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/navlist.hxx>
  12
  13 #include "dme.hxx"
  14
  15
  16 /**
  17  * Adjust the range.
  18  *
  19  * Start by calculating the radar horizon based on the elevation
  20  * difference, then clamp to the maximum, then add a fudge for
  21  * borderline reception.
  22  */
  23 static double
  24 adjust_range (double transmitter_elevation_ft, double aircraft_altitude_ft,
  25               double max_range_nm)
  26 {
  27     double delta_elevation_ft =
  28         fabs(aircraft_altitude_ft - transmitter_elevation_ft);
  29     double range_nm = 1.23 * sqrt(delta_elevation_ft);
  30     if (range_nm > max_range_nm)
  31         range_nm = max_range_nm;
  32     else if (range_nm < 20.0)
  33         range_nm = 20.0;
  34     double rand = sg_random();
  35     return range_nm + (range_nm * rand * rand);
  36 }
  37
  38
  39 DME::DME ()
  40     : _last_distance_nm(0),
  41       _last_frequency_mhz(-1),
  42       _time_before_search_sec(0),
  43       _transmitter_valid(false),
  44       _transmitter_elevation_ft(0),
  45       _transmitter_range_nm(0),
  46       _transmitter_bias(0.0)
  47 {
  48 }
  49
  50 DME::~DME ()
  51 {
  52 }
  53
  54 void
  55 DME::init ()
  56 {
  57     _longitude_node = fgGetNode("/position/longitude-deg", true);
  58     _latitude_node = fgGetNode("/position/latitude-deg", true);
  59     _altitude_node = fgGetNode("/position/altitude-ft", true);
  60     _serviceable_node = fgGetNode("/instrumentation/dme/serviceable", true);
  61     _electrical_node = fgGetNode("/systems/electrical/outputs/dme", true);
  62     _source_node = fgGetNode("/instrumentation/dme/frequencies/source", true);
  63     _frequency_node =
  64         fgGetNode("/instrumentation/dme/frequencies/selected-mhz", true);
  65
  66     _in_range_node = fgGetNode("/instrumentation/dme/in-range", true);
  67     _distance_node =
  68         fgGetNode("/instrumentation/dme/indicated-distance-nm", true);
  69     _speed_node =
  70         fgGetNode("/instrumentation/dme/indicated-ground-speed-kt", true);
  71     _time_node =
  72         fgGetNode("/instrumentation/dme/indicated-time-min", true);
  73 }
  74
  75 void
  76 DME::update (double delta_time_sec)
  77 {
  78                                 // Figure out the source
  79     const char * source = _source_node->getStringValue();
  80     if (source[0] == '\0') {
  81         source = "/instrumentation/dme/frequencies/selected-mhz";
  82         _source_node->setStringValue(source);
  83     }
  84
  85                                 // Get the frequency
  86     double frequency_mhz = fgGetDouble(source, 108.0);
  87     if (frequency_mhz != _last_frequency_mhz) {
  88         _time_before_search_sec = 0;
  89         _last_frequency_mhz = frequency_mhz;
  90     }
  91
  92                                 // Get the aircraft position
  93     double longitude_rad =
  94         _longitude_node->getDoubleValue() * SGD_DEGREES_TO_RADIANS;
  95     double latitude_rad =
  96         _latitude_node->getDoubleValue() * SGD_DEGREES_TO_RADIANS;
  97     double altitude_m =
  98         _altitude_node->getDoubleValue() * SG_FEET_TO_METER;
  99
 100                                 // On timeout, scan again
 101     _time_before_search_sec -= delta_time_sec;
 102     if (_time_before_search_sec < 0)
 103         search(frequency_mhz, longitude_rad,
 104                latitude_rad, altitude_m);
 105
 106                                 // If it's off, don't bother.
 107     if (!_serviceable_node->getBoolValue() ||
 108         !_electrical_node->getBoolValue() ||
 109         !_transmitter_valid) {
 110         _last_distance_nm = 0;
 111         _in_range_node->setBoolValue(false);
 112         _distance_node->setDoubleValue(0);
 113         _speed_node->setDoubleValue(0);
 114         _time_node->setDoubleValue(0);
 115         return;
 116     }
 117
 118                                 // Calculate the distance to the transmitter
 119     Point3D location =
 120         sgGeodToCart(Point3D(longitude_rad, latitude_rad, altitude_m));
 121     double distance_nm = _transmitter.distance3D(location) * SG_METER_TO_NM;
 122     double range_nm = adjust_range(_transmitter_elevation_ft,
 123                                    altitude_m * SG_METER_TO_FEET,
 124                                    _transmitter_range_nm);
 125     if (distance_nm <= range_nm) {
 126         double speed_kt = (fabs(distance_nm - _last_distance_nm) *
 127                            ((1 / delta_time_sec) * 3600.0));
 128         _last_distance_nm = distance_nm;
 129
 130         _in_range_node->setBoolValue(true);
 131         _distance_node->setDoubleValue(distance_nm - _transmitter_bias);
 132         _speed_node->setDoubleValue(speed_kt);
 133         _time_node->setDoubleValue(distance_nm/speed_kt*60.0);
 134
 135     } else {
 136         _last_distance_nm = 0;
 137         _in_range_node->setBoolValue(false);
 138         _distance_node->setDoubleValue(0);
 139         _speed_node->setDoubleValue(0);
 140         _time_node->setDoubleValue(0);
 141     }
 142 }
 143
 144 void
 145 DME::search (double frequency_mhz, double longitude_rad,
 146              double latitude_rad, double altitude_m)
 147 {
 148     // reset search time
 149     _time_before_search_sec = 1.0;
 150
 151     // try the ILS list first
 152     FGNavRecord *dme
 153         = globals->get_dmelist()->findByFreq( frequency_mhz, longitude_rad,
 154                                               latitude_rad, altitude_m);
 155
 156     _transmitter_valid = (dme != NULL);
 157
 158     if ( _transmitter_valid ) {
 159         _transmitter = Point3D(dme->get_x(), dme->get_y(), dme->get_z());
 160         _transmitter_elevation_ft = dme->get_elev_ft();
 161         _transmitter_range_nm = dme->get_range();
 162         _transmitter_bias = dme->get_multiuse();
 163     }
 164 }
 165
 166 // end of dme.cxx