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Home > Air Travel > How planes are kept in contact with the air traffic controllers over the Ocean?
How planes are kept in contact with the air traffic controllers over the Ocean?
Flying - Air Travel
Tuesday, 15 July 2014 15:37

Funny ATC (VHF Communication) as President Obama Departs Boston

Hi Captain Lim,

I am flying from Toronto to London and I was wondering how planes are kept in contact with the air traffic controllers once over the Atlantic Ocean.

Thank you.


Hi Ryan,

The pilots of the planes are always in contact with the air traffic controllers (and the airlines personnel on the ground as well). Both of them are following and tracking the planes all the times.

How they go about doing that?

Well, various medium are used depending on where you are. The most common ones are the radios, either VHF or HF (Very High Frequency or High Frequency). VHF has a range of around 250-300 miles (based on line-of-sight range). It is very clear and used all along the route from Toronto to London as long as the plane is within 250 miles from any ground stations.

The distance of Toronto to London is around 3550 miles (5713 kilometers, 3085 nautical miles)

Once the plane is out of VHF radio range, then the pilot can use the HF radios. The HF does not depend on line-of-sight transmission but can follow the curvature of the earth. Although it has longer range, it suffers from night effect and become not very clear at times – so not a very good alternative.

Today most modern planes use more advanced communications (CPDLC, ADS, SATCOM – Google them for the technical explanations) to talk to the ground. CPDLC is quite similar to you using the SMS function of your phone.

Hence normal communication over the ocean is through text messages. However voice communication via the satellites (SATCOM) is also available. Since the cost of this communication is more expensive than text messages, SATCOM is restricted to urgent or emergency communications

Additionally, your plane from Toronto is also tracked all the way by radar as well as by ADS (Automatic Dependant Surveillance – a technology that allows the air traffic controllers to see traffic with more precision)

However, radar range is limited – similar to VHF radio range. Further tracking of the plane along the 3550 miles route would now be done by the ADS once radar coverage becomes unavailable.

So within the 250 miles when the plane is tracked on radar, for civil planes, this is performed with the transponder that allows the air traffic controller to see, for instance, how fast or how high the plane is flying at.

I believe there was some confusion on the understanding of the transponder operation in one recent event.

Could the transponder be switched off in flight?

Yes, it is possible. On the ground, this is done firstly to avoid cluttering up air traffic control radar when the plane is parked at the gate. It is then switch on shortly before taxiing out and then switched off again (on standby mode) after docking in at the next destination.

In case of an emergency such as a fire or electrical system problem, the transponder can be switched off (isolated) through a circuit breaker.

So the transponder, when it is in in the air, would provide all its function only within radar coverage. Once out of radar range, its function would be taken over by the ADS.

Hence, your trip from Toronto to London is within a call away from the ATC at anytime. Furthermore, your plane is positively tracked by the ground ATC as you passed by the various airspaces.

How Radar and Transponder work?


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thanks for answering my question!
ryan , 09 Aug, 2014

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