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 #ifdef HAVE_WINDOWS_H
  29 #  include <windows.h>
  30 #endif
  31
  32 #include <math.h>
  33
  34 #include <plib/sg.h>
  35
  36 #include <simgear/constants.h>
  37 #include <simgear/debug/logstream.hxx>
  38 #include <simgear/math/interpolater.hxx>
  39 #include <simgear/environment/visual_enviro.hxx>
  40
  41 #include "environment.hxx"
  42
  43
  44 \f
  45 ////////////////////////////////////////////////////////////////////////
  46 // Atmosphere model.
  47 ////////////////////////////////////////////////////////////////////////
  48
  49 // Copied from YASim Atmosphere.cxx, with m converted to ft, degK
  50 // converted to degC, Pa converted to inHG, and kg/m^3 converted to
  51 // slug/ft^3; they were then converted to deltas from the sea-level
  52 // defaults (approx. 15degC, 29.92inHG, and 0.00237slugs/ft^3).
  53
  54 // Original comment from YASim:
  55
  56 // Copied from McCormick, who got it from "The ARDC Model Atmosphere"
  57 // Note that there's an error in the text in the first entry,
  58 // McCormick lists 299.16/101325/1.22500, but those don't agree with
  59 // R=287.  I chose to correct the temperature to 288.20, since 79F is
  60 // pretty hot for a "standard" atmosphere.
  61
  62 // Elevation (ft), temperature factor (degK), pressure factor (inHG)
  63 static double atmosphere_data[][3] = {
  64  { 0.00, 1.00, 1.000 },
  65  { 2952.76, 0.98, 0.898 },
  66  { 5905.51, 0.96, 0.804 },
  67  { 8858.27, 0.94, 0.719 },
  68  { 11811.02, 0.92, 0.641 },
  69  { 14763.78, 0.90, 0.570 },
  70  { 17716.54, 0.88, 0.506 },
  71  { 20669.29, 0.86, 0.447 },
  72  { 23622.05, 0.84, 0.394 },
  73  { 26574.80, 0.82, 0.347 },
  74  { 29527.56, 0.80, 0.304 },
  75  { 32480.31, 0.78, 0.266 },
  76  { 35433.07, 0.76, 0.231 },
  77  { 38385.83, 0.75, 0.201 },
  78  { 41338.58, 0.75, 0.174 },
  79  { 44291.34, 0.75, 0.151 },
  80  { 47244.09, 0.75, 0.131 },
  81  { 50196.85, 0.75, 0.114 },
  82  { 53149.61, 0.75, 0.099 },
  83  { 56102.36, 0.75, 0.086 },
  84  { 59055.12, 0.75, 0.075 },
  85  { 62007.87, 0.75, 0.065 },
  86  { -1, -1, -1 }
  87 };
  88
  89 static SGInterpTable * _temperature_degc_table = 0;
  90 static SGInterpTable * _pressure_inhg_table = 0;
  91
  92 static void
  93 _setup_tables ()
  94 {
  95   if (_temperature_degc_table != 0)
  96       return;
  97
  98   _temperature_degc_table = new SGInterpTable;
  99   _pressure_inhg_table = new SGInterpTable;
 100
 101   for (int i = 0; atmosphere_data[i][0] != -1; i++) {
 102     _temperature_degc_table->addEntry(atmosphere_data[i][0],
 103                                       atmosphere_data[i][1]);
 104     _pressure_inhg_table->addEntry(atmosphere_data[i][0],
 105                                    atmosphere_data[i][2]);
 106   }
 107 }
 108
 109
 110 \f
 111 ////////////////////////////////////////////////////////////////////////
 112 // Implementation of FGEnvironment.
