Wednesday, April 3, 2024

Project Paper 793 - Third Generation Aero Satcom

Project Paper 793 is under development by the AEEC/SAE-ITC Ku/Ka-band Satcom Subcommittee. PP793 is the third generation satcom system, following ARINC 791 and ARINC 792. PP793 provides support for multibeam antennas through the use of fiber optic interconnections that replace all coaxial cables and quadrax (Ethernet) cables.

ARINC 792 Baseline Modman Interfaces

ARINC 792 provides for two coaxial interfaces to the Outside Antenna Equipment (OAE), one for Tx and one for Rx. 

A baseband frequency reference (10, 25, 50, 100 MHz) can be communicated alongside an Intermediate Frequency (IF) carrier. The frequency reference is provided within restricted limits of phase noise and drift.

The IF band for Tx is typically 950-1450 MHz (500 MHz bandwidth). The IF band for Rx is typically 950-2150 MHz (1100 MHz bandwidth). RF block up/down converters translate the Tx and Rx frequency as required. 

The Modman communicates with the KPSU across eight manufacturer-specific discrete wire interfaces (twisted-shielded pairs), a single discrete wire for transmit mute, and a 100BaseTx four-wire Ethernet interface. The Ethernet interface is for control and maintenance applications only. 

The Modman receives an ARINC 429 broadcast from an Inertial Reference System which may include GNSS updates. There is an optional ARINC 429 interface for GNSS interface. 

The Modman receives Weight-on-Wheels discrete.

The Modman provides discrete interfaces to a control panel to annunciate Data Link and System Available and for discrete command interfaces for Ground Transmit Enable, Passenger Service Disable, Data Load Enable, and Bench Mode Enable.

The Modman interfaces to other avionics systems with eight-wire Gb Ethernet, one for Passenger Information and Entertainment Services, one for Airplane Information Services, and two for Passenger Owned Devices. The Modman also has an optional 100BaseTx interface to a control/maintenance panel.

Figure 1 ARINC 792 Modman Interfaces

ARINC 792 abandoned the ARINC 791 KRFU (High Power Amplifier). The KRFU was installed as close to the OAE bulkhead penetrations as possible, to minimize the transmission line loss. Ka-band applications benefitting from installing the HPA as part of the OAE.  ARINC 792 also promoted active phased array antennas, where each element includes an integrated HPA (distributed HPA), rather than a passive antenna that relies on a single-ended external HPA input. 

ARINC 792 also abandoned the beam-steering functionality that was included in the KANDU and instead promoted the KPSU, effectively a power supply. Active phased array antennas also increased power demand, and so the KPSU was provided a three-phase AC power input and wiring capable of delivering 1000W to each of the Rx and Tx apertures.  The heat generated by such great power within the OAE has created significant concerns around system reliability and causing damage to the underlying aircraft skin. 

Project Paper 793

ARINC 792 supplement 1 introduced the Auxiliary Modem Unit (AMU). The AMU provides the Modman expansion room for additional modems. The coaxial interface was daisy-chained from traditional OAE to Modman to OAE to AMU (to AMU2) to Modman. The operational concept was that only one modem was active, whether within the Modman or AMU. Providing support for multi-beam (multi-modem) would require stacked IF carriers.

Project Paper 793 is in development to address multi-beam, multi-carrier operation. Rather than relying on coaxial interfaces, 793 favors fiber optic strands organized broadly around 12 strand MT ferrules. Furthermore, Ethernet interfaces are also offered over fiber.  Multi-mode fiber optic strands are dedicated to a single wavelength (unidirectional) but could be used with wavelength division modulation (WDM) for bidirectional application.

IF is abandoned, instead utilizing DIFI consortium baseband signaling.  Every modem interface is provided two Rx fiber strands (to accommodate 500+ Msps carriers), one Tx fiber strand, and a shared pool of Frequency Reference strands.  Please see DIFI attachment for more information. Ethernet interfaces are assumed to support 10GbE. 

Frequency references are communicated “analog over fiber”.  Phase noise accumulation across the fiber interface is expected to support GEO modulations of 8PSK or less, while LEO/MEO modulations of 16QAM or higher will likely require a local slave oscillator to achieve acceptable phase noise.

