Radar Principles — Page 198, Lesson 185

Let’s pick this up right where the radar picture gets interesting. We’re looking at a specific problem with Moving Target Indication, or MTI, radars, and then we’re moving into the antennae that make radar work. First, the MTI problem. An MTI radar is designed to show only moving targets, like aircraft, and to filter out stationary clutter like buildings or the ground. But there’s a nasty failure mode called a second trace return. Here’s what happens. The radar sends out a pulse, and it selects a maximum range to display. If there’s a target beyond that selected maximum range, its echo can arrive back at the receiver after the radar has already sent out the next pulse. So that late-arriving echo from the far-away target appears during the period of the next pulse, and the radar misinterprets it as a moving target within the selected range. That’s a false target — a ghost on the screen. The fix is elegant. The radar changes the PRI — that’s the Pulse Repetition Interval, the time between consecutive pulses — between one pulse and the next. By varying that interval, the radar makes it impossible for a second trace return to line up consistently. This technique is called jittering the PRF, the Pulse Repetition Frequency. Jittering the PRF breaks the false target’s consistency, so the MTI processing can reject it. Now let’s move to the 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. The microwave horn is very often used as a feed for large parabolic reflectors — it’s the source that illuminates the dish. The parabolic reflector and the flat plate antenna both generate main lobes as well as side lobes. A main lobe is the primary beam of radiation; side lobes are smaller, unwanted beams that radiate in other directions. Most radars incorporate circuits for side lobe suppression, so that echoes coming in from the side lobes don’t interfere with the main pulse returns. Now, the slotted planar array has a real advantage. It produces a narrower beam with much smaller side lobes. That means less power is required, and the resolution — the ability to distinguish two closely spaced targets as separate — is improved. Let me show you the antennae and the radiation pattern. Now we shift to Ground Radar. Air Traffic Control services use ground radars extensively, and they employ both primary radar and secondary radar techniques. Primary Radar is used to detect aircraft that are not equipped with a Secondary Radar Transponder — that’s the transponder that responds to interrogation. Primary radar may incorporate MTI, which we just discussed. The services that Air Traffic Controllers can offer are Information, Surveillance, or Guidance. The primary radar systems used by ATC include five specific types. Area Surveillance Radar, or ASR. Terminal Area Surveillance Radar, or TAR. Aerodrome Surveillance Radar. Precision Approach Radar, or PAR. And Airport Surface Movement Radar, or ASMR. Let’s look at the first one in detail. Area Surveillance Radars, the ASRs, are long-range radars, operating from 200 to 300 nautical miles. They’re used for airway surveillance — that is, watching aircraft along the airways — and they provide range and bearing of aircraft. Additional information, like identity and altitude, is provided by Secondary Surveillance Radar, the SSR. The figures show the locations and coverage of the London ACC and Scottish ACC radars, and the UK Airways structure. So to tie it together: we’ve got the MTI false-target problem and its fix through jittering the PRF, then the three antenna types and the side-lobe issue, and now we’re into the ground radar family, starting with the long-range Area Surveillance Radar. That’s the foundation for the rest of the ground radar systems.