 113 ////////////////////////////////////////////////////////////////////////
 114
 115 FGEnvironment::FGEnvironment()
 116   : elevation_ft(0),
 117     visibility_m(32000),
 118     temperature_sea_level_degc(15),
 119     temperature_degc(15),
 120     dewpoint_sea_level_degc(5), // guess
 121     dewpoint_degc(5),
 122     pressure_sea_level_inhg(29.92),
 123     pressure_inhg(29.92),
 124     turbulence_magnitude_norm(0),
 125     turbulence_rate_hz(1),
 126     wind_from_heading_deg(0),
 127     wind_speed_kt(0),
 128     wind_from_north_fps(0),
 129     wind_from_east_fps(0),
 130     wind_from_down_fps(0),
 131     altitude_half_to_sun_m(1000),
 132     altitude_tropo_top_m(10000)
 133 {
 134   _setup_tables();
 135   _recalc_density();
 136   _recalc_relative_humidity();
 137 }
 138
 139 FGEnvironment::FGEnvironment (const FGEnvironment &env)
 140 {
 141     FGEnvironment();
 142     copy(env);
 143 }
 144
 145 FGEnvironment::~FGEnvironment()
 146 {
 147 }
 148
 149 void
 150 FGEnvironment::copy (const FGEnvironment &env)
 151 {
 152     elevation_ft = env.elevation_ft;
 153     visibility_m = env.visibility_m;
 154     temperature_sea_level_degc = env.temperature_sea_level_degc;
 155     temperature_degc = env.temperature_degc;
 156     dewpoint_sea_level_degc = env.dewpoint_sea_level_degc;
 157     dewpoint_degc = env.dewpoint_degc;
 158     pressure_sea_level_inhg = env.pressure_sea_level_inhg;
 159     wind_from_heading_deg = env.wind_from_heading_deg;
 160     wind_speed_kt = env.wind_speed_kt;
 161     wind_from_north_fps = env.wind_from_north_fps;
 162     wind_from_east_fps = env.wind_from_east_fps;
 163     wind_from_down_fps = env.wind_from_down_fps;
 164     turbulence_magnitude_norm = env.turbulence_magnitude_norm;
 165     turbulence_rate_hz = env.turbulence_rate_hz;
 166 }
 167
 168 static inline bool
 169 maybe_copy_value (FGEnvironment * env, const SGPropertyNode * node,
 170                   const char * name, void (FGEnvironment::*setter)(double))
 171 {
 172     const SGPropertyNode * child = node->getNode(name);
 173                                 // fragile: depends on not being typed
 174                                 // as a number
 175     if (child != 0 && child->hasValue() &&
 176         child->getStringValue()[0] != '\0') {
 177         (env->*setter)(child->getDoubleValue());
 178         return true;
 179     } else {
 180         return false;
 181     }
 182 }
 183
 184 void
 185 FGEnvironment::read (const SGPropertyNode * node)
 186 {
 187     maybe_copy_value(this, node, "visibility-m",
 188                      &FGEnvironment::set_visibility_m);
 189
 190     if (!maybe_copy_value(this, node, "temperature-sea-level-degc",
 191                           &FGEnvironment::set_temperature_sea_level_degc))
 192         maybe_copy_value(this, node, "temperature-degc",
 193                          &FGEnvironment::set_temperature_degc);
 194
 195     if (!maybe_copy_value(this, node, "dewpoint-sea-level-degc",
 196                           &FGEnvironment::set_dewpoint_sea_level_degc))
 197         maybe_copy_value(this, node, "dewpoint-degc",
 198                          &FGEnvironment::set_dewpoint_degc);
 199
 200     if (!maybe_copy_value(this, node, "pressure-sea-level-inhg",
 201                           &FGEnvironment::set_pressure_sea_level_inhg))
 202         maybe_copy_value(this, node, "pressure-inhg",
 203                          &FGEnvironment::set_pressure_inhg);
