Enhanced GPWS - Additional logic R Figure 025
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Non-Standart Altitude To support the Geometric Altitude function, the Enhanced GPWS computes a non-standard altitude using the hydrostatic equation relating changes in height to changes in pressure and temperature. Non-standard altitude uses static pressure derived from standard altitude, along with static air temperature, to continuously accumulate changes in Geometric Altitude. Since the non-standard altitude algorithm incorporates actual atmospheric temperature, it does not suffer from errors due to non-standard temperatures. Non-standard altitude is highly accurate for measuring relative vertical changes over short periods of time and distance, such as during take off and approach. Non-standard altitude does not provide an absolute altitude and is prone to significant errors over extended periods of time and distance due to the effects of pressure gradients and long-term integration errors. Due to these limitations, non-standard altitude is not used directly, but is calibrated using additional signals and data to produce a set of component altitudes for use in the final altitude solution.
Computed component altitudes The Enhanced GPWS generates three component altitudes that are combined, along with corrected altitude if available, to produce Geometric Altitude. These component altitudes are runway calibrated altitude, GPS calibrated altitude, and radio altitude calibrated altitude.
-Runway calibrated altitude is a one-time calibration of non-standard altitude during take off run. A correction factor for non-standard is computed using the runway elevation from the Enhanced GPWS runway database while the aircraft is on the ground. Runway calibrated altitude is used during the take off and climb-out portions of flight. VFOM of runway calibrated altitude is estimated based on changes in altitude since calibration, time since calibration, and distance from the runway.
-GPS calibrated altitude is produced by combining GPS altitude and non-standard altitude through a complementary filter. The complementary filter is dynamically optimized to reduce errors in GPS altitude while minimizing pressure gradient and drift errors of non-standard altitude. GPS calibrated altitude is accurate through all phases of flight and is the primary altitude source during the cruise portion of flight. GPS calibrated altitude VFOM is estimate using GPS VFOM and estimated non-standard altitude drift errors.
-Radio altitude calibrated altitude is a calibration of non-standard altitude during approach using an altitude
R 1EFF : 106-149, 220-299, 301-399, 401-499, 1 34-48-00Page A49 1 1 Config-1 Aug 01/05 1 1 1CES 1 derived from radio altitude (height above terrain) and the terrain elevation data stored in the Enhanced GPWS terrain database. This calibration is performed during the approach phase of flight when the aircraft is within a minimum distance and elevation of any runway. Once a correction factor is determined, it is applied to non-standard altitude until the aircraft lands. VFOM of radio altitude calibrated altitude is based on the accuracy of the calibration as estimated from the resolution of the terrain data and flatness of the terrain. The altitude is re-calibrated if a correction with a higher estimated accuracy is computed. An estimated VFOM for corrected barometric altitude is computed in order to determine its weight in the final altitude. VFOM of corrected barometric altitude is based on aircraft altitude above and distance from the nearest runway, with the accuracy assumed to be the highest close to runway.
3 Blending and reasonableness checking
_ The final Geometric Altitude is computed by combining the three computed component altitudes with optional corrected barometric altitude. The weighting of each altitude in the final solution is based on the corresponding estimated VFOM. The blending algorithm gives the most weight to altitudes with a higher estimated accuracy, reducing the effect of less accurate altitudes on the final computed altitude. Each component altitude is also checked for reasonableness using a window monitor computed from GPS altitude and GPS VFOM. Altitudes that are invalid, not available, or fall outside the reasonableness window are not included in the final blended altitude.
(4) The Terrain Clearance Floor (TCF) The Terrain Clearance Floor (TCF) alert function adds an additional element of protection to the standard Ground Proximity Warning System. It creates an increasing terrain clearance envelope around the airport runway to provide protection against Controlled Flight Into Terrain (CFIT) situations beyond that which is currently provided. TCF alerts are based on current aircraft location, nearest runway center point position and radio altitude. TCF is active during takeoff, cruise and final approach. This alert mode complements the existing Mode 4 protection by providing an alert based on insufficient terrain clearance even when in landing configuration. Alerts for TCF illuminate GPWS cockpit lamps and produce aural messages.
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