Technical data

Product Specification
HIF-2121/R5 CAGE CODE: 97896 SCALE: NONE SIZE: A DWG NO: 965-1176-601 REV: D SHEET 100
6.8 Envelope Modulation
Revision History
Effectivity
Date - Modified By Description of the Updates App. Cfg.
29-NOV-99 Peter Bateman Initial release and entry into PVCS -001 -001
23-MAY-00 Susie Wright SCR 4796: Update Envelope Modulation to use Geometric Altitude when
available. General document cleanup.
-003 -003
06-JUN-00 – Susie Wright Document only change – Deleted proprietary note from footer. -003 -003
27-FEB-01 – M. Calhoun Document only change – Highlighted the fact that if the system is not
configured to bring in Localizer, then some Envelope Modulation cases
will not be available.
--
02-JUL-01 – Susie Wright Added review comments. -008 -008
During the past 20 years, experience with Ground Proximity Warning Systems has shown that some approaches to certain
airports can be incompatible with the normal alert/warning envelopes and signal filtering.
Honeywell has developed a number of enhancements to the envelopes and filters during this time in an attempt to
accommodate these few airports, without compromising the overall GPWS effectiveness for all the other “normal” airport
approaches. However, there remain a limited number of cases where problems persist despite these efforts.
All of the noticeable problems have been due to nuisance alerts/warnings for approaches and departures at particular airports.
The majority of nuisance alerts/warnings involve Mode 2 closure rate due to terrain under the approach path or rising terrain
just before the runway threshold. Others involve Mode 4 terrain clearance alerts during initial approach. A few Mode 1
alerts/warnings are the result of steeper than normal approaches over terrain which slopes down to the runway at some
airports.
A different type of problem is inadequate protection during ILS approaches because Mode 5 is limited to less than 1000 feet
radio altitude. There are airports located at a significantly higher altitude than the surrounding terrain. In some instances this
difference is over 1000 feet, thus requiring the aircraft to be below the runway elevation before a Mode 5 alert is possible
during most of the approach.
The Envelope Modulation feature provides improved alert/warning protection at some key locations throughout the world,
while improving nuisance margins at others. This is made possible with the use of navigational signals from GPS or FMS
navigation equipment. All navigational position data is cross checked to ground based navigational aids, altimeter and
heading information, and stored terrain characteristics prior to being accepted for Envelope Modulation purposes. This
guards against possible navigational position errors.
After recognizing the approach to or departure from one of these airports, it is also important to verify the aircraft is at a
reasonable altitude before desensitizing any warning criteria. If the aircraft is already low, further warning reduction is not
desirable. Geometric Altitude data is used for this purpose.
The Geometric Altitude is verified in one of two ways:
1. For ILS approaches, the glideslope deviation is used to establish that adequate terrain clearance exists (i.e. a “normal”
approach). Consequently, errors in altitude data will not enable Envelope Modulation during an unsafe condition.
2. When ILS information is not available, stored terrain elevation data is matched against computed elevation data (i.e.
Geometric Altitude - radio altitude) to verify altitude. This is done for a “snapshot” location immediately prior to the
Envelope Modulation area.
If Geometric Altitude is not of high integrity, corrected barometric altitude is used.
The following additional input data is used to cross check the navigational and altitude information:
Localizer deviation
Magnetic track/heading
Selected runway heading or selected course
Radio altitude