Figure: Aeronautical SATVOICE system overview, from SVGM, Figure 2-1.

Eddie Sez:

The use of satellite voice systems for air traffic control has evolved over the years but has been made complicated by the variety of systems on aircraft versus systems that have proliferated in space. In other words, if your manufacturer guessed wrong about the way things were headed, your aircraft may not be as prepared as others.

First some terminology. We grew up with Satellite Communications (SATCOM) to mean picking up a phone and making a call via satellite. But now we have a myriad of systems that use the same satellites to transmit and receive all sorts of data. So SATCOM includes the data and the voice; we speak of SATVOICE to denote the picking up the phone bit.

Can you use SATVOICE to make a position report? In most parts of the world, yes. Can you use SATVOICE to replace the requirement for an HF? No, you still need at least one HF. Can you plan on using SATVOICE for air traffic services with an HF as backup? If the region you are flying has made such provisions and you have a qualified system, yes. But how do you know your system qualifies? The easiest way to tell is to look at your MMEL. More about that: LRCS to be Carried on Board

Finally, just how many HF's do you need? The answer used to be just one, unless your country or the region you flew said more. Up until 2014, the U.S. only implied you needed "a" HF radio and no region specified more than that. In 2012 the ICAO implied you needed at least two long range communications systems and one of those had to be an HF. In 2014, the FAA issued a global change to all master minimum equipment lists that codifies that. So the answer is now "at least one, perhaps two." More about that: Communications / HF Requirement for Oceanic / Remote Areas.

What follows are quotes from various sources, listed below, as well as my comments in blue:


Historical Background

We've come a long way with satellite communications and there are changes in our near future . . .

[ICAO SVGM, pg. vi]


SATVOICE System Overview

[14 CFR 1] Long-range communication system (LRCS). A system that uses satellite relay, data link, high frequency, or another approved communication system which extends beyond line of sight.

[ICAO SVGM, ¶2.2.2] Satellite communication systems are defined by three different altitude orbits; low earth orbit (LEO), medium earth orbit (MEO) and geosynchronous earth orbit (GEO). The altitude of the orbit determines the area illuminated by the satellite. The higher the orbit the weaker the signal is from the satellite, but it has a much larger footprint. Propagation loss is overcome by increased complexity of the antenna systems along with higher transmitter power. Conversely, a LEO satellite's footprint is much smaller requiring a higher number of satellites to provide coverage, but the antennas used are much simpler along with reduced radio frequency power requirement on the subscriber end. Also, the lifetime of a LEO satellite is less due to drag caused by the close proximity of earth.

Iridium

[ICAO SVGM, ¶2.3] Iridium uses a constellation of 66 satellites at an altitude of 780 km (450 miles) in six orbital planes, with eleven satellites in each orbital plane, providing global coverage. Additionally there are a number of spare satellites to replace any in-orbit failures. At that altitude each satellite covers a circular area of 4,700 km (2,900 miles) and is in-view to a location on the ground for approximately 9 minutes. Using a small amount of overlap in coverage between each satellite, the Iridium network hands-off the call to the next satellite coming into view to the ground location. This is similar to a GSM cellular telephone system where the subscriber moves from one cell site to another except that the satellite is the moving vehicle. The speed of the aircraft appears almost stationary compared to the speed the satellite moves.

Iridium operates in low earth orbit, which means it doesn't cost as much to get their satellites in space. The cost to install on an aircraft is also relatively cheap compared to earlier systems.

Inmarsat

[ICAO SVGM, ¶2.4]

Public Switched Telephone Network (PSTN)

[ICAO SVGM, ¶2.6] The aeronautical SATVOICE system uses the public switched telephone network (PSTN) and/or dedicated networks to route calls between the aircraft and the appropriate ground party. Dedicated network access switches locate the aircraft anywhere in the world regardless of the satellite and ground earth station (GES) to which the aircraft is logged on.

There is a fair amount of magic going on here, but all of that was handled by the vendor who installed the equipment on your aircraft, the satellite service provider, and the person who set up your aircraft accounts. From the pilot's perspective, it is only important to remember that when you pick up the phone and the necessary connections are made, you are talking to someone on the other end who picked up a telephone. You cannot do this as a matter of making normal position reports and other air traffic services unless the air traffic services unit on the other end has made provisions to use these SATVOICE systems.


SATVOICE as a Long Range Communications System

Is SATVOICE a valid Long Range Communications System (LRCS)?

[ICAO SVGM, ¶2.1.4] SATVOICE could potentially be considered a LRCS as defined by State MMEL/MEL policies. When approving reduced carriage requirements for HF radio, States may allow aircraft to operate with only one serviceable HF radio. However, airspace requirements will take precedence over the MMEL/MEL requirements.

The answer is "It depends." It depends if your state and the airspace in which you fly allow it. If you are flying an N-numbered airplane your state does allow it, but it is more complicated than that. See LRCS to be Carried on Board.

