Aircraft fly from a departure airport to a destination airport along a route that is ideally a great circle connecting the two airports. Weather and other aircraft traffic, along with other considerations, can lead an airplane across a range of territory when flying the route from day to day.
Airplanes inherently favor flying wings-level while not maneuvering (as opposed to a slip.) This leads to greatest passenger satisfaction and can avoid troubling issues with fluids. As a result, the orientation of the airplane changes with respect to satellites operating along the geostationary orbit (GSO). The relative orientation between satellite and antenna leads to beam steering (azimuth, elevation, polarization skew) commands and also has an influence upon the airplane antenna performance.
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| Skew angle and Elevation angle from various satellite positions, flying Los Angeles to Sao Paulo, Multi-gimbal antenna |
Antenna Gain Vs. Satellite Elevation
Beam steering in azimuth is accomplished without significant changes in performance for any candidate antenna.
Beam steering in elevation has a profound effect when using a flat panel antenna, and has no significant effect with a multi-gimbal antenna.
In general, a 30" flat panel antenna offers as much as three times the useful aperture as multi-gimbal antennas when the satellite is directly overhead (elevation = 90 degrees). The 30" flat aperture provides comparable performance to competing multi-gimbal antennas when the satellite is around 15-20 degrees elevation. The 30" flat aperture performance falls off rapidly as satellite elevation falls below 10 degrees elevation.
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| Relative size of Flat Panel Versus other antenna types |
ThinKom 30" Ku aperture (Gogo 2KU) provided the following plot of figure of merit (G/T) from 10 to 90 degrees elevation. The rolloff closely follows ideal trigonometry due to the continuous transverse nature of the beam forming inherent in this aperture.
ThinKom models a particularly small competing multi-gimbal antenna. A more competitive antenna G/T of 12.5 would cross the ThinKom line at about 15 degrees.
ThinKom touts that the roll-off is less severe when compared to other "discrete" phased arrays. The assertion is that other antennas gain will roll-off more rapidly, and the cross-over with a multi-gimbal antenna would be at a higher elevation (as well, they would fall more deeply at low elevation.)
"Most Flat-Panel antennas suffer an additional directivity (gain) degradation owing to “active impedance” (aka “mutual coupling”) mismatch at the radiating element level (in addition to the cosine roll-off). With many radiators or pixels densely packed (typically 0.5 freespace wavelength) the emissions from one element begin to be undesirably “absorbed” by adjacent elements and my “reciprocity” this applies to both the transmit and receive functions. This additional gain loss contribution is on the order of Cosine^0.3 to Cosine^0.5 depending on the particular antenna technology and scan angle range, so the net roll-off is more like Cosine^1.3 to Cosine^1.5
In the (special!) case of the VICTS array, with its unique continuous (rather than discrete) radiator geometry, this second “mutual coupling” roll-off is absent, and the VICTS array therefore holds more strictly to a Cosine^1.0 (projected area loss only) out to very large scan (very low elevation) angles. At large scan angles (75 degree scan for example) this can make for as much as 3 dB of additional gain advantage to the VICTS array as compared to other Phased-Array/ESA solutions." Bill Milroy, ThinKom
Multi-gimbal antennas can steer even to negative elevation angles. This offers the ability to extend coverage while in a roll away from the satellite, during a turn.
In general, satellite signals are degraded by noise and multipath when the satellite elevation approaches the horizon. As a rule, satellite signals are not normally expected when below five degrees elevation from the horizon. Flat antennas operate poorer than multi-gimbal antennas in the range of 5-15/20 degrees elevation, and better in the range of 15/20-90 degrees elevation.
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| Flat Panel Aperture size is a function of satellite elevation angle |
Skew Angle
Polarization skew is applied in Ku satellite systems to account for difference in the linear polarization between satellite and antenna (skew angle), whether Vertical or Horizontal. There may be slight differences in performance depending on the antenna feed network and aperture.
Skew angle is also used to describe the orientation of an elliptical beam pattern with respect to the GSO. Satellite emissions are limited in the direction of adjacent satellites to the target satellite. Satellite receivers are illuminated by adjacent satellites to the target satellite, leading to co-channel interference.
