Question about CPDLC

[ogopogo]

Also posted this over on ATS, hope that's OK.

Hello everyone:

For a research project I am doing, I have a question about CPDLC -- specifically what rf/radio systems are used as the communications link - 1) over land masses and 2) over oceans or non-radar areas. VHF? HF? Satellite? In what order of priority? If Satellite, which system? Does it vary from carrier to carrier?

To the pilot, this is probably transparent, as the avionics takes care of things, but I'd like to know specifically what route the signals take arriving and leaving the aircraft.

Thanx in advance.

[Tim]

http://en.wikipedia.org/wiki/Controller ... unications

[hst]

Initial log on to an ATSU (air traffic services unit) is done by pilots sending a logon request (text message) which uses data link communication from the aircraft to the appropriate ATSU. No VHF/HF or satellite but data link must be operational. As long as there is overlap between ATSU's (with CPDLC capability) the transfer is automatic with a text message sent to the aircraft indication such.

Here is a more thorough explanation on CPDLC.

http://members.optusnet.com.au/~cjr/connections.htm

Flash movie http://members.optusnet.com.au/~cjr/Gnd%20Tfr.gif

Having lost data link communications recently on an oceanic crossing I certainly do appreciate CPDLC.

[ahramin]

As for order of priority, it's always the same: cheapest to most expensive. Satcom is most expensive.

[ogopogo]

Sorry folks, but still not getting the answer to my question - unless I missed it in the above. The data link has to involve a radio somewhere! I realize it's all automatic, and you don't really care how it all works, ie on what frequency, but I'd still like to know - specifically over non-radar areas.

[pika]

VHF first, HF second. Both radios.

[ogopogo]

Never satellite?

[hst]

Ogopogo, does the transponder use a radio? On my aircraft (B777) I believe it works in a similar fashion but I may be wrong. There is no need to dial in frequencies (VHF, HF or link via satcom). I do not believe it works thru VHF ACARS (usually preset on data function of 3rd radio) as I have cycled data on and off when logged on CPDLC without losing the CPDLC connection. It may depend on the aircraft type and installation. What I do know is that it works automatically and is a gem on long haul flights

Why not use your internet search function?

[CID]

You're talking about a relatively new service that many here just don't use. Satellite FANS and ATS isn't very widespread and the standards are still being developed. Even Airbus and Boeing have different standards (FANS 1 versus FANS A) and much of the guidance and standards has either been only recently released or amended.

There is a Eurocontrol mandate for equipage but it doesn't affect many operators and the Gander GAATS system, although configured for CPDLC about 6 years ago, hasn't had much use until recently because it wasn't compulsory. (and it still isn't in Canada). The GAATS and the ADS-B airspace in the Hudson Bay area are generally only known to high fliers and airline pilots.

From what I know, there currently are no land route CPDLC services in the world, just oceanic for now and yes, they use VHF (primary) or Satellite (Secondary) and in some cases HF as an alternate.

Satellite CPDLC is conducted via the Inmarsat system. At least that's the only system I've seen that is compatible. You can't use just any data capable Inmarsat SATCOM system. There are a few additional technical requirements to make SATCOM systems compatible with CPDLC and the SATCOM manufacturers are a little behind in developing a wide range of products.

Although development and operation of CPDLC systems has been around for more than a decade, it's certainly not widespread but implementation is starting to heat up and more information will be available to the general public as standards solidify and more products are available.

[CID]

Umm...the transponder IS a radio.

[hst]

Umm...the transponder IS a radio.

My point exactly CID.

If CPDLC uses a VHF/HF or Satcom to operate it is completely hidden to the operator. How the logon signals get to ATC is PFM I guess. When I logon out over the pacific we are well out of VHF range. Not the answer you are looking for ogopogo but these articles may help.

http://www.sita.aero/file/2171/CPDLC_AT ... torial.pdf

http://www.paris.icao.int/documents_ope ... ory_id=112

[ogopogo]

hst and CID, tanks very much for your responses.

I used the term 'radio' in the sense of a transceiver somewhere in the a/c, and not something on the panel you adjust, sorry. It's the frequency of operation that I'm after. Obviously, out over the ocean VHF isn't going to work since it is line-of-sight. Unless there are specific shore stations set up for this?? (The radio horizon at FL390 is about 250miles)

If CPDLC uses a VHF/HF or Satcom to operate it is completely hidden to the operator -- exactly. This is what I meant by 'transparent'. I appreacite as pilots, all of this is automatic and you're busy looking at a screen and not worrying about how the info got there.

