flightgear/Simulator/Autopilot/autopilot.cxx

// autopilot.cxx -- autopilot subsystem
//
// Written by Jeff Goeke-Smith, started April 1998.
//
// Copyright (C) 1998  Jeff Goeke-Smith, jgoeke@voyager.net
//
// 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., 675 Mass Ave, Cambridge, MA 02139, USA.
//
// $Id$


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

#include <assert.h>
#include <stdlib.h>

#include <Scenery/scenery.hxx>

#include "autopilot.hxx"

#include <Include/fg_constants.h>
#include <Debug/logstream.hxx>
#include <Main/options.hxx>

#include <plib/pu.h>

// The below routines were copied right from hud.c ( I hate reinventing
// the wheel more than necessary)

// The following routines obtain information concerntin the aircraft's
// current state and return it to calling instrument display routines.
// They should eventually be member functions of the aircraft.
//


static double get_speed( void )
{
    return( current_aircraft.fdm_state->get_V_equiv_kts() );
}

static double get_aoa( void )
{
    return( current_aircraft.fdm_state->get_Gamma_vert_rad() * RAD_TO_DEG );
}

static double fgAPget_roll( void )
{
    return( current_aircraft.fdm_state->get_Phi() * RAD_TO_DEG );
}

static double get_pitch( void )
{
    return( current_aircraft.fdm_state->get_Theta() );
}

double fgAPget_heading( void )
{
    return( current_aircraft.fdm_state->get_Psi() * RAD_TO_DEG );
}

static double fgAPget_altitude( void )
{
    return( current_aircraft.fdm_state->get_Altitude() * FEET_TO_METER );
}

static double fgAPget_climb( void )
{
    // return in meters per minute
    return( current_aircraft.fdm_state->get_Climb_Rate() * FEET_TO_METER * 60 );
}

static double get_sideslip( void )
{
    return( current_aircraft.fdm_state->get_Beta() );
}

static double fgAPget_agl( void )
{
    double agl;

    agl = current_aircraft.fdm_state->get_Altitude() * FEET_TO_METER
	- scenery.cur_elev;

    return( agl );
}

// End of copied section.  ( thanks for the wheel :-)

// Local Prototype section

double LinearExtrapolate( double x,double x1, double y1, double x2, double y2);
double NormalizeDegrees( double Input);

// End Local ProtoTypes

fgAPDataPtr APDataGlobal;       // global variable holding the AP info
// I want this gone.  Data should be in aircraft structure


bool fgAPHeadingEnabled( void )
{
    fgAPDataPtr APData;
    
    APData = APDataGlobal;
    
    // heading hold enabled?
    return APData->heading_hold;
}

bool fgAPAltitudeEnabled( void )
{
    fgAPDataPtr APData;
    
    APData = APDataGlobal;
    
    // altitude hold or terrain follow enabled?
    return APData->altitude_hold;
}

bool fgAPTerrainFollowEnabled( void )
{
    fgAPDataPtr APData;
    
    APData = APDataGlobal;
            
    // altitude hold or terrain follow enabled?
    return APData->terrain_follow ;
}

bool fgAPAutoThrottleEnabled( void )
{
    fgAPDataPtr APData;
    
    APData = APDataGlobal;

    // autothrottle enabled?
    return APData->auto_throttle;
}

void fgAPAltitudeAdjust( double inc )
{
    // Remove at a later date
    fgAPDataPtr APData;
    APData = APDataGlobal;
    // end section

    double target_alt, target_agl;

    if ( current_options.get_units() == fgOPTIONS::FG_UNITS_FEET ) {
	target_alt = APData->TargetAltitude * METER_TO_FEET;
	target_agl = APData->TargetAGL * METER_TO_FEET;
    } else {
	target_alt = APData->TargetAltitude;
	target_agl = APData->TargetAGL;
    }

    target_alt = (int)(target_alt / inc) * inc + inc;
    target_agl = (int)(target_agl / inc) * inc + inc;

    if ( current_options.get_units() == fgOPTIONS::FG_UNITS_FEET ) {
	target_alt *= FEET_TO_METER;
	target_agl *= FEET_TO_METER;
    }

