Airlines flying routes connecting European airports fly far enough north to limit satellite elevation to less than 45 degrees. Installing a Ku/Ka band satellite antenna in the Airbus-nominated position for A320 series creates a blockage from the tail (shadowing) that can extend more than 30 degrees in elevation and about 4 degrees wide. This combination can lead to loss of service when the airplane is flying directly away from the servicing satellite. The problem is made worse by using satellites that are not well situated.
ARINC 791 Part 1 provides a summary of the blockages that may be encountered if installed at a particular location. In particular, the Airbus narrow-body installation blockages are shown below for the installation centered on C60.
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| Installation at C60 |
The Vertical Stabilizer and the wing/winglets can intrude on the line of sight antenna-satellite.
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| A791 Blockage map (note Antenna location should be C60, blockages e and h would enlarge relatively) |
The vertical stabilizer represents a blockage from 27 to 36 degrees elevation, from 178 to 182 degrees azimuth. These blockages are quoted from ARINC 791 Part 1, and noting that the listed location is forward of the stated C60. In the worst case, the blockages would enlarge.
The wingtip comes above the horizon when the airplane is rolled more than 5 degrees away from the satellite.
The potential blockage by the Vertical Stabilizer is a function of heading, flying radially away from a particular satellite. Any heading along the 356/360 headings would be unaffected.
With more than one satellite in view, an alternative satellite can be used that is not similarly blocked. Overlapping coverage is critical to network flexibility.
The wingtip issue would be a transient affair, noting that in cruise roll angles rarely exceed five degrees. The potential for blockage due to "terminal area" maneuvering, which can exceed even 30 degrees roll angle, is inconsequential, as every satcom system suffers when the satellite steering falls below about five degrees elevation.
In order to evaluate the situation, a fictitious route was constructed connecting
Helsinki-Lisbon-Athens-Kiev-Shannon.
Using this route, the elevation and skew angle were calculated for a nominated satellite position about every 11 nm along the route. The data is plotted as a function of the route distance, which proceeded to criss-cross across Europe along its borders.
Inmarsat operates two of the three GX (5th generation) "global" satellites for coverage into Europe.
The designated European coverage satellites are I-5F1 (63E) and I-5F2 (55W).
While the note makes mention that I-5 F4 position may change, it apparently is not shown in the correct location.
From what I can gather, I-5 F4 is at 11E, which is an ideal position for European coverage. I will assume this is the case, but I have no Inmarsat confirmation or commentary I can find, and note it may not be in-service at this point in time.
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| https://www.satbeams.com/satellites?norad=42698 |
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| Inmarsat I-5 F4 positioned at 11E would bring significant benefit to European flights |
Inmarsat can offset the tail-blockage issues facing some European flights by switching between three satellites servicing the region. The third satellite boosts the favorable steering angles to all of Europe, and adds considerably more capacity.
European airline routes challenge lies in their mid-latitude position, taking even the most favorable satellite no higher than about 45 degrees elevation and as low as 25 degrees in the best situation. Use of a satellite offset from the serviced longitudes quickly falls to where the best circumstance is 25 degrees elevation, with most much lower. The issues facing European routes are encountered with any Geostationary satcom network.
Stay tuned!
Peter Lemme
peter @ satcom.guru
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Peter Lemme has been a leader in avionics engineering for 37 years. He offers independent consulting services largely focused on avionics and L, Ku, and Ka band satellite communications to aircraft. Peter chairs the SAE-ITC AEEC Ku/Ka-band satcom subcommittee, developing ARINC 791 and 792 characteristics and contributes to the Network Infrastructure and Interfaces (NIS) subcommittee developing Project Paper 848, standard for Media Independent Secure Offboard Network.
Peter was Boeing avionics supervisor for 767 and 747-400 data link recording, data link reporting, and satellite communications. He was an FAA designated engineering representative (DER) for ACARS, satellite communications, DFDAU, DFDR, ACMS and printers. Peter was lead engineer for Thrust Management System (757, 767, 747-400), also supervisor for satellite communications for 777, and was manager of terminal-area projects (GLS, MLS, enhanced vision).
An instrument-rated private pilot, single engine land and sea, Peter has enjoyed perspectives from both operating and designing airplanes. Hundreds of hours of flight test analysis and thousands of hours in simulators have given him an appreciation for the many aspects that drive aviation; whether tandem complexity, policy, human, or technical; and the difficulties and challenges to achieving success.
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