 204
 205     maybe_copy_value(this, node, "wind-from-heading-deg",
 206                      &FGEnvironment::set_wind_from_heading_deg);
 207
 208     maybe_copy_value(this, node, "wind-speed-kt",
 209                      &FGEnvironment::set_wind_speed_kt);
 210
 211     maybe_copy_value(this, node, "elevation-ft",
 212                      &FGEnvironment::set_elevation_ft);
 213
 214     maybe_copy_value(this, node, "turbulence/magnitude-norm",
 215                      &FGEnvironment::set_turbulence_magnitude_norm);
 216
 217     maybe_copy_value(this, node, "turbulence/rate-hz",
 218                      &FGEnvironment::set_turbulence_rate_hz);
 219 }
 220
 221
 222 double
 223 FGEnvironment::get_visibility_m () const
 224 {
 225   return visibility_m;
 226 }
 227
 228 double
 229 FGEnvironment::get_temperature_sea_level_degc () const
 230 {
 231   return temperature_sea_level_degc;
 232 }
 233
 234 double
 235 FGEnvironment::get_temperature_degc () const
 236 {
 237   return temperature_degc;
 238 }
 239
 240 double
 241 FGEnvironment::get_temperature_degf () const
 242 {
 243   return (temperature_degc * 9.0 / 5.0) + 32.0;
 244 }
 245
 246 double
 247 FGEnvironment::get_dewpoint_sea_level_degc () const
 248 {
 249   return dewpoint_sea_level_degc;
 250 }
 251
 252 double
 253 FGEnvironment::get_dewpoint_degc () const
 254 {
 255   return dewpoint_degc;
 256 }
 257
 258 double
 259 FGEnvironment::get_pressure_sea_level_inhg () const
 260 {
 261   return pressure_sea_level_inhg;
 262 }
 263
 264 double
 265 FGEnvironment::get_pressure_inhg () const
 266 {
 267   return pressure_inhg;
 268 }
 269
 270 double
 271 FGEnvironment::get_density_slugft3 () const
 272 {
 273   return density_slugft3;
 274 }
 275
 276 double
 277 FGEnvironment::get_relative_humidity () const
 278 {
 279   return relative_humidity;
 280 }
 281
 282 double
 283 FGEnvironment::get_density_tropo_avg_kgm3 () const
 284 {
 285   return density_tropo_avg_kgm3;
 286 }
 287
 288 double
 289 FGEnvironment::get_altitude_half_to_sun_m () const
 290 {
 291   return altitude_half_to_sun_m;
 292 }
 293
 294 double
 295 FGEnvironment::get_altitude_tropo_top_m () const
 296 {
 297   return altitude_tropo_top_m;
 298 }
 299
 300 double
 301 FGEnvironment::get_wind_from_heading_deg () const
 302 {
 303   return wind_from_heading_deg;
 304 }
 305
 306 double
 307 FGEnvironment::get_wind_speed_kt () const
 308 {
 309   return wind_speed_kt;
 310 }
 311
 312 double
 313 FGEnvironment::get_wind_from_north_fps () const
 314 {
 315   return wind_from_north_fps;
 316 }
 317
 318 double
 319 FGEnvironment::get_wind_from_east_fps () const
 320 {
 321   return wind_from_east_fps;
 322 }
 323
 324 double
 325 FGEnvironment::get_wind_from_down_fps () const
 326 {
 327   return wind_from_down_fps;
 328 }
 329
 330 double
 331 FGEnvironment::get_turbulence_magnitude_norm () const
 332 {
 333   if( sgEnviro.get_turbulence_enable_state() )
 334     if (fgGetBool("/environment/params/real-world-weather-fetch") == true)
 335       return sgEnviro.get_cloud_turbulence();
 336   return turbulence_magnitude_norm;
 337 }
 338
 339 double
 340 FGEnvironment::get_turbulence_rate_hz () const
 341 {
 342   return turbulence_rate_hz;
 343 }
 344
 345 double
 346 FGEnvironment::get_elevation_ft () const
 347 {
 348   return elevation_ft;
 349 }
 350
 351 void
 352 FGEnvironment::set_visibility_m (double v)
 353 {
 354   visibility_m = v;
 355 }
 356
 357 void