    APData->TargetAltitude = target_alt;
    APData->TargetAGL = target_agl;
}

void fgAPHeadingAdjust( double inc )
{
    fgAPDataPtr APData;
    APData = APDataGlobal;

    double target = (int)(APData->TargetHeading / inc) * inc + inc;

    APData->TargetHeading = NormalizeDegrees(target);
}

void fgAPAutoThrottleAdjust( double inc )
{
    fgAPDataPtr APData;
    APData = APDataGlobal;

    double target = (int)(APData->TargetSpeed / inc) * inc + inc;

    APData->TargetSpeed = target;
}

void fgAPReset(void)
{
    if( fgAPTerrainFollowEnabled() )
	fgAPToggleTerrainFollow( );

    if( fgAPAltitudeEnabled() )
	fgAPToggleAltitude();
		
    if( fgAPHeadingEnabled() )
	fgAPToggleHeading();
		
    if( fgAPAutoThrottleEnabled() )
	fgAPToggleAutoThrottle();
}

#define mySlider puSlider

/// These statics will eventually go into the class
/// they are just here while I am experimenting -- NHV :-)

static double MaxRollAdjust;       // MaxRollAdjust       = 2 * APData->MaxRoll;
static double RollOutAdjust;       // RollOutAdjust       = 2 * APData->RollOut;
static double MaxAileronAdjust;    // MaxAileronAdjust    = 2 * APData->MaxAileron;
static double RollOutSmoothAdjust; // RollOutSmoothAdjust = 2 * APData->RollOutSmooth;

static float MaxRollValue;         // 0.1 -> 1.0
static float RollOutValue;
static float MaxAileronValue;
static float RollOutSmoothValue;

static float TmpMaxRollValue;      // for cancel operation
static float TmpRollOutValue;
static float TmpMaxAileronValue;
static float TmpRollOutSmoothValue;

static puDialogBox     *APAdjustDialog;
static puFrame         *APAdjustFrame;
static puText          *APAdjustDialogMessage;

static int DialogX = 40;
static int DialogY = 100;

static puOneShot       *APAdjustOkButton;
static puOneShot       *APAdjustResetButton;
static puOneShot       *APAdjustCancelButton;

//static puButton        *APAdjustDragButton;

static puText          *APAdjustMaxRollTitle;
static puText          *APAdjustRollOutTitle;
static puText          *APAdjustMaxAileronTitle;
static puText          *APAdjustRollOutSmoothTitle;

static puText          *APAdjustMaxAileronText;
static puText          *APAdjustMaxRollText;
static puText          *APAdjustRollOutText;
static puText          *APAdjustRollOutSmoothText;

static mySlider        *APAdjustHS0;
static mySlider        *APAdjustHS1;
static mySlider        *APAdjustHS2;
static mySlider        *APAdjustHS3;

static char SliderText[4][8];


#define fgAP_CLAMP(val,min,max) \
( (val) = (val) > (max) ? (max) : (val) < (min) ? (min) : (val) )

    static void maxroll_adj(puObject *hs)
{
    float val ;
    fgAPDataPtr APData;
    APData = APDataGlobal;
    
    hs -> getValue ( &val ) ;
    fgAP_CLAMP ( val, 0.1, 1.0 ) ;
    //    printf ( "maxroll_adj( %p ) %f %f\n", hs, val, MaxRollAdjust * val ) ;
    APData->MaxRoll = MaxRollAdjust * val;
    sprintf(SliderText[0],"%05.2f", APData->MaxRoll );
    APAdjustMaxRollText -> setLabel ( SliderText[0] ) ;
}

static void rollout_adj(puObject *hs)
{
    float val ;
    fgAPDataPtr APData;
    APData = APDataGlobal;
    
    hs -> getValue ( &val ) ;
    fgAP_CLAMP ( val, 0.1, 1.0 ) ;
    //    printf ( "rollout_adj( %p ) %f %f\n", hs, val, RollOutAdjust * val ) ;
    APData->RollOut = RollOutAdjust * val;
    sprintf(SliderText[1],"%05.2f", APData->RollOut );
    APAdjustRollOutText -> setLabel ( SliderText[1] );
}

