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

Let's start with the special purpose codes, because these are the ones that can save your life and the lives of others. These are codes reserved internationally, and they are selected by the pilot in specific situations. The first is 7700, which indicates an emergency condition. You should select this code as soon as is practicable after declaring an emergency, but with due regard for the over-riding importance of controlling the aircraft and containing the emergency. In plain terms, don't fumble with the transponder while the aircraft is still in a critical state — fly the aircraft first, then set the code. There's an important exception here: if you are 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 is not always mandatory if you're already being positively identified and handled. Next is 7600, which indicates a radio failure. Simple and clear. Then 7500, which indicates unlawful interference with the planned operation of the flight — that's your hijack code. Note the qualifier: you use 7500 unless circumstances warrant the operation of code 7700. So if the unlawful interference escalates into an emergency, 7700 takes precedence. Now, the Conspicuity Code. When operating under VFR rules at and above FL100 — that's Flight Level 100, 10,000 feet on the standard pressure setting — in European airspace, you should select code 7000 + Mode C. In North America, the equivalent is 1200 + Mode C. There are two exceptions: when given a different setting by an ATS unit, and when circumstances require the use of one of the special purpose codes. When operating below FL100, you still select the Conspicuity Code and Mode C, subject to the same exceptions. And importantly, Mode C should be operated with all of the above codes — so altitude reporting is always on. Let me clarify what Mode A and Mode C mean, because they come up right after this. Mode A is the identification code — that's the four-digit squawk you set. Mode C is altitude reporting — the transponder transmits your pressure altitude to the ground. So when we say "7000 + Mode C," we mean the conspicuity squawk with altitude reporting active. Now let's move to the disadvantages of SSR. Air Traffic Services in Europe have increased their reliance on SSR, which provides data on an aircraft's position, identification, altitude, speed, and track. But the existing civil Mode A and Mode C system is reaching the limits of its operational capability. There are two key disadvantages you need to know. The first is 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. Here's the physics: 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 within that 1.7 NM range on nearly the same bearing, their replies overlap in time and the ground station can't separate them — the signals garble together. The second disadvantage is fruiting. This is interference at one interrogator caused by replies from a transponder responding to interrogations from another. In other words, your transponder replies to every interrogator that asks, not just the one you're working with. Those replies can be picked up by a different ground station as noise, corrupting its picture. Finally, there's the availability of codes. Only 4096 identification codes are available in Mode A. That's because the code is four digits, each ranging from 0 to 7 — octal digits — giving 8 to the power of 4, which is 4096. With that many aircraft in busy airspace, the code pool gets tight, which is part of why the system is reaching its limits. Let me show you how this all fits together on the radar picture. Here you can see primary and secondary radar used for ATC — primary radar bounces energy off the aircraft skin, while secondary radar relies on the transponder replying to the interrogation. And here's a radar display showing positions of aircraft in the London TMA — that's the Terminal Control Area. Finally, note that SSR operates in the UHF band — Ultra High Frequency. So to tie it together: you have your special codes — 7700 for emergency, 7600 for radio failure, 7500 for unlawful interference — plus the conspicuity codes of 7000 in Europe and 1200 in North America, all with Mode C altitude reporting. And you understand the two big limitations of the system: garbling from overlapping replies within 1.7 NM, and fruiting from cross-interrogator interference, all constrained by the 4096 available Mode A codes.