 You've seen how the A, K, and J scopes work. But remember we said there was still need for another type of scope. For instance, here's a newly established beachhead on a Pacific Island. Because of its advanced position, enemy bombers can attack the island from any point on the compass. Radar coverage is being handled by this SCR 545. Here it is, searching through a full 360 degrees. And there's a target. See it there on the search scope at the right. To find the target's range, you have to stop the antenna and bring it back onto the target. Then crank the pip into the notch on the range scope. To get the azimuth, the pip must be split and balanced on the azimuth scope. Only after you've made these adjustments can you report the range and azimuth of the target. But while you're doing all this with every target you pick up, what about planes in other sectors? As long as the set is looking in one direction, it's blind to everything else around it. So you can see that there was need for a new type of scope. A more versatile one. A scope that would measure both range and azimuth at the same time. One that would enable you to view the whole area at once and thus follow several targets simultaneously. Out of this need came the PPI, Plan Position Indicator. With this scope, no matter how many planes there are in the sky around you or from what direction they approach, your beam is continually searching and will pick up each one of them. The PPI paints a map for you of the surrounding area. Each target appears as a glowing blob of light at a point on your scope corresponding to its position in the sky. Each time the antenna beam passes over the target, the baseline, which is synchronized with the antenna, repaints the target blips on your scope. Thus, you can watch the movement of any number of targets at the same time without trouble or delay. Here's a sample of a PPI scope. And to show you how accurate a picture it presents of the surrounding area, here's the actual map. In developing this PPI scope, a new type of cathode ray tube had to be worked out. In order to project both range and azimuth at the same time, the deflecting plates inside the tube were replaced by a pair of coils around the neck of the tube on the outside. This is the other type of tube we mentioned earlier, the electromagnetic cathode ray tube. Let's look at the tube and coils from the front. When current is sent through the coils, it sets up a magnetic field between them. That deflects the electron beam away from center. As more current flows through the coils, the magnetic field between them gets stronger and the farther off center the beam is deflected. As the current decreases, the electron beam returns to its normal center position. The deflection of the beam from center is always proportional to the amount of current flowing through the coils. And of course, just as on the ascope, the deflection of the beam is synchronized with each transmitted pulse. When you apply current in the form of sawtooth voltage, you get the familiar trace and snapback movement of the electron beam. When it is speeded up, there's your PPI baseline, composed as it was on the ascope, of an electron beam sweeping back and forth so rapidly, it creates the illusion of a solid line. When a target signal reaches the indicator circuits, it causes a greater number of electrons to be fired through the tube. The increased flow lasts for only an instant. Then the stream returns to normal, but the result on the scope is an intense glow of light at that point on the baseline, which represents the range of the target. Since each time the antenna is pointing at the target, there are several thousand pulses sent out, and several thousand echoes return, the increase in electron flow at that point happens so fast that it forms a steady glow of light on the baseline at the range of the target. Range is measured out from the center of the scope, just as though a range scale were placed beneath the baseline. That blob of light in the center is the same as the main bang on an ascope. The second blip is the target signal, and range is the distance between them. By the way, the process that makes the signals appear as blobs of light instead of pips is called intensity modulation. Okay, you know how the target signal is formed and how to measure range on this PPI. But how about azimuth? Well, those coils that cause the electron beam to sweep back and forth to form the baseline are designed to rotate freely around the neck of the tube. Since it was the current flowing through the coils that produced the baseline, when the coils rotate, the baseline sweeps around the scope. The rotation of the coils, and hence the sweep of the baseline, is synchronized with the antenna so that the baseline is always pointing in the same direction as the antenna. So by calibrating the edge of the scope, you can read the azimuth of the target at a glance. For instance, if a target shows up here, it's no trick at all to report its azimuth as 40 degrees. Notice too that since the glow of the target signal persists for some time after the baseline is painted it, you can make very accurate reports. For ease in operation, some sets have range markers as an integral part of the scope presentation, like this PPI on the AN-NPG-1. Some sets, like this AN-CPS-1, a microwave early warning radar, employ both range and azimuth markers on the PPI presentation, like this. If you want to examine a particular sector closer, it's possible to expand the picture by moving the sweep off-center in any direction. Now one quadrant is blown up to fill the whole scope. This presentation is called the EPI, or Expanded Position Indicator. On other sets, like this searchlight radar, the AN-TPL-1, the pattern of geographical grids is placed over the PPI scope so you can locate the target in terms of grid coordinates. Another method of PPI operation employs two sets of deflecting coils which do not rotate as the antenna turns. Instead, the flow of current through the coils is regulated so that the resulting electromagnetic force causes the baseline to rotate. For instance, if current flows through these two coils with this polarity, the electron beam or baseline is deflected to one side, so. But if the polarity on the coils is reversed, the baseline is deflected to the other side, like this. Same thing is true of the other set of coils. Current flowing through them one way will deflect the baseline upwards, and a change in polarity will deflect the beam downwards. By synchronizing the flow of current through these coils with the antenna, the baseline will always point in the same direction as the antenna. Thus, by altering the flow of current through the four coils, like this, we can rotate the baseline without rotating the coils, which means we can forget about movable parts, bearings, and the added weight that goes with them. The lightness of this type of PPI makes it very useful for airborne sets. Echoes picked up on an airborne set enable the PPI to project a bird's-eye view of the ground. The scope will indicate shorelines, rivers, railroads, ships in a harbor, boys, even the projecting periscopes of a submerged sub. So the PPI is unusually valuable on bombing missions. In fact, all of the overcast and night bombing raids on both Germany and Japan were made possible by radar. But what about the night fighters that protect the bombers, or that go out to clear the sky of enemy bombers or night raiders? A night fighter pilot is after enemy planes rather than ground targets, and since long-range radar is pretty bulky, a radar-equipped night fighter usually operates with an early warning ground set. This MEW usually operates as a GCI set, Ground Control Interception, and often uses this ANCPS-4, known as Beaver Tail, or Big Adner, to supply accurate height data. When a new target is picked up, challenged, and doesn't identify itself as friendly, the ground station vectors the night fighter to it. The controller watches the blip of the night fighter, guides him toward the target signal by radio. When the controller sees that the night fighter is close enough to the enemy plane so that its own shorter range, highly accurate radar can take over, he tips off the pilot.