src/Environment/environment.cxx

   1 // environment.cxx -- routines to model the natural environment
   2 //
   3 // Written by David Megginson, started February 2002.
   4 //
   5 // Copyright (C) 2002  David Megginson - david@megginson.com
   6 //
   7 // This program is free software; you can redistribute it and/or
   8 // modify it under the terms of the GNU General Public License as
   9 // published by the Free Software Foundation; either version 2 of the
  10 // License, or (at your option) any later version.
  11 //
  12 // This program is distributed in the hope that it will be useful, but
  13 // WITHOUT ANY WARRANTY; without even the implied warranty of
  14 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  15 // General Public License for more details.
  16 //
  17 // You should have received a copy of the GNU General Public License
  18 // along with this program; if not, write to the Free Software
  19 // Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
  20 //
  21 // $Id$
  22
  23
  24 #ifdef HAVE_CONFIG_H
  25 #  include <config.h>
  26 #endif
  27
  28 #include <math.h>
  29
  30 #include <boost/tuple/tuple.hpp>
  31
  32 #include <simgear/constants.h>
  33 #include <simgear/debug/logstream.hxx>
  34 #include <simgear/math/interpolater.hxx>
  35 #include <simgear/props/props.hxx>
  36 #include <simgear/environment/visual_enviro.hxx>
  37
  38 #include <Main/fg_props.hxx>
  39 #include <signal.h>
  40
  41 #include "environment.hxx"
  42 #include "atmosphere.hxx"
  43
  44 \f
  45 ////////////////////////////////////////////////////////////////////////
  46 // Atmosphere model.
  47 ////////////////////////////////////////////////////////////////////////
  48
  49 #ifdef USING_TABLES
  50
  51 // Calculated based on the ISA standard day, as found at e.g.
  52 // http://www.av8n.com/physics/altimetry.htm
  53
  54 // Each line of data has 3 elements:
  55 //   Elevation (ft),
  56 //   temperature factor (dimensionless ratio of absolute temp),
  57 //   pressure factor (dimensionless ratio)
  58 static double atmosphere_data[][3] = {
  59  {  -3000.00,   1.021,  1.1133 },
  60  {      0.00,   1.000,  1.0000 },
  61  {   2952.76,   0.980,  0.8978 },
  62  {   5905.51,   0.959,  0.8042 },
  63  {   8858.27,   0.939,  0.7187 },
  64  {  11811.02,   0.919,  0.6407 },
  65  {  14763.78,   0.898,  0.5697 },
  66  {  17716.54,   0.878,  0.5052 },
  67  {  20669.29,   0.858,  0.4468 },
  68  {  23622.05,   0.838,  0.3940 },
  69  {  26574.80,   0.817,  0.3463 },
  70  {  29527.56,   0.797,  0.3034 },
  71  {  32480.31,   0.777,  0.2649 },
  72  {  35433.07,   0.756,  0.2305 },
  73  {  38385.83,   0.752,  0.2000 },
  74  {  41338.58,   0.752,  0.1736 },
  75  {  44291.34,   0.752,  0.1506 },
  76  {  47244.09,   0.752,  0.1307 },
  77  {  50196.85,   0.752,  0.1134 },
  78  {  53149.61,   0.752,  0.0984 },
  79  {  56102.36,   0.752,  0.0854 },
  80  {  59055.12,   0.752,  0.0741 },
  81  {  62007.87,   0.752,  0.0643 },
  82  {  65000.00,   0.752,  0.0557 },
  83  {  68000.00,   0.754,  0.0482 },
  84  {  71000.00,   0.758,  0.0418 },
  85  {  74000.00,   0.761,  0.0362 },
  86  {  77000.00,   0.764,  0.0314 },
  87  {  80000.00,   0.767,  0.0273 },
  88  {  83000.00,   0.770,  0.0237 },
  89  {  86000.00,   0.773,  0.0206 },
  90  {  89000.00,   0.777,  0.0179 },
  91  {  92000.00,   0.780,  0.0156 },
  92  {  95000.00,   0.783,  0.0135 },
  93  {  98000.00,   0.786,  0.0118 },
  94  { 101000.00,   0.789,  0.0103 },
  95  { -1, -1, -1 }
  96 };
  97
  98 static SGInterpTable * _temperature_degc_table = 0;
  99 static SGInterpTable * _pressure_inhg_table = 0;
 100
 101 static void
 102 _setup_tables ()
 103 {
 104   if (_temperature_degc_table != 0)
 105       return;
 106
 107   _temperature_degc_table = new SGInterpTable;
 108   _pressure_inhg_table = new SGInterpTable;
 109
 110   for (int i = 0; atmosphere_data[i][0] != -1; i++) {
 111     _temperature_degc_table->addEntry(atmosphere_data[i][0],
 112                                       atmosphere_data[i][1]);
 113     _pressure_inhg_table->addEntry(atmosphere_data[i][0],
 114                                    atmosphere_data[i][2]);
 115   }
 116 }
 117 #endif
 118
 119 \f
 120 ////////////////////////////////////////////////////////////////////////
 121 // Implementation of FGEnvironment.
