Control of Aircraft — Page 326, Lesson 431

Let's start with the very foundation here. The system that gives you that radar return you see on a screen is called Primary Surveillance Radar, or PSR. That's the raw echo — the blip — that comes back from the aircraft itself. Now, every radar system can be enhanced with something called Secondary Surveillance Radar, or SSR. This is the key part: SSR doesn't just show a blip, it gives you flight-specific identification and altitude information. So while PSR shows you something is there, SSR tells you who it is and how high it is. And radar isn't just for surveillance. It can also be used to provide precision or non-precision approaches — we'll get into what those mean shortly. Now let's look at Radar Services — Section 17.6. At its most basic level, radar is used to update the information on the flight strips. Think of flight strips as the paper or electronic records ATC uses to track each aircraft. In this basic mode, radar is just augmenting a procedural system — it's helping confirm what the procedures already tell you. But here's where it gets powerful. If the radar response from a specific aircraft can be individually determined — that is, identified — then you can provide separation from other radar contacts. This is the big shift: now the radar-derived information is used directly to provide separation, and it gives you a much greater degree of accuracy than the procedural system alone. Let me walk you through the four types of radar service: First, Radar Control — this is for controlled en route aircraft. That's aircraft flying between airports in controlled airspace. Second, Approach Radar Control — this is for arriving and departing controlled traffic. So it handles the aircraft coming into and leaving an aerodrome. Third, Radar vectoring — this is the provision of navigation instructions to an aircraft to achieve a specific aim. For example, positioning an aircraft to intercept the ILS localizer — that's the Instrument Landing System's lateral guidance beam. Fourth, PAR and SRA as instrument approach systems. PAR is Precision Approach Radar, and SRA is Surveillance Radar Approach. These are radar-based approaches — PAR is precision, SRA is non-precision. Now, there's a crucial limitation here. As with all ATC procedures, a radar service is only provided inside CAS — that's Controlled Airspace. But ICAO does permit radar to be used outside CAS for one specific purpose: to obtain information to enable the provision of an FIS — a Flight Information Service. Radar can also assist in providing information as part of the Advisory ATC service in Class F airspace. That's the advisory service — it gives information and advice, not full control. And here's a subtle but important point about aerodromes: radar may be used by the aerodrome controller to determine the separation between departing aircraft — but not to actually apply that separation. So the controller can see the separation on radar, but the actual application of separation is done through other means. Let me show you what these radar heads actually look like — Figure 17.1 shows the ATC radar head at Clee Hill in Shropshire, and Figure 17.2 shows the one at London Heathrow. So to tie it all together: PSR gives you the raw blip, SSR adds identity and altitude, and the radar services — Radar Control, Approach Radar Control, vectoring, and PAR/SRA — all use that information to provide separation and guidance, but only within the proper airspace classifications and with the proper limitations.