Friday, June 24, 2016

Cybersecurity and Phased Arrays - PP848 and PP792

The Ku/Ka satcom subcommittee met the last two weeks to progress the latest AEEC standards and characterstics defining secure broadband connections and satcom equipment.  Airbus expects to use PP848 for securing non-safety communications, bringing an unexpected urgency to completion of IPSec end-end functional definitions. PP848 will offer an optional means to segregate both passenger and non-safety aeronautical communications with a commercial broadband radio, such as Ku/Ka satcom.  PP792 builds upon ARINC 791 to characterize emerging flat-panel antennas, especially phased array antennas, as well supplier-specific guidance for installing antenna systems using either 791 or PP792.

PP848 is built around a defense-in-depth architecture that includes the commercial broadband Internet access, PP848 end-end network-layer security and domain segmentation, and an upper-layer application security beyond the scope of PP848.  Quality of Service is managed intelligently by abstracting all traffic such that each broadband radio may apply resources efficiently and when needed.

ARINC 791 has always favored minimizing outside antenna equipment, as replacing a failed component under a radome may not be possible for a week or more.  The high-power amplifier (HPA) has been kept "inside" to make its replacement possible in an "overnight", as well to offer it conditioned cooling air in most cases. Most antennas today use a single-input from an HPA, allowing it to be separated by a transmission line.  The transmission line is usually a coaxial cable, but ARINC 791 also permits use of waveguide to span the "pressure bulkhead".  Regardless, losses on the transmission line, especially at 30 GHz Ka-band transmit frequencies, are a challenge.

Thus, as with all things aeronautical, for eight years, as a subcommittee, we have been conflicted with wanting to put the HPA outside but fearing its failure and service-disruption.  With the Boeing 787, we faced the prospect that there was no way to install the HPA near-enough to the antenna connectors, and that the HPA must be moved outside.

Antennas may include the HPA as an embedded part of the aperture itself, inseparable from the outside equipment.  A new class of antenna, a phased-array, may include the amplifier as an integral part of each element.   Even multi-gimbal antennas may include a set of HPAs distributed across the aperture rather than an external single-input.

Space-Vehicles utilize fail-operative amplifiers, where any individual amplifier can fail without any loss to the remaining amplifiers.  A fail-operative design allows for continued operation in a degraded manner, buying time to repair the assembly in an aeronautical application.

The loss of a passenger satcom system due to equipment under the radome should be improbable, from a practical sense.  A standard for that would be a loss-of-function target of once in 100,000 hours, or about 99.999% reliable.  Loss-of-function can be minimized by fail-operative features combined with high reliability single-path components.  For example, an aperture that can reconfigure the beam patterns around failed elements may have slightly less receive figure-of-merit (G/T) or radiated power (EIRP) or a larger beamwidth leading to less efficient operation.  But it may continue to operate progressively through single failures.

With a high-reliability assumption that permits moving the HPA outside, PP792 is faced with the option to remove one of the ARINC 791 line-replaceable units (LRU) with a simplified wiring interface.  The following are the options under review, but noting that removing the KRFU has gained consensus.

The ARINC 791 KRFU normally is installed within one meter of the antenna RF bulkhead penetrations, to keep the transmission line as short as possible.  The overhead, crown location is difficult to access and is subject to heat accumulation and high temperatures.  In the case of removing the ARINC 791 Modman, the modem moves to the KRFU crown position (vacated in the space left by moving the HPA outside).  The manager function follows suit into the KRFU.  The KANDU is simplified into just a power supply, with the antenna steering function (stabilization and tracking) function and sensors moving outside as well.  The block-upconvertor (BUC) and Low-Noise Amplifier/Block-Down Converter (LNB) also are now outside.

A variation of the KRFU configuration incorporates the power-supply, effectively removing the KANDU LRU as well as the Modman.  

The need to support more than one modem is driven by a number of factors.  Supporting both television broadcast and Internet access takes two modems.  Support for highly-customized regional networks may require carrying a modem for each network.  The Modman is best suited to host multiple modem cards.   

This configuration removes the KRFU, leaving a low-loss L-band coaxial interface between the Modman and the antenna (through the bulkhead).  The KANDU is mounted nearby the antenna to deliver conditioned power to the antenna components.

Some antennas may have simplified steering requirements.  Some antennas may favor the use of ships sensors (Inertial Reference Unit).  For these antennas, the KANDU may serve to deliver the beam steering information to the outside antenna equipment.  Unlike ARINC 791, PP792 will not support the discrete motor control interface from the KANDU, as the beam steering is not mechanical.  For all PP792 installation, this allows for removing about 50 discrete contacts in the control-bulkhead connector.

PP792 will support the simplest installation of large flat-panel antennas.  A panel 30" wide by 50" long will perform well down to five degree elevation and deliver extraordinary performance under most normal conditions.

