/***************************************************************************

	TITLE:	ls_step
	
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	FUNCTION:	Integration routine for equations of motion 
			(vehicle states)

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	MODULE STATUS:	developmental

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	GENEALOGY:	Written 920802 by Bruce Jackson.  Based upon equations
			given in reference [1] and a Matrix-X/System Build block
			diagram model of equations of motion coded by David Raney
			at NASA-Langley in June of 1992.

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	DESIGNED BY:	Bruce Jackson
	
	CODED BY:	Bruce Jackson
	
	MAINTAINED BY:	

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	MODIFICATION HISTORY:
	
	DATE	PURPOSE						BY

	921223  Modified calculation of Phi and Psi to use the "atan2" routine
	        rather than the "atan" to allow full circular angles.
		"atan" limits to +/- pi/2.                      EBJ
		
	940111	Changed from oldstyle include file ls_eom.h; also changed
		from DATA to SCALAR type.			EBJ

	950207  Initialized Alpha_dot and Beta_dot to zero on first pass; calculated
		thereafter.					EBJ

	950224	Added logic to avoid adding additional increment to V_east
		in case V_east already accounts for rotating earth. 
								EBJ

	CURRENT RCS HEADER:

$Header$
$Log$
Revision 1.3  1998/07/12 03:11:04  curt
Removed some printf()'s.
Fixed the autopilot integration so it should be able to update it's control
  positions every time the internal flight model loop is run, and not just
  once per rendered frame.
Added a routine to do the necessary stuff to force an arbitrary altitude
  change.
Gave the Navion engine just a tad more power.

Revision 1.2  1998/01/19 18:40:28  curt
Tons of little changes to clean up the code and to remove fatal errors
when building with the c++ compiler.

Revision 1.1  1997/05/29 00:09:59  curt
Initial Flight Gear revision.

 * Revision 1.5  1995/03/02  20:24:13  bjax
 * Added logic to avoid adding additional increment to V_east
 * in case V_east already accounts for rotating earth. EBJ
 *
 * Revision 1.4  1995/02/07  20:52:21  bjax
 * Added initialization of Alpha_dot and Beta_dot to zero on first
 * pass; they get calculated by ls_aux on next pass...  EBJ
 *
 * Revision 1.3  1994/01/11  19:01:12  bjax
 * Changed from DATA to SCALAR type; also fixed header files (was ls_eom.h)
 *
 * Revision 1.2  1993/06/02  15:03:09  bjax
 * Moved initialization of geocentric position to subroutine ls_geod_to_geoc.
 *
 * Revision 1.1  92/12/30  13:16:11  bjax
 * Initial revision
 * 

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	REFERENCES:
	
		[ 1]	McFarland, Richard E.: "A Standard Kinematic Model
			for Flight Simulation at NASA-Ames", NASA CR-2497,
			January 1975
			 
		[ 2]	ANSI/AIAA R-004-1992 "Recommended Practice: Atmos-
			pheric and Space Flight Vehicle Coordinate Systems",
			February 1992
			

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	CALLED BY:

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	CALLS TO:	None.

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	INPUTS:	State derivatives

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	OUTPUTS:	States

--------------------------------------------------------------------------*/

#include "ls_types.h"
#include "ls_constants.h"
#include "ls_generic.h"
#include "ls_accel.h"
#include "ls_aux.h"
#include "ls_model.h"
#include "ls_step.h"
#include "ls_geodesy.h"
#include "ls_gravity.h"
/* #include "ls_sim_control.h" */
#include <math.h>

extern SCALAR Simtime;		/* defined in ls_main.c */

void ls_step( SCALAR dt, int Initialize ) {
	static	int	inited = 0;
		SCALAR	dth;
	static	SCALAR	v_dot_north_past, v_dot_east_past, v_dot_down_past;
	static	SCALAR	latitude_dot_past, longitude_dot_past, radius_dot_past;
	static	SCALAR	p_dot_body_past, q_dot_body_past, r_dot_body_past;
		SCALAR	p_local_in_body, q_local_in_body, r_local_in_body;
		SCALAR	epsilon, inv_eps, local_gnd_veast;
		SCALAR	e_dot_0, e_dot_1, e_dot_2, e_dot_3;
	static	SCALAR	e_0, e_1, e_2, e_3;
	static	SCALAR	e_dot_0_past, e_dot_1_past, e_dot_2_past, e_dot_3_past;

/*  I N I T I A L I Z A T I O N   */


	if ( (inited == 0) || (Initialize != 0) )
	{
/* Set past values to zero */
	v_dot_north_past = v_dot_east_past = v_dot_down_past      = 0;
	latitude_dot_past = longitude_dot_past = radius_dot_past  = 0;
	p_dot_body_past = q_dot_body_past = r_dot_body_past       = 0;
	e_dot_0_past = e_dot_1_past = e_dot_2_past = e_dot_3_past = 0;
	