 122 ////////////////////////////////////////////////////////////////////////
 123
 124 void FGEnvironment::_init()
 125 {
 126     elevation_ft = 0;
 127     visibility_m = 32000;
 128     temperature_sea_level_degc = 15;
 129     temperature_degc = 15;
 130     dewpoint_sea_level_degc = 5; // guess
 131     dewpoint_degc = 5;
 132     pressure_sea_level_inhg = 29.92;
 133     pressure_inhg = 29.92;
 134     turbulence_magnitude_norm = 0;
 135     turbulence_rate_hz = 1;
 136     wind_from_heading_deg = 0;
 137     wind_speed_kt = 0;
 138     wind_from_north_fps = 0;
 139     wind_from_east_fps = 0;
 140     wind_from_down_fps = 0;
 141     thermal_lift_fps = 0;
 142     ridge_lift_fps= 0;
 143     local_weather_lift_fps=0;
 144     altitude_half_to_sun_m = 1000;
 145     altitude_tropo_top_m = 10000;
 146 #ifdef USING_TABLES
 147     _setup_tables();
 148 #endif
 149     _recalc_density();
 150     _recalc_relative_humidity();
 151     live_update = true;
 152 }
 153
 154 FGEnvironment::FGEnvironment()
 155 {
 156     _init();
 157 }
 158
 159 FGEnvironment::FGEnvironment (const FGEnvironment &env)
 160 {
 161     _init();
 162     copy(env);
 163 }
 164
 165 FGEnvironment::~FGEnvironment()
 166 {
 167 }
 168
 169 void
 170 FGEnvironment::copy (const FGEnvironment &env)
 171 {
 172     elevation_ft = env.elevation_ft;
 173     visibility_m = env.visibility_m;
 174     temperature_sea_level_degc = env.temperature_sea_level_degc;
 175     temperature_degc = env.temperature_degc;
 176     dewpoint_sea_level_degc = env.dewpoint_sea_level_degc;
 177     dewpoint_degc = env.dewpoint_degc;
 178     pressure_sea_level_inhg = env.pressure_sea_level_inhg;
 179     wind_from_heading_deg = env.wind_from_heading_deg;
 180     wind_speed_kt = env.wind_speed_kt;
 181     wind_from_north_fps = env.wind_from_north_fps;
 182     wind_from_east_fps = env.wind_from_east_fps;
 183     wind_from_down_fps = env.wind_from_down_fps;
 184     thermal_lift_fps = env.thermal_lift_fps;
 185     ridge_lift_fps= env.ridge_lift_fps;
 186     local_weather_lift_fps = env.local_weather_lift_fps;
 187     turbulence_magnitude_norm = env.turbulence_magnitude_norm;
 188     turbulence_rate_hz = env.turbulence_rate_hz;
 189 }
 190
 191 static inline bool
 192 maybe_copy_value (FGEnvironment * env, const SGPropertyNode * node,
 193                   const char * name, void (FGEnvironment::*setter)(double))
 194 {
 195     const SGPropertyNode * child = node->getNode(name);
 196                                 // fragile: depends on not being typed
 197                                 // as a number
 198     if (child != 0 && child->hasValue() &&
 199         child->getStringValue()[0] != '\0') {
 200         (env->*setter)(child->getDoubleValue());
 201         return true;
 202     } else {
 203         return false;
 204     }
 205 }
 206
 207 void
 208 FGEnvironment::read (const SGPropertyNode * node)
 209 {
 210     bool live_update = set_live_update( false );
 211     maybe_copy_value(this, node, "visibility-m",
 212                      &FGEnvironment::set_visibility_m);
 213
