Radar Principles — Page 198, Lesson 185

Let’s pick this up right where the radar picture gets interesting. We’ve just dealt with a nasty problem in Moving Target Indication, or MTI, radar. Here’s the situation: an MTI receiver can produce false targets because of what we call second trace returns. Let me unpack that phrase carefully, because it’s a classic exam trap. Imagine a radar that sends out a pulse, then waits for the echo, then sends the next pulse. The time between pulses is the PRI — the Pulse Repetition Interval. Now, suppose there’s a target that is beyond the maximum range you’ve selected on the radar. That target is so far away that its echo doesn’t come back during the waiting period of the pulse that hit it. Instead, that echo arrives late — during the waiting period of the next pulse. So the radar thinks it’s seeing a target within the selected range, and because the target is moving, it looks like a moving target inside your range. That’s the false target. That’s the second trace return — literally the return from the preceding pulse, arriving during the period of the next pulse. How do we get rid of it? The technique is called jittering the PRF. Here’s the idea: if the radar changes the PRI between consecutive pulses — that is, it varies the time between pulses from one pulse to the next — then a second trace return won’t line up consistently. A genuine target within range will always appear at the same range on successive sweeps, but a second trace return will appear to jump around, because the timing between pulses keeps changing. So the MTI processor can recognise it as false and remove it. That’s the whole point of jittering the PRF — it removes second trace returns. Now let’s move on to the hardware — the Radar Antennae. There are three popular types shown in the figures: the microwave horn, the parabolic reflector, and the slotted planar array, which is also called the flat plate antenna. These are used extensively in radar and satellite systems. Let me explain each one. The microwave horn is very often used as a feed for large parabolic reflectors. Think of it as the source of the radio energy that the big dish then focuses. The parabolic reflector is that classic dish shape — it collects the energy from the feed and shapes it into a beam. The slotted planar array, or flat plate antenna, is a flat surface with slots cut into it that radiate the energy. Here’s a key point about the beam pattern. Both the parabolic reflector and the flat plate antenna generate main lobes as well as side lobes. The main lobe is the primary beam of energy — that’s the useful one that illuminates the target. The side lobes are smaller, unwanted beams of energy that radiate in other directions. They’re a problem because they can pick up echoes from directions you’re not actually looking at, and those echoes can interfere with the main pulse returns. So most radars incorporate circuits for side lobe suppression. The purpose is to make sure that echoes coming in via the side lobes don’t interfere with the main pulse returns. In other words, the radar can tell the difference between a strong echo in the main lobe and a weak echo in a side lobe, and it suppresses the side lobe ones. Now, here’s the contrast between the two. The slotted planar array produces a narrower beam with much smaller side lobes. What does that buy you? Two things: it reduces the power required, and it improves the resolution. A narrower beam means the energy is concentrated in a tighter area, so you need less power to get a usable return. And better resolution means you can distinguish between two targets that are close together — they don’t blur into one. So the flat plate antenna is the more efficient, higher-resolution option. Let me just make sure you’ve got the figure references straight. Figure 11.8 shows the radar antennae — the microwave horn, parabolic reflector, and slotted planar array. Figure 11.9 shows an airborne weather radar antenna. And Figure 11.10 shows a typical radiation pattern, with the main and side lobes of a parabolic reflector clearly visible. Now we shift to Ground Radar. This is where we start talking about how Air Traffic Control actually uses radar. ATC services use ground radars extensively to serve a large number of requirements and users. And they employ both primary radar and secondary radar techniques. Let me define those clearly. Primary Radar is used to detect aircraft that are not equipped with a Secondary Radar Transponder. So primary radar works purely on the echo reflected off the aircraft — no cooperation from the aircraft needed. It may incorporate Moving Target Indication, or MTI, which is that technique we just discussed for filtering out stationary clutter and false targets. The services that Air Traffic Controllers can offer are Information, Surveillance, or Guidance. Those are the three categories of ATC service. Now, the primary radar systems used by ATC include five specific types. Let me list them for you, because you need to know each one by name: - Area Surveillance Radar, abbreviated ASR - Terminal Area Surveillance Radar, abbreviated TAR - Aerodrome Surveillance Radar - Precision Approach Radar, abbreviated PAR - Airport Surface Movement Radar, abbreviated ASMR Let me focus on the first one in detail, because it’s the one the excerpt develops. Area Surveillance Radars, or ASR, are long range radars — specifically 200 to 300 nautical miles in range. They’re used for airway surveillance, to provide range and bearing of aircraft. So they tell the controller how far away an aircraft is and in what direction. And note this: additional information is provided by Secondary Surveillance Radar, abbreviated SSR. So the primary radar gives you range and bearing, and the SSR adds extra data on top of that. The figures referenced here — Figures 12.1 and 12.2 — show the locations and coverage of the London ACC and Scottish ACC radars, and Figure 12.3 shows the UK Airways structure. Those are the operational pictures of where these ASRs sit and what they cover. So to tie it all together: we’ve got the antenna types — horn, parabolic reflector, and flat plate array — each with their beam characteristics and side lobe behaviour. We’ve got the MTI problem of second trace returns and the jittering solution. And we’ve moved into ground radar, where primary radar with MTI is the workhorse for detecting non-transponder aircraft, with ASR being the long-range airway surveillance tool giving range and bearing out to 300 nautical miles. That’s the foundation we’ll build on as we go deeper into the radar systems.