/* Initialize geocentric position from geodetic latitude and altitude */

	ls_geod_to_geoc( Latitude, Altitude, &Sea_level_radius, &Lat_geocentric);
	Earth_position_angle = 0;
	Lon_geocentric = Longitude;
	Radius_to_vehicle = Altitude + Sea_level_radius;

/* Correct eastward velocity to account for earths' rotation, if necessary */

	local_gnd_veast = OMEGA_EARTH*Sea_level_radius*cos(Lat_geocentric);
	if( fabs(V_east - V_east_rel_ground) < 0.8*local_gnd_veast )
    	V_east = V_east + local_gnd_veast;

/* Initialize quaternions and transformation matrix from Euler angles */

	    e_0 = cos(Psi*0.5)*cos(Theta*0.5)*cos(Phi*0.5) 
		+ sin(Psi*0.5)*sin(Theta*0.5)*sin(Phi*0.5);
	    e_1 = cos(Psi*0.5)*cos(Theta*0.5)*sin(Phi*0.5) 
		- sin(Psi*0.5)*sin(Theta*0.5)*cos(Phi*0.5);
	    e_2 = cos(Psi*0.5)*sin(Theta*0.5)*cos(Phi*0.5) 
		+ sin(Psi*0.5)*cos(Theta*0.5)*sin(Phi*0.5);
	    e_3 =-cos(Psi*0.5)*sin(Theta*0.5)*sin(Phi*0.5) 
		+ sin(Psi*0.5)*cos(Theta*0.5)*cos(Phi*0.5);
	    T_local_to_body_11 = e_0*e_0 + e_1*e_1 - e_2*e_2 - e_3*e_3;
	    T_local_to_body_12 = 2*(e_1*e_2 + e_0*e_3);
	    T_local_to_body_13 = 2*(e_1*e_3 - e_0*e_2);
	    T_local_to_body_21 = 2*(e_1*e_2 - e_0*e_3);
	    T_local_to_body_22 = e_0*e_0 - e_1*e_1 + e_2*e_2 - e_3*e_3;
	    T_local_to_body_23 = 2*(e_2*e_3 + e_0*e_1);
	    T_local_to_body_31 = 2*(e_1*e_3 + e_0*e_2);
	    T_local_to_body_32 = 2*(e_2*e_3 - e_0*e_1);
	    T_local_to_body_33 = e_0*e_0 - e_1*e_1 - e_2*e_2 + e_3*e_3;

/*	Calculate local gravitation acceleration	*/

		ls_gravity( Radius_to_vehicle, Lat_geocentric, &Gravity );

/*	Initialize vehicle model 			*/

		ls_aux();
		ls_model(0.0, 0);

/* 	Calculate initial accelerations */

		ls_accel();
		
/* Initialize auxiliary variables */

		ls_aux();
		Alpha_dot = 0.;
		Beta_dot = 0.;

/* set flag; disable integrators */

		inited = -1;
		dt = 0;
		
	}

/* Update time */

	dth = 0.5*dt;
	Simtime = Simtime + dt;

/*  L I N E A R   V E L O C I T I E S   */

/* Integrate linear accelerations to get velocities */
/*    Using predictive Adams-Bashford algorithm     */

    V_north = V_north + dth*(3*V_dot_north - v_dot_north_past);
    V_east  = V_east  + dth*(3*V_dot_east  - v_dot_east_past );
    V_down  = V_down  + dth*(3*V_dot_down  - v_dot_down_past );
    
/* record past states */

    v_dot_north_past = V_dot_north;
    v_dot_east_past  = V_dot_east;
    v_dot_down_past  = V_dot_down;
    
/* Calculate trajectory rate (geocentric coordinates) */

    if (cos(Lat_geocentric) != 0)
    	Longitude_dot = V_east/(Radius_to_vehicle*cos(Lat_geocentric));
    	
    Latitude_dot = V_north/Radius_to_vehicle;
    Radius_dot = -V_down;
    	
/*  A N G U L A R   V E L O C I T I E S   A N D   P O S I T I O N S  */
    
/* Integrate rotational accelerations to get velocities */

    P_body = P_body + dth*(3*P_dot_body - p_dot_body_past);
    Q_body = Q_body + dth*(3*Q_dot_body - q_dot_body_past);
    R_body = R_body + dth*(3*R_dot_body - r_dot_body_past);

/* Save past states */

    p_dot_body_past = P_dot_body;
    q_dot_body_past = Q_dot_body;
    r_dot_body_past = R_dot_body;
    