 214     if (!maybe_copy_value(this, node, "temperature-sea-level-degc",
 215                           &FGEnvironment::set_temperature_sea_level_degc))
 216         maybe_copy_value(this, node, "temperature-degc",
 217                          &FGEnvironment::set_temperature_degc);
 218
 219     if (!maybe_copy_value(this, node, "dewpoint-sea-level-degc",
 220                           &FGEnvironment::set_dewpoint_sea_level_degc))
 221         maybe_copy_value(this, node, "dewpoint-degc",
 222                          &FGEnvironment::set_dewpoint_degc);
 223
 224     if (!maybe_copy_value(this, node, "pressure-sea-level-inhg",
 225                           &FGEnvironment::set_pressure_sea_level_inhg))
 226         maybe_copy_value(this, node, "pressure-inhg",
 227                          &FGEnvironment::set_pressure_inhg);
 228
 229     maybe_copy_value(this, node, "wind-from-heading-deg",
 230                      &FGEnvironment::set_wind_from_heading_deg);
 231
 232     maybe_copy_value(this, node, "wind-speed-kt",
 233                      &FGEnvironment::set_wind_speed_kt);
 234
 235     maybe_copy_value(this, node, "elevation-ft",
 236                      &FGEnvironment::set_elevation_ft);
 237
 238     maybe_copy_value(this, node, "turbulence/magnitude-norm",
 239                      &FGEnvironment::set_turbulence_magnitude_norm);
 240
 241     maybe_copy_value(this, node, "turbulence/rate-hz",
 242                      &FGEnvironment::set_turbulence_rate_hz);
 243
 244     // calculate derived properties here to avoid duplicate expensive computations
 245     _recalc_ne();
 246     _recalc_alt_pt();
 247     _recalc_alt_dewpoint();
 248     _recalc_density();
 249     _recalc_relative_humidity();
 250
 251     set_live_update(live_update);
 252 }
 253
 254
 255 double
 256 FGEnvironment::get_visibility_m () const
 257 {
 258   return visibility_m;
 259 }
 260
 261 double
 262 FGEnvironment::get_temperature_sea_level_degc () const
 263 {
 264   return temperature_sea_level_degc;
 265 }
 266
 267 double
 268 FGEnvironment::get_temperature_degc () const
 269 {
 270   return temperature_degc;
 271 }
 272
 273 double
 274 FGEnvironment::get_temperature_degf () const
 275 {
 276   return (temperature_degc * 9.0 / 5.0) + 32.0;
 277 }
 278
 279 double
 280 FGEnvironment::get_dewpoint_sea_level_degc () const
 281 {
 282   return dewpoint_sea_level_degc;
 283 }
 284
 285 double
 286 FGEnvironment::get_dewpoint_degc () const
 287 {
 288   return dewpoint_degc;
 289 }
 290
 291 double
 292 FGEnvironment::get_pressure_sea_level_inhg () const
 293 {
 294   return pressure_sea_level_inhg;
 295 }
 296
 297 double
 298 FGEnvironment::get_pressure_inhg () const
 299 {
 300   return pressure_inhg;
 301 }
 302
 303 double
 304 FGEnvironment::get_density_slugft3 () const
 305 {
 306   return density_slugft3;
 307 }
 308
 309 double
 310 FGEnvironment::get_relative_humidity () const
 311 {
 312   return relative_humidity;
 313 }
 314
 315 double
 316 FGEnvironment::get_density_tropo_avg_kgm3 () const
 317 {
 318   return density_tropo_avg_kgm3;
 319 }
 320
 321 double
 322 FGEnvironment::get_altitude_half_to_sun_m () const
