Secondary Surveillance Radar (SSR) — Page 235, Lesson 223

Let's start with the special purpose codes, because these are the ones you'll actually be selecting in the cockpit when things go wrong. First, code 7700 — this indicates an emergency condition. You select it as soon as is practicable after declaring an emergency, and you have due regard for the overriding importance of controlling the aircraft and containing the emergency. But here's the nuance: if you're already transmitting a discrete code and receiving an air traffic service, that code may be retained at the discretion of either the pilot or the controller. So 7700 isn't always mandatory — it's the default for an emergency, but a discrete code already in use can be kept if either party decides it's better. Next, 7600 — this indicates a radio failure. Simple and direct. Then 7500 — this indicates unlawful interference with the planned operation of the flight, unless circumstances warrant the operation of code 7700. So if you're being hijacked, 7500 is your code, unless the situation is such that 7700 is more appropriate. Now, the Conspicuity Code. When you're operating under VFR rules at and above FL100 in European airspace, you should select code 7000 + Mode C. In North America, that same conspicuity code is 1200 + Mode C. There are two exceptions: when you're given a different setting by an ATS unit, and when circumstances require one of the special purpose codes. Below FL100, you still select the Conspicuity Code and Mode C, subject to the same exceptions. And note this important line: Mode C should be operated with all of the above codes — so altitude reporting is on regardless of which code you're using. Now let's talk about the disadvantages of SSR. Air Traffic Services in Europe have increased their reliance on SSR — it gives data on an aircraft's position, identification, altitude, speed, and track. But the existing civil system — Mode A for identification and Mode C for altitude reporting — is reaching the limits of its operational capability. There are two key disadvantages. First, garbling. This is caused by overlapping replies from two or more transponders on nearly the same bearing from the ground station, and within a distance of 1.7 NM from each other, measured on a line from the antenna. The reply pulses from the aircraft are transmitted over a period of 20.3 microseconds, which relates to a distance of just under 1.7 NM in terms of radar miles. So if two aircraft are that close on the same bearing, their replies overlap and the ground station can't separate them. Second, fruiting. This is interference at one interrogator caused by replies from a transponder responding to interrogations from another. So your transponder replies to a different ground station, and that reply gets picked up by the station that's trying to interrogate you — that's fruiting. Finally, the availability of codes. Only 4096 identification codes are available in Mode A. That's the total pool — and it's a big reason why the system is reaching its limits. Let me show you the radar display so you can see how this all fits together in practice. And here's the key point about the frequency band — SSR operates in the UHF band. So to tie it together: you've got your special codes for emergencies — 7700, 7600, 7500 — your conspicuity code for normal VFR operations — 7000 in Europe, 1200 in North America — and you've got the system's limitations: garbling from overlapping replies, fruiting from cross-interrogation, and only 4096 codes available in Mode A. That's the full picture of SSR as it stands today.