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

Let’s start with Mode C, because it’s the heart of this page. When the aircraft receives a Mode C interrogation, the transponder produces an ICAO-determined code that corresponds to its height, referenced to 1013 hPa. That reference is critical — it means the code is based on the standard pressure setting, not whatever the pilot has dialled into the altimeter. So the code is totally independent of the altimeter’s pressure setting and independent of the code selected on the transponder. The Mode C code is determined by an encoder, which is mechanically actuated by the altimeter’s aneroid capsule. The aneroid capsule is the little bellows inside the altimeter that expands and contracts with pressure — so the encoder is physically driven by that capsule, which is why it can’t be fooled by a pressure setting. The system provides Automatic Altitude Telemetering up to 128,000 ft. That’s the maximum altitude it can report. The output changes based upon 50 ft increments or decrements, but the change is transmitted every 100 ft. So the encoder senses in 50 ft steps, but the code updates only every 100 ft. This gives the controller the aircraft’s Flight Level or Altitude. For example, if an aircraft is flying at an allocated level of FL65, then 065 will be displayed on the screen. Now, if the aircraft drifts downwards, as it passes from 6450 ft to 6445 ft, the coded transmission changes and results in 064 being indicated at the controller’s console. So you see the step — it doesn’t show every foot, it shows the 100 ft step. Now the SSR Operating Procedure — these are the pilot’s obligations. 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. Second, you may select or reselect codes, or switch off the equipment when airborne, only when instructed by an ATS unit. Third, you must acknowledge code setting instructions by reading back the code to be set. Fourth, you must select Mode C simultaneously with Mode A unless otherwise instructed by an ATS unit. And fifth, when reporting vertical levels under routine procedures or when requested by ATC, you must read the current altimeter reading to the nearest 100 ft. This is to assist in the verification of Mode C data transmitted by the aircraft. Now the notes. Note 1: if, on verification, there is a difference of more than 300 ft 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. That’s the ICAO specification. Note 2: a standard of ±200 ft is applied in the UK and other countries. So the tolerance is tighter in the UK — ±200 ft — versus the 300 ft threshold in the ICAO note. Let me show you the radar picture. — that’s primary and secondary radar used for ATC. — that’s a radar display showing aircraft positions in the London TMA. And — that shows SSR operates in the UHF band. So the key takeaway: Mode C gives automatic altitude reporting, independent of the pilot’s altimeter setting, driven by the aneroid capsule, stepping every 100 ft up to 128,000 ft. And the operating procedure is all about discipline — maintain codes, change only when instructed, read back, select Mode C with Mode A, and report levels to the nearest 100 ft for verification.