flightgear/src/Instrumentation/dme.cxx

// dme.cxx - distance-measuring equipment.
// Written by David Megginson, started 2003.
//
// This file is in the Public Domain and comes with no warranty.

#ifdef HAVE_CONFIG_H
#  include <config.h>
#endif

#include <simgear/compiler.h>
#include <simgear/math/sg_geodesy.hxx>
#include <simgear/math/sg_random.h>

#include <Main/fg_props.hxx>
#include <Navaids/navlist.hxx>

#include "dme.hxx"


/**
 * Adjust the range.
 *
 * Start by calculating the radar horizon based on the elevation
 * difference, then clamp to the maximum, then add a fudge for
 * borderline reception.
 */
static double
adjust_range (double transmitter_elevation_ft, double aircraft_altitude_ft,
              double max_range_nm)
{
    double delta_elevation_ft =
        fabs(aircraft_altitude_ft - transmitter_elevation_ft);
    double range_nm = 1.23 * sqrt(delta_elevation_ft);
    if (range_nm > max_range_nm)
        range_nm = max_range_nm;
    else if (range_nm < 20.0)
        range_nm = 20.0;
    double rand = sg_random();
    return range_nm + (range_nm * rand * rand);
}


DME::DME ( SGPropertyNode *node )
    : _last_distance_nm(0),
      _last_frequency_mhz(-1),
      _time_before_search_sec(0),
      _transmitter_valid(false),
      _transmitter_elevation_ft(0),
      _transmitter_range_nm(0),
      _transmitter_bias(0.0),
      _name(node->getStringValue("name", "dme")),
      _num(node->getIntValue("number", 0))
{
}

DME::~DME ()
{
}

void
DME::init ()
{
    string branch;
    branch = "/instrumentation/" + _name;

    SGPropertyNode *node = fgGetNode(branch.c_str(), _num, true );

    _longitude_node = fgGetNode("/position/longitude-deg", true);
    _latitude_node = fgGetNode("/position/latitude-deg", true);
    _altitude_node = fgGetNode("/position/altitude-ft", true);
    _serviceable_node = node->getChild("serviceable", 0, true);
    _electrical_node = fgGetNode("/systems/electrical/outputs/dme", true);
    SGPropertyNode *fnode = node->getChild("frequencies", 0, true);
    _source_node = fnode->getChild("source", 0, true);
    _frequency_node = fnode->getChild("selected-mhz", 0, true);
    _in_range_node = node->getChild("in-range", 0, true);
    _distance_node = node->getChild("indicated-distance-nm", 0, true);
    _speed_node = node->getChild("indicated-ground-speed-kt", 0, true);
    _time_node = node->getChild("indicated-time-min", 0, true);
}

void
DME::update (double delta_time_sec)
{
                                // Figure out the source
    const char * source = _source_node->getStringValue();
    if (source[0] == '\0') {
        string branch;
        branch = "/instrumentation/" + _name + "/frequencies/selected-mhz";
        _source_node->setStringValue(branch.c_str());
        source = _source_node->getStringValue();
    }
                                // Get the frequency

    double frequency_mhz = fgGetDouble(source, 108.0);
    if (frequency_mhz != _last_frequency_mhz) {
        _time_before_search_sec = 0;
        _last_frequency_mhz = frequency_mhz;
    }
    _frequency_node->setDoubleValue(frequency_mhz);

                                // Get the aircraft position
    double longitude_rad =
        _longitude_node->getDoubleValue() * SGD_DEGREES_TO_RADIANS;
    double latitude_rad =
        _latitude_node->getDoubleValue() * SGD_DEGREES_TO_RADIANS;
    double altitude_m =
        _altitude_node->getDoubleValue() * SG_FEET_TO_METER;

                                // On timeout, scan again
    _time_before_search_sec -= delta_time_sec;
    if (_time_before_search_sec < 0)
        search(frequency_mhz, longitude_rad,
               latitude_rad, altitude_m);

                                // If it's off, don't bother.
    if (!_serviceable_node->getBoolValue() ||
        !_electrical_node->getBoolValue() ||
        !_transmitter_valid) {
        _last_distance_nm = 0;
        _in_range_node->setBoolValue(false);
        _distance_node->setDoubleValue(0);
        _speed_node->setDoubleValue(0);
        _time_node->setDoubleValue(0);
        return;
    }

                                // Calculate the distance to the transmitter
    SGGeod geod = SGGeod::fromRadM(longitude_rad, latitude_rad, altitude_m);
    SGVec3d location = SGVec3d::fromGeod(geod);
    
    double distance_nm = dist(_transmitter, location) * SG_METER_TO_NM;

    double range_nm = adjust_range(_transmitter_elevation_ft,
                                   altitude_m * SG_METER_TO_FEET,
                                   _transmitter_range_nm);

    if (distance_nm <= range_nm) {
        double speed_kt = (fabs(distance_nm - _last_distance_nm) *
                           ((1 / delta_time_sec) * 3600.0));
        _last_distance_nm = distance_nm;

        _in_range_node->setBoolValue(true);
        double tmp_dist = distance_nm - _transmitter_bias;
        if ( tmp_dist < 0.0 ) {
            tmp_dist = 0.0;
        }
        _distance_node->setDoubleValue( tmp_dist );
        _speed_node->setDoubleValue(speed_kt);
        if (SGLimitsd::min() < fabs(speed_kt))
          _time_node->setDoubleValue(distance_nm/speed_kt*60.0);
        
    } else {
        _last_distance_nm = 0;
        _in_range_node->setBoolValue(false);
        _distance_node->setDoubleValue(0);
        _speed_node->setDoubleValue(0);
        _time_node->setDoubleValue(0);
    }
}

void
DME::search (double frequency_mhz, double longitude_rad,
             double latitude_rad, double altitude_m)
{
    // reset search time
    _time_before_search_sec = 1.0;

    // try the ILS list first
    
    FGNavRecord *dme = globals->get_dmelist()->findByFreq( frequency_mhz,
      SGGeod::fromRadM(longitude_rad, latitude_rad, altitude_m));

    _transmitter_valid = (dme != NULL);

    if ( _transmitter_valid ) {
        _transmitter = dme->cart();
        _transmitter_elevation_ft = dme->get_elev_ft();
        _transmitter_range_nm = dme->get_range();
        _transmitter_bias = dme->get_multiuse();
    }
}

// end of dme.cxx