static void maxaileron_adj( puObject *hs )
{
    float val ;
    fgAPDataPtr APData;
    APData = APDataGlobal;
    
    hs -> getValue ( &val ) ;
    fgAP_CLAMP ( val, 0.1, 1.0 ) ;
    //    printf ( "maxaileron_adj( %p ) %f %f\n", hs, val, MaxAileronAdjust * val ) ;
    APData->MaxAileron = MaxAileronAdjust * val;
    sprintf(SliderText[3],"%05.2f", APData->MaxAileron );
    APAdjustMaxAileronText -> setLabel ( SliderText[3] );
}

static void rolloutsmooth_adj( puObject *hs )
{
    float val ;
    fgAPDataPtr APData;
    APData = APDataGlobal;
    
    hs -> getValue ( &val ) ;
    fgAP_CLAMP ( val, 0.1, 1.0 ) ;
    //    printf ( "rolloutsmooth_adj( %p ) %f %f\n", hs, val, RollOutSmoothAdjust * val ) ;
    APData->RollOutSmooth = RollOutSmoothAdjust * val;
    sprintf(SliderText[2],"%5.2f", APData->RollOutSmooth );
    APAdjustRollOutSmoothText -> setLabel ( SliderText[2] );
	
}

static void goAwayAPAdjust (puObject *)
{
    puPopLiveInterface ( ) ;
    //	puPopInterface ( ) ;
    puPopGroup ( ) ;	
    APAdjustDialog -> hide();
}

void cancelAPAdjust(puObject *)
{
    fgAPDataPtr APData  = APDataGlobal;
	
    APData->MaxRoll       = TmpMaxRollValue;
    APData->RollOut       = TmpRollOutValue;
    APData->MaxAileron    = TmpMaxAileronValue;
    APData->RollOutSmooth = TmpRollOutSmoothValue;
	
    puPopLiveInterface ( ) ;
    //	puPopInterface ( ) ;
    puPopGroup ( ) ;	

	
    APAdjustDialog -> hide();
}

void resetAPAdjust(puObject *self)
{
    fgAPDataPtr APData  = APDataGlobal;
	
    APData->MaxRoll       = MaxRollAdjust / 2;
    APData->RollOut       = RollOutAdjust / 2;
    APData->MaxAileron    = MaxAileronAdjust / 2;
    APData->RollOutSmooth = RollOutSmoothAdjust / 2;
	
    puPopLiveInterface ( ) ;
    //	puPopInterface ( ) ;
    puPopGroup ( ) ;	

    APAdjustDialog -> hide();
    fgAPAdjust(self);
}

void fgAPAdjust( puObject * )
{
    fgAPDataPtr APData  = APDataGlobal;
	
    TmpMaxRollValue        = APData->MaxRoll;
    TmpRollOutValue        = APData->RollOut;
    TmpMaxAileronValue     = APData->MaxAileron;
    TmpRollOutSmoothValue  = APData->RollOutSmooth;

    MaxRollValue        = APData->MaxRoll       / MaxRollAdjust;
    RollOutValue        = APData->RollOut       / RollOutAdjust;
    MaxAileronValue     = APData->MaxAileron    / MaxAileronAdjust;
    RollOutSmoothValue  = APData->RollOutSmooth / RollOutSmoothAdjust;

    APAdjustHS0 -> setValue ( MaxRollValue ) ;
    APAdjustHS1 -> setValue ( RollOutValue ) ;
    APAdjustHS2 -> setValue ( RollOutSmoothValue ) ;
    APAdjustHS3 -> setValue ( MaxAileronValue ) ;
	
    //	puPushInterface     ( APAdjustDialog ) ;
    puPushGroup     ( APAdjustDialog ) ;	
    puPushLiveInterface ( APAdjustDialog ) ;
	
    APAdjustDialog -> reveal();
}

#ifdef NOT_USED
void dragAPAdjust(puObject *self)
{

    return;
			
    int LastX, LastY;
    int DeltaX, DeltaY, X, Y;
    int button,  count;