 323 {
 324   return altitude_half_to_sun_m;
 325 }
 326
 327 double
 328 FGEnvironment::get_altitude_tropo_top_m () const
 329 {
 330   return altitude_tropo_top_m;
 331 }
 332
 333 double
 334 FGEnvironment::get_wind_from_heading_deg () const
 335 {
 336   return wind_from_heading_deg;
 337 }
 338
 339 double
 340 FGEnvironment::get_wind_speed_kt () const
 341 {
 342   return wind_speed_kt;
 343 }
 344
 345 double
 346 FGEnvironment::get_wind_from_north_fps () const
 347 {
 348   return wind_from_north_fps;
 349 }
 350
 351 double
 352 FGEnvironment::get_wind_from_east_fps () const
 353 {
 354   return wind_from_east_fps;
 355 }
 356
 357 double
 358 FGEnvironment::get_wind_from_down_fps () const
 359 {
 360   return wind_from_down_fps;
 361 }
 362
 363 double
 364 FGEnvironment::get_thermal_lift_fps () const
 365 {
 366   return thermal_lift_fps;
 367 }
 368
 369 double
 370 FGEnvironment::get_ridge_lift_fps () const
 371 {
 372   return ridge_lift_fps;
 373 }
 374
 375 double
 376 FGEnvironment::get_local_weather_lift_fps () const
 377 {
 378   return local_weather_lift_fps;
 379 }
 380
 381 double
 382 FGEnvironment::get_turbulence_magnitude_norm () const
 383 {
 384   if( sgEnviro.get_turbulence_enable_state() )
 385     if (fgGetBool("/environment/params/real-world-weather-fetch") == true)
 386       return sgEnviro.get_cloud_turbulence();
 387   return turbulence_magnitude_norm;
 388 }
 389
 390 double
 391 FGEnvironment::get_turbulence_rate_hz () const
 392 {
 393   return turbulence_rate_hz;
 394 }
 395
 396 double
 397 FGEnvironment::get_elevation_ft () const
 398 {
 399   return elevation_ft;
 400 }
 401
 402 void
 403 FGEnvironment::set_visibility_m (double v)
 404 {
 405   visibility_m = v;
 406 }
 407
 408 void
 409 FGEnvironment::set_temperature_sea_level_degc (double t)
 410 {
 411   temperature_sea_level_degc = t;
 412   if (dewpoint_sea_level_degc > t)
 413       dewpoint_sea_level_degc = t;
 414   if( live_update ) {
 415     _recalc_alt_pt();
 416     _recalc_density();
 417   }
 418 }
 419
 420 void
 421 FGEnvironment::set_temperature_degc (double t)
 422 {
 423   temperature_degc = t;
 424   if( live_update ) {
 425     _recalc_sl_temperature();
 426     _recalc_sl_pressure();
 427     _recalc_alt_pt();
 428     _recalc_density();
 429     _recalc_relative_humidity();
 430   }
 431 }
 432
 433 void
 434 FGEnvironment::set_dewpoint_sea_level_degc (double t)
 435 {
 436   dewpoint_sea_level_degc = t;
 437   if (temperature_sea_level_degc < t)
 438       temperature_sea_level_degc = t;
 439   if( live_update ) {
 440     _recalc_alt_dewpoint();
 441     _recalc_density();
 442   }
 443 }
 444
 445 void
 446 FGEnvironment::set_dewpoint_degc (double t)
 447 {
 448   dewpoint_degc = t;
 449   if( live_update ) {
 450     _recalc_sl_dewpoint();
 451     _recalc_density();
 452     _recalc_relative_humidity();
 453   }
 454 }
 455
 456 void
 457 FGEnvironment::set_pressure_sea_level_inhg (double p)
 458 {
 459   pressure_sea_level_inhg = p;
 460   if( live_update ) {
 461     _recalc_alt_pt();
 462     _recalc_density();
 463   }
 464 }
 465
 466 void