 358 FGEnvironment::set_temperature_sea_level_degc (double t)
 359 {
 360   temperature_sea_level_degc = t;
 361   if (dewpoint_sea_level_degc > t)
 362       dewpoint_sea_level_degc = t;
 363   _recalc_alt_temperature();
 364   _recalc_density();
 365 }
 366
 367 void
 368 FGEnvironment::set_temperature_degc (double t)
 369 {
 370   temperature_degc = t;
 371   _recalc_sl_temperature();
 372   _recalc_density();
 373   _recalc_relative_humidity();
 374 }
 375
 376 void
 377 FGEnvironment::set_dewpoint_sea_level_degc (double t)
 378 {
 379   dewpoint_sea_level_degc = t;
 380   if (temperature_sea_level_degc < t)
 381       temperature_sea_level_degc = t;
 382   _recalc_alt_dewpoint();
 383   _recalc_density();
 384 }
 385
 386 void
 387 FGEnvironment::set_dewpoint_degc (double t)
 388 {
 389   dewpoint_degc = t;
 390   _recalc_sl_dewpoint();
 391   _recalc_density();
 392   _recalc_relative_humidity();
 393 }
 394
 395 void
 396 FGEnvironment::set_pressure_sea_level_inhg (double p)
 397 {
 398   pressure_sea_level_inhg = p;
 399   _recalc_alt_pressure();
 400   _recalc_density();
 401 }
 402
 403 void
 404 FGEnvironment::set_pressure_inhg (double p)
 405 {
 406   pressure_inhg = p;
 407   _recalc_sl_pressure();
 408   _recalc_density();
 409 }
 410
 411 void
 412 FGEnvironment::set_wind_from_heading_deg (double h)
 413 {
 414   wind_from_heading_deg = h;
 415   _recalc_ne();
 416 }
 417
 418 void
 419 FGEnvironment::set_wind_speed_kt (double s)
 420 {
 421   wind_speed_kt = s;
 422   _recalc_ne();
 423 }
 424
 425 void
 426 FGEnvironment::set_wind_from_north_fps (double n)
 427 {
 428   wind_from_north_fps = n;
 429   _recalc_hdgspd();
 430 }
 431
 432 void
 433 FGEnvironment::set_wind_from_east_fps (double e)
 434 {
 435   wind_from_east_fps = e;
 436   _recalc_hdgspd();
 437 }
 438
 439 void
 440 FGEnvironment::set_wind_from_down_fps (double d)
 441 {
 442   wind_from_down_fps = d;
 443   _recalc_hdgspd();
 444 }
 445
 446 void
 447 FGEnvironment::set_turbulence_magnitude_norm (double t)
 448 {
 449   turbulence_magnitude_norm = t;
 450 }
 451
 452 void
 453 FGEnvironment::set_turbulence_rate_hz (double r)
 454 {
 455   turbulence_rate_hz = r;
 456 }
 457
 458 void
 459 FGEnvironment::set_elevation_ft (double e)
 460 {
 461   elevation_ft = e;
 462   _recalc_alt_temperature();
 463   _recalc_alt_dewpoint();
 464   _recalc_alt_pressure();
 465   _recalc_density();
 466   _recalc_relative_humidity();
 467 }
 468
 469 void
 470 FGEnvironment::set_altitude_half_to_sun_m (double alt)
 471 {
 472  altitude_half_to_sun_m = alt;
 473  _recalc_density_tropo_avg_kgm3();
 474 }
 475
 476 void
 477 FGEnvironment::set_altitude_tropo_top_m (double alt)
 478 {
 479  altitude_tropo_top_m = alt;
 480  _recalc_density_tropo_avg_kgm3();
 481 }
 482
 483
 484 void
 485 FGEnvironment::_recalc_hdgspd ()
 486 {
 487   double angle_rad;
 488
 489   if (wind_from_east_fps == 0) {
 490     angle_rad = (wind_from_north_fps >= 0 ? SGD_PI_2 : -SGD_PI_2);
 491   } else {
 492     angle_rad = atan(wind_from_north_fps/wind_from_east_fps);
 493   }
 494   wind_from_heading_deg = angle_rad * SGD_RADIANS_TO_DEGREES;
 495   if (wind_from_east_fps >= 0)
 496     wind_from_heading_deg = 90 - wind_from_heading_deg;
 497   else
 498     wind_from_heading_deg = 270 - wind_from_heading_deg;
 499
 500 #if 0
 501   // FIXME: Windspeed can become negative with these formulas.
 502   //        which can cause problems for animations that rely
 503   //        on the windspeed property.
