atmosphere.hxx

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// atmosphere.hxx -- routines to model the air column
//
// Written by David Megginson, started February 2002.
// Modified by John Denker to correct physics errors in 2007
//
// Copyright (C) 2002  David Megginson - david@megginson.com
//
// This program is free software; you can redistribute it and/or
// modify it under the terms of the GNU General Public License as
// published by the Free Software Foundation; either version 2 of the
// License, or (at your option) any later version.
//
// This program is distributed in the hope that it will be useful, but
// WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
// General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program; if not, write to the Free Software
// Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
//
// $Id$
 
 
#ifndef _ATMOSPHERE_HXX
#define _ATMOSPHERE_HXX
 
#include <simgear/compiler.h>
#include <simgear/math/interpolater.hxx>
 
#include <cmath>
#include <utility>

 
#define SCD(name,val) const double name(val)
namespace atmodel {
    SCD(g, 9.80665);                // [m/s/s] acceleration of gravity
    SCD(mm, .0289644);             // [kg/mole] molar mass of air (dry?)
    SCD(Rgas, 8.31432);          // [J/K/mole] gas constant
    SCD(inch, 0.0254);           // [m] definition of inch
    SCD(foot, 12 * inch);    // [m]
    SCD(inHg, 101325.0 / 760 * 1000 * inch);  // [Pa] definition of inHg
    SCD(mbar, 100.);            // [Pa] definition of millibar
    SCD(freezing, 273.15);   // [K] centigrade - kelvin offset
    SCD(nm, 1852);         // [m] nautical mile (NIST)
    SCD(sm, 5280*foot);                // [m] nautical mile (NIST)
 
    namespace ISA {
        SCD(P0, 101325.0);                 // [pascals] ISA sea-level pressure
        SCD(T0, 15. + freezing);   // [K] ISA sea-level temperature
        SCD(lam0, .0065);                // [K/m] ISA troposphere lapse rate
    }
}
#undef SCD
 
 
 
class ISA_layer {
public:
  double height;
  double temp;
  double lapse;
  ISA_layer(int, double h, double, double, double, double t, double,
                    double l=-1, double=0)
   :  height(h),        // [meters]
      temp(t),          // [kelvin]
      lapse(l)          // [K/m]
  {}
};
 
extern const ISA_layer ISA_def[];
 
std::pair<double,double> PT_vs_hpt(
          const double hh, 
          const double _p0 = atmodel::ISA::P0,
          const double _t0 = atmodel::ISA::T0);
 
double P_layer(const double height, const double href,
  const double Pref, const double Tref, const double lapse );
 
double T_layer(const double height, const double href,
  const double Pref, const double Tref,  const double lapse );
 
// The base class is little more than a namespace.
// It has no constructor, no destructor, and no variables.
class FGAtmo {
public:
    double a_vs_p(const double press, const double qnh = atmodel::ISA::P0);
    double fake_T_vs_a_us(const double h_ft, 
                const double Tsl = atmodel::ISA::T0) const;
    double fake_dp_vs_a_us(const double dpsl, const double h_ft);
    void check_one(const double height);
 
// Altimeter setting _in pascals_ 
//  ... caller gets to convert to inHg or millibars
// Field elevation in m
// Field pressure in pascals
// Valid for fields within the troposphere only.
    double QNH(const double field_elev, const double field_press);
    static double fieldPressure(const double field_elev, const double qnh);
    
    static double ISATemperatureKAtAltitudeFt(const double alt,
                                             const double Tsl = atmodel::ISA::T0);
    
    static double CSMetersPerSecondAtAltitudeFt(const double alt,
                                 const double Tsl = atmodel::ISA::T0);
    
    static double densityAtAltitudeFt(const double alt, const double Tsl = atmodel::ISA::T0);
    
    static double machFromKnotsAtAltitudeFt(const double knots,
                                                    const double altFt,
                                                    const double Tsl = atmodel::ISA::T0);
    
    static double knotsFromMachAtAltitudeFt(const double mach,
                                            const double altFt,
                                            const double Tsl = atmodel::ISA::T0);
};
 
 
 
class FGAtmoCache : FGAtmo {
    friend class FGAltimeter;
    SGInterpTable * a_tvs_p;    // _tvs_ means "tabulated versus"
 
public:
    FGAtmoCache();
    ~FGAtmoCache();
    void tabulate();
    void cache();
    void check_model();  // debug
};
 
 
 
class FGAltimeter : public FGAtmoCache {
public:
    FGAltimeter();
    double reading_ft(const double p_inHg,
            const double set_inHg = atmodel::ISA::P0/atmodel::inHg);
    inline double press_alt_ft(const double p_inHg) {
        return a_tvs_p->interpolate(p_inHg);
    }
    inline double kollsman_ft(const double set_inHg) {
        return a_tvs_p->interpolate(set_inHg);
    }
 
    // debug
    void dump_stack();
    void dump_stack1(const double Tref);
};
 
#endif // _ATMOSPHERE_HXX