 467 FGEnvironment::set_pressure_inhg (double p)
 468 {
 469   pressure_inhg = p;
 470   if( live_update ) {
 471     _recalc_sl_pressure();
 472     _recalc_density();
 473   }
 474 }
 475
 476 void
 477 FGEnvironment::set_wind_from_heading_deg (double h)
 478 {
 479   wind_from_heading_deg = h;
 480   if( live_update ) {
 481     _recalc_ne();
 482   }
 483 }
 484
 485 void
 486 FGEnvironment::set_wind_speed_kt (double s)
 487 {
 488   wind_speed_kt = s;
 489   if( live_update ) {
 490     _recalc_ne();
 491   }
 492 }
 493
 494 void
 495 FGEnvironment::set_wind_from_north_fps (double n)
 496 {
 497   wind_from_north_fps = n;
 498   if( live_update ) {
 499     _recalc_hdgspd();
 500   }
 501 }
 502
 503 void
 504 FGEnvironment::set_wind_from_east_fps (double e)
 505 {
 506   wind_from_east_fps = e;
 507   if( live_update ) {
 508     _recalc_hdgspd();
 509   }
 510 }
 511
 512 void
 513 FGEnvironment::set_wind_from_down_fps (double d)
 514 {
 515   wind_from_down_fps = d;
 516   if( live_update ) {
 517     _recalc_hdgspd();
 518   }
 519 }
 520
 521 void
 522 FGEnvironment::set_thermal_lift_fps (double th)
 523 {
 524   thermal_lift_fps = th;
 525   if( live_update ) {
 526     _recalc_updraft();
 527   }
 528 }
 529
 530 void
 531 FGEnvironment::set_ridge_lift_fps (double ri)
 532 {
 533   ridge_lift_fps = ri;
 534   if( live_update ) {
 535     _recalc_updraft();
 536   }
 537 }
 538
 539 void
 540 FGEnvironment::set_local_weather_lift_fps (double lwl)
 541 {
 542   local_weather_lift_fps = lwl;
 543   if( live_update ) {
 544     _recalc_updraft();
 545   }
 546 }
 547
 548 void
 549 FGEnvironment::set_turbulence_magnitude_norm (double t)
 550 {
 551   turbulence_magnitude_norm = t;
 552 }
 553
 554 void
 555 FGEnvironment::set_turbulence_rate_hz (double r)
 556 {
 557   turbulence_rate_hz = r;
 558 }
 559
 560 void
 561 FGEnvironment::set_elevation_ft (double e)
 562 {
 563   elevation_ft = e;
 564   if( live_update ) {
 565     _recalc_alt_pt();
 566     _recalc_alt_dewpoint();
 567     _recalc_density();
 568     _recalc_relative_humidity();
 569   }
 570 }
 571
 572 void
 573 FGEnvironment::set_altitude_half_to_sun_m (double alt)
 574 {
 575   altitude_half_to_sun_m = alt;
 576   if( live_update ) {
 577     _recalc_density_tropo_avg_kgm3();
 578   }
 579 }
 580
 581 void
 582 FGEnvironment::set_altitude_tropo_top_m (double alt)
 583 {
 584   altitude_tropo_top_m = alt;
 585   if( live_update ) {
 586     _recalc_density_tropo_avg_kgm3();
 587   }
 588 }
 589
 590
 591 void
 592 FGEnvironment::_recalc_hdgspd ()
 593 {
 594   double angle_rad;
 595
 596   if (wind_from_east_fps == 0) {
 597     angle_rad = (wind_from_north_fps >= 0 ? SGD_PI_2 : -SGD_PI_2);
 598   } else {
 599     angle_rad = atan(wind_from_north_fps/wind_from_east_fps);
 600   }
 601   wind_from_heading_deg = angle_rad * SGD_RADIANS_TO_DEGREES;
 602   if (wind_from_east_fps >= 0)
 603     wind_from_heading_deg = 90 - wind_from_heading_deg;
 604   else
 605     wind_from_heading_deg = 270 - wind_from_heading_deg;
 606
 607 #if 0
 608   // FIXME: Windspeed can become negative with these formulas.
