Advanced Avionics
By John C. Colwell, Bishop & Associates Inc.

The gestation period for highly complex systems, such as military and commercial aircraft, can take as much as a decade, or five generations of Moore’s Law. As a result, the initial conceptualization of systems and performance requirements become moving targets. In the case of the F-22 Raptor, the Intel processors initially selected for the common integrated processor (CIP) were already out of production by the time the aircraft itself entered production. Clearly, a new approach was needed.

In the traditional acquisition model, the system context—or conceptualization—was defined first, and then driven primarily by the mission requirements. The architecture and design phase would follow, and finally, the program would move into the implementation phase. The new model involves the dynamic interaction, from program start to finish, between the system context, the system architecture and design, and the commercial items available in the marketplace.

Traditional vs. COTS System Acquisition Model

The key advantage of this approach is that the avionics system designer can leverage the latest technology and performance advantages that are independently driven by the commercial industry sectors, in particular, telecommunications and computers. The elegant evolution of VME backplane standards and Ethernet represent classic examples of this process.

From an electronic packaging and interconnection viewpoint, commercial off-the-shelf procurement (COTS) takes us to a standards-based imbedded systems approach to system architecture for avionics, mission computing, and similar applications. The VPX (VITA 46), and more specifically, its hardened version (VITA 48) standard is the platform of choice for the F-22 Raptor, because of its ruggedness, ability to handle a diverse array of system challenges, and its extensive I/O handling capabilities.

Also known as VPX-REDI (ruggedized enhanced design implementation), VITA 48 has three versions: V48.1 air-cooled, V48.2 conduction-cooled, and V48.3 liquid-cooled. In its current implementation, the F-22 utilizes the liquid flow through (LFT) method of cooling. A pair of liquid distribution rails provides polyalphaolefin (PAO), or similar coolant, to the modules by means of quick disconnects, or QDs. The backplane interconnect system for VPX (VITA 46) and VPX-REDI (VITA 48) is the multi-gig MT 2 product developed and manufactured by Tyco Electronics Inc.

There are three versions of VPX-REDI modules based on the method of cooling. There are two module sizes: 3U and 6U by 160mm. There are also mezzanine boards.

The LFT cooling system also requires a method of interconnection between the backplane distribution rail and the module. The coolant interconnect is referred to as a quick disconnect, and is illustrated at the right.

The backplane interconnect system was chosen by VITA because of its high bandwidth capability, ESD and grounding characteristics, signal density, and ruggedness. These attributes are essential to military avionics systems due to the rapidly increasing number of very high-speed on-board sensors, as well as the growing level of communications with ground, in-flight, and space-based resources. 

While the switching fabrics, processors, and backplanes are COTS, the I/O and cable assemblies are typically not. The interconnect cabling between the backplane and the enclosure I/O is generally custom designed for the intended application environment. Flexible circuit assemblies are commonly employed in this application. The same applies to external cable assemblies that link the computer to sensors and other black boxes.

And in spite of the fact that the communication protocol may be Avionics Full DupleX Switched Ethernet (AFDX), introduced by Airbus in its A380 commercial craft, or Fibre Channel, which is being deployed in the F-35 Joint Strike Fighter (JSF), the I/O interconnects remain mil-spec circular in nature. The F-22 utilizes a mix of Category 5 shielded twisted-pair and fiber optic cables, whereas the F-35 utilizes a predominantly fiber optic network. Both aircraft will continue to support legacy systems and sensors using the older MIL-STD-1553 copper bus networks.

Harris Corp. Government Communications Systems Division in Melbourne, Florida, holds the contract for the F-35’s advanced avionics design from Northrop Grumman. The contract includes fiber optic transceivers and physical hardware to provide four channels of data transmission and reception at a data rate of two gigabits per second over the aircraft's extensive fiber optic network. Other significant avionics contractors include Boeing, Mercury Computer Systems, Curtiss-Wright, and GE Fanuc Embedded Systems.

John Colwell Director, Telecom, Medical and Instrumentation, Bishop & Associates Inc. John Colwell’s background includes 10 years at Nortel Networks‑Cable Group, where he directed the U.S. premises cable marketing effort. In addition, Colwell directed Nortel's global product development group. Prior to joining Nortel, Colwell held positions in engineering, business planning and development at Amphenol Corporation.