Ground Radar — Page 198, Lesson 189

Let’s start with the big picture. We’re looking at ground-based radar — the radars that sit on the ground and watch aircraft. I want to walk you through the three main families of these radars, because each one is built for a different job, and the numbers tell you exactly what job it’s doing. First, long range radars. These are the ones that see traffic far out, and the numbers are all about reaching out a long way. The wavelengths are 10 to 50 cm, and the pulse lengths are 2 to 4 microseconds. A microsecond is one millionth of a second. The longer pulse length matters because it keeps the target illuminated for long enough to give a good, strong return — the radar needs to "paint" the target with energy for a decent chunk of time to hear it back clearly. Now, the PRF and the scan rate are low. PRF is pulse repetition frequency — how many pulses the radar sends out per second. Here it’s 300 to 400 pulses per second. The scan rate is how fast the antenna rotates, and that’s 5 to 6 revolutions per minute. Why so slow? Because the target is far away. The next pulse must not be transmitted until the first one has had enough time to travel out to the long range target and come back. If you fired pulses too quickly, you’d be sending a new pulse before the echo from the old one returned, and you’d get confused returns. So low PRF and slow scan are the price you pay for long range. Next, Terminal Surveillance Area Radars. These are medium range — up to 75 nautical miles — and they’re used for controlling traffic in TMAs, that’s Terminal Manoeuvring Areas, the busy airspace around airports. Note that additional information is provided by Secondary Surveillance Radar, SSR — that’s the transponder-based system that gives identity and altitude, but the primary radar here is the one we’re focused on. Typical wavelengths here are 10 cm, 23 cm, and 50 cm, with pulse widths of 1 to 3 microseconds. There’s a very practical operational point attached to this radar type. In the UK, horizontal radar separation minima may be reduced to 3 nautical miles — that’s 5.6 km — within 40 nautical miles of the radar head, or in certain circumstances 60 nautical miles, and below FL245, that’s flight level 245, which is 24,500 feet. But only where the procedure has been officially approved. So the radar’s accuracy and coverage allow tighter separation, but only under those specific conditions. Now the third family: Aerodrome Surveillance Approach Radars. These are short range — up to 25 nautical miles — and they provide positional information for aircraft near the aerodrome. Their wavelengths are 3 cm or 10 cm, with pulse widths of 0.5 to 1 microsecond. They provide three specific services. First, positional information and control of aircraft in the aerodrome vicinity — that’s Approach Radar, abbreviated RAD. Second, radar vectoring to the ILS — the Instrument Landing System — so the controller steers the aircraft onto the localiser and glideslope. Third, Surveillance Radar Approach, abbreviated SRA, which is a radar-guided approach in its own right. Now let’s move to a very different beast: Airport Surface Movement Radar, ASMR. This is also known as the Airfield Surface Movement Indicator, ASMI. It’s installed at major airfields to give a very accurate radar display — in all weathers and all conditions of visibility — of the aerodrome infrastructure: taxiways, runways, aprons, and also vehicular traffic and aircraft that are stationary, taxiing, landing, or taking off. The whole point of ASMI is to give a detailed, bright, flicker-free display of all aircraft and vehicles on runways and taxiways. Why? So that Air Traffic Control Officers can be certain that runways are clear of traffic before landings or take-offs, and so they can ensure the safe and orderly movement of traffic on taxiways. The processing can remove selected fixed features, leaving just the targets on runways and taxiways clearly visible. That’s shown in Figure 12.4 — the aircraft taking off there is a DC9. Now, to get that very high definition, the radar is designed with very specific parameters. A very narrow beam, in the order of 0.2° to 1°. A scanner rotation rate of 60 rpm — that’s ten times faster than the long range radar, because you need to update the picture constantly for moving traffic on the ground. A PRF in the order of 4000 to 20,000 pulses per second. Pulse widths in the order of 0.03 microseconds — extremely short. Frequencies of 15 to 17 GHz, which is SHF, Super High Frequency, corresponding to wavelengths of 2 to 1.76 cm. And ranges of 2.5 to 6 nautical miles in light precipitation. Here’s the trade-off, and it’s important. The frequencies required for ASMI — those very high GHz frequencies — result in the transmissions being increasingly attenuated and absorbed as the intensity of precipitation increases. That means rain and heavy weather literally soak up the signal. So the very thing that gives you the fine detail — the short wavelength — is also what makes the radar vulnerable to weather. That’s the fundamental tension in this design, and it’s exactly the kind of limitation you need to understand as a professional. Let me pause there. That’s the full picture of ground radar: long range for en-route surveillance, terminal radars for the TMA with reduced separation minima, approach radars for the final few miles, and ASMI for the surface