visual-reference-point-extensions.xml

<?xml version="1.0"?>
<!--

  Extension properties for the JSBSim visual reference point (VRP).

    Copyright (C) 2015 - 2023  Anders Gidenstam  (anders(at)gidenstam.org)

    This library is free software; you can redistribute it and/or
    modify it under the terms of the GNU Lesser General Public
    License as published by the Free Software Foundation; either
    version 2.1 of the License, or (at your option) any later version.

    This library is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
    Lesser General Public License for more details.

    You should have received a copy of the GNU Lesser General Public
    License along with this library; if not, write to the Free Software
    Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301
    USA

-->
<system name="visual-reference-point-extensions">

 <channel name="position">

  <fcs_function name="position/vrp-cg-altitude-offset-ft">
   <description>
    Altitude offset of the visual reference point w.r.t. the centre of gravity.
    FIXME: Verify the roll dependence.
   </description>
   <function>
    <sum>
     <product>
      <value>-0.083333333</value>
      <difference>
       <property>metrics/visualrefpoint-x-in</property>
       <property>inertia/cg-x-in</property>
      </difference>
      <sin>
       <property>attitude/pitch-rad</property>
      </sin>
     </product>
     <product>
      <value>0.083333333</value>
      <difference>
       <property>metrics/visualrefpoint-z-in</property>
       <property>inertia/cg-z-in</property>
      </difference>
      <cos>
       <property>attitude/pitch-rad</property>
      </cos>
     </product>
     <product>
      <value>-0.083333333</value>
      <difference>
       <property>metrics/visualrefpoint-y-in</property>
       <property>inertia/cg-y-in</property>
      </difference>
      <sin>
       <property>attitude/roll-rad</property>
      </sin>
     </product>
     <!-- product>
      <value>0.083333333</value>
      <difference>
       <property>metrics/visualrefpoint-z-in</property>
       <property>inertia/cg-z-in</property>
      </difference>
      <cos>
       <property>attitude/roll-rad</property>
      </cos>
     </product -->
    </sum>
   </function>
  </fcs_function>

  <fcs_function name="position/vrp-altitude-ft">
   <description>
    Altitude of the visual reference point.
   </description>
   <function>
    <sum>
     <property>position/h-sl-ft</property>
     <property>position/vrp-cg-altitude-offset-ft</property>
    </sum>
   </function>
  </fcs_function>

  <fcs_function name="position/vrp-altitude-agl-ft">
   <description>
    Distance between the visual reference point and the ground surface.
   </description>
   <function>
    <sum>
     <property>position/h-agl-ft</property>
     <property>position/vrp-cg-altitude-offset-ft</property>
    </sum>
   </function>
  </fcs_function>

  <fcs_function name="position/vrp-latitude-deg">
   <description>
    Latitude of the visual reference point.

    Based on the geocentric to geodetic conversion
      vrp-latitude-deg = atan(tan(vrp-gc-latitude_deg)/(1-f)^2))
    where for WGS 84 f = 1.0/298.25722.
   </description>
   <function>
    <product>
     <value>57.29578</value>
     <atan>
      <product>
       <tan>
        <product>
         <value>0.017453293</value>
         <property>position/vrp-gc-latitude_deg</property>
        </product>
       </tan>
       <value>1.0067395</value>
      </product>
     </atan>
    </product>
   </function>
  </fcs_function>

  <fcs_function name="position/vrp-longitude-deg">
   <description>
    Longitude of the visual reference point.
   </description>
   <function>
    <property>position/vrp-longitude_deg</property>
   </function>
  </fcs_function>

  <fcs_function name="position/latitude-unit-distance-ft_deg">
   <description>
    Feet per degree of latitude at the present position.

    The conversion is based on
    http://gis.stackexchange.com/questions/75528/length-of-a-degree-where-do-the-terms-in-this-formula-come-from
    but further truncated.
   </description>
   <function>
    <product>
     <value>3.2808399</value>
     <!-- Approximate WGS 84 meters per latitude deg. -->
     <sum>
      <value>111132.92</value>
      <product>
       <value>-559.82</value>
       <cos>
        <product>
         <value>2.0</value>
         <value>0.017453293</value>
         <property>position/vrp-latitude-deg</property>
        </product>
       </cos>
      </product>
     </sum>
    </product>
   </function>
  </fcs_function>

  <fcs_function name="position/longitude-unit-distance-ft_deg">
   <description>
    Feet per degree of longitude at the present position.

    The conversion is based on
    http://gis.stackexchange.com/questions/75528/length-of-a-degree-where-do-the-terms-in-this-formula-come-from
    but further truncated.
   </description>
   <function>
    <product>
     <value>3.2808399</value>
     <!-- Approximate WGS 84 meters per longitude deg. -->
     <sum>
      <product>
       <value>111412.84</value>
       <cos>
        <product>
         <value>0.017453293</value>
         <property>position/vrp-latitude-deg</property>
        </product>
       </cos>
      </product>
      <product>
       <value>-93.5</value>
       <cos>
        <product>
         <value>3.0</value>
         <value>0.017453293</value>
         <property>position/vrp-latitude-deg</property>
        </product>
       </cos>
      </product>
     </sum>
    </product>
   </function>
  </fcs_function>

 </channel>

 <channel name="velocities">
  <!-- NOTE:
         Computing the velocity by derivating the position is probably not
         the best way.
  -->

  <pid name="velocities/vrp-v-north-deg_sec">
   <input>position/vrp-latitude-deg</input>
   <kp>0.0</kp>
   <ki>0.0</ki>
   <kd>1.0</kd>
  </pid>

  <fcs_function name="velocities/vrp-v-north-fps">
   <description>
    The conversion from degrees of latitude per second to ft/sec is based on
    http://gis.stackexchange.com/questions/75528/length-of-a-degree-where-do-the-terms-in-this-formula-come-from
    but further truncated.
   </description>
   <function>
    <product>
     <property>position/latitude-unit-distance-ft_deg</property>
     <property>velocities/vrp-v-north-deg_sec</property>
    </product>
   </function>
  </fcs_function>

  <pid name="velocities/vrp-v-east-deg_sec">
   <input>position/vrp-longitude-deg</input>
   <kp>0.0</kp>
   <ki>0.0</ki>
   <kd>1.0</kd>
  </pid>

  <fcs_function name="velocities/vrp-v-east-fps">
   <description>
    The conversion from degrees of longitude per second to ft/sec is based on
    http://gis.stackexchange.com/questions/75528/length-of-a-degree-where-do-the-terms-in-this-formula-come-from
    but further truncated.
   </description>
   <function>
    <product>
     <property>position/longitude-unit-distance-ft_deg</property>
     <property>velocities/vrp-v-east-deg_sec</property>
    </product>
   </function>
  </fcs_function>

  <pid name="velocities/vrp-v-down-fps">
   <input>position/vrp-altitude-ft</input>
   <kp>0.0</kp>
   <ki>0.0</ki>
   <kd>-1.0</kd>
  </pid>

 </channel>

</system>