/* Calculate local axis frame rates due to travel over curved earth */

    P_local =  V_east/Radius_to_vehicle;
    Q_local = -V_north/Radius_to_vehicle;
    R_local = -V_east*tan(Lat_geocentric)/Radius_to_vehicle;
    
/* Transform local axis frame rates to body axis rates */

    p_local_in_body = T_local_to_body_11*P_local + T_local_to_body_12*Q_local + T_local_to_body_13*R_local;
    q_local_in_body = T_local_to_body_21*P_local + T_local_to_body_22*Q_local + T_local_to_body_23*R_local;
    r_local_in_body = T_local_to_body_31*P_local + T_local_to_body_32*Q_local + T_local_to_body_33*R_local;
    
/* Calculate total angular rates in body axis */

    P_total = P_body - p_local_in_body;
    Q_total = Q_body - q_local_in_body;
    R_total = R_body - r_local_in_body;
    
/* Transform to quaternion rates (see Appendix E in [2]) */

    e_dot_0 = 0.5*( -P_total*e_1 - Q_total*e_2 - R_total*e_3 );
    e_dot_1 = 0.5*(  P_total*e_0 - Q_total*e_3 + R_total*e_2 );
    e_dot_2 = 0.5*(  P_total*e_3 + Q_total*e_0 - R_total*e_1 );
    e_dot_3 = 0.5*( -P_total*e_2 + Q_total*e_1 + R_total*e_0 );

/* Integrate using trapezoidal as before */

	e_0 = e_0 + dth*(e_dot_0 + e_dot_0_past);
	e_1 = e_1 + dth*(e_dot_1 + e_dot_1_past);
	e_2 = e_2 + dth*(e_dot_2 + e_dot_2_past);
	e_3 = e_3 + dth*(e_dot_3 + e_dot_3_past);
	
/* calculate orthagonality correction  - scale quaternion to unity length */
	
	epsilon = sqrt(e_0*e_0 + e_1*e_1 + e_2*e_2 + e_3*e_3);
	inv_eps = 1/epsilon;
	
	e_0 = inv_eps*e_0;
	e_1 = inv_eps*e_1;
	e_2 = inv_eps*e_2;
	e_3 = inv_eps*e_3;

/* Save past values */

	e_dot_0_past = e_dot_0;
	e_dot_1_past = e_dot_1;
	e_dot_2_past = e_dot_2;
	e_dot_3_past = e_dot_3;
	
/* Update local to body transformation matrix */

	T_local_to_body_11 = e_0*e_0 + e_1*e_1 - e_2*e_2 - e_3*e_3;
	T_local_to_body_12 = 2*(e_1*e_2 + e_0*e_3);
	T_local_to_body_13 = 2*(e_1*e_3 - e_0*e_2);
	T_local_to_body_21 = 2*(e_1*e_2 - e_0*e_3);
	T_local_to_body_22 = e_0*e_0 - e_1*e_1 + e_2*e_2 - e_3*e_3;
	T_local_to_body_23 = 2*(e_2*e_3 + e_0*e_1);
	T_local_to_body_31 = 2*(e_1*e_3 + e_0*e_2);
	T_local_to_body_32 = 2*(e_2*e_3 - e_0*e_1);
	T_local_to_body_33 = e_0*e_0 - e_1*e_1 - e_2*e_2 + e_3*e_3;
	
/* Calculate Euler angles */

	Theta = asin( -T_local_to_body_13 );

	if( T_local_to_body_11 == 0 )
	Psi = 0;
	else
	Psi = atan2( T_local_to_body_12, T_local_to_body_11 );

	if( T_local_to_body_33 == 0 )
	Phi = 0;
	else
	Phi = atan2( T_local_to_body_23, T_local_to_body_33 );

/* Resolve Psi to 0 - 359.9999 */

	if (Psi < 0 ) Psi = Psi + 2*PI;

/*  L I N E A R   P O S I T I O N S   */

/* Trapezoidal acceleration for position */

	Lat_geocentric    = Lat_geocentric    + dth*(Latitude_dot  + latitude_dot_past );
	Lon_geocentric    = Lon_geocentric    + dth*(Longitude_dot + longitude_dot_past);
	Radius_to_vehicle = Radius_to_vehicle + dth*(Radius_dot    + radius_dot_past );
	Earth_position_angle = Earth_position_angle + dt*OMEGA_EARTH;
	
/* Save past values */

	latitude_dot_past  = Latitude_dot;
	longitude_dot_past = Longitude_dot;
	radius_dot_past    = Radius_dot;
	
/* end of ls_step */
}
/*************************************************************************/