Consideration for using a software-defined radio approach to accommodating multiple modems was not viewed as practical (commercially), but viable technically.  The issue is around the difficulties in getting competing service providers to cooperate on a common design standard, where the market opportunity is not significant and the technical solution may be counter to their proprietary interests.

The ARINC 791 antenna-modem interface protocol (791-AMIP) will be revised in response to the need to support flat panel antennas where their beam-pattern changes as a function of both steered elevation-angle and skew angle.  791-AMIP accounts for multi-gimbaled antennas whose beam pattern changes as a function of skew angle only.  791-AMIP protocol permits real-time transmit optimizations in response to the actual antenna beam-pattern.  Today, many systems rely on "worst-case" link-budgets that under-perform in favorable skew conditions and fail-to-perform beyond artificial skew limits.

Ongoing work has been started to review and revise DO-160 categories and also to agree and document the expected aircraft dynamic behaviors.  The goal of this activity is to agree on the minimum safe qualification standards and to ensure that antenna systems are designed to operate in all phases of flight, including ground operations.  Operations on the ground include taxi (where the aircraft is subjected to the most severe movements) and when sitting at the gate (solar heating and no air exchange, the greenhouse, subjects the equipment to the hottest environment).

Proposed Aircraft Dynamic Limits

Ku/Ka band satellite communications rely on an antenna subsystem to communicate with a satellite transponder and a modem to communicate with a ground-based teleport hub for network access.  ARINC 791 part 1 defines the form and fit for an aeronautical terminal used on regional jet and air-transport aircraft.  791 was developed around the concept of a steerable horn-array, a multi-gimbaled antenna that physically moves in both elevation and azimuth to square a fixed-beam aperture.  The movement of the aperture assembly, on a positioning platform, creates the "swept volume", or the space needed for the aperture to move freely.  A radome is place over the swept volume to seal the space from the environment, to divert lightning, to sustain impacts from foreign objects, to not build up ice, and to minimize drag.  The aperture and radome are fastened to the airplane in a manner to take the loads presented by the assembly, including a decompression event, using up to seven fittings with attachments positioned relative to each other as defined in 791.  In all cases, a seal is pressed around the periphery, but otherwise the assembly does not touch the skin of the airplane.

There are other methods to install the radome to the aircraft. One notable method uses different locations proprietary to Boeing.  Whether Boeing or 791, both methods rely on fittings attached to a plate that holds the equipment and the radome.  Only two fittings take longitudinal loads (X-along the airplane fuselage, only two fittings take lateral loads (Y-across the fuselage), all fittings take loads in the up direction (Z).

While Airbus prefers seven ARINC 791 fitting locations and Boeing prefers its proprietary (CbB) eight fitting locations, no set of fittings has been optimized for flat panel antennas, and particularly where two tandem apertures are used instead of a single aperture.  In the figure below, the ARINC 791 fitting and connector penetration locations shown with a single aperture centered between fittings #3 and  #4.  A set of 30" apertures are highlighted, revealing that the connector penetrations and fittings #5 and #6 are covered by the aft aperture assembly.
Gogo developed a special adapter plate to account for this challenge with the ThinKom 2Ku antenna. Their strategy ended up raising the overall assembly to allow for the overlapping aft components while staying compliant to ARINC 791 fitting locations and connector penetrations.

Delta and Gogo are preparing a proposal for alternative fitting locations to vacate the space for large flat-panel antennas.  Moving fittings #5 and #6 outboard is expected, where ARINC 791 may profess two alternative locations for those fittings.  Airplane provisions (pad-up, strengthening) may account for both locations, allowing a customer to use either set as applicable, without substantial weight or cost penalty.  A glimmer of hope is rising that maybe Airbus and Boeing can agree on a common set of fitting locations for flat-panel antennas.

The other installation method relies on separating the radome assembly from the equipment mountings.  This approach does not use an external adapter plate to tie the radome to the equipment mounting.  791 does not discuss this style of installation.  PP792 will describe these other methods of installation and offer commentary or guidance on suitability and benefits when not using slip-fittings and whenever a large number of fastener penetrations are utilized considering maintenance and retrofit of a new antenna.


Interested parties are encouraged to contribute to PP848, ARINC 791 and PP792 development.  Monthly telecons and face-face meetings offer opportunity to submit draft proposals, general commentary, and helpful suggestions. 

Stay tuned,

Peter Lemme
peter @

Follow me on twitter: @Satcom_Guru

Copyright 2016 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 PP848, ARINC 791, and PP792 standards and characteristics. 

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 also lead engineer for Thrust Management System (757, 767, 747-400), supervisor for satellite communications for 777, and manager of terminal-area projects.

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