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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?

Teodorism
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    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) – mins Oct 09 '17 at 23:59
  • @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? – Teodorism Oct 10 '17 at 00:21
  • @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. – jamesqf Oct 10 '17 at 05:27
  • "Are ground transmitters located along the travel path of the aircraft", see how the different types of ground control stations work together. – mins Oct 10 '17 at 05:29
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    RADAR has nothing to do with communication. Communication is done using radio. Satellite communication is in no way normal. – 60levelchange Oct 10 '17 at 05:40
  • @jamesqf: There is a need for a big change in ATM, both in coverage , digitalization and automation, as traffic is increasing. ESA is currently paving the way for satellite (Immarsat) assisted ATM with Iris, like Aireon/Thales are now deploying the global Iridium based ADS-B. Backward compatibility is a requirement taken into account, but the change has to be disruptive. Current system is at its limits, and not far from being archaic. Cf. AF447/MH370. – mins Oct 10 '17 at 06:26
  • Is there a lag on the satellite comms? Imagine a controller instructing an aircraft to go-around on short final and the flight crew not getting the message for a few seconds – Dave Gremlin Oct 10 '17 at 08:52
  • @Teodorism if you are interested, you can listen to ATC chatter at www.liveatc.net – Dave Gremlin Oct 10 '17 at 08:54
  • "Is there a lag on the satellite comms?" If the satellite is in low orbit (300-600 km), there is a lag equivalent to the one between an interstate land line. But ATC managing landings will not use a satellite to talk to an aircraft at 0.5 NM. Would you use this solution if you were a communication engineer in charge of designing such system? – mins Oct 10 '17 at 18:54
  • @mins: Why do you think ATC is the problem? The system works quite well as it is. If increasing air traffic is causing problems, they seem to be related to needed spacing (which is often governed by physical things like wake turbulence) and available landing slots at busy airports/times, rather than any defficiency in communication. – jamesqf Oct 11 '17 at 03:18
  • @jamesqf: The lack of communication and tracking over e.g. oceans leads to considerable problems, as demonstrated by the effort and money invested to search for AF447/MH370, not mentioning the humans aspects of being in the dark for weeks. Then in busy areas, adding UAV or aircraft (traffic is expected to double in next decade) without permanent links and automation will lead to the ATM incapacity to absorb the traffic and keeping wait time, noise and pollution at the desired levels. – mins Oct 11 '17 at 06:17
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    @ 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. – jamesqf Oct 12 '17 at 05:22

2 Answers2

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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.

BowlOfRed
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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.

CrossRoads
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