Technical data

Product Specification
HIF-2121/R5 CAGE CODE: 97896 SCALE: NONE SIZE: A DWG NO: 965-1176-601 REV: D SHEET 98
6.7.8.2.1 Non-Standard Altitude
To support the Geometric Altitude function the EGPWS 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 (uncorrected barometric altitude), along with SAT, 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.
6.7.8.2.2 Computed Component Altitudes
The EGPWS 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 roll. A correction factor for
non-standard is computed using the runway elevation from the EGPWS 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 produce by combining GPS altitude and non-standard altitude through a complementary filter.
The complimentary filter is dynamically optimized to reduce errors in GPS altitude caused by selective availability 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 estimated
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 derived from
radio altitude (height above terrain) and the terrain elevation data stored in the EGPWS 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.
6.7.8.2.3 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.
6.7.8.3 Input Failures and Reversionary Operation
The Geometric Altitude algorithm is designed to allow continued operation when one or more of the altitude components are
unavailable. Component altitudes that are unavailable due to a failed input signal or flagged as unreasonable are not used,
with the final blended altitude comprised of the remaining, valid signals. If all component altitudes are invalid or
unreasonable, then GPS Altitude is used directly for the Terrain Awareness functions. If GPS altitude is invalid then the
Terrain Awareness functions operate using corrected altitude when available, otherwise, a Terrain Awareness INOP results.
For installations without SAT or if the SAT input fails, standard altitude is use in place of computed non-standard altitude.
Under such conditions, all computed component altitudes normally requiring non-standard altitude use standard altitude with