Sidelobes create sensitivity towards adjacent satellites as well as the main boresite lobe. Adjacent Channel Interference results from this sensitivity. ThinKom plot of the Carrier/Interference for the case of two degree satellite separation shows how interference (degradation) increases with skew angle and by satellite elevation. Notably, C/I of -6 dB or higher may greatly diminish the received signal if exposed to adjacent satellites having overlapping coverage.
ThinKom models a competing multi-gimbal antenna a bit smaller than I would; the 90 deg beamwidth may be around 7.5 degrees. Regardless, this shows is how much more severe the skew effects with multi-gimbal antennas, and the limited range of satellite elevations that bring effects to the flat panel antenna.
Note that C/I issues on the forward channel are realized only if there are overlapping, neighboring satellites. Return channel issues are because of regulatory compliance to a spectral mask, largely immune to specific neighboring satellites. Return channel issues follow the same beamwidth trends shown here as for the forward channel.
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| ThinKom 30" aperture estimate of C/I as a function of skew angle and satellite elevation. |
Aero antennas are low profile to limit drag; they are short and wide. The short elevation aperture leads a wide elevation beamwidth. The wide azimuth aperture leads to narrow azimuth beamwidth.
Zero skew angle describes the scenario where the azimuth beamwidth is aligned to the GSO (most favorable.)
90 degree skew angle describes the scenario where the elevation beamwidth is aligned to the GSO (least favorable.)
Airplanes operating due north/south of the longitude assigned to the target satellite slot are at zero skew angle.
Airplanes operating along the equator, east/west of the longitude assigned to the target satellite are at 90 degree skew angle.
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| Skew Angle issues may limit performance in the areas shaded in red |
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| 30" Flat Panel Antenna (FPA) encounters skew angle degradation only while satellite is low in elevation |
Airplane actual attitude may adjust the skew angle slightly, and maneuvering can contribute significantly to skew angle.
Airlines need to be diligent evaluating their route structure against proposed satellite locations. A satellite is ideally suited when it's longitude is between the departure and destination, or two satellites are used, one sited near the departure longitude and the other near the destination longitude.
Boston to Los Angeles
Inherently, some airlines may operate in regions that are bounded by a large land mass. For example, the 48 continental United States (CONUS) landmass has clearly defined latitude and longitude boundaries. Notably, the northern and southern borders avoid both tropics and arctic latitudes.
Boston to Los Angeles provides a demanding flight route that pushes the boundaries of CONUS coverage.
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| Boston to Los Angeles Great Circle Flight Route |
WISP servicing satellites might be anywhere from 180W to 10W.
Totaport has a tool that takes the coordinates of a candidate flight (in this case Boston to Los Angeles) and calculates the elevation and skew angle to a given GSO satellite stationed at a given slot (longitude). The following GIF cycles through slots at 10 degree increments. From each slot, the elevation and skew are calculated as a function of airplane position along the route (shown by longitude on the x-axis, latitude is not plotted.)
Skew-Elevation trigger set to 90 degrees is applicable to multi-gimbal antennas, where there is no relief from skew angle effect. I have set a limit of 55 degrees in skew as the level at which service is severely degraded. The Skew Mute factor indicates what percentage of the flight route encounters skew issues.
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| Boston to Los Angeles, Satellites 10W - 180W, Skew& Elevation angle, Multi-Gimbal Antenna |
Elevation angle (in blue) falls below the horizon in some cases and peaks around 50 degrees. Average elevation angle maximum is 42 degrees.
Skew angle (in orange) is plotted +90 to -90, where the sign just refers to which side the satellite is relative to the airplane. Skew effects are about the same at a given angle, whether plus or minus. Skew angle stays in between -60 and +60 degrees.
First, from a satellite at 130W, average elevation was 31 degrees (ranging from 15-50 degrees) and skew angle from 15-45 degrees.
Second, from 100W, average elevation was 42 degrees (ranging from 30-50 degrees) and skew angle from -30 to +30 degrees.
Third, from 60W, average elevation was 30 degrees (ranging from 20-40 degrees) and skew angle from -10 to -50 degrees.
