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     double rand = sg_random();
  34     return range_nm + (range_nm * rand * rand);
  35 }
  36
  37
  38 DME::DME ()
  39     : _last_distance_nm(0),
  40       _last_frequency_mhz(-1),
  41       _time_before_search_sec(0),
  42       _transmitter_valid(false),
  43       _transmitter_elevation_ft(0),
  44       _transmitter_range_nm(0)
  45 {
  46 }
  47
  48 DME::~DME ()
  49 {
  50 }
  51
  52 void
  53 DME::init ()
  54 {
  55     _longitude_node = fgGetNode("/position/longitude-deg", true);
  56     _latitude_node = fgGetNode("/position/latitude-deg", true);
  57     _altitude_node = fgGetNode("/position/altitude-ft", true);
  58     _serviceable_node = fgGetNode("/instrumentation/dme/serviceable", true);
  59     _electrical_node = fgGetNode("/systems/electrical/outputs/dme", true);
  60     _source_node = fgGetNode("/instrumentation/dme/frequencies/source", true);
  61     _frequency_node =
  62         fgGetNode("/instrumentation/dme/frequencies/selected-mhz", true);
  63
  64     _in_range_node = fgGetNode("/instrumentation/dme/in-range", true);
  65     _distance_node =
  66         fgGetNode("/instrumentation/dme/indicated-distance-nm", true);
  67     _speed_node =
  68         fgGetNode("/instrumentation/dme/indicated-ground-speed-kt", true);
  69     _time_node =
  70         fgGetNode("/instrumentation/dme/indicated-time-min", true);
  71 }
  72
  73 void
  74 DME::update (double delta_time_sec)
  75 {
  76                                 // Figure out the source
  77     const char * source = _source_node->getStringValue();
  78     if (source[0] == '\0') {
  79         source = "/instrumentation/dme/frequencies/selected-mhz";
  80         _source_node->setStringValue(source);
  81     }
  82
  83                                 // Get the frequency
  84     double frequency_mhz = fgGetDouble(source, 108.0);
  85     if (frequency_mhz != _last_frequency_mhz) {
  86         _time_before_search_sec = 0;
  87         _last_frequency_mhz = frequency_mhz;
  88     }
  89
  90                                 // Get the aircraft position
  91     double longitude_rad =
  92         _longitude_node->getDoubleValue() * SGD_DEGREES_TO_RADIANS;
  93     double latitude_rad =
  94         _latitude_node->getDoubleValue() * SGD_DEGREES_TO_RADIANS;
  95     double altitude_m =
  96         _altitude_node->getDoubleValue() * SG_FEET_TO_METER;
  97
  98                                 // On timeout, scan again
  99     _time_before_search_sec -= delta_time_sec;
 100     if (_time_before_search_sec < 0)
 101         search(frequency_mhz, longitude_rad,
 102                latitude_rad, altitude_m);
 103
 104                                 // If it's off, don't bother.
 105     if (!_serviceable_node->getBoolValue() ||
 106         !_electrical_node->getBoolValue() ||
 107         !_transmitter_valid) {
 108         _last_distance_nm = 0;
 109         _in_range_node->setBoolValue(false);
 110         _distance_node->setDoubleValue(0);
 111         _speed_node->setDoubleValue(0);
 112         _time_node->setDoubleValue(0);
 113         return;
 114     }
 115
 116                                 // Calculate the distance to the transmitter
 117     Point3D location =
 118         sgGeodToCart(Point3D(longitude_rad, latitude_rad, altitude_m));
 119     double distance_nm = _transmitter.distance3D(location) * SG_METER_TO_NM;
 120     double range_nm = adjust_range(_transmitter_elevation_ft,
 121                                    altitude_m * SG_METER_TO_FEET,
 122                                    _transmitter_range_nm);
 123     if (distance_nm <= range_nm) {
 124         double speed_kt = (fabs(distance_nm - _last_distance_nm) *
 125                            ((1 / delta_time_sec) * 3600.0));
 126         _last_distance_nm = distance_nm;
 127
 128         _in_range_node->setBoolValue(true);
 129         _distance_node->setDoubleValue(distance_nm);
 130         _speed_node->setDoubleValue(speed_kt);
 131         _time_node->setDoubleValue(distance_nm/speed_kt*60.0);
 132
 133     } else {
 134         _last_distance_nm = 0;
 135         _in_range_node->setBoolValue(false);
 136         _distance_node->setDoubleValue(0);
 137         _speed_node->setDoubleValue(0);
 138         _time_node->setDoubleValue(0);
 139     }
 140 }
 141
 142 void
 143 DME::search (double frequency_mhz, double longitude_rad,
 144              double latitude_rad, double altitude_m)
 145 {
 146                                 // reset search time
 147     _time_before_search_sec = 1.0;
 148
 149                                 // try the ILS list first
 150     FGILS * ils = current_ilslist->findByFreq(frequency_mhz,
 151                                               longitude_rad,
 152                                               latitude_rad,
 153                                               altitude_m);
 154     if (ils != 0 && ils->get_has_dme()) {
 155         _transmitter_valid = true;
 156         _transmitter = Point3D(ils->get_dme_x(),
 157                                ils->get_dme_y(),
 158                                ils->get_dme_z());
 159         _transmitter_elevation_ft = ils->get_gselev() * SG_METER_TO_FEET;
 160         _transmitter_range_nm = 50; // arbitrary
 161         return;
 162     }
 163
 164                                 // try the VORs next
 165     FGNav * nav = current_navlist->findByFreq(frequency_mhz,
 166                                               longitude_rad,
 167                                               latitude_rad,
 168                                               altitude_m);
 169     if (nav != 0)
 170     if (nav != 0 && nav->get_has_dme()) {
 171         _transmitter_valid = true;
 172         _transmitter = Point3D(nav->get_x(),
 173                                nav->get_y(),
 174                                nav->get_z());
 175         _transmitter_elevation_ft = nav->get_elev() * SG_METER_TO_FEET;
 176         _transmitter_range_nm = nav->get_range(); // fixme
 177         return;
 178     }
 179
 180     _transmitter_valid = false;
 181 }
 182
 183 // end of dme.cxx