Finished off the atmosphere model (Troposphere and lower

Stratosphere).  The atmospheric properties are as follow:

/environment/temperature-sea-level-degc
/environment/temperature-degc
/environment/pressure-sea-level-inhg
/environment/pressure-inhg
/environment/density-sea-level-slugft3
/environment/density-slugft3

Setting either the sea-level or altitude value automatically sets the
other value appropriate, except for temperature at altitude above the
Troposphere (where there's no reliable way to back-calculate it).  The
atmosphere model appears in the atmosphere_data array in
environment.cxx, and can easily be extended into the upper
stratosphere and beyond.

These are not yet tied into the FDMs or steam module.

src/Environment/environment.cxx

@@ -35,38 +35,52 @@
 
 #include <simgear/constants.h>
 #include <simgear/debug/logstream.hxx>
+#include <simgear/math/interpolater.hxx>
 
 #include <Main/fg_props.hxx>
 #include "environment.hxx"
 
 
 FGEnvironment::FGEnvironment()
-  : visibility_m(32000),
+  : _temperature_degc_table(new SGInterpTable),
+    _pressure_inhg_table(new SGInterpTable),
+    _density_slugft3_table(new SGInterpTable),
+    visibility_m(32000),
     temperature_sea_level_degc(20),
     pressure_sea_level_inhg(28),
+    density_sea_level_slugft3(0.0024),
     wind_from_heading_deg(0),
     wind_speed_kt(0),
     wind_from_north_fps(0),
     wind_from_east_fps(0),
     wind_from_down_fps(0)
 {
+  _setup_tables();
 }
 
 FGEnvironment::FGEnvironment (const FGEnvironment &env)
-  : elevation_ft(env.elevation_ft),
+  : _temperature_degc_table(new SGInterpTable),
+    _pressure_inhg_table(new SGInterpTable),
+    _density_slugft3_table(new SGInterpTable),
+    elevation_ft(env.elevation_ft),
     visibility_m(env.visibility_m),
     temperature_sea_level_degc(env.temperature_sea_level_degc),
     pressure_sea_level_inhg(env.pressure_sea_level_inhg),
+    density_sea_level_slugft3(env.density_sea_level_slugft3),
     wind_from_heading_deg(env.wind_from_heading_deg),
     wind_speed_kt(env.wind_speed_kt),
     wind_from_north_fps(env.wind_from_north_fps),
     wind_from_east_fps(env.wind_from_east_fps),
     wind_from_down_fps(env.wind_from_down_fps)
 {
+  _setup_tables();
 }
 
 FGEnvironment::~FGEnvironment()
 {
+  delete _temperature_degc_table;
+  delete _pressure_inhg_table;
+  delete _density_slugft3_table;
 }
 
 
@@ -100,6 +114,18 @@ FGEnvironment::get_pressure_inhg () const
   return pressure_inhg;
 }
 
+double
+FGEnvironment::get_density_sea_level_slugft3 () const
+{
+  return density_sea_level_slugft3;
+}
+
+double
+FGEnvironment::get_density_slugft3 () const
+{
+  return density_slugft3;
+}
+
 double
 FGEnvironment::get_wind_from_heading_deg () const
 {
@@ -136,8 +162,6 @@ FGEnvironment::get_elevation_ft () const
   return elevation_ft;
 }
 
-
-
 void
 FGEnvironment::set_visibility_m (double v)
 {
@@ -148,28 +172,42 @@ void
 FGEnvironment::set_temperature_sea_level_degc (double t)
 {
   temperature_sea_level_degc = t;
-  _recalc_alt_temp();
+  _recalc_alt_temperature();
 }
 
 void
 FGEnvironment::set_temperature_degc (double t)
 {
   temperature_degc = t;
-  _recalc_sl_temp();
+  _recalc_sl_temperature();
 }
 
 void
 FGEnvironment::set_pressure_sea_level_inhg (double p)
 {
   pressure_sea_level_inhg = p;
-  _recalc_alt_press();
+  _recalc_alt_pressure();
 }
 
 void
 FGEnvironment::set_pressure_inhg (double p)
 {
   pressure_inhg = p;
-  _recalc_sl_press();
+  _recalc_sl_pressure();
+}
+
+void
+FGEnvironment::set_density_sea_level_slugft3 (double d)
+{
+  density_sea_level_slugft3 = d;
+  _recalc_alt_density();
+}
+
+void
+FGEnvironment::set_density_slugft3 (double d)
+{
+  density_slugft3 = d;
+  _recalc_sl_density();
 }
 
