EGPWS Modulation
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1 Background
EGPWS contains some advanced modes to prevent CFIT. Unfortunately, these modes can create some nuisance alerts when terrain is present close to the airport. To prevent these “false” warnings, a modulation database needs to be defined to control the mode or to simply disable it. This database is proprietary information and needs to be manually defined
The modulation database is made of a single xml file named egpws.xml and located in ProgramData\ProSim-AR\Database folder
2 XML example
<?xml version="1.0" encoding="utf-8"?>
<egpwsDatabase refreshThreshold = "30">
<tcf enabled="true">
<bias>0</bias>
</tcf>
<rfcf enabled="true">
<bias>0</bias>
</rfcf>
<lookAhead enabled="true">
<inhibitRange>2</inhibitRange>
<warningLookAheadSeconds>30</warningLookAheadSeconds>
<cautionLookAheadSeconds>40</cautionLookAheadSeconds>
<warningLookUpSeconds>60</warningLookUpSeconds>
<cautionLookUpSeconds>80</cautionLookUpSeconds>
</lookAhead>
<airport id="MROC">
<tcf enabled="true"/>
<rfcf enabled="true"/>
<lookAhead enabled="true">
<inhibitRange>2</inhibitRange>
<warningLookAheadSeconds>30</warningLookAheadSeconds>
<cautionLookAheadSeconds>40</cautionLookAheadSeconds>
<warningLookUpSeconds>60</warningLookUpSeconds>
<cautionLookUpSeconds>80</cautionLookUpSeconds>
</lookAhead>
<runway id="07">
<tcf enabled="false" />
<rfcf enabled="false" />
<lookAhead enabled="false" />
</runway>
</airport>
<airport id="MMMX">
<runway id="32L">
<tcf enabled="false" />
<rfcf enabled="false" />
<lookAhead enabled="false" />
</runway>
<runway id="5R">
<tcf enabled="false" />
<rfcf enabled="false" />
<lookAhead enabled="false" />
</runway>
</airport>
</egpwsDatabase>
3 General
GPWC will search for nearest airports when airplane moves more than 50 NM (default). This may cause issues in areas where we have several airports in the 50NM radius.
This range can be defined using the refreshThreshold parameter. e.g: <egpwsDatabase refreshThreshold = "30">
There are 3 modes that can be modulated. TCF, RFCF and Look Ahead terrain.
Modulation can be done globally, by airport, runway or a combination of both.
Global modulation:
<tcf enabled="true">
<bias>0</bias>
</tcf>
<rfcf enabled="true">
<bias>0</bias>
</rfcf>
<lookAhead enabled="true">
<inhibitRange>2</inhibitRange>
<warningLookAheadSeconds>30</warningLookAheadSeconds>
<cautionLookAheadSeconds>40</cautionLookAheadSeconds>
<warningLookUpSeconds>60</warningLookUpSeconds>
<cautionLookUpSeconds>80</cautionLookUpSeconds>
</lookAhead>
The following example disables all 3 modes for runway 07 only:
<airport id="MROC">
<runway id="07">
<tcf enabled="false" />
<rfcf enabled="false" />
<lookAhead enabled="false" />
</runway>
</airport>
To disable all 3 modes for all MROC runways, just use:
<airport id="MROC">
<tcf enabled="false" />
<rfcf enabled="false" />
<lookAhead enabled="false" />
</airport>
And this will disable all modes for all runways, except TCF for runway 07:
<airport id="MROC">
<tcf enabled="false" />
<rfcf enabled="false" />
<lookAhead enabled="false" />
<runway id="07">
<tcf>
<bias>2</bias>
</tcf>
</runway>
</airport>
4 Terrain Clearance Floor
The Terrain Clearance Floor (TCF) function (enabled with TAD) enhances the basic GPWS Modes by alerting the pilot of descent below a defined “Terrain Clearance Floor” regardless of the aircraft configuration. The TCF alert is a function of the aircraft’s Radio Altitude and distance (calculated from latitude/longitude position) relative to the center of the nearest runway in the database (all runways greater than 3500 feet in length). The TCF envelope is defined for all runways as illustrated below and extends to infinity, or until it meets the envelope of another runway. The envelope bias factor is typically 1/2 to 2 nm and varies as a function of position accuracy.
