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I have a question on voice/data communication between air traffic control and an aircraft.

I read somewhere that control communicates with an aircraft directly, through a direct radio link, while it's within the line-of-sight. After that control communicates through satellite.

My questions are:

  1. Why doesn't control use satellite throughout the flight?

  2. Are direct radio links used only for voice services? or are they used for data as well?

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    $\begingroup$ For 1, the problem is about who pays for the satellite communication and the equipment. As the answer is nobody wants to ask the passengers to pay, then they use good old ground transmitters. But transponder broadcast is going to be relayed by satellite soon, to track aircraft over the globe (10 new Iridium satellites have been launched in orbit this afternoon by SpaceX) $\endgroup$
    – mins
    Oct 9, 2017 at 23:59
  • $\begingroup$ @mins Thanks. Are ground transmitters located along the travel path of the aircraft? or is there only one based at the airport that adjusts its transmit power as the aircraft moves close to or away from the airport? Aren't there security issues or power safety issues involved in ground to air transmissions? $\endgroup$
    – Teodorism
    Oct 10, 2017 at 0:21
  • $\begingroup$ @mins: Re "nobody wants to ask the passengers to pay", you're perhaps overlooking the large number of non-commercial aircraft out there. Making passengers (or people using air freight) pay for equipment on commercial flights would give you two different, largely incompatible, systems, with all the possibilities for confusion that implies. $\endgroup$
    – jamesqf
    Oct 10, 2017 at 5:27
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    $\begingroup$ RADAR has nothing to do with communication. Communication is done using radio. Satellite communication is in no way normal. $\endgroup$
    – 60levelchange
    Oct 10, 2017 at 5:40
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    $\begingroup$ @ mins: Over the oceans (and above 18K ft) is not really what I (and I expect most of us) think of as ATC. That's more like a few tens of miles/km from the runway. Communication there is an entirely separate problem, and one that's pretty much irrelevant to those of us who fly Pipers, Cessnas, and so on. $\endgroup$
    – jamesqf
    Oct 12, 2017 at 5:22

2 Answers 2

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1) Why doesn't the tower use satellite throughout the flight?

The narrow answer is that the control tower is responsible for a small geographic area. The "tower" doesn't want to talk to the craft at other points in the flight, so the need is not there. It only wants to talk to the plane when it is close to the tower's airspace.

Other components of air traffic control that do need to talk with airplanes in flight also divide up regions of the airspace and have radio equipment situated to talk to the necessary craft.

A larger answer is that ground control and aircraft communications were under major development for decades before satellite communications were available. The system we use today is heavily based on line-of-sight VHF communication radio over most populated areas. Since these systems are already installed in many thousands of aircraft, changing to something different like satellite communication requires a significant benefit, someone to pay for it, and a lot of time to allow for the change.

2) Does a control tower only send voice services through the direct link? or does it send data as well?

Data is beginning to be sent via CPDLC. You can search for CPDLC on this site and find several questions and answers about it, but it is considered only a supplement to the voice communication of ATC today.

Aren't there security issues or power safety issues involved in ground to air transmissions?

Unauthorized transmissions are possible, but they are managed and haven't traditionally been a big problem. There's another question about What prevents someone from hijacking a radio frequency mid-air?. But much of the answer is that since ATC is monitoring the frequency, unauthorized transmissions would be heard and responded to. I'm sure that future digital delivery will change some of these issues, but that's likely to be true whether the message is transmitted via local radio transmitters or satellite.

Not sure if that's what you meant by security issues. I'm not familiar with power safety issues.

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We smaller private planes, the 2-4 seat Pipers, Cessnas, etc. are usually flying lower (except by the Rockies) below 12-14000 feet. Above that special needs come into consideration for Oxygen as the planes are not pressurized (after some time duration above one altitude, and full time above another). We would generally talk to 3 layers of ATC - the ground/tower layer near an airport, then Approach as we start climbing up, and finally Center as we depart the airport area and start cross country at higher altitudes (otherwise, bouncing from one Approach to the next if lower down). I've never been above 12,500 myself, crossing the Rockies and talking to Kasper, WY area ATC (with mountains at 11,500! Nice day for it anyway). I do a lot of east coast region flying, and Approach and Center are the main folks I talk to via VHF radio.

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