    //	button = guiGetMouseButton();
    guiGetMouse(&LastX, &LastY);

    button = 0;
    printf("button %d  LastX %d  LastY %d\n", button, LastX, LastY);

    //	count = 0;
    //	while( guiGetMouseButton() == button )
    for(count=0;count<1000;count++)
	{
	    guiGetMouse(&X, &Y);
	    printf("    X %d  Y %d\n", X, Y);
	}
	
    DeltaX = X - LastX;
    DeltaY = Y - LastY;

    // LiveInterface is Dialog Box
    puInterface *Interface = puGetBaseLiveInterface();
    puObject *dlist = Interface->getFirstChild() ;
    //	printf("%s %p  %s %p\n",
    //		   Interface->getTypeString(), Interface, dlist->getTypeString(), dlist);
	
    Interface -> getPosition ( &X, &Y ) ;
    Interface -> setPosition ( X + DeltaX, Y + DeltaY ) ;
    //	Interface -> recalc_bbox();
	
    for ( puObject *bo = dlist ; bo != NULL ; bo = bo->next )
	{
	    bo -> getPosition ( &X, &Y ) ;
	    bo -> setPosition ( X + DeltaX, Y + DeltaY ) ;
	    bo -> recalc_bbox();
	    //		printf("%s %p  X %d  Y %d\n", bo->getTypeString(), bo, X, Y);
	}

    Interface->recalc_bbox();
	
    puPopLiveInterface ( ) ;
    //	puPopInterface ( ) ;
    puPopGroup ( ) ;	

    APAdjustDialog -> hide();
    fgAPAdjust(dlist);
}
#endif // NOT_USED

// Done once at system initialization
void fgAPAdjustInit( void )
{
#define HORIZONTAL  FALSE

    fgAPDataPtr APData  = APDataGlobal;
	
    TmpMaxRollValue        = APData->MaxRoll;
    TmpRollOutValue        = APData->RollOut;
    TmpMaxAileronValue     = APData->MaxAileron;
    TmpRollOutSmoothValue  = APData->RollOutSmooth;

    MaxRollValue        = APData->MaxRoll       / MaxRollAdjust;
    RollOutValue        = APData->RollOut       / RollOutAdjust;
    MaxAileronValue     = APData->MaxAileron    / MaxAileronAdjust;
    RollOutSmoothValue  = APData->RollOutSmooth / RollOutSmoothAdjust;

    APAdjustDialog = new puDialogBox (DialogX, DialogY);
    {
        int horiz_slider_height =  puGetStringHeight () +
	    puGetStringDescender () +
	    PUSTR_TGAP + PUSTR_BGAP+5;

        APAdjustFrame  =  new puFrame (0,0,230, 85+4*horiz_slider_height);
		
        APAdjustDialogMessage =  new puText   (40, 52 + 4*horiz_slider_height);
        APAdjustDialogMessage ->     setLabel ("AutoPilot Adjust");
        
	//        APAdjustDragButton =  new puButton    (0, 75+4*horiz_slider_height,
	//											   230, 85+4*horiz_slider_height);
	//        APAdjustDragButton -> setLegend   ("*");
	//        APAdjustDragButton -> setCallback (dragAPAdjust);
		
        int slider_y = 55;
        APAdjustHS0  =   new mySlider    ( 10, slider_y, 210, HORIZONTAL ) ;
        APAdjustHS0  ->      setDelta    ( 0.1 ) ;
        APAdjustHS0  ->      setValue    ( MaxRollValue ) ;
        APAdjustHS0  ->      setCBMode   ( PUSLIDER_DELTA ) ;
        APAdjustHS0  ->      setCallback ( maxroll_adj ) ;
		
	sprintf(SliderText[0],"%05.2f", APData->MaxRoll );
        APAdjustMaxRollTitle =  new puText   ( 15, slider_y ) ;
        APAdjustMaxRollTitle ->     setLabel ( "MaxRoll" ) ;
	APAdjustMaxRollText  =  new puText   ( 160, slider_y ) ;
	APAdjustMaxRollText  ->     setLabel ( SliderText[0] ) ;

        slider_y += horiz_slider_height;
        