 void
@@ -211,8 +249,63 @@ void
 FGEnvironment::set_elevation_ft (double e)
 {
   elevation_ft = e;
-  _recalc_alt_temp();
+  _recalc_alt_temperature();
-  _recalc_alt_press();
+  _recalc_alt_pressure();
+  _recalc_alt_density();
+}
+
+// Atmosphere model.
+
+// Copied from YASim Atmosphere.cxx, with m converted to ft, degK
+// converted to degC, Pa converted to inHG, and kg/m^3 converted to
+// slug/ft^3.
+
+// Original comment from YASim:
+
+// Copied from McCormick, who got it from "The ARDC Model Atmosphere"
+// Note that there's an error in the text in the first entry,
+// McCormick lists 299.16/101325/1.22500, but those don't agree with
+// R=287.  I chose to correct the temperature to 288.20, since 79F is
+// pretty hot for a "standard" atmosphere.
+
+// Elevation (ft), temperature (degC), pressure (inHG), density (slug/ft^3)
+static double atmosphere_data[][4] = {
+  0, 0.00, 0.000, 0.000000,
+  2952, -5.89, -3.058, -0.000198,
+  5905, -11.74, -5.856, -0.000384,
+  8858, -17.58, -8.413, -0.000556,
+  11811, -23.43, -10.745, -0.000717,
+  14763, -29.27, -12.867, -0.000867,
+  17716, -35.11, -14.794, -0.001006,
+  20669, -40.95, -16.541, -0.001136,
+  23622, -46.79, -18.120, -0.001255,
+  26574, -52.62, -19.544, -0.001366,
+  29527, -58.46, -20.826, -0.001467,
+  32480, -64.29, -21.976, -0.001561,
+  35433, -70.12, -23.005, -0.001647,
+  38385, -71.54, -23.916, -0.001739,
+  41338, -71.54, -24.708, -0.001822,
+  44291, -71.54, -25.395, -0.001894,
+  47244, -71.54, -25.991, -0.001957,
+  50196, -71.54, -26.509, -0.002012,
+  53149, -71.54, -26.959, -0.002059,
+  56102, -71.54, -27.349, -0.002100,
+  59055, -71.54, -27.687, -0.002136,
+  62007, -71.54, -27.981, -0.002167,
+  -1, -1, -1, -1
+};
+
+void
+FGEnvironment::_setup_tables ()
+{
+  for (int i = 0; atmosphere_data[i][0] != -1; i++) {
+    _temperature_degc_table->addEntry(atmosphere_data[i][0],
+				      atmosphere_data[i][1]);
+    _pressure_inhg_table->addEntry(atmosphere_data[i][0],
+				   atmosphere_data[i][2]);
+    _density_slugft3_table->addEntry(atmosphere_data[i][0],
+				     atmosphere_data[i][3]);
+  }
 }
 
 void
@@ -251,47 +344,50 @@ FGEnvironment::_recalc_ne ()
 }
 
 void
-FGEnvironment::_recalc_sl_temp ()
+FGEnvironment::_recalc_sl_temperature ()
 {
-  // Earth atmosphere model from
+  if (elevation_ft < 38000) {
-  // http://www.grc.nasa.gov/WWW/K-12/airplane/atmos.html
-
-  // Stratospheric temperatures are not really reversible, so use 15degC.
-
-  if (elevation_ft < 36152)	// Troposphere
     temperature_sea_level_degc =
-      temperature_degc + (.00649 * SG_FEET_TO_METER * elevation_ft);
+      temperature_degc - _temperature_degc_table->interpolate(elevation_ft);
+  }
   // If we're in the stratosphere, leave sea-level temp alone
 }
 
 void
-FGEnvironment::_recalc_alt_temp ()
+FGEnvironment::_recalc_alt_temperature ()
 {
-  // Earth atmosphere model from
-  // http://www.grc.nasa.gov/WWW/K-12/airplane/atmos.html
-
-  if (elevation_ft < 36152)	// Troposphere
   temperature_degc =
-      temperature_sea_level_degc - (.00649 * SG_FEET_TO_METER * elevation_ft);
+    temperature_sea_level_degc +
-  else if (elevation_ft < 82345) // Lower Stratosphere
+    _temperature_degc_table->interpolate(elevation_ft);
-    temperature_degc = -56.46;
-  else
-    temperature_degc = -131.21 + (.00299 * SG_FEET_TO_METER * elevation_ft);
 }
 