 504   if (angle_rad == 0)
 505     wind_speed_kt = fabs(wind_from_east_fps
 506                          * SG_METER_TO_NM * SG_FEET_TO_METER * 3600);
 507   else
 508     wind_speed_kt = (wind_from_north_fps / sin(angle_rad))
 509       * SG_METER_TO_NM * SG_FEET_TO_METER * 3600;
 510 #else
 511   wind_speed_kt = sqrt(wind_from_north_fps * wind_from_north_fps +
 512                        wind_from_east_fps * wind_from_east_fps)
 513                   * SG_METER_TO_NM * SG_FEET_TO_METER * 3600;
 514 #endif
 515 }
 516
 517 void
 518 FGEnvironment::_recalc_ne ()
 519 {
 520   double speed_fps =
 521     wind_speed_kt * SG_NM_TO_METER * SG_METER_TO_FEET * (1.0/3600);
 522
 523   wind_from_north_fps = speed_fps *
 524     cos(wind_from_heading_deg * SGD_DEGREES_TO_RADIANS);
 525   wind_from_east_fps = speed_fps *
 526     sin(wind_from_heading_deg * SGD_DEGREES_TO_RADIANS);
 527 }
 528
 529 void
 530 FGEnvironment::_recalc_sl_temperature ()
 531 {
 532   // If we're in the stratosphere, leave sea-level temp alone
 533   if (elevation_ft < 38000) {
 534     temperature_sea_level_degc = (temperature_degc + 273.15)
 535         / _temperature_degc_table->interpolate(elevation_ft)
 536       - 273.15;
 537   }
 538 }
 539
 540 void
 541 FGEnvironment::_recalc_alt_temperature ()
 542 {
 543   if (elevation_ft < 38000) {
 544     temperature_degc = (temperature_sea_level_degc + 273.15) *
 545         _temperature_degc_table->interpolate(elevation_ft) - 273.15;
 546   } else {
 547     temperature_degc = -56.49;  // Stratosphere is constant
 548   }
 549 }
 550
 551 void
 552 FGEnvironment::_recalc_sl_dewpoint ()
 553 {
 554                                 // 0.2degC/1000ft
 555                                 // FIXME: this will work only for low
 556                                 // elevations
 557   dewpoint_sea_level_degc = dewpoint_degc + (elevation_ft * .0002);
 558   if (dewpoint_sea_level_degc > temperature_sea_level_degc)
 559     dewpoint_sea_level_degc = temperature_sea_level_degc;
 560 }
 561
 562 void
 563 FGEnvironment::_recalc_alt_dewpoint ()
 564 {
 565                                 // 0.2degC/1000ft
 566                                 // FIXME: this will work only for low
 567                                 // elevations
 568   dewpoint_degc = dewpoint_sea_level_degc + (elevation_ft * .0002);
 569   if (dewpoint_degc > temperature_degc)
 570     dewpoint_degc = temperature_degc;
 571 }
 572
 573 void
 574 FGEnvironment::_recalc_sl_pressure ()
 575 {
 576   pressure_sea_level_inhg =
 577     pressure_inhg / _pressure_inhg_table->interpolate(elevation_ft);
 578 }
 579
 580 void
 581 FGEnvironment::_recalc_alt_pressure ()
 582 {
 583   pressure_inhg =
 584     pressure_sea_level_inhg * _pressure_inhg_table->interpolate(elevation_ft);
 585 }
 586
 587 void
 588 FGEnvironment::_recalc_density ()
 589 {
 590   double pressure_psf = pressure_inhg * 70.7487;
 591
 592   // adjust for humidity
 593   // calculations taken from USA Today (oops!) at
 594   // http://www.usatoday.com/weather/basics/density-calculations.htm
 595   double temperature_degk = temperature_degc + 273.15;
 596   double pressure_mb = pressure_inhg * 33.86;
 597   double vapor_pressure_mb =
 598     6.11 * pow(10.0, 7.5 * dewpoint_degc / (237.7 + dewpoint_degc));
 599   double virtual_temperature_degk = temperature_degk / (1 - (vapor_pressure_mb / pressure_mb) * (1.0 - 0.622));
 600   double virtual_temperature_degr = virtual_temperature_degk * 1.8;
 601
 602   density_slugft3 = pressure_psf / (virtual_temperature_degr * 1718);
 603   _recalc_density_tropo_avg_kgm3();
 604 }
 605
 606 // This is used to calculate the average density on the path
 607 // of sunlight to the observer for calculating sun-color
