used by a ship or a coastal site to track other ships, since their altitude was of course at sea level, but not so good if the search radar was tracking aircraft.

The search radar didn’t really need to determine altitude by itself. Its major function was just to provide a warning, and to do that it was best designed to generate a long-range beam in a “fan” configuration that was very tall and thin, with the radar essentially throwing out a cylindrical “wall” of radio waves with each sweep, through which intruders must pass. Once an intruder was located, a separate, steerable “height-finder” radar could be pointed in the direction given by the search radar to determine the intruder’s altitude. The height-finder radar generated a beam that was very short but wide, exactly the opposite of the search radar. The two radars effectively formed “crosshairs” that pinned down the precise coordinates of the target. Height finders were often designed to “nod” up and down to search for a target. The US Navy “Mark 22” or “Lil’ Abner” radar was a classic example of such a height-finding radar, with a vertical antenna like a peel of a slice of an orange and nodding operation.

Of course, over the long run improvements in radar technology allowed development of a single radar that could determine both the azimuth and altitude of an intruder. Such a radar is of course known as a “three-dimensional” or “3D” radar.