        APAdjustHS1  = new mySlider    ( 10, slider_y, 210, HORIZONTAL ) ;
        APAdjustHS1  ->    setDelta    ( 0.1 ) ;
        APAdjustHS1  ->    setValue    ( RollOutValue ) ;
        APAdjustHS1  ->    setCBMode   ( PUSLIDER_DELTA ) ;
        APAdjustHS1  ->    setCallback ( rollout_adj ) ;

	sprintf(SliderText[1],"%05.2f", APData->RollOut );
        APAdjustRollOutTitle = new puText   ( 15, slider_y ) ;
        APAdjustRollOutTitle ->    setLabel ( "AdjustRollOut" ) ;
	APAdjustRollOutText  = new puText   ( 160, slider_y ) ;
	APAdjustRollOutText  ->    setLabel ( SliderText[1] );
	
        slider_y += horiz_slider_height;
        
        APAdjustHS2  = new mySlider    ( 10, slider_y, 210, HORIZONTAL ) ;
        APAdjustHS2  ->    setDelta    ( 0.1 ) ;
        APAdjustHS2  ->    setValue    ( RollOutSmoothValue ) ;
        APAdjustHS2  ->    setCBMode   ( PUSLIDER_DELTA ) ;
        APAdjustHS2  ->    setCallback ( rolloutsmooth_adj ) ;
		
	sprintf(SliderText[2],"%5.2f", APData->RollOutSmooth );
        APAdjustRollOutSmoothTitle = new puText   ( 15, slider_y ) ;
        APAdjustRollOutSmoothTitle ->    setLabel ( "RollOutSmooth" ) ;
	APAdjustRollOutSmoothText  = new puText   ( 160, slider_y ) ;
	APAdjustRollOutSmoothText  ->    setLabel ( SliderText[2] );

        slider_y += horiz_slider_height;
        
        APAdjustHS3  = new mySlider    ( 10, slider_y, 210, HORIZONTAL ) ;
        APAdjustHS3  ->    setDelta    ( 0.1 ) ;
        APAdjustHS3  ->    setValue    ( MaxAileronValue ) ;
        APAdjustHS3  ->    setCBMode   ( PUSLIDER_DELTA ) ;
        APAdjustHS3  ->    setCallback ( maxaileron_adj ) ;
        
	sprintf(SliderText[3],"%05.2f", APData->MaxAileron );
        APAdjustMaxAileronTitle = new puText   ( 15, slider_y ) ;
        APAdjustMaxAileronTitle ->    setLabel ( "MaxAileron" ) ;
	APAdjustMaxAileronText =  new puText   ( 160, slider_y ) ;
	APAdjustMaxAileronText ->     setLabel ( SliderText[3] );
	
        APAdjustOkButton =  new puOneShot         (10, 10, 60, 50);
        APAdjustOkButton ->     setLegend         ("OK");
        APAdjustOkButton ->     makeReturnDefault (TRUE );
        APAdjustOkButton ->     setCallback       (goAwayAPAdjust);
		
        APAdjustCancelButton =  new puOneShot         (70, 10, 150, 50);
        APAdjustCancelButton ->     setLegend         ("Cancel");
        APAdjustCancelButton ->     makeReturnDefault (TRUE );
        APAdjustCancelButton ->     setCallback       (cancelAPAdjust);
		
        APAdjustResetButton =  new puOneShot         (160, 10, 220, 50);
        APAdjustResetButton ->     setLegend         ("Reset");
        APAdjustResetButton ->     makeReturnDefault (TRUE );
        APAdjustResetButton ->     setCallback       (resetAPAdjust);
    }
    APAdjustDialog -> close();
    //  APAdjustDialog -> reveal();
    // Take it off the Stack 
    puPopLiveInterface ( ) ;
    //	puPopInterface ( ) ;
    puPopGroup ( ) ;	
#undef HORIZONTAL
}

void fgAPInit( fgAIRCRAFT *current_aircraft )
{
    fgAPDataPtr APData ;

    FG_LOG( FG_AUTOPILOT, FG_INFO, "Init AutoPilot Subsystem" );