CONUS service was best from 100W. The average elevation angle of 42 degrees provides at least a 4-5 dB (2KU) improvement in G/T, figure of merit. from a 30" aperture compared to a multi-gimbal antenna. The skew angle range is not an issue for any antenna.
CONUS service from 130W or 60W reduces average elevation to about 30 degrees, which brings a 30" aperture about a 3 dB (2KU) benefit compared to multi-gimbal antennas. Skew angle extremes may have some effect on Ku antennas, while Ka antennas are likely to operate without any degradation.
Los Angeles to Sao Paulo
Flights through the tropics have the gift of high satellite elevation and the curse of extreme skew angles.
Los Angeles to Sao Paulo provides a typical route that crosses through the equator.
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| Great Circle route, Los Angeles to Sao Paulo |
WISP servicing satellites might be anywhere from 180W to 20E.
Skew angles of +/- 90 degrees are encountered in this route, as shown in the following GIF. Note that the route is plotted by latitude (reflecting is mostly north/south direction.)
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| Los Angeles to Sao Paulo, Satellites 20E - 180W, Skew& Elevation angle, Multi-gimbal antenna |
Elevation angle (in blue) falls below the horizon in some cases and at nearly 90 degrees. Average elevation angle maximum is 61 degrees.
Skew angle crosses both -90 and +90 degrees for every satellite (at the equatorial route crossing at 0 degree latitude.
Looking at three specific examples.
First, from a satellite at 110W, average elevation was 58 degrees (ranging from 15-60 degrees). Multi-gimbal antennas encounter skew issues for 63% percent of the route. A 30" flat panel antenna encounters skew issues for 16% of the route.
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| Los Angeles to Sao Paulo, Satellite 110W, Skew & Elevation angle, Multi-gimbal antenna |
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| Los Angeles to Sao Paulo, Satellite 110W, Skew & Elevation angle, 30" Flat Panel Antenna |
Second, from 70W, average elevation was 45 degrees (ranging from 25-85 degrees). The 30" flat aperture antenna is not expected to have any skew angle issues due to the high satellite elevation angle during the equatorial crossing. The multi-gimbal antenna encounters skew issues for 18% of the route.
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| Los Angeles to Sao Paulo, Satellite 70W, Skew & Elevation angle, Multi-gimbal antenna |
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| Los Angeles to Sao Paulo, Satellite 70W, Skew & Elevation angle, 30" Flat Panel Antenna |
Third, from 50W, average elevation was 27 degrees (ranging from 10-70 degrees). Multi-gimbal antennas encounter skew issues for 72% percent of the route. A 30" flat panel antenna encounters skew issues for 19% of the route.
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| Los Angeles to Sao Paulo, Satellite 50W, Skew & Elevation angle, Multi-gimbal antenna |
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| Los Angeles to Sao Paulo, Satellite 50W, Skew & Elevation angle, 30" Flat Panel Antenna |
Tropical service was best from 70W. The average elevation angle of 45 degrees provides a 4-5 dB G/T figure of merit gain benefit, from a 30" flat aperture compared to a multi-gimbal antenna. The 30" flat aperture had no skew issues at all, while the multi-gimbal antenna may not be able to operate for 18% of the flight route.
Conclusion
The combination of an airline route structure, antenna patterns, servicing satellite, and adjacent satellites creates variations in system performance, including regions without any service.
A 30" flat panel antenna has moderate gain benefits over multi-gimbal antennas when looking at most routes, and can have significant gain benefits when operating in the tropical regions.
Aircraft operating outside of the temperate zones are not normally exposed to high skew angles. Airlines that operate airplanes exclusively above about 30 degrees latitude should not have to be concerned significantly over skew angles.
Airlines that operate airplanes below 30 degrees latitude should note the significant benefits from a 30" flat aperture as compared to multi-gimbal antennas in service coverage and performance.
Stay tuned!
Peter Lemme
peter @ satcom.guru
Follow me on twitter: @Satcom_Guru
Copyright 2017 satcom.guru All Rights Reserved
Peter Lemme has been a leader in avionics engineering for 35 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 Secure Broadband IP Air/Ground Interface.
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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