Project Paper 793 currently utilizes the same lug layout as ARINC 792 (see discussion around small form factor lug layout below). 

Project Paper 793 replaces the Modman with an Expanded Functionality Modman (EFnModman). The EFnModman size may range from the existing 4 MCU to either 6 MCU or 8 MCU (to be finalized). The larger form factor increases power, cooling, and space to accommodate additional modems and processing without the need for an AMU (effectively, incorporating the AMU). 

As a scaling concept, six carriers (beams) are provided for:

1. LEO servicing satellite (Tx, Rx)

2. LEO servicing/handoff satellite (Tx, Rx)

3. GEO servicing satellite (Tx, Rx)

4. GEO servicing/handoff satellite (Tx, Rx)

5. GEO Wideband broadcast (Rx) – live television

6. GEO Wideband AIS/AOC/ATS (Tx, Rx) - conceptional

Every installation can configure the fiber stands (cooperatively) between the OAE and Modman as desired.

A pair of strands (Tx, Rx) are provided for another OAE radio (L-band satcom, cellular, T-WAP).

A single strand is provided for analog over fiber GNSS RF signal. The Modman would have a GNSS receiver and can produce a 1PPS signal for internal use. 

A pair of strands (Tx, Rx) are provided between the Modman and the Tx and Rx aperture “managers”. This ethernet interface is for control and maintenance only, replacing the interface that was formerly hosted in the ARINC 792 KPSU.

Figure 2 Project Paper 793 OAE-Modman Fiber Strands

Project Paper 793 Modman Interfaces

In addition to the fiber interfaces to the OAE described above, the Modem provides fiber interfaces for growth to next-gen airplane systems. 

The benefits from fiber optic interfaces include weight, bandwidth, EMI.  

Fiber optic interconnections are still being defined, with the expectation to utilize unmodified MT ferrules incorporated into ARINC 600 and D38999 circular connector inserts.

The Modman retains two 10GbE interfaces for passenger devices (via WAP interfaces), one 10GbE PIES interface and one 10GbE interface for AIS, plus one optional 100BaseTx interface to a control panel.

Figure 3  PP793 Modman Interfaces

PP793 KPSU Interface

The PP793 KPSU does not utilize an Ethernet interface to the OAE.

Figure 4 PP793 OAE


ARINC 792 six-lug layout was modified from ARINC 791 seven-lug layout to accommodate antennas with two discrete apertures.  At this time, PP793 is following the ARINC 792 lug layout.

Figure 5 ARINC 792 Six-Lug Layout

To address small antennas, a special six-lug layout is shown below along with the load axis for each lug.  In this case, lugs 3-4 are deleted and two sets of lugs (1-2) are provided. 

 Figure 6 ARINC 792-1 Lug Layout – Small Form Factor Antenna

The shortest extent (fore and aft lugs) is about 62” on the x-axis, 15” on the y-axis. For this configuration, the defined Rx and Tx cutouts are abandoned, along with lugs in position 3 and 4. Instead, the installer is expected to define a connector cutout location for the fiber optic strands (48) from the Modman and the power/discrete interface from the KPSU, likely in the forward space between four forward lugs.

Figure 7  ARINC 792-1 Six-Lug Layout, Small Form Factor Antenna 
removes connector cutouts and lugs in positions 3 and 4

While a four-lug layout is possible, it creates serious concerns regarding fault tolerance that have left it unacceptable by at least one aircraft manufacturer. This layout is 50” between fore and aft lug.

 Figure 8 ARINC 792-1 four lug layout (see attachment 12 for concerns)

Please refer to 792 attachment 12 for a complete discussion.

Stay tuned!

Peter Lemme

peter @
Copyright 2024 All Rights Reserved

Peter Lemme has been a leader in avionics engineering for 43 years. He offers independent consulting services largely focused on avionics and L, Ku, and Ka band satellite communications to aircraft. Peter chaired the SAE-ITC AEEC Ku/Ka-band satcom subcommittee for more than ten years, developing ARINC 791 and 792 characteristics, and continues as a member. He also contributes to the Network Infrastructure and Interfaces (NIS) subcommittee.

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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