Long Distance Communications

by on June 22, 2009

There has been some discussion recently about long distance communications, specifically in relation to what could or should have been transmitted from Air France flight 447 before it disappeared on it’s flight from Rio De Janeiro (GIG) to Paris (CDG) on June 1st. Simeiz RT-22 telescope (2005-09-285) by Argenberg There appear to be some misconceptions about what types of communication are used between aircraft and Air Traffic Control and a confusion between Air Traffic Control and radar contact on longhaul flights. I thought a little information about how my international flights are conducted might clear up some of the confusion.

The majority of my flights  pass in and out of radar contact several times between take-off and landing. We start out the flight using the VHF radio to talk to Air Traffic Control (ATC). While we are using VHF communications we keep our company updated on our progress by manually sending position reports directly to them using the ACARS datalink system at approximately 90 minute intervals. Eventually we fly out of VHF (line-of-sight) range and switch to the High Frequency (HF) radio for long-range communications.  At that point we are also usually out of radar contact. Radar, like VHF communications,  is a ground-based and line-of-sight system. Once an aircraft is beyond the reach of the radar signals the separation of aircraft flying on the same route is accomplished through position reporting to an ATC facility.

When we switch to the HF radio we are usually no longer talking directly to Air Traffic Control. Some remote areas over land use HF for communication, however the majority of our HF work is accomplished in the overwater portion of our flights. We pass position reports and altitude change requests to an operator working for ARINC (Aeronautical Radio, Incorporated). ARINC then passes the position information to both the agency controlling the flight (ATC) and to the company operating the aircraft. Altitude or routing change requests are relayed to ATC through ARINC.   ATC then advises ARINC that a requested change is either permissible or not and issues a clearance for the aircraft. ARINC  then calls the aircraft and relays the clearance, requests a readback confirmation and, if it is an altitude change or weather deviation clearance,  requires a call when the altitude change has been completed or when the weather deviation is no longer necessary and the aircraft is back on its assigned course.

The aircraft HF radio has a Selective Calling (SELCAL) feature that permits ARINC to activate a chime on the flight deck by transmitting the aircraft’s specific SELCAL code. The SELCAL feature allows the pilots to turn down the HF radio reception and know that if ARINC needs to pass information the chime will indicate an incoming call. Radio reception on the HF radio is effected by any type of electrical disturbance from solar flares to static build-up on the aircraft due to flying through clouds. Listening to the HF radio is like turning your am radio dial to a place where there is no radio station and listening to the white-noise hash. HF radio signals can travel around the curvature of the earth by bouncing off the ionosphere. Single frequencies can be used over huge areas for position reporting. My last flight used a Polar route between the U.S. and Shanghai, China. Flying through the area north of Alaska we were making our HF position reports to the same ARINC operator who was also handling all of the traffic in the northwestern part of the Atlantic for flights between the U.S. and Europe.

Since ARINC passes HF radio position reports to our company, duplicate ACARS position reports are not required. The ACARS system on our aircraft has the ability to use either the VHF, HF or SATCOM, so we are never without datalink capability. We also have the ability to use SATCOM communications to speak directly with our company dispatcher or maintenance control to discuss enroute conditions.

Newer aircraft have a system referred to as CPDLC or Controller-Pilot Datalink Communications. When an aircraft would normally switch to HF communications, CPDLC contact between Air Traffic Control and the aircraft is established and verified. Position reports can then be automatically or manually sent to the controlling agency without using voice communications and the pilots can make altitude or routing requests directly to ATC using CPDLC messaging.

This is an excerpt from a manual covering some of the capabilities of the CPDLC system used in one of the newer long-haul transport aircraft.

Automatic Dependent Surveillance (ADS)
Automatic Dependant Surveillance (ADS) automatically downlinks position, speed, heading, altitude to ATC in areas where no radar coverage exists. ADS provides essentially the same information to ATC as they would have if the aircraft was in radar contact. ADS allows real time tracking of aircraft positions with reduced separation and better ATC services than is normally possible in areas without ATC radar coverage. Using ADS, the aircraft automatically transmits position reports, meteorology reports, and other data relevant to the flight to as many as four connected ATC facilities.

Different types of ADS downlinks exist:
• Periodic – Data is sent at periodic time intervals.
• On demand – Data is sent only when ATC requests it.
• On event – Data is sent whenever a specified event occurs (e.g., waypoint passage, altitude or heading changes, direct to, excess vertical speed).
• Emergency mode – Activated by the crew, data is automatically sent every minute.
The content and the frequency of the downlinks are defined by ATC to satisfy their operational needs. There is no way for the crew to know when a downlink occurs.
Automatic position reports using CPDLC are only necessary if ADS is not available or is lost. Loss of ADS is not necessarily caused by an aircraft malfunction. It can also be caused by reception or ground network interconnection problems. The crew will be notified by ATC if ADS is no longer available.
Position reports can be made using one or more of the following five methods:
• ADS generated position reports (no crew action required)
• In response to an uplinked ATC position report request
• Automatic position reporting using CPDLC
• Manual position reporting using CPDLC
• Traditional voice reports

I don’t know what type of communication system that the Air France flight or the ground station had available. It could be that the aircraft was equipped with ADS but the ground station was not or the electrical interference due to the storms in the area may have prevented the system from operating normally. It is also possible that the aircraft had as much as an hour between mandatory position reporting points.

As a side note, ADS is a portion of the ADS-B system that has been in operation in Alaska and is now being installed in facilities along the eastern coast of the U.S. The ‘B’ in ADS-B stands for Broadcast. When the system is fully operational and your aircraft is equipped with ADS-B transmitters/receivers/displays your cockpit display will show the same traffic information that the ATC controller has available.
I hope that this gives you a little more information about how communications are conducted on the long haul flights.

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