LOW-OBSERVABLE AIR superiority places severe requirements on avionics that can only be met by the degree of integration evident in the F-22, says Marty Broadwell, deputy avionics team-leader. "We are collecting snippets of information, with minimum illumination, pencil beams, sensors that are passive or barely on. Alone, none is adequate, so we have to share and fuse information to create a coherent picture," he says.
To Broadwell, integration means three things: what the pilot sees; how the system operates; and how it is put together. The low-observability air-superiority mission cannot be carried out without integration at all three levels, he believes. For the pilot, integration means seeing all the information on a single screen, but that requires massive functional and hardware integration to create and use the coherent picture.
Avionics, stealth and supercruise requirements compete, says Broadwell. "Apertures and conductors on the outside of the aircraft conflict with low observability while, with supercruise, we have have to look a long way ahead, which means our [transmitter] power and [receiver] gains have to be high," he says. Integration of the F-22's avionics provides the pilot with situational awareness "compatible with the mission", while addressing other issues such as reliability and maintainability, he says.
The F-22 avionics concept is based on the correlation and fusion of sensor data to create a coherent picture which is used by the system for situation assessment, for display to the pilot, and for tasking the sensors to collect more information. Onboard sensors are:
- the Northrop Grumman APG-77 multi-mode radar, with its active electronically scanned array of some 2,000 transmit/receive modules;
n- he Lockheed Martin Sanders electronic-warfare (EW) system, which includes radar warning and missile-launch detection;
- the TRW communication/navigation/ identification (CNI) system, which includes the intra-flight datalink (IFDL), Joint Tactical Information Distribution System (JTIDS) datalink, and MkXII identification friend-or-foe.
All sensor signal and data processing is performed by the F-22's two Hughes-developed common integrated processors (CIPs). Each CIP is a rack of plug-in modules, including signal- and data-processing elements. Front-end support electronics at each sensor send a digitised waveform over fibre-optic bus to the CIP, where it is stored in global bulk-memory. A signal-processing element takes the waveform and transforms it for the data-processing element responsible for the fusion of sensor tracks. This creates a coherent track file which is passed to another module for display to the pilot, and made available via databus to any CIP module that needs the data. For example, says Broadwell, the track file is used to control the aircraft's sensors.
Sharing track, mission and system data between avionics functions is made possible by using common hardware modules and common operating-system software. There are just seven module types, which plug into identical backplanes using standard connectors. The two CIPs are linked by high-speed fibre-optic bus, and weight, space and power for a third CIP has been provided, although Broadwell suspects that this will not be necessary. About 20 of the 66 module slots in each CIP are still available for growth.
For security, flight software is stored in encrypted form in the data-transfer cartridge which the pilot carries out to the aircraft. At start-up, the software is decrypted by a Motorola KOV-5 module and broadcast over high-speed fibre-optic databus to the load the modules. "Everything is erased at shutdown," says Broadwell. A similar approach is used to reconfigure the fault-tolerant avionics when a module fails. Each CIP has at least one data-processing element which is not in use and which can be reloaded with software from the failed processor. If the failure is more extensive, the pilot is given options on which software is reloaded - radar and weapons to get out of the fight, or navigation and defensive to return to base, for example.
Software is being developed in blocks corresponding to increasing capability. The first avionics-test F-22, aircraft 4004, is scheduled to be flown in August 1999 with Block 1 software, which is primarily for the radar, with some CNI but no EW. This will comprise over 50% of the 1.7 million lines of avionics code, and will allow end-to-end testing of the sensor-to-pilot data flow. Block 2 will add EW, increase CNI and expand radar modes to permit data fusion, and is scheduled to be completed before 4004 is flown, allowing almost all flight-testing to be accomplished with data-fusion operational. Block 3 will include all sensor modes and add electronic counter-countermeasures. Two further steps have been added: Block 3.1 will add JDAM and JTIDS-receive capability; while Block 4 will add JTIDS-transmit, the Joint Helmet-Mounted Cueing System and AIM-9X and is scheduled to be in the F-22 when it enters service in 2004.