 609   //        which can cause problems for animations that rely
 610   //        on the windspeed property.
 611   if (angle_rad == 0)
 612     wind_speed_kt = fabs(wind_from_east_fps
 613                          * SG_METER_TO_NM * SG_FEET_TO_METER * 3600);
 614   else
 615     wind_speed_kt = (wind_from_north_fps / sin(angle_rad))
 616       * SG_METER_TO_NM * SG_FEET_TO_METER * 3600;
 617 #else
 618   wind_speed_kt = sqrt(wind_from_north_fps * wind_from_north_fps +
 619                        wind_from_east_fps * wind_from_east_fps)
 620                   * SG_METER_TO_NM * SG_FEET_TO_METER * 3600;
 621 #endif
 622 }
 623
 624 void
 625 FGEnvironment::_recalc_ne ()
 626 {
 627   double speed_fps =
 628     wind_speed_kt * SG_NM_TO_METER * SG_METER_TO_FEET * (1.0/3600);
 629
 630   wind_from_north_fps = speed_fps *
 631     cos(wind_from_heading_deg * SGD_DEGREES_TO_RADIANS);
 632   wind_from_east_fps = speed_fps *
 633     sin(wind_from_heading_deg * SGD_DEGREES_TO_RADIANS);
 634 }
 635
 636 void
 637 FGEnvironment::_recalc_updraft ()
 638 {
 639   wind_from_down_fps = thermal_lift_fps + ridge_lift_fps + local_weather_lift_fps ;
 640 }
 641
 642 // Intended to help with the interpretation of METAR data,
 643 // not for random in-flight outside-air temperatures.
 644 void
 645 FGEnvironment::_recalc_sl_temperature ()
 646 {
 647
 648 #if 0
 649   {
 650     SG_LOG(SG_GENERAL, SG_DEBUG, "recalc_sl_temperature: using "
 651       << temperature_degc << " @ " << elevation_ft << " :: " << this);
 652   }
 653 #endif
 654
 655   if (elevation_ft >= ISA_def[1].height) {
 656     SG_LOG(SG_GENERAL, SG_ALERT, "recalc_sl_temperature: "
 657         << "valid only in troposphere, not " << elevation_ft);
 658     return;
 659   }
 660
 661 // Clamp: temperature of the stratosphere, in degrees C:
 662   double t_strato = ISA_def[1].temp - atmodel::freezing;
 663   if (temperature_degc < t_strato) temperature_sea_level_degc = t_strato;
 664   else temperature_sea_level_degc =
 665       temperature_degc + elevation_ft * atmodel::foot * ISA_def[0].lapse;
 666
 667 // Alternative implemenation:
 668 //  else temperature_sea_level_inhg = T_layer(0., elevation_ft * foot,
 669 //      pressure_inhg * inHg, temperature_degc + freezing, ISA_def[0].lapse) - freezing;
 670 }
 671
 672 void
 673 FGEnvironment::_recalc_sl_dewpoint ()
 674 {
 675                                 // 0.2degC/1000ft
 676                                 // FIXME: this will work only for low
 677                                 // elevations
 678   dewpoint_sea_level_degc = dewpoint_degc + (elevation_ft * .0002);
 679   if (dewpoint_sea_level_degc > temperature_sea_level_degc)
 680     dewpoint_sea_level_degc = temperature_sea_level_degc;
 681 }
 682
 683 void
 684 FGEnvironment::_recalc_alt_dewpoint ()
 685 {
 686                                 // 0.2degC/1000ft
 687                                 // FIXME: this will work only for low
 688                                 // elevations
 689   dewpoint_degc = dewpoint_sea_level_degc + (elevation_ft * .0002);
 690   if (dewpoint_degc > temperature_degc)
 691     dewpoint_degc = temperature_degc;
 692 }
 693
 694 void
 695 FGEnvironment::_recalc_sl_pressure ()
 696 {
 697   using namespace atmodel;
 698 #if 0
 699   {
 700     SG_LOG(SG_GENERAL, SG_ALERT, "recalc_sl_pressure: using "
 701       << pressure_inhg << " and "
 702       << temperature_degc << " @ " << elevation_ft << " :: " << this);
 703   }
 704 #endif
 705   pressure_sea_level_inhg = P_layer(0., elevation_ft * foot,
 706       pressure_inhg * inHg, temperature_degc + freezing, ISA_def[0].lapse) / inHg;
 707 }
 708
 709 // This gets called at frame rate, to account for the aircraft's
 710 // changing altitude.
