Did you know that your airplane has a distinctive identification number that is not your N number? And depending on what type of transponder you have, that special number may be broadcast automatically without you even knowing it.
The special identification number is assigned to every registered airplane. The number is sometimes called the ICAO (International Civil Aviation Organization) number, or can be labeled the Mode S number. The reason it is called the ICAO identification is because unlike the registration (N number) it is unique for the world, not just a single country.
The eight-digit ICAO number is assigned to the airplane and the owner. The ICAO number is essentially invisible; it is like the VIN number on your car while the N number is the license plate. The authority that issues the N number or other registration number matches it with the ICAO number the same way the DMV has methods to make sure the VIN and license plate numbers match the records.
Because different countries use combinations of letters and numbers to identify airplanes there was no easy way to translate something like an N number, or a C and letters from Canada, into an electronic code. The 8 digit ICAO number can be unique around the globe no matter what the registration or call sign may be.
The ICAO identification number system dates back to sometime in the 1970s when the parameters for Mode S transponders were being established. Mode S stands for selectable, as compared to the other transponder Modes of A and C. Mode A is the four-digit code you dial into the transponder, and Mode C is your pressure altitude measured by the altitude encoder.
Modes A and C are really pretty dumb modes. Whenever a ground based radar or an airborne traffic warning system sends out an interrogation a Mode A/C transponder just yells back with its code and altitude. A Mode S transponder can respond with Mode A/C in the same way, but it also has a smart “selectable” mode that waits to be called by name.
The reason Mode S was developed is because it looked like radars interrogating, and Mode A/C transponders all replying, would be stepping all over each other the same way a crowded Unicom frequency becomes unusable when too many pilots transmit at once. Mode S, on the other hand, only responds when it is called by name.
The idea was that the controller’s radar would periodically “call the roll” by sending a general interrogation asking which transponders are within range and all transponders would respond. After the roll call the radar would interrogate specific Mode S transponders by name only as often as needed to keep pace with changes in relative aircraft positions. That would free up a lot of talk time on the transponder frequency.
The FAA planned to phase out Mode A/C transponders by the 1980s, and then by the 1990s and allow only Mode S. A number of other countries, mostly in Europe, actually did require Mode S for much of the airspace, but in the U.S. the transponder frequency crowding didn’t reach the crisis levels that were predicted in the 1970s, so the proposed Mode S requirement deadlines came and went and were never were enforced.
However, Mode S found a new and crucial use when TCAS, the collision avoidance system required on jets, came along. TCAS operates by interrogating transponders on other nearby airplanes. The TCAS calculates the range to the other transponder by timing the round trip of the interrogation and reply, and uses directional ADF-type antenna technology to calculate a bearing to the other airplane. TCAS range is quite accurate, but the bearing is less so.
TCAS knows the range – distance if you want to think of it that way – to an airplane that may be a collision threat, and it knows the altitude of the other airplane because of transponder Mode C, so TCAS can calculate a vertical escape maneuver to avoid a collision threat. TCAS creates what is called a resolution advisory (RA) telling a crew to either climb or descend, and how fast to go up or down, to resolve a collision threat. TCAS can’t calculate a turn to escape because the bearing accuracy just isn’t good enough to provide the time necessary to initiate and complete a heading change.
As you can imagine, when two TCAS-equipped airplanes are a threat to each other the TCAS in each could decide that a descent, for example, is the best avoidance maneuver. But that is a potential nightmare if both TCAS computers tell the crews to escape in the same vertical direction. As both descend or climb together, the box would calculate that an even greater vertical rate of descent or climb is needed and the collision risk would actually increase, not be resolved.
The solution to that problem is to use Mode S and the ICAO identification of each airplane. The TCAS in each airplane asks for and receives the ICAO number of the other over Mode S. Now TCAS knows the name, as it were, of the threat airplane and can make a deal. TCAS in one airplane says, “Hey, you, No. 51010010 (or whatever the number is), you tell your pilots to climb, and I’ll tell my pilots to descend.” Without Mode S to selectively address the specific threat airplane, and without the unique ICAO identification number to know for sure who is who, it would not be possible for TCAS to resolve a potential collision threat between two TCAS-equipped airplanes.
The ICAO number will now be a big part of ADS-B that is slated to replace ground-based radar in 2020. ADS-B will broadcast your ICAO number along with a host of position and velocity information so each and every ADS-B-equipped airplane is positively identified.
Many of us already have Mode S transponders and the ICAO number is programmed (by the avionics installer) into the transponder even if we don’t have TCAS or ADS-B installed. You can see your ICAO (Mode S) number on your aircraft registration card if the card has been issued in the past few years. The number begins with a 5 for U.S. airplanes. When we key the mic we still call ourselves by our registration number or flight number, but before long that almost-invisible eight-digit global identification will be telling controllers and other airplanes in the area exactly who we are without us even knowing that it is happening.