    APData  = (fgAPDataPtr)calloc(sizeof(fgAPData),1);
    
    if (APData == NULL) {
	// I couldn't get the mem.  Dying
	FG_LOG( FG_AUTOPILOT, FG_ALERT, "No ram for Autopilot. Dying.");
	exit(-1);
    }
    
    APData->heading_hold = false ;     // turn the heading hold off
    APData->altitude_hold = false ;    // turn the altitude hold off

    APData->TargetHeading = 0.0;    // default direction, due north
    APData->TargetAltitude = 3000;  // default altitude in meters
    APData->alt_error_accum = 0.0;

    // These eventually need to be read from current_aircaft somehow.

#if 0
    // Original values
    // the maximum roll, in Deg
    APData->MaxRoll = 7;
    // the deg from heading to start rolling out at, in Deg
    APData->RollOut = 30;
    // how far can I move the aleron from center.
    APData->MaxAileron= .1;
    // Smoothing distance for alerion control
    APData->RollOutSmooth = 10;
#endif

    // the maximum roll, in Deg
    APData->MaxRoll = 20;

    // the deg from heading to start rolling out at, in Deg
    APData->RollOut = 20;

    // how far can I move the aleron from center.
    APData->MaxAileron= .2;

    // Smoothing distance for alerion control
    APData->RollOutSmooth = 10;

    //Remove at a later date
    APDataGlobal = APData;
    
#if !defined( USING_SLIDER_CLASS )
    MaxRollAdjust       = 2 * APData->MaxRoll;
    RollOutAdjust       = 2 * APData->RollOut;
    MaxAileronAdjust    = 2 * APData->MaxAileron;
    RollOutSmoothAdjust = 2 * APData->RollOutSmooth;
#endif // !defined( USING_SLIDER_CLASS )

    fgAPAdjustInit( ) ;
};

int fgAPRun( void )
{
    // Remove the following lines when the calling funcitons start
    // passing in the data pointer

    fgAPDataPtr APData;
    
    APData = APDataGlobal;
    // end section
    
    // heading hold enabled?
    if ( APData->heading_hold == true ) {
	double RelHeading;
	double TargetRoll;
	double RelRoll;
	double AileronSet;
	
	RelHeading =  
	    NormalizeDegrees( APData->TargetHeading - fgAPget_heading());
	// figure out how far off we are from desired heading
	
	// Now it is time to deterime how far we should be rolled.
	FG_LOG( FG_AUTOPILOT, FG_DEBUG, "RelHeading: " << RelHeading );
	
	
	// Check if we are further from heading than the roll out point
	if ( fabs(RelHeading) > APData->RollOut ) {
	    // set Target Roll to Max in desired direction
	    if (RelHeading < 0 ) {
		TargetRoll = 0-APData->MaxRoll;
	    } else {
		TargetRoll = APData->MaxRoll;
	    }
	} else {
	    // We have to calculate the Target roll

	    // This calculation engine thinks that the Target roll
	    // should be a line from (RollOut,MaxRoll) to (-RollOut,
	    // -MaxRoll) I hope this works well.  If I get ambitious
	    // some day this might become a fancier curve or
	    // something.

	    TargetRoll = LinearExtrapolate( RelHeading, -APData->RollOut,
					    -APData->MaxRoll, APData->RollOut,
					    APData->MaxRoll );
	}
	
	// Target Roll has now been Found.

	// Compare Target roll to Current Roll, Generate Rel Roll

	FG_LOG( FG_COCKPIT, FG_BULK, "TargetRoll: " << TargetRoll );
	
	RelRoll = NormalizeDegrees(TargetRoll - fgAPget_roll());