Can SATVOICE replace the HF?

[ICAO SVGM, ¶2.1.4] SATVOICE is not a replacement for ADS-C/CPDLC or HF/VHF voice via a radio operator.


LRCS to be Carried on Board

[ICAO SVGM, ¶3.3.2]

[FAA MMEL Policy Letter (PL)106]

Figure: FAA LRCS MMEL Policy, from FAA MMEL Policy Letter (PL)106, page 3.

This is the first document from the FAA that says you need two LRCS. You could get away with just one HF as your only means of long range communications until this came out. Now, if you don't have a qualified SATVOICE or CPDLC, you need two HFs.


Flight Crew Procedures

[ICAO SVGM, ¶5.1]


Example Aircraft SATVOICE LRCS Discussion

You cannot assume a satellite telephone installed in your aircraft qualifies as a suitable LRCS that can take the place of one HF. The satellite telephone must pass several security and Required Communications Performance (RCP) tests. The easiest way for you, the pilot, to tell if your system qualifies is to check your MMEL. The GV series MMEL, for example, was modified as a result as follows:

Figure: GV MMEL HF Systems, from GV MMEL, page 23-14.

Gulfstream has stated SATCOM Voice or Data Link can substitute as a backup to normal HF communications, so with this series of aircraft we are good to go.

What if you don't have such a statement in your MEL?

Figure: DA-2000 MMEL HF Systems, from DA-2000 MMEL, page 1-23-1.

This is an older copy of the DA-2000 MMEL for sake of illustration, they might have a qualified system and this may have changed. But for the purpose of this discussion, if what you see is something that doesn't mention your satellite phone system or an LRCS at all, you might not have a qualified system. You should ask your aircraft manufacturer for guidance. If they are clueless, send them the Appendices to the ICAO Satellite Voice Guidance Material, available for download below.


ARINC SATCOM Voice Procedures

[ARINC Handbook, ¶2.6]


Data Link on VHF vs. SATCOM

[G450 Aircraft Operating Manual, §2B-21-40, ¶1.B.] By default, the PLANEVIEW CMF communicates by way of the land-based ACARS VHF network, which includes the Aeronautical Radio, Inc. (ARINC) and SITA subnetworks. Based on position information provided by the aircraft FMSs, the CMF automatically tunes to the appropriate subnetwork. In areas where VHF coverage is unavailable, the CMF may use the Inmarsat Aero-H, Aero-H+, or Aero-I satellite UHF networks. This provides both packet mode (datalink) and circuit mode (voice and data) capabilities to the aircraft. The CMF switches to and from the satellite UHF network based on the availability of land-based VHF network coverage.

The G450, as an example, automatically switches to SATCOM for datalink when VHF coverage is unavailable.


Line of Sight and Curvature of the Earth Issues

Figure: INMARSAT Line of Sight, from Eddie's Notes.

[G450 Aircraft Operating Manual, §2B-21-40, ¶1.D.]

  • All PLANEVIEW CMF transmissions, whether VHF or satellite, require line of sight to a VHF ground station or Inmarsat satellite, respectively.

  • Transmitting by way of satellite while on the ground is generally reliable. Although, line of sight issues may still arise due to surrounding terrain and man made structures because the Inmarsat satellites are in an equatorial geostationary orbit. In flight, the curvature of the Earth is a concern only at latitudes greater than 70° North or South. Except at these high latitudes, satellite coverage while in flight is seamless.

Because INMARSAT satellites are in geostationary orbits over the equator, the curvature of the earth limits their use at the poles. While the G450 manual says you may have issues above 70° latitude, it is said that SATCOM is available for voice and datalink up to 82°N.

[G450 Aircraft Operating Manual, §2B-22-30] The airborne SATCOM equipment links to one of the geostationary International Maritime Satellite Organization (INMARSAT) satellites, which in turn links to a ground earth station. The ground earth station then connects to a long distance telephone company, and then to a local telephone company. Once these connections are established, the airborne communication equipment communicates with the ground based equipment using the telephone company services.


Book Notes

Portions of this page can be found in the book International Flight Operations, Part IV, Chapter 4.


References

14 CFR 1, Title 14: Aeronautics and Space, Definitions and Abbreviations, Federal Aviation Administration, Department of Transportation

Advisory Circular 91-70A, Oceanic and International Operations, 8/12/10, U.S. Department of Transportation

ARINC Voice Services Operating Procedures Handbook, ARINC, Annapolis, MD, 13393 Rev. R, September 27, 2006

FAA MMEL Policy Letter (PL)106, Revision 5 GC, June 6, 2014

Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.

Gulfstream GV Master Minimum Equipment List (MMEL), GV GV-SP (G550), GV-SP (G500), GIV-X (G450), GIV-X (G350), Revision 8, 11/07/2014

ICAO Satellite Voice Guidance Material (SVGM), International Civil Aviation Organization, First Edition, 24 July 2012