ASTi has often provided the audio replay component of many subsystems commonly found in a simulation environment, such as warning tones, voice alerts, and Ground Proximity Warning Systems (GPWS). This has typically taken the form of an ASTi audio processing platform coupled via a high-level, parameter-driven interface to a simulation host computer providing audio demand flags. A recent project, however, has moved the subsystem boundary to become an entirely self-contained package. ASTi's customer was contracted to provide a simulated Ground Proximity Warning System (GPWS) as part of a larger trainer upgrade package.

This specific situation involved a legacy trainer, and all of the potential difficulties related to updating older equipment. Typically, these include issues with computing system capacity, operating system limitations, minimal or nonexistent support documentation, I/O availability, and so on. In this case, the customer deemed it preferable to avoid adding substantial code to the simulation host. ASTi was then contracted to provide a "single box" solution, providing not only the audio generation capability, but also the GPWS subsystem simulation itself. Through use of the ASTi DDI module, additional I/O to drive the required alert and status mode indicators was also added to the existing system.

The system architecture is identical to most common ASTi communication processing platforms, using the TDM DSP audio processing card, and, in this case, a single RIU to provide the audio output. In addition to the standard Model Builder application software, however, a custom Dynamic Linked Library (DLL) was provided that implements the full GPWS subsystem simulation model. Standard GPWS interface parameters are passed to the ASTi system via the network in the same way as existing host data. This interface is highly standardized per the real-world GPWS, and includes the following sampling of GPWS parameters:

• Validity flags for radar altimeter
• AOA
• Glideslope and attitude
• ARINC inputs
• System and sensor inputs for radio altitude, roll angle, pitch angle, true airspeed, flap extension, etc.

A complete Interface Control Document (ICD) detailing these parameters and more is available from ASTi upon request.

The DLL software is called at the start-up of Model Builder and runs as a routine called from the internal Model Builder executive timing routine. All 7 operational modes, plus the test mode of the GPWS system, are simulated including:

• Mode 1: Excessive sink rate
• Mode 2: Excessive terrain closure rate
• Mode 3: Altitude loss after takeoff
• Mode 4: Unsafe terrain clearance
• Mode 5: Excessive deviation below glideslope
• Mode 6: Advisory callouts
• Mode 7: Windshear alerting
• Self-test

The ASTi GPWS system takes the interface parameters from the host simulation and passes them internally through to the DLL software, which performs the necessary mode determination, and, if applicable, envelope penetration determination. Based in these calculations, the system will prioritize alerts, warnings, and callouts; and trigger the appropriate audio and visual annunciators. The system can be configured for either "Business" or "Air Transport" operational modes of GPWS operation, and standard or audio-declutter operation may be selected.

The GPWS system may be used as a stand-alone system, or incorporated as a building block as part of a larger communications system simulation.