 608 void
 609 FGEnvironment::_recalc_density_tropo_avg_kgm3 ()
 610 {
 611   double pressure_mb = pressure_inhg * 33.86;
 612   double vaporpressure = 6.11 * pow(10, ((7.5 * dewpoint_degc) / (237.7 + dewpoint_degc)));
 613
 614   double virtual_temp = (temperature_degc + 273.15) / (1 - 0.379 * (vaporpressure/pressure_mb));
 615
 616   double density_half = (100 * pressure_mb * exp(-altitude_half_to_sun_m / 8000))
 617       / (287.05 * virtual_temp);
 618   double density_tropo = (100 * pressure_mb * exp((-1 * altitude_tropo_top_m) / 8000))
 619       / ( 287.05 * virtual_temp);
 620
 621   density_tropo_avg_kgm3 = ((density_slugft3 * 515.379) + density_half + density_tropo) / 3;
 622 }
 623
 624 void
 625 FGEnvironment::_recalc_relative_humidity ()
 626 {
 627   double vaporpressure = 6.11 * pow(10, ((7.5 * dewpoint_degc) / ( 237.7 + dewpoint_degc)));
 628   double sat_vaporpressure = 6.11 * pow(10, ((7.5 * temperature_degc)
 629       / ( 237.7 + temperature_degc)) );
 630   relative_humidity = 100 * vaporpressure / sat_vaporpressure ;
 631 }
 632
 633
 634
 635 ////////////////////////////////////////////////////////////////////////
 636 // Functions.
 637 ////////////////////////////////////////////////////////////////////////
 638
 639 static inline double
 640 do_interp (double a, double b, double fraction)
 641 {
 642     double retval = (a + ((b - a) * fraction));
 643     return retval;
 644 }
 645
 646 static inline double
 647 do_interp_deg (double a, double b, double fraction)
 648 {
 649     a = fmod(a, 360);
 650     b = fmod(b, 360);
 651     if (fabs(b-a) > 180) {
 652         if (a < b)
 653             a += 360;
 654         else
 655             b += 360;
 656     }
 657     return fmod(do_interp(a, b, fraction), 360);
 658 }
 659
 660 void
 661 interpolate (const FGEnvironment * env1, const FGEnvironment * env2,
 662              double fraction, FGEnvironment * result)
 663 {
 664     result->set_visibility_m
 665         (do_interp(env1->get_visibility_m(),
 666                    env2->get_visibility_m(),
 667                    fraction));
 668
 669     result->set_temperature_sea_level_degc
 670         (do_interp(env1->get_temperature_sea_level_degc(),
 671                    env2->get_temperature_sea_level_degc(),
 672                    fraction));
 673
 674     result->set_dewpoint_degc
 675         (do_interp(env1->get_dewpoint_sea_level_degc(),
 676                    env2->get_dewpoint_sea_level_degc(),
 677                    fraction));
 678
 679     result->set_pressure_sea_level_inhg
 680         (do_interp(env1->get_pressure_sea_level_inhg(),
 681                    env2->get_pressure_sea_level_inhg(),
 682                    fraction));
 683
 684     result->set_wind_from_heading_deg
 685         (do_interp_deg(env1->get_wind_from_heading_deg(),
 686                        env2->get_wind_from_heading_deg(),
 687                        fraction));
 688
 689     result->set_wind_speed_kt
 690         (do_interp(env1->get_wind_speed_kt(),
 691                    env2->get_wind_speed_kt(),
 692                    fraction));
 693
 694     result->set_elevation_ft
 695         (do_interp(env1->get_elevation_ft(),
 696                    env2->get_elevation_ft(),
 697                    fraction));
 698
 699     result->set_turbulence_magnitude_norm
 700         (do_interp(env1->get_turbulence_magnitude_norm(),
 701                    env2->get_turbulence_magnitude_norm(),
 702                    fraction));
 703
 704     result->set_turbulence_rate_hz
 705         (do_interp(env1->get_turbulence_rate_hz(),
 706                    env2->get_turbulence_rate_hz(),
 707                    fraction));
 708 }
 709
 710 // end of environment.cxx