	// Check if we are further from heading than the roll out smooth point
	if ( fabs(RelRoll) > APData->RollOutSmooth ) {
	    // set Target Roll to Max in desired direction
	    if (RelRoll < 0 ) {
		AileronSet = 0-APData->MaxAileron;
	    } else {
		AileronSet = APData->MaxAileron;
	    }
	} else {
	    AileronSet = LinearExtrapolate( RelRoll, -APData->RollOutSmooth,
					    -APData->MaxAileron,
					    APData->RollOutSmooth,
					    APData->MaxAileron );
	}
	
	controls.set_aileron( AileronSet );
	controls.set_rudder( 0.0 );
    }

    // altitude hold or terrain follow enabled?
    if ( APData->altitude_hold || APData->terrain_follow ) {
	double speed, max_climb, error;
	double prop_error, int_error;
	double prop_adj, int_adj, total_adj;

	if ( APData->altitude_hold ) {
	    // normal altitude hold
	    APData->TargetClimbRate = 
		(APData->TargetAltitude - fgAPget_altitude()) * 8.0;
	} else if ( APData->terrain_follow ) {
	    // brain dead ground hugging with no look ahead
	    APData->TargetClimbRate = 
		( APData->TargetAGL - fgAPget_agl() ) * 16.0;
	} else {
	    // just try to zero out rate of climb ...
	    APData->TargetClimbRate = 0.0;
	}

	speed = get_speed();

	if ( speed < 90.0 ) {
	    max_climb = 0.0;
	} else if ( speed < 100.0 ) {
	    max_climb = (speed - 90.0) * 20;
	} else {
	    max_climb = ( speed - 100.0 ) * 4.0 + 200.0;
	}

	if ( APData->TargetClimbRate > max_climb ) {
	    APData->TargetClimbRate = max_climb;
	}

	if ( APData->TargetClimbRate < -400.0 ) {
	    APData->TargetClimbRate = -400.0;
	}

	error = fgAPget_climb() - APData->TargetClimbRate;

	// accumulate the error under the curve ... this really should
	// be *= delta t
	APData->alt_error_accum += error;

	// calculate integral error, and adjustment amount
	int_error = APData->alt_error_accum;
	// printf("error = %.2f  int_error = %.2f\n", error, int_error);
	int_adj = int_error / 8000.0;
	
	// caclulate proportional error
	prop_error = error;
	prop_adj = prop_error / 2000.0;

	total_adj = 0.9 * prop_adj + 0.1 * int_adj;
	if ( total_adj >  0.6 ) { total_adj =  0.6; }
	if ( total_adj < -0.2 ) { total_adj = -0.2; }

	controls.set_elevator( total_adj );
    }

    // auto throttle enabled?
    if ( APData->auto_throttle ) {
	double error;
	double prop_error, int_error;
	double prop_adj, int_adj, total_adj;

	error = APData->TargetSpeed - get_speed();

	// accumulate the error under the curve ... this really should
	// be *= delta t
	APData->speed_error_accum += error;
	if ( APData->speed_error_accum > 2000.0 ) {
	    APData->speed_error_accum = 2000.0;
	}
	if ( APData->speed_error_accum < -2000.0 ) {
	    APData->speed_error_accum = -2000.0;
	}

	// calculate integral error, and adjustment amount
	int_error = APData->speed_error_accum;

	// printf("error = %.2f  int_error = %.2f\n", error, int_error);
	int_adj = int_error / 200.0;
	
	// caclulate proportional error
	prop_error = error;
	prop_adj = 0.5 + prop_error / 50.0;

	total_adj = 0.9 * prop_adj + 0.1 * int_adj;
	if ( total_adj > 1.0 ) { total_adj = 1.0; }
	if ( total_adj < 0.0 ) { total_adj = 0.0; }

	controls.set_throttle( FGControls::ALL_ENGINES, total_adj );
    }

    /*
    if (APData->Mode == 2) // Glide slope hold
    {
	double RelSlope;
	double RelElevator;
		
	// First, calculate Relative slope and normalize it
	RelSlope = NormalizeDegrees( APData->TargetSlope - get_pitch());
	
	// Now calculate the elevator offset from current angle
	if ( abs(RelSlope) > APData->SlopeSmooth )
	{
	    if ( RelSlope < 0 )		//  set RelElevator to max in the correct direction
		RelElevator = -APData->MaxElevator;
	    else
		RelElevator = APData->MaxElevator;
	}
		
	else
	    RelElevator = LinearExtrapolate(RelSlope,-APData->SlopeSmooth,-APData->MaxElevator,APData->SlopeSmooth,APData->MaxElevator);
		
