src/Instrumentation/dme.cxx

   1 // dme.cxx - distance-measuring equipment.
   2 // Written by David Megginson, started 2003.
   3 //
   4 // This file is in the Public Domain and comes with no warranty.
   5
   6 #include <simgear/compiler.h>
   7 #include <simgear/math/sg_geodesy.hxx>
   8 #include <simgear/math/sg_random.h>
   9
  10 #include <Main/fg_props.hxx>
  11 #include <Navaids/ilslist.hxx>
  12 #include <Navaids/navlist.hxx>
  13
  14 #include "dme.hxx"
  15
  16
  17 /**
  18  * Adjust the range.
  19  *
  20  * Start by calculating the radar horizon based on the elevation
  21  * difference, then clamp to the maximum, then add a fudge for
  22  * borderline reception.
  23  */
  24 static double
  25 adjust_range (double transmitter_elevation_ft, double aircraft_altitude_ft,
  26               double max_range_nm)
  27 {
  28     double delta_elevation_ft =
  29         fabs(aircraft_altitude_ft - transmitter_elevation_ft);
  30     double range_nm = 1.23 * sqrt(delta_elevation_ft);
  31     if (range_nm > max_range_nm)
  32         range_nm = max_range_nm;
  33     else if (range_nm < 20.0)
  34         range_nm = 20.0;
  35     double rand = sg_random();
  36     return range_nm + (range_nm * rand * rand);
  37 }
  38
  39
  40 DME::DME ()
  41     : _last_distance_nm(0),
  42       _last_frequency_mhz(-1),
  43       _time_before_search_sec(0),
  44       _transmitter_valid(false),
  45       _transmitter_elevation_ft(0),
  46       _transmitter_range_nm(0),
  47       _transmitter_bias(0.0)
  48 {
  49 }
  50
  51 DME::~DME ()
  52 {
  53 }
  54
  55 void
  56 DME::init ()
  57 {
  58     _longitude_node = fgGetNode("/position/longitude-deg", true);
  59     _latitude_node = fgGetNode("/position/latitude-deg", true);
  60     _altitude_node = fgGetNode("/position/altitude-ft", true);
  61     _serviceable_node = fgGetNode("/instrumentation/dme/serviceable", true);
  62     _electrical_node = fgGetNode("/systems/electrical/outputs/dme", true);
  63     _source_node = fgGetNode("/instrumentation/dme/frequencies/source", true);
  64     _frequency_node =
  65         fgGetNode("/instrumentation/dme/frequencies/selected-mhz", true);
  66
  67     _in_range_node = fgGetNode("/instrumentation/dme/in-range", true);
  68     _distance_node =
  69         fgGetNode("/instrumentation/dme/indicated-distance-nm", true);
  70     _speed_node =
  71         fgGetNode("/instrumentation/dme/indicated-ground-speed-kt", true);
  72     _time_node =
  73         fgGetNode("/instrumentation/dme/indicated-time-min", true);
  74 }
  75
  76 void
  77 DME::update (double delta_time_sec)
  78 {
  79                                 // Figure out the source
  80     const char * source = _source_node->getStringValue();
  81     if (source[0] == '\0') {
  82         source = "/instrumentation/dme/frequencies/selected-mhz";
  83         _source_node->setStringValue(source);
  84     }
  85
  86                                 // Get the frequency
  87     double frequency_mhz = fgGetDouble(source, 108.0);
  88     if (frequency_mhz != _last_frequency_mhz) {
  89         _time_before_search_sec = 0;
  90         _last_frequency_mhz = frequency_mhz;
  91     }
  92
  93                                 // Get the aircraft position
  94     double longitude_rad =
  95         _longitude_node->getDoubleValue() * SGD_DEGREES_TO_RADIANS;
  96     double latitude_rad =
  97         _latitude_node->getDoubleValue() * SGD_DEGREES_TO_RADIANS;
  98     double altitude_m =
  99         _altitude_node->getDoubleValue() * SG_FEET_TO_METER;
 100
 101                                 // On timeout, scan again
 102     _time_before_search_sec -= delta_time_sec;
 103     if (_time_before_search_sec < 0)
 104         search(frequency_mhz, longitude_rad,
 105                latitude_rad, altitude_m);
 106
 107                                 // If it's off, don't bother.
 108     if (!_serviceable_node->getBoolValue() ||
 109         !_electrical_node->getBoolValue() ||
 110         !_transmitter_valid) {
 111         _last_distance_nm = 0;
 112         _in_range_node->setBoolValue(false);
 113         _distance_node->setDoubleValue(0);
 114         _speed_node->setDoubleValue(0);
 115         _time_node->setDoubleValue(0);
 116         return;
 117     }
 118
 119                                 // Calculate the distance to the transmitter
 120     Point3D location =
 121         sgGeodToCart(Point3D(longitude_rad, latitude_rad, altitude_m));
 122     double distance_nm = _transmitter.distance3D(location) * SG_METER_TO_NM;
 123     double range_nm = adjust_range(_transmitter_elevation_ft,
 124                                    altitude_m * SG_METER_TO_FEET,
 125                                    _transmitter_range_nm);
 126     if (distance_nm <= range_nm) {
 127         double speed_kt = (fabs(distance_nm - _last_distance_nm) *
 128                            ((1 / delta_time_sec) * 3600.0));
 129         _last_distance_nm = distance_nm;
 130
 131         _in_range_node->setBoolValue(true);
 132         _distance_node->setDoubleValue(distance_nm - _transmitter_bias);
 133         _speed_node->setDoubleValue(speed_kt);
 134         _time_node->setDoubleValue(distance_nm/speed_kt*60.0);
 135
 136     } else {
 137         _last_distance_nm = 0;
 138         _in_range_node->setBoolValue(false);
 139         _distance_node->setDoubleValue(0);
 140         _speed_node->setDoubleValue(0);
 141         _time_node->setDoubleValue(0);
 142     }
 143 }
 144
 145 void
 146 DME::search (double frequency_mhz, double longitude_rad,
 147              double latitude_rad, double altitude_m)
 148 {
 149     // reset search time
 150     _time_before_search_sec = 1.0;
 151
 152     // try the ILS list first
 153     FGNavRecord *dme
 154         = globals->get_dmelist()->findByFreq( frequency_mhz, longitude_rad,
 155                                               latitude_rad, altitude_m);
 156
 157     _transmitter_valid = (dme != NULL);
 158
 159     if ( _transmitter_valid ) {
 160         _transmitter = Point3D(dme->get_x(), dme->get_y(), dme->get_z());
 161         _transmitter_elevation_ft = dme->get_elev_ft();
 162         _transmitter_range_nm = dme->get_range();
 163         _transmitter_bias = dme->get_multiuse();
 164     }
 165 }
 166
 167 // end of dme.cxx