Secondary Surveillance Radar (SSR) — Page 235, Lesson 220

We're starting a fresh topic now: Secondary Surveillance Radar, or SSR. I want to walk you through the Mode C function first, because that's where this excerpt begins. Mode C is the part of the transponder that reports altitude. When the aircraft receives a Mode C interrogation, the transponder produces an ICAO-determined code that corresponds to its height. Now, here's the critical part: that height is referenced to 1013 hPa, regardless of the pressure setting on the altimeter and regardless of the code selected on the transponder. So the Mode C code is completely independent of what you've dialled into your altimeter sub-scale. How does it work? The Mode C code is determined by an encoder, and that encoder is mechanically actuated by the altimeter's aneroid capsule. The aneroid capsule is the pressure-sensitive element inside the altimeter. Because it's mechanically driven, the whole system is totally independent of the altimeter's pressure setting. That's the key contrast: your displayed altitude changes with QNH, but Mode C always transmits pressure altitude referenced to 1013 hPa. The system provides Automatic Altitude Telemetering up to 128,000 feet. That's the upper limit of the reporting range. The output changes based upon 50-foot increments or decrements, but it updates every 100 feet. So the code steps in 100-foot intervals, and each step represents a 50-foot change in the underlying value. Let me give you the example from the text. If an aircraft is flying at an allocated level of FL65, then 065 will be displayed on the controller's screen. Now, if the aircraft drifts downwards, as it passes from 6450 feet to 6445 feet, the coded transmission changes and results in 064 being indicated at the controller's console. So you see, a 5-foot drift across a 100-foot boundary flips the displayed flight level. Now let's move to the SSR Operating Procedure. These are the pilot's obligations, and I want you to treat them as a checklist of duties. First, if you're proceeding from an area where a specific code has been assigned to the aircraft by an ATS unit, you must maintain that code setting unless otherwise instructed. ATS is Air Traffic Services. Second, you select or reselect codes, or switch off the equipment when airborne, only when instructed by an ATS unit. So you don't touch the transponder code in the air on your own initiative. Third, you acknowledge code setting instructions by reading back the code to be set. That's the read-back requirement. Fourth, you select Mode C simultaneously with Mode A unless otherwise instructed by an ATS unit. Mode A is the identity code; Mode C is the altitude. They go together by default. Fifth, when reporting vertical levels under routine procedures or when requested by ATC, you read the current altimeter reading to the nearest 100 feet. This is to assist in the verification of Mode C data transmitted by the aircraft. Now there are two notes on verification. Note 1: if, on verification, there is a difference of more than 300 feet between the level read-out and the reported level, the pilot will normally be instructed to switch off Mode C. If independent switching of Mode C is not possible, the pilot will be instructed to select Code 0000 to indicate transponder malfunction. And the text notes that this is the ICAO specification. Note 2: a standard of plus or minus 200 feet is applied in the UK and other countries. So the ICAO threshold is 300 feet, but the UK applies a tighter standard of 200 feet. Let me tie that together. The Mode C encoder gives you pressure altitude in 100-foot steps, referenced to 1013 hPa. The controller compares that read-out against what you report. If the difference exceeds the threshold, you either switch off Mode C or squawk 0000 to flag the malfunction. That's the whole verification loop. I've got a figure here that shows the difference between primary and secondary radar used for ATC — that's . And there's another showing an SSR display in the London TMA at . The band information is in .