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

Right, let’s pick this up exactly where we left off. We’d just worked out that the second pulse can only leave 1246 microseconds after the first. Now I want to show you what that number actually buys us, because it gives us the pulse repetition frequency, the PRF. PRF stands for pulse repetition frequency, and it’s measured in pulses per second, abbreviated pps. It’s simply the reciprocal of the pulse repetition interval, the PRI. So PRF equals 1 divided by PRI. With our PRI of 1246 microseconds, that’s 1 divided by 1246 microseconds. Since there are one million microseconds in a second, we write that as 1,000,000 divided by 1246, which comes out to 802 pulses per second. 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 us 802 pps. The 300,000,000 is the speed of light in metres per second, and the 374,000 is the total distance in metres the pulse travels out and back. So you see, the same PRF can be derived either from the time interval between pulses or from the range and the speed of the signal. Now, I want to be clear about something important. 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 — that’s the cathode ray tube, the display screen — needs a period of time to return to the point of origin after each sweep. That return period is called the fly-back, or the dead time. During fly-back, returning echoes cannot be displayed. So even though the radar could theoretically receive echoes during that period, it can’t show them, and that reduces the range you can actually achieve for a given PRF. So always remember: practical range is less than theoretical range because of fly-back, or dead time. Now let’s move on to the primary radar itself. The pulses are concentrated into the beam dimensions designed for that particular radar. The beam uses two principles. It uses the ‘echo’ principle to determine range, and the ‘searchlight’ principle to indicate bearing or height. So range comes from timing the echo, and bearing or height comes from where the beam is pointing, just like a searchlight. Here we have Figure 11.4, which shows the Plan Position Indicator display — that’s the PPI — and Figure 11.5 shows the ATC radar antennae. ATC stands for air traffic control. Notice the long structures at the top of the primary radar antennae — those are the secondary radar antennae. So the primary antenna carries a secondary antenna on top of it. Now, a key point about the hardware: the transmitter and receiver share the same antenna. The receiver is energised to accept echoes from objects in the pulses’ path as soon as the transmitter pulse exits the antenna. And here’s the catch — the reflected pulses are very weak, because they’ve made a double journey. The pulse goes out to the target and comes all the way back, so by the time it returns, it’s lost a lot of energy. Finally, the shape and size of the radar antennae determine the size of the main and side lobes, as well as the width of the radar beam generated by the system. The larger the aerial, the narrower the beam. So a bigger antenna gives you a tighter, more focused beam, which is generally what you want for better resolution. So to tie it all together: we have the PRF of 802 pps derived from the PRI, we know the practical range is limited by fly-back on the CRT, and we know the primary radar uses echo for range and searchlight for bearing, with a shared antenna and a beam width set by the aerial size. That’s the core of primary radar principles.