All modern aircraft are assigned a unique ICAO bit address or informally Mode-S "hex code" upon national registration and this address becomes a part of the aircraft's Certificate of Registration. Normally, the address is never changed, however, the transponders are reprogrammable and, occasionally, are moved from one aircraft to another presumably for operational or cost purposes , either by maintenance or by changing the appropriate entry in the aircraft's FMS system.
There are 16,, unique ICAO bit addresses hex codes available. The ICAO bit address can be represented in three digital formats: hexadecimal , octal , and binary. These addresses can be decoded and converted amongst each other online using tools such as those at Airframes. Whichever format is used, the same information is carried through the signal. These all correlate to the same aircraft registration , NNA. Mode S TIS Secondary surveillance radar SSR , or Traffic Information Service, allows a radar installation to send information about nearby traffic back to aircraft, which then displays it on the moving map.
Mode S TIS is only available when the aircraft is within radar range of a radar installation that supports it. Mode S capable transponders are also a building block for next generation air traffic control systems, as they can be used to transmit location information for ADS-B and potentially other air traffic control communications.
One major issue with Mode S transponders is that pilots have frequently been entering the wrong "flight identity" i. A pilot may be requested to squawk a given code by the air traffic controller via the radio, using a phrase such as "Cessna AB, squawk ". The pilot then selects the code on their transponder and the track on the radar screen of the air traffic controller will become correctly associated with their identity. Because primary radar generally gives bearing and range position information, but lacks altitude information, mode C and mode S transponders also report pressure altitude.
Around busy airspace there is often a regulatory requirement that all aircraft be equipped with an altitude-reporting mode C or mode S transponders. In the United States, this is known as a Mode C veil.
Mode S transponders are compatible with transmitting the mode C signal, and have the capability to report in 25 foot increments. Without the pressure altitude reporting, the air traffic controller has no display of accurate altitude information, and must rely on the altitude reported by the pilot via radio.
This limitation has resulted in at least one accident. On 31 August , a Piper Archer with a pilot and two passengers had inadvertently penetrated the 6,foot floor of controlled airspace without a clearance and collided with Aeromexico Flight , a DC-9 with 58 passengers and 6 crew at an altitude of 6, feet. The Archer had only a mode A squawk reporting capability and the air traffic controller assumed it was below the controlled airspace.
All mode A, C, and S transponders include an "ident" button, which activates a special "thirteenth" bit on the mode A reply known as Ident, short for Identify. When radar equipment receives the Ident bit, it results in the aircraft's blip "blossoming" on the radar scope. This is often used by the controller to locate the aircraft amongst others by requesting the ident function from the pilot i.
Ident can also be used in case of a reported or suspected radio failure to determine if the failure is only one way and whether the pilot can still transmit or receive but not both i. Transponder codes are four digit numbers transmitted by the transponder in an aircraft in response to a secondary surveillance radar interrogation signal to assist air traffic controllers in traffic separation.
A discrete transponder code often called a squawk code is assigned by air traffic controllers to uniquely identify an aircraft. This allows easy identity of the aircraft on radar. Squawk codes are four-digit octal numbers; the dials on a transponder read from zero to seven inclusive. Thus the lowest possible squawk is and the highest is There are combinations of these four digit codes, which is why they are often called " code transponders.
For example, when changing from to an assigned ATC squawk , one might turn the second wheel to a 5 thus , and then rotate the first wheel backwards in the sequence to get to 6. This would momentarily have the transponder squawking a hijack code , which might lead to more attention than one desires. Pilots are instructed not to place the transponder in "standby mode" while changing the codes as it causes the loss of target information on the ATC radar screen, but instead to carefully change codes to avoid inadvertently selecting an emergency code.
The aircraft automatically responds with a series of short pulses that let air traffic control know the identity of the plane and its altitude. Then air traffic control can compare the identity code to flight plans to identify the aircraft. The time taken between the interrogation transmission and the received code lets us know the distance between the radar and the aircraft. Air traffic control computer systems use this information, the direction of the interrogation signal, and the altitude to determine exactly where the aircraft is.
Other navigation and airspace management systems have been developed over the years. Secondary surveillance radar was an important development in the safety of aviation and remains a key element of airspace management today. Read more: Curious Kids: what's it like to be a fighter pilot? Hello, curious kids!
Ask an adult to send your question to us. They can:. Please tell us your name, age and which city you live in. You can send an audio recording of your question too, if you want. Send as many questions as you like! It is incumbent on the pilot to know they must retain their discrete code.
ADS-B is the next step in the evolution of the transponder. By combining satellite information with the transponder output, ATC now has more precise positional information. So there is a good chance that you may encounter ground stations where coverage is less than ideal or not available at all, especially at lower altitudes.
As more aircraft become ADS-B equipped you will begin to see marked improvements in air traffic flow management as ATC has more accurate information on the aircraft in its airspace. Richard Lanning Ph.
Naval Academy and a pilot for more than 30 years. For more great content like this, subscribe to IFR Refresher! Log in to leave a comment. First Domestic Drone Attack Documented. Two Bernies Who Saved Aviation. Is A 5G Showdown Looming? Silent Show-Stealer: Honeywell Anthem.
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