I am going to have to assume that all a/c using CPDLC have an Inmarsat terminal on board, and that this is the 'radio'. Better buy shares in Inmarsat, I guess.

I'll tell you why I'm asking -- I am in the satellite industry and am involved in trying to bring ADS-B to you in non-radar areas (NRA). Satellite is probably the only answer. Large oceans are a good example. The end goal of ADS-B as you know is to space a/c closer together (among many other goals). I am trying to understand how comms (voice, data or both) might be brought into the equation without having to equip modern aircraft with another set of avionics.

Perhaps I should have asked "does CPDLC work in all NRAs?". I realize from a pilot's point of view it's the 'data link' and you actually don't give a hoot how the data comes and goes into the a/c. Is Inmarsat the only way? Do all long range a/c have Inmarsat? Do all use CPDLC?

Inmarsat's satellites are in geosync orbit and above 70deg LAT the look angles become pretty bad -- does CPDLC work over the poles? If so, is it Inmarsat? HF?

Interesting discussion, folks.

[hst]

Yes a good discussion, and worth looking into the stock tip

Perhaps I should have asked "does CPDLC work in all NRAs?".

The answer at the moment is no, only those ATSU's that have CPDLC capability. There are exceptions. As an example, CZEG (Edmonton) has you logon to CPDLC to enable ADS position reports but does not have functioning CPDLC.

Is Inmarsat the only way? Do all long range a/c have Inmarsat? Do all use CPDLC?

Most older generation (B767, B747's (older -400 series) do not have CPDLC.

does CPDLC work over the poles? If so, is it Inmarsat? HF?

Yes, GDXB (Magadan, Russia), PAZA (Anchorage) CZQX (Gander) all have CPDLC in the polar region.

[CID]

It's not the worst stock tip in the world but keep in mind that the Inmarsat system has been around for about 3 decades so it's not exactly a fledgling start-up.

Also, and most importantly, like any satellite service, it's one big solar flare away from going down the toilet. Satellite technology requires a great deal of capital for the space segment and extensive investment for the ground segment if you want any sort of reasonable network capability. In fact, stock prices have been "settling" after the announcement that "Earth" is entering the peak of the 11 year solar flare cycle.

Many satellite service companies have gone under because of the incredible amount of money and coordination needed. The Iridium system went bankrupt a few years ago and came back after a while after someone came in and bought the pieces at a discount price.

In my opinion, investment in satellite providers is for the experienced investor that has the ability to quickly place money in and out of the system.

[TopperHarley]

http://www.smartcockpit.com/pdf/flightops/navigation/2

Check out that link, it has lots of info about FANS and CPDLC.

[ogopogo]

CID -- was just kidding about the stock... any time a company has a lock on something as a sole-source provider, it usually means success. For sure, from a user point of view, Inmarsat is an expensive service -- because they can be.

I have been in the satellite business (space segment) for many years; Inmarsat is currently buying its 5th generation of satellites (Boeing is building), so don't assume they're at a standstill. Satellites, especially in geosync, are pretty reliable and typical lifetime is 15 years.

Anyway, I am still learning about how all this satellite ATC thing fits together!

[complexintentions]

We use CPDLC and ADS-B and ADS-C all over the world. This by no means makes me an expert, as much of the operation is transparent, but I have used it in virtually every place it can be used from Maastricht to Mumbai to Melbourne. There are lots of variations between the capabilities worldwide.

Speaking of Maastricht, (one of the German FIR's), they do use CPDLC over land. You log on and they will issue clearances via datalink. VHF voice is still primary though and clearances involving aircraft movement changes (heading, altitude) must be verified by voice. Things such as frequency changes don't need to be read back via voice, you just change frequencies and check in.

There seems to be some confusion about the relationship between CPDLC and ADS. CPDLC is simply the link between the controllers and aircraft, whether by VHF or Satcom. But ADS (Automatic Dependent Surveillance) can be a couple of different things. ADS-C (C stands for Contract) is initiated by logging into the appropriate ATS facility and position reports are transmitted by the same protocol that the ACARS uses. Thus reports can only be sent within the parameters of the contract with an ATS facility. But ADS-B (B for Broadcast) emits through the Mode S transponder and doesn't depend on ACARS. The operation is supposed to be automatic and transparent to the flight crew, but it is correct that you have to log into Edmonton for some reason to initiate the ADS-B. Don't know why. ADS-B can update every second to ground stations so to the controller display looks very similar to a rotating radar display. The data package for ADS-B transmits (squits) on 1090MHz, if I recall.

