1 /**************************************************************************
2 * fg_geodesy.c -- routines to convert between geodetic and geocentric
5 * Copied and adapted directly from LaRCsim/ls_geodesy.c
7 * See below for the complete original LaRCsim comments.
10 * (Log is kept at end of this file)
11 **************************************************************************/
16 #include "fg_geodesy.h"
17 #include "../constants.h"
20 /* ONE_SECOND is pi/180/60/60, or about 100 feet at earths' equator */
21 #define ONE_SECOND 4.848136811E-6
24 /* fgGeocToGeod(lat_geoc, radius, *lat_geod, *alt, *sea_level_r)
26 * lat_geoc Geocentric latitude, radians, + = North
27 * radius C.G. radius to earth center, ft
30 * lat_geod Geodetic latitude, radians, + = North
31 * alt C.G. altitude above mean sea level, ft
32 * sea_level_r radius from