	// set the elevator
	fgElevMove(RelElevator);
		
    }
    */
    
    // Ok, we are done
    return 0;

}

/*
void fgAPSetMode( int mode)
{
    //Remove the following line when the calling funcitons start passing in the data pointer
    fgAPDataPtr APData;
    
    APData = APDataGlobal;
    // end section
    
    fgPrintf( FG_COCKPIT, FG_INFO, "APSetMode : %d\n", mode );
    
    APData->Mode = mode;  // set the new mode
}
*/

void fgAPToggleHeading( void )
{
    // Remove at a later date
    fgAPDataPtr APData;

    APData = APDataGlobal;
    // end section

    if ( APData->heading_hold ) {
	// turn off heading hold
	APData->heading_hold = false;
    } else {
	// turn on heading hold, lock at current heading
	APData->heading_hold = true;
	APData->TargetHeading = fgAPget_heading();
    }

    FG_LOG( FG_COCKPIT, FG_INFO, " fgAPSetHeading: (" 
	    << APData->heading_hold << ") " << APData->TargetHeading );
}


void fgAPToggleAltitude( void )
{
    // Remove at a later date
    fgAPDataPtr APData;

    APData = APDataGlobal;
    // end section

    if ( APData->altitude_hold ) {
	// turn off altitude hold
	APData->altitude_hold = false;
    } else {
	// turn on altitude hold, lock at current altitude
	APData->altitude_hold = true;
	APData->terrain_follow = false;
	APData->TargetAltitude = fgAPget_altitude();
	APData->alt_error_accum = 0.0;
	// alt_error_queue.erase( alt_error_queue.begin(), 
	//                        alt_error_queue.end() );
    }

    FG_LOG( FG_COCKPIT, FG_INFO, " fgAPSetAltitude: (" 
	    << APData->altitude_hold << ") " << APData->TargetAltitude );
}


void fgAPToggleAutoThrottle ( void )
{
    // Remove at a later date
    fgAPDataPtr APData;

    APData = APDataGlobal;
    // end section

    if ( APData->auto_throttle ) {
	// turn off altitude hold
	APData->auto_throttle = false;
    } else {
	// turn on terrain follow, lock at current agl
	APData->auto_throttle = true;
	APData->TargetSpeed = get_speed();
	APData->speed_error_accum = 0.0;
    }

    FG_LOG( FG_COCKPIT, FG_INFO, " fgAPSetAutoThrottle: (" 
	    << APData->auto_throttle << ") " << APData->TargetSpeed );
}

void fgAPToggleTerrainFollow( void )
{
    // Remove at a later date
    fgAPDataPtr APData;

    APData = APDataGlobal;
    // end section

    if ( APData->terrain_follow ) {
	// turn off altitude hold
	APData->terrain_follow = false;
    } else {
	// turn on terrain follow, lock at current agl
	APData->terrain_follow = true;
	APData->altitude_hold = false;
	APData->TargetAGL = fgAPget_agl();
	APData->alt_error_accum = 0.0;
    }

    FG_LOG( FG_COCKPIT, FG_INFO, " fgAPSetTerrainFollow: ("
	    << APData->terrain_follow << ") " << APData->TargetAGL );
}

double LinearExtrapolate( double x,double x1,double y1,double x2,double y2)
{
    // This procedure extrapolates the y value for the x posistion on a line defined by x1,y1; x2,y2
    //assert(x1 != x2); // Divide by zero error.  Cold abort for now
    
    double m, b, y; 	// the constants to find in y=mx+b
    m=(y2-y1)/(x2-x1);	// calculate the m
    b= y1- m * x1;	// calculate the b
    y = m * x + b;	// the final calculation
	
    return y;
};

double NormalizeDegrees(double Input)
{
    // normalize the input to the range (-180,180]
    // Input should not be greater than -360 to 360.  Current rules send the output to an undefined state.
    if (Input > 180)
	Input -= 360;
    if (Input <= -180)
	Input += 360;
    
    return (Input);
};