 711 // Called by set_elevation_ft() which is called by FGEnvironmentMgr::update
 712
 713 void
 714 FGEnvironment::_recalc_alt_pt ()
 715 {
 716   using namespace atmodel;
 717 #if 0
 718   {
 719     static int count(0);
 720     if (++count % 1000 == 0) {
 721       SG_LOG(SG_GENERAL, SG_ALERT,
 722            "recalc_alt_pt for: " << elevation_ft
 723         << "  using "  << pressure_sea_level_inhg
 724         << "  and "  << temperature_sea_level_degc
 725         << " :: " << this
 726         << "  # " << count);
 727     }
 728   }
 729 #endif
 730   double press, temp;
 731   boost::tie(press, temp) = PT_vs_hpt(elevation_ft * foot,
 732         pressure_sea_level_inhg * inHg, temperature_sea_level_degc + freezing);
 733   temperature_degc = temp - freezing;
 734   pressure_inhg = press / inHg;
 735 }
 736
 737 void
 738 FGEnvironment::_recalc_density ()
 739 {
 740   double pressure_psf = pressure_inhg * 70.7487;
 741
 742   // adjust for humidity
 743   // calculations taken from USA Today (oops!) at
 744   // http://www.usatoday.com/weather/basics/density-calculations.htm
 745   double temperature_degk = temperature_degc + 273.15;
 746   double pressure_mb = pressure_inhg * 33.86;
 747   double vapor_pressure_mb =
 748     6.11 * pow(10.0, 7.5 * dewpoint_degc / (237.7 + dewpoint_degc));
 749   double virtual_temperature_degk = temperature_degk / (1 - (vapor_pressure_mb / pressure_mb) * (1.0 - 0.622));
 750   double virtual_temperature_degr = virtual_temperature_degk * 1.8;
 751
 752   density_slugft3 = pressure_psf / (virtual_temperature_degr * 1718);
 753   _recalc_density_tropo_avg_kgm3();
 754 }
 755
 756 // This is used to calculate the average density on the path
 757 // of sunlight to the observer for calculating sun-color
 758 void
 759 FGEnvironment::_recalc_density_tropo_avg_kgm3 ()
 760 {
 761   double pressure_mb = pressure_inhg * 33.86;
 762   double vaporpressure = 6.11 * pow(10.0, ((7.5 * dewpoint_degc) / (237.7 + dewpoint_degc)));
 763
 764   double virtual_temp = (temperature_degc + 273.15) / (1 - 0.379 * (vaporpressure/pressure_mb));
 765
 766   double density_half = (100 * pressure_mb * exp(-altitude_half_to_sun_m / 8000))
 767       / (287.05 * virtual_temp);
 768   double density_tropo = (100 * pressure_mb * exp((-1 * altitude_tropo_top_m) / 8000))
 769       / ( 287.05 * virtual_temp);
 770
 771   density_tropo_avg_kgm3 = ((density_slugft3 * 515.379) + density_half + density_tropo) / 3;
 772 }
 773
 774 void
 775 FGEnvironment::_recalc_relative_humidity ()
 776 {
 777 /*
 778   double vaporpressure = 6.11 * pow(10.0, ((7.5 * dewpoint_degc) / ( 237.7 + dewpoint_degc)));
 779   double sat_vaporpressure = 6.11 * pow(10.0, ((7.5 * temperature_degc)
 780       / ( 237.7 + temperature_degc)) );
 781   relative_humidity = 100 * vaporpressure / sat_vaporpressure ;
 782
 783   with a little algebra, this gets the same result and spares two multiplications and one pow()
 784 */
 785   double a = (7.5 * dewpoint_degc)    / ( 237.7 + dewpoint_degc);
 786   double b = (7.5 * temperature_degc) / ( 237.7 + temperature_degc);
 787   relative_humidity = 100 * pow(10.0,a-b);
 788 }
 789
 790 bool
 791 FGEnvironment::set_live_update( bool _live_update )
 792 {
 793   bool b = live_update;
 794   live_update = _live_update;
 795   return b;
 796 }
 797
 798
 799 ////////////////////////////////////////////////////////////////////////
 800 // Functions.