 void
-FGEnvironment::_recalc_sl_press ()
+FGEnvironment::_recalc_sl_pressure ()
 {
-  // FIXME: calculate properly
+  pressure_sea_level_inhg =
-  pressure_sea_level_inhg = pressure_inhg;
+    pressure_inhg - _pressure_inhg_table->interpolate(elevation_ft);
 }
 
 void
-FGEnvironment::_recalc_alt_press ()
+FGEnvironment::_recalc_alt_pressure ()
 {
-  // FIXME: calculate properly
+  pressure_inhg =
-  pressure_inhg = pressure_sea_level_inhg;
+    pressure_sea_level_inhg + _pressure_inhg_table->interpolate(elevation_ft);
+}
+
+void
+FGEnvironment::_recalc_sl_density ()
+{
+  density_sea_level_slugft3 =
+    density_slugft3 - _density_slugft3_table->interpolate(elevation_ft);
+}
+
+void
+FGEnvironment::_recalc_alt_density ()
+{
+  density_slugft3 =
+    density_sea_level_slugft3 +
+    _density_slugft3_table->interpolate(elevation_ft);
 }
 
 // end of environment.cxx
-

src/Environment/environment.hxx

@@ -34,6 +34,8 @@
 #  include <math.h>
 #endif
 
+class SGInterpTable;
+
 
 /**
  * Model the natural environment.
@@ -53,10 +55,14 @@ public:
   virtual ~FGEnvironment();
   
   virtual double get_visibility_m () const;
+
   virtual double get_temperature_sea_level_degc () const;
   virtual double get_temperature_degc () const;
   virtual double get_pressure_sea_level_inhg () const;
   virtual double get_pressure_inhg () const;
+  virtual double get_density_sea_level_slugft3 () const;
+  virtual double get_density_slugft3 () const;
+
   virtual double get_wind_from_heading_deg () const;
   virtual double get_wind_speed_kt () const;
   virtual double get_wind_from_north_fps () const;
@@ -64,10 +70,14 @@ public:
   virtual double get_wind_from_down_fps () const;
 
   virtual void set_visibility_m (double v);
+
   virtual void set_temperature_sea_level_degc (double t);
   virtual void set_temperature_degc (double t);
   virtual void set_pressure_sea_level_inhg (double p);
   virtual void set_pressure_inhg (double p);
+  virtual void set_density_sea_level_slugft3 (double d);
+  virtual void set_density_slugft3 (double d);
+
   virtual void set_wind_from_heading_deg (double h);
   virtual void set_wind_speed_kt (double s);
   virtual void set_wind_from_north_fps (double n);
@@ -83,20 +93,33 @@ protected:
 
 private:
 
+  void _setup_tables ();
+
   void _recalc_hdgspd ();
   void _recalc_ne ();
-  void _recalc_sl_temp ();
+
-  void _recalc_alt_temp ();
+  void _recalc_sl_temperature ();
-  void _recalc_sl_press ();
+  void _recalc_alt_temperature ();
-  void _recalc_alt_press ();
+  void _recalc_sl_pressure ();
+  void _recalc_alt_pressure ();
+  void _recalc_sl_density ();
+  void _recalc_alt_density ();
+
+  SGInterpTable * _temperature_degc_table;
+  SGInterpTable * _pressure_inhg_table;
+  SGInterpTable * _density_slugft3_table;
 
   double elevation_ft;
 
   double visibility_m;
+
+				// Atmosphere
   double temperature_sea_level_degc;
   double temperature_degc;
   double pressure_sea_level_inhg;
   double pressure_inhg;
+  double density_sea_level_slugft3;
+  double density_slugft3;
 
   double wind_from_heading_deg;
   double wind_speed_kt;

src/Environment/environment_mgr.cxx

@@ -72,6 +72,14 @@ FGEnvironmentMgr::bind ()
 	&FGEnvironment::get_pressure_inhg,
 	&FGEnvironment::set_pressure_inhg);
   fgSetArchivable("/environment/pressure-inhg");
+  fgTie("/environment/density-sea-level-slugft3", _environment,
+	&FGEnvironment::get_density_sea_level_slugft3,
+	&FGEnvironment::set_density_sea_level_slugft3);
+  fgSetArchivable("/environment/density-sea-level-slugft3");
+  fgTie("/environment/density-slugft3", _environment,
+	&FGEnvironment::get_density_slugft3,
+	&FGEnvironment::set_density_slugft3);
+  fgSetArchivable("/environment/density-inhg");
   fgTie("/environment/wind-from-heading-deg", _environment,
 	&FGEnvironment::get_wind_from_heading_deg,
 	&FGEnvironment::set_wind_from_heading_deg);