For much of the transpolar flying we do, there is no CPDLC depending on the route and the availability of comms. Even the Satcom coverage is pretty spotty that far north, it's not unusual to be without any comms at all for awhile. We route via Murmansk westbound usually and they don't have CPDLC so we don't have it at all until we get to Edmonton FIR and even then it's really only for the ADS reporting. But we use it all the time for crossing the Indian Ocean and the flights to Oz, Asia, and Africa.

[ogopogo]

Thank you, complex. Clear explanation.

My interest is in developing a method to implement ADS-B in Non-Radar Areas, via satellite, such as Iridium is proposing to do. (<- clickie)

While this seems feasible, it occurred to me that if a/c are going to be spaced that much closer together as a result, (such is the case currently with developing ADS-B systems over land masses) then some sort of reliable comms link would be needed. (Maybe not?) Hence my questions about CPDLC and its coverage.

[AirSatOne]

"what rf/radio systems are used as the communications link - 1) over land masses and 2) over oceans or non-radar areas. VHF? HF? Satellite? In what order of priority? If Satellite, which system? Does it vary from carrier to carrier?
To the pilot, this is probably transparent, as the avionics takes care of things, but I'd like to know specifically what route the signals take arriving and leaving the aircraft."

Over land masses VDL2 is used for CPDLC. Once the aircraft in remote areas (oceanic regions) the aircraft will use either Iridium or Inmarsat for both CPDLC and ADS-C. The aircraft will have a Communications Management Unit (CMU) or sometimes called a Data Management Unit (DMU) that is connected (usually) to both a VHF Data Link Radio (VDL2) and a SatCom system. Sometimes the VHF Data Link radio is part of the CMU/DMU - sometimes it is a separate box. The DMU/CMU will automatically switch when it detects it has lost VHF Datalink Communications or the pilot will initiate a FANS Login from the CDU establishing communications with ATC. The messages travel from the aircraft systems particularly the CDU and FMS to the CDU/CMU which routes it to the VHF VDL-2 or the Iridium SBD (Short Burst Data Transceiver) or Inmarsat (Inmarsat must be Aero H or H+ - SwiftBroadband coming soon). For the SatCom side the message travels to and from the Iridium or Inmarsat satellite system to the ground earth station where FANS 1/A messages are picked out, routed to the backbone provided by SITA who then routes the message to ATC.

[AirSatOne]

FYI: CPDLC and ADS-C are both part of FANS 1/A. CPDLC is basically text messaging between the pilot and controller and ADS-C gives the controller your position information - ATC will set parameters as to when to report. For example ATC will tell the aircraft to give a position report every 5 minutes and tell it to report of the aircraft deviates from the assigned altitude, airspeed and heading - this is basically the contract which is where the C comes from in ADS-C. ATC can space aircraft tighter because of more accurate and timely position reports - it can also predict trajectory based on the position report which includes airspeed, altitude, heading etc. If the aircraft deviates it will also give a report letting ATC know they need to start separating aircraft to accommodate the deviation - they may kick the deviateor our of FANS airspace

CPDLC replaces VHF communications over land via VHF Data Link and replaces HF communications over remote / oceanic regions. ADS-C replaces voice position reporting. In the FANS environment obviously the flight crew needs to communicate (CPDLC) and also needs to give position reports (ADS-C).

http://www.veengle.com/s/CPDLC;datalink;air.html
Jo Kremsreiter - AirSatOne

[pelmet]

I believe that CPDLC automatically switches over to sattelite communication once you are out of range of VHF range of an appropriate antenna. We have two satellite comms on the aircraft I fly. You check their status on the preflight.

There seems to be three satellite providers according to the new ICAO flight plan codes. Inmarsat, MTSAT(from Japan) and Iridium.

By the way, you might find operators voluntarily using satcom instead of ACARS VHF com over China and Japan due to the very high fees that those countries impose. The pilots just manually turn off their ACARS VHF Com. This of course is for non-ATC stuff such as company com and weather etc. not ATC communications.