 801 ////////////////////////////////////////////////////////////////////////
 802
 803 static inline double
 804 do_interp (double a, double b, double fraction)
 805 {
 806     double retval = (a + ((b - a) * fraction));
 807     return retval;
 808 }
 809
 810 static inline double
 811 do_interp_deg (double a, double b, double fraction)
 812 {
 813     a = fmod(a, 360);
 814     b = fmod(b, 360);
 815     if (fabs(b-a) > 180) {
 816         if (a < b)
 817             a += 360;
 818         else
 819             b += 360;
 820     }
 821     return fmod(do_interp(a, b, fraction), 360);
 822 }
 823
 824 void
 825 interpolate (const FGEnvironment * env1, const FGEnvironment * env2,
 826              double fraction, FGEnvironment * result)
 827 {
 828     // don't calculate each internal property every time we set a single value
 829     // we trigger that at the end of the interpolation process
 830     bool live_update = result->set_live_update( false );
 831
 832     result->set_visibility_m
 833         (do_interp(env1->get_visibility_m(),
 834                    env2->get_visibility_m(),
 835                    fraction));
 836
 837     result->set_temperature_sea_level_degc
 838         (do_interp(env1->get_temperature_sea_level_degc(),
 839                    env2->get_temperature_sea_level_degc(),
 840                    fraction));
 841
 842     result->set_dewpoint_sea_level_degc
 843         (do_interp(env1->get_dewpoint_sea_level_degc(),
 844                    env2->get_dewpoint_sea_level_degc(),
 845                    fraction));
 846
 847     result->set_pressure_sea_level_inhg
 848         (do_interp(env1->get_pressure_sea_level_inhg(),
 849                    env2->get_pressure_sea_level_inhg(),
 850                    fraction));
 851
 852     result->set_wind_from_heading_deg
 853         (do_interp_deg(env1->get_wind_from_heading_deg(),
 854                        env2->get_wind_from_heading_deg(),
 855                        fraction));
 856
 857     result->set_wind_speed_kt
 858         (do_interp(env1->get_wind_speed_kt(),
 859                    env2->get_wind_speed_kt(),
 860                    fraction));
 861
 862     result->set_elevation_ft
 863         (do_interp(env1->get_elevation_ft(),
 864                    env2->get_elevation_ft(),
 865                    fraction));
 866
 867     result->set_turbulence_magnitude_norm
 868         (do_interp(env1->get_turbulence_magnitude_norm(),
 869                    env2->get_turbulence_magnitude_norm(),
 870                    fraction));
 871
 872     result->set_turbulence_rate_hz
 873         (do_interp(env1->get_turbulence_rate_hz(),
 874                    env2->get_turbulence_rate_hz(),
 875                    fraction));
 876
 877     // calculate derived properties here to avoid duplicate expensive computations
 878     result->_recalc_ne();
 879     result->_recalc_alt_pt();
 880     result->_recalc_alt_dewpoint();
 881     result->_recalc_density();
 882     result->_recalc_relative_humidity();
 883
 884     result->set_live_update(live_update);
 885 }
 886
 887 // end of environment.cxx