Ground Radar — Page 198, Lesson 189

We're starting a new chapter now — Ground Radar. And I want to begin by setting the scene, because this chapter is all about the different types of radar that sit on the ground and serve air traffic control. Let's start with the long-range radars. These are the big en-route surveillance radars. Their wavelengths and pulse lengths are relatively long — wavelengths of 10 to 50 centimetres, and pulse lengths of 2 to 4 microseconds. Now, why the long pulse length? Because the longer the pulse, the longer the target is illuminated, and that gives you a good, strong return echo. The PRF and the antenna rotation rate — the scan rate — are both low. PRF is pulse repetition frequency, the number of pulses transmitted per second. Here it's 300 to 400 pulses per second. And the antenna rotates at 5 to 6 revolutions per minute. The reason for those low values is critical: you must not transmit the next pulse until the first one has had sufficient time to travel out to a long-range target and come back. If you fired pulses too quickly, you'd get confused returns. So low PRF and slow scan rate are the deliberate design choices for long range. Now, moving in closer, we have Terminal Surveillance Area Radars. These are medium-range radars, out to 75 nautical miles, used for controlling traffic in TMAs — Terminal Manoeuvring Areas. And I should mention that additional information here is provided by Secondary Surveillance Radar, SSR — that's the transponder-based system that works alongside the primary radar. Typical wavelengths here are 10 centimetres, 23 centimetres, and 50 centimetres, with pulse widths of 1 to 3 microseconds. And there's an operational point worth noting for the UK: horizontal radar separation minima may be reduced to 3 nautical miles — that's 5.6 kilometres — within 40 nautical miles of the radar head, or in certain circumstances 60 nautical miles, and below flight level 245, where the procedure has been officially approved. So that's a specific separation standard tied to this radar type. Next we come to Aerodrome Surveillance Approach Radars. These are short-range radars providing positional information out to 25 nautical miles. Their wavelengths are 3 centimetres or 10 centimetres, with pulse widths of 0.5 to 1 microsecond. And 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. And third, Surveillance Radar Approach, SRA. So this radar is the workhorse for bringing aircraft in on approach. Then we have the Airport Surface Movement Radar, ASMR. This is also known as the Airfield Surface Movement Indicator, ASMI. It's installed at major airfields to provide a very accurate radar display — in all weathers and all conditions of visibility — of the aerodrome infrastructure: the taxiways, runways, aprons, and so on, plus vehicular traffic and aircraft that are stationary, taxiing, landing, or taking off. The design goal is a detailed, bright, flicker-free display of all aircraft and vehicles on runways and taxiways, so that Air Traffic Control Officers can be certain runways are clear before landings or take-offs, and to ensure safe, orderly movement on the taxiways. The processing can remove selected fixed features, leaving the moving targets on runways and taxiways clearly visible. Now, achieving that very high definition requires some very specific radar parameters. The beam must be very narrow — on the order of 0.2 to 1 degree. The scanner rotation rate is 60 rpm — much faster than the long-range radar. The PRF is on the order of 4000 to 20,000 pulses per second. Pulse widths are extremely short — on the order of 0.03 microseconds. The frequencies are 15 to 17 gigahertz, which is SHF — Super High Frequency — corresponding to wavelengths of 2 to 1.76 centimetres. And the ranges are 2.5 to 6 nautical miles in light precipitation. And there's an important limitation here. Those high frequencies required for ASMI mean the transmissions are increasingly attenuated and absorbed as the intensity of precipitation increases. So in heavy rain, the performance of this radar degrades — that's a direct consequence of the frequency choice needed for the high definition. So the pattern across this chapter is clear: the closer the radar's job is to the ground and the finer the detail needed, the shorter the wavelength, the shorter the pulse, the higher the PRF, and the faster the scan. Long range buys you reach at the cost of detail; surface movement buys you detail at the cost of range and weather penetration.