TCF modulation:
<tcf>
<bias>2</bias>
</tcf>
The enable = “true” can be suppressed as it is implicit when creating the modulation entry
Bias [integer]: moves the envelope away from the runway by the value (in NM) entered.
5 Runway Field Clearance Floor
This is similar to the TCF feature except that RFCF is based on the current aircraft position and height above the destination runway, using Geometric Altitude (in lieu of Radio Altitude). This provides improved protection at locations where the runway is significantly higher than the surrounding terrain as illustrated below. With version -218-218 and later models, the RFCF envelope is moved from 1nm to 1/2nm if runway and position data is of high integrity.
RFCF modulation:
<rfcf>
<bias>2</bias>
</rfcf>
The enable = “true” can be suppressed as it is implicit when creating the modulation entry
Bias [integer]: moves the envelope away from the runway by the value (in NM) entered.
6 Look Ahead Terrain
Another enhancement provided by the internal terrain database, is the ability to look ahead of the aircraft and detect terrain or obstacle conflicts with greater alerting time.
This is accomplished (when enabled) based on aircraft position, flight path angle, track, and speed relative to the terrain database image forward the aircraft.
Through sophisticated look ahead algorithms, both caution and warning alerts are generated if terrain or an obstacle conflict with “ribbons” projected forward of the aircraft (see following illustration). These ribbons project down, forward, then up from the aircraft with a width starting at 1/4 nm and extending out at ±3º laterally, more if turning. The look-down and up angles are a function of the aircraft flight path angle, and the look-down distance a function of the aircraft’s altitude with respect to the nearest or destination runway. This relationship prevents undesired alerts when taking off or landing. The look-ahead distance is a function of the aircraft’s speed, and distance to the nearest runway.
A terrain conflict intruding into the caution ribbon activates EGPWS caution lights and the aural message “CAUTION TERRAIN, CAUTION TERRAIN” or “TERRAIN AHEAD, TERRAIN AHEAD”. An obstacle conflict provides a “CAUTION OBSTACLE, CAUTION OBSTACLE” or “OBSTACLE AHEAD, OBSTACLE AHEAD” message.1 The caution alert is given typically 60 seconds ahead of the terrain/obstacle conflict and is repeated every seven seconds as long as the conflict remains within the caution area. When the warning ribbon is intruded (typically 30 seconds prior to the terrain/obstacle conflict), EGPWS warning lights activate and the aural message “TERRAIN, TERRAIN, PULL UP” or “OBSTACLE, OBSTACLE, PULL UP” is enunciated with “PULL UP” repeating continuously while the conflict is within the warning area.
Look Ahead Modulation:
<lookAhead>
<inhibitRange>2</inhibitRange>
<warningLookAheadSeconds>30</ warningLookAheadSeconds >
<cautionLookAheadSeconds>40</cautionLookAheadSeconds>
<warningLookUpSeconds>60</warningLookUpSeconds>
<cautionLookUpSeconds>80</cautionLookUpSeconds>
</lookAhead>
The enable = “true” can be suppressed as it is implicit when creating the modulation entry
inhibitRange [double]: Inhibits the mode if within the distance from the runway threshold
warningLookAheadSeconds [integer]: Time in seconds to red band’s first step
cautionLookAheadSeconds [integer]: Time in seconds to yellow band’s first step entered.
warningLookUpSeconds [integer]: Time in seconds to red band’s second step
cautionLookUpSeconds [integer]: Time in seconds to yellow band’s second step.
1ProSim does not support OBSTACLE alerts
