Thus the second pulse can only leave 1246 µs after the first — Page 192, Lesson 177

Let me walk you through this. We've just worked out that the second pulse can only leave 1246 microseconds after the first. Now I want to show you how that number becomes the heartbeat of the whole radar system. The time between pulses is called the PRI — the Pulse Repetition Interval. That's the 1246 microseconds. The PRF — the Pulse Repetition Frequency — is simply the reciprocal of the PRI. So PRF equals 1 divided by PRI. Let me do that arithmetic with you. 1 divided by 1246 microseconds. Since there are one million microseconds in a second, that's 1,000,000 divided by 1246, which gives us 802 pulses per second — 802 pps. There's an alternative way to arrive at the same figure. We can say PRF equals 300,000,000 divided by 374,000, and that also gives 802 pps. The 300,000,000 is the speed of light in metres per second — the speed at which the radar pulse travels. The 374,000 is the total distance in metres the pulse must cover — out to the target and back. So both routes give us the same 802 pulses per second. Now, I want to introduce a crucial distinction. That 802 pps is the maximum theoretical range for this radar. But the practical range is less than the maximum theoretical range. Why? Because the trace on the CRT — the cathode ray tube, the display screen — needs a period of time to return to the point of origin after each sweep. That period is called the fly-back, or dead time. During fly-back, returning echoes cannot be displayed. So even though the radar could theoretically receive an echo during that time, the display simply can't show it. That reduces the range achievable for a given PRF. Let me now turn to how the radar actually works. The pulses are concentrated into a beam whose dimensions are designed for the particular radar. The beam uses two principles. The 'echo' principle determines range — how far away the object is. The 'searchlight' principle indicates bearing or height — which direction the object lies. So range comes from timing the echo, and direction comes from where the beam is pointing. Here's Figure 11.4 — the Plan Position Indicator, the PPI display, showing primary raw radar. And Figure 11.5 shows the typical ATC radar antennae. Notice the long structures at the top of the primary radar antennae — those are the secondary radar antennae, a separate system mounted on the same structure. Now, a key design point: the transmitter and receiver share the same antenna. The receiver is energized to accept echoes from objects in the pulses' path as soon as the transmitter pulse exits the antenna. So the moment the pulse leaves, the antenna switches to listening mode. The reflected pulses are very weak — and that's because they've made a double journey. The pulse travels out to the object and all the way back, losing energy over that entire round trip. Finally, the shape and size of the radar antennae determine three things: the size of the main lobe, the size of the side lobes, and the width of the radar beam generated by the system. And here's the relationship to remember: the larger the aerial, the narrower the beam. A big antenna focuses the energy into a tighter, more precise beam. So to tie it all together: the PRF of 802 pps sets the maximum range, the fly-back time reduces that to the practical range, the echo principle gives you range, the searchlight principle gives you bearing or height, and the antenna size shapes the beam that makes it all possible.