Aircraft design today involves far more than thrust and aerodynamic surfaces. Modern aircraft must support high volumes of data exchange among communications systems, weapons systems, flight-critical systems, and more. Whether they're jets like the F-35 Joint Strike Fighter or unmanned aerial vehicles (UAVs,) successful operation of today's military aircraft requires robust, high-speed data buses. The IEEE 1394b standard provides a solution.
The logical high-speed interfaces defined by the IEEE 1394b standard can link together multiple devices to minimize wiring and, hence, weight, always a factor in avionic systems. IEEE-1394b-compliant data buses like FireWire provide a range of topology choices, including tree architectures, daisy chains, and even loops to support redundancy and hot swaps. The standard has a track record of success, supporting the vehicle management system (VMS) in the F-35, for example, which facilitates communications among more than 70 separate 1394 devices that deliver critical operational data on engine and flight controls, weapons systems, mission details, and communications.
When it came time for the X-47B Unmanned Combat Air System Demonstration (UCAS-D), a 1394b bus was the natural choice. Launched as part of the Joint Unmanned Combat Air Systems (J-UCAS) program of the US Defense Advanced Research Projects Agency (DARPA) and taken over by the U.S. Navy, the X-47B is a large-frame, tailless unmanned aerial vehicle designed for carrier-based operations. The X-47B required a network backbone to provide guaranteed quality of service with predictable latencies in real-time control applications, which it achieves with the help of SAE-defined AS5643, the military version of the 1394b standard.
Supported by IEEE-1394b-compliant data buses, the X-47B Unmanned Combat Air System is a tailless unmanned aerial vehicle designed for operation on aircraft carriers. (Courtesy of Northrop Grumman Corp.)
The X-47B leverages stealth technology and has a wingspan of just over 62 ft and a length of 38.2 ft. Capable of altitudes greater than 40,000 ft and a range greater 2,100 nautical miles, this autonomously air-refueled UAV can stay in theater for days while carrying weapons payloads of nearly 4,500 lbs. Powered by a Pratt & Whitney F100-PW-220U jet engine, the X-47B achieves high subsonic speeds and is equipped with the latest in electro-optical; infrared; synthetic-aperture radar; intelligence, surveillance, and reconnaissance (ISAR); ground moving target indicator (GMTI)’ maritime moving target indicator (MMTI); and electronic support measures (ESM) sensor technology.
Because of the critical nature of the VMS, the X-47B implements three redundant buses, each with its own vehicle management computer (VMC), which delivers the type of critical information handled in the F-35 program. For both projects, 1394b was chosen based on its deterministic behavior, speed, bandwidth, fault tolerance, and long-distance capabilities. In addition, the interface allows enables operational software to be remotely downloaded to network modules without removing them.
Main X-47B flight control and subsystem processing is completed in a triplex network of the VMCs, which act as the master for each 1394b bus. The three VMCs are cross channeled and data linked together to provide redundancy. In addition to the triplexed VMCs, the X-47B takes advantage of 1394b’s loop topology to provide additional redundancy, protecting against single port or cable failures. If a single port/cable fails, the 1394b bus will automatically reconfigure using the alternate path for communication.
When coupled with AS5643’s fixed-frame-rate synchronization, the 1394b bus provides predictable latencies that allow the VMC to house all flight-control algorithms and utilities in a highly centralized structure, while interfacing easily with legacy buses such as those compliant with MIL-STD-1553. The architecture also makes use of independent controllers for applications that require dedicated, high-bandwidth control loops, according to Northrop Grumman engineer Matthew Pugh.
The VMC incorporates guidance, navigation, and control as well as subsystem processing that on legacy aircraft was performed se ofparately, says Pugh. Components residing on the 1394b network serve the following systems:
Serving X-47B's distance requirements
- Vehicle systems processing, VMC, and nine remote input/output units (RIOs);
- Guidance, navigation, and control with all flight control surfaces, including ailerons, elevons, and spoilers; use of data from the air data probes; and inertial electronics;
- Subsystems such as weapons bay door drives, precision navigation, and pump and valve control for the fuel and hydraulic systems;
- Propulsion systems engine interface units (EIUs) and prognostics health area managers;
- Mission systems including interface with the mission operator, waypoint management, mission planning and validation, command and control data link path with the VMS;
- Flight test instrumentation in the form of a high-speed data acquisition unit on each bus for capturing flight test data.
Commercial implementations of the IEEE 1394b standard are typically limited to 4.5 m between devices. To robustly operate at distances up to 10 m, the X-47B utilizes AS5643/1-specified active transformers, quad cabling, connectors, and termination methods. These enhancements also ensure optimal operation in the harsh temperature and vibration environments that characterize safety- and mission-critical applications for military and aerospace vehicles.
To meet the special test requirements for the X-47B design, test tool providers were able to use existing and commercially available technology to provide electrical signaling and protocol level tools. For example, the signal quality tester from Quantum Parametric (Colorado Springs, CO) provides transmit signal integrity and receiver sensitivity testing, and the FireSpy 1394 protocol analyzer from Dap Technology (Oldenzaal, Netherlands) has been widely used in the program. Both test systems are used as part of system debug and subsystem qualification, as well as module acceptance testing.
The success of 1394b-compliant interfaces in this high-profile, mission-critical program reflects the bandwidth, distance, and quality of service features enabled by the standard. The guaranteed quality of service and predictable latencies provided by the standard make the data bus well-suited to military and aerospace applications in general, making it likely that we will see additional implementations in the future.About the author
Richard Mourn is vice president of Test and Measurement at Astek Corp. (Colorado Springs, CO). He was a developer of the 1394 standard while at Texas Instruments Corp. (Dallas, TX), where he helped design the first two production link-layer controllers. He continued working on 1394 while at NCR Corp. before co-founding Quantum Parametrics in 2000.
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