COMMUNICATION METHODS IN AVIATION
I am sure that many of us think that how to an aircraft in the sky communicates with the ground and other aircraft during the flight. When you consider that although there are hundred of thousands flights all over the world, crash percentage is really less than other transportation way, the question is really logical. Airmen know that there is no safety flight without the communication. Pilots should give and take informations about route, weather, position or unexpected situations to perform a safe flight by acting in accordance with the specific communication rules and instructions. Let we initially start with the history of communication.
History of The Communication in Aviation
At the first time in aviation, the communication was conducting by flags, colored objects, hand signs or other visual aids. These firstly used methods were useful for ground crew because they could send the message to pilot but there was no backing messages from the pilot. After a while, Morse code had been the first wireless ground to air and later air to ground communication method. When calendars show the year of 1915, the first air to ground voice transmission took place thanks to radio. In 1938, the first point-to-point establish was used in WW II. Now, there are fortunately various tools both in aircrafts and on the ground to communicate.
- Voice Communications
Verbal speaking has been spreading day-by-day in aviation since the invention of radio technology. It is taken place in ionosphere layer of the atmosphere as ground-to-ground, ground-to-air and air-to-air via phone and radio communication devices without any code. Whereas there are more ionized particules in the ionosphere during day, less particules during the night. Consequently, higher frequency communications are more suitable in daytime and lower frequency is better in nighttime. Besides, effectivity of a verbal communication is also related to the range and obstacles.
When you look at above formula, frequency and wavelength are inversely proportional to each other so the higher frequency, the less coverage area in the communication.
UHF (Ultra High Frequency)
UHF frequency band is between 225.0 and 399.95 MHz. Its primary usage area is military aircrafts. They have special UHF band for air-to-air and air-to-ground communications. Additionally, it is used in ILS (Instrument Landing System) and DME (Distance Measuring Equipment) operations. Though its quality is more effective, its range is narrower than other frequencies due to limited UHF coverage.
VHF (Very High Frequency)
New generation airplanes have three VHFs identified as “VHF Left”, “VHF Center”, and “VHF Right” in order to provide using in different frequencies and compensate a VHF failure. It is probably a mostly used telecommunication method in aviation. VHF band uses the frequency from 108 to 137 MHz. Low frequency band range between 108–117.95 MHz is used for ILS and VOR, high frequency band range between 118–137 MHz is also used for the communication. Its coverage area can be affected by obstacles such as mountains and hills. In addition to this, because VHF has more energy, it is able to go out to space throughout the atmosphere. That is why VHF is affected by the curvature of Earth. To calculate VHF range, below formula could be applied theoretically.
In the formula, unit of the range is in nautical miles, Ht is height of transmitter in feet, and Hr is the height of receiver in feet. So, if an aircraft flies at higher level or the antenna at higher location, the voice would be probably clearer.
HF (High Frequency)
Modern aircrafts have two HF communication as HF-L and HF-R. Its band is 3 MHz to 30 MHz. Its mainly usage area is transatlantic flights. For example, while an airplane is flying over areas where VHF covering is sufficient, ground-air communication would be easily carried out; however, that aircraft flies over the oceans, it would probably need at least one HF radio. Moreover, HF method is used for remote sensing devices such as UAV (Unmanned Aerial Vehicle) and drones. So, which parameter(s) does HF make appropriate facility method for a long range communication?
According to above formula, HF is possessing lower energy, therefore ionosphere that consists of multi-layers formed by Sun UV reflects HF radio waves to the atmosphere. Because of which, it is more spreaded out than UHF and VHF radio waves throughout the atmosphere. As the result, an aircraft easily takes radio signals.
SELCAL (Selective Calling)
SELCAL that comes from ‘Selective Calling’ is a ground-to-air radiotelephony communication. It works in VHF and HF radio bands. It is critical to remember that both the ground station and the aircraft need to be operating on the same HF or VHF frequency for the system to operate properly. It consists of two pair codes and totally four letters. It is made up from A to S letters excluding I, N, and O. Repetitive (KK-LM) and non-order letters (AB-DC) are not allowed to use. Each codes are converted to different audio tones by a decoder. So, codes represent an audio voice, additionally each pair has 1 second calling voice and the silence time between the pairs is 0.2 second. When there is an incoming message from ground stations, SELCAL would be alerted. It is not always turned on because the main aim is to avoid continuous listening in HF and VHF frequencies.
In first years, SELCAL technology was under the control of ICAO in 1957. After a while, ARINC (Aeronautical Radio Incorporated) became official registrar of SELCAL. In 2006, it was transferred to ASRI (Aviation Spectrum Resources, Inc). ASRI is now the registrar of SELCAL all over the world.
Due to limited SELCAL code, now there are 16 tones; therefore, some aircrafts can carry same codes. Consequently, the biggest problem in SELCAL is duplicate codes for aircrafts that is why flight number and aircraft registration are able to be used as a SELCAL code in order to reduce the confusion. The radio operator considers geographical separations to prevent code conflicts because to be of aircrafts that have a same SELCAL code is not always possible at the same FIR or same HF frequency.
SATCOM (Satellite Communication)
Satellite communication is more technological, basic and lively method in the telecommunication. If an airplane is out of the range of VHF coverage or HF, the voice quality could be very low. In this case, SATCOM would be an alternative for both of them.
As you see the image, SATCOM allows to reach ATC or to make and take calls to ground stations and data link communication wherever you are. It is performed by more than 10 geostationary satellites weaker over polar regions. Its benefits are also mainly clear communication, to get weather info and updated flight plans for crews. As well as for passengers, the system provides internet connection, live TV and SMS messaging during the flight.
2. Data Communications
When there is no verbal communication possibility, data link communications are able to be as an alternative. Data communications will importantly reduce workloads of both pilots and controllers. It could be performed in order to communicate with ATC to take the clearance, and prevent high delays and ground departments such as OCC and MCC. This communication methods have a few devices for writing a message.
ACARS (Aircraft Communications, Addressing and Reporting System)
ACARS provides character based connectivity in the communication. An aircraft on the ground or in the air keeps in touch with ground stations via air band radio or SATCOM facilities while using ACARS. It was designed in 1978 by ARINC (Aeronautical radio Inc.) in the telex messaging format. Now it is based on aircraft equipments, service providers, and ground processing systems. Aircrafts must have CMU (Communication Management Unit) and printer or most advanced system CDU (Communication Display Unit) that consists of seven various systems such as FMS. Additionally, CDU could be integrated with EFBs with new technologies. The aircraft easily takes updates of flight plan, WX, and NOTAM by the way of FMS. Moreover, if there is a mechanical problem or an aircraft did a hard landing, ACARS directly sends a message to the ground station and then responsible team would be ready to fix it. Its mainly usage phases are called OOOI (out of the gate, off the ground, on the ground, into the gate). These OOOI events are detected by sensors where doors, landing gears, and parking brakes of aircrafts.
It has three messaging formats such Air Traffic Control, Aeronautical Operation Control, and Airline Administrative Control. Nowadays, there are mainly two service providers for the system as ARINC based on Chicago and SITA based on Singapore. ARINC is very effective in the USA and SITA is widely used for Europe. Ping messages are test communications in order to know whether the system runs or not. Finally, manual messages can be used by pilots to take and send messages to the ground.
CPDLC (Controller Pilot Data Link Communications)
Since initial times of aviation, amount of air traffic has been increasing day by day. This increment impacts on ATC controllers, so their workloads are higher than previous times due to more traffic flows. CPDLC is a system that was created in order to decrease workloads of ATC, provide more secure and clear communications, less communication on the ATC frequency, and increase sector capabilities. It is an air-to-ground communication method for the communication between ATC controllers and pilots by using satellites. It is a secondly alternative to communicate instead of VHF over lands or HF over oceans. The communication carried out by writing in free text format between ATC and the aircraft includes ATC clearance, heading, deviation, speed, increase or decrease to FLXXX etc… There are two main application ways of CPLDC: FANS (Future Air Navigation System), which is ACARS based system, was developed by Boeing and Airbus for wide bodied aircrafts of long haul and oceanic flights; ATN (Aeronautical Telecommunication Network)/CPDLC operational at Eurocontrol FIRs should be equipped on the aircraft. Now, CPDLC is required for flights above FL285 over Eurocontrol areas but servicing altitude can be different regarding to the FIR. To set the system up, the pilot firstly log on to the ATC unit the pilot wish to connect to. Each unit has own four letter code like EGTT, EDUU, ESOS, and LKAA.
TRANSPONDER
Transponder is a device that receives and sends data link to or from ground stations and other aircraft. It is the abbreviation of the words of transmitter and responder. Transponder was firstly invented to identify friend or foe (IFF)aircrafts in military aviation during the World War II. Right after, it has been used in civil aviation as well. New generation aircrafts carry two transponders, one of them is operative and other one is back-up position. Before the flight, ATC assigns a discrete transponder code known as a Squawk code to uniquely identify and call them in a FIR (Flight Information Region). Pilots enter the squawk code into the transponder and then ATC gets informations such as position, altitude, speed, and special cases of an aircraft. Squawk code should be a four digit number between 0000 and 7777 and it is called like PC620, squawk 1234.
Transponder has also special codes such 1200 (Under VFR), 7500 (hi-jacking), 7600 (Radio Failure), and 7700 (Emergency). In addition to this, transponder has three modes as Mode A, Mode C, and Mode S. All modes have an IDENT phase. There is no altitude information in Mode A except close areas because it uses primary radar. Mode C gives additionally the altitude information as 100 ft by the way of SSR (Secondary Surveillance Radar). Mode S comes from the term of ‘Mode Select’ and it was developed to overcome Mode A/C limitations. It has more than 17 million 24-bit aircraft addresses. It gives 25 ft intervals of altitude thanks to SSR as well. It is used for TCAS and ACAS II functions and it is integrated with ADS-B system; moreover, Mode S is mandatory for busy ATC areas to fly over.
ADS-B (Automatic Dependent Surveillance — Broadcast)
Today many of aircrafts are tracked by ground radars. This tracking is easily monitored by ATC, thanks to transponders. However, the tracking based on transponders is insufficient over oceans, polar regions, mountain regions, and deserts where lack of ATC facilities. As a result, approximately 30 % of aircrafts are able to be followed. So, it is really hard to detect the position of an aircraft over those areas in case of a crash. After the Malaysia B777 crash, developments of the telecommunication that depends on satellites have accelerated. ADS-B system broadcasts identity, position (latitude and longitude), altitude, and speed of an airplane, drone, UAV, or a vehicle two times in a second by the way of satellites. ADS-B Out messages are captured by ground stations and other aircrafts (ADS-B In). Thus, ATC gets the information about ADS-B Out aircrafts and consequently it easily separates from each other to prevent a collision. To use ADS-B system, aircrafts must have GPS or GNSS receiver and an ADS-B data link. The most common data link in the usage is Mode S transponders operated at 1090 MHz and 978 MHz and additionally new generation aircrafts have transponders adapted to ADS-B. Thereby, operators do not have to equip an aircraft with an additional device. In a short, as long as the transponder is turn-on, that aircraft is tracked all over the world. To improve a non-turned-off transponder is particularly considered in order to prevent the loss of the aircraft tracking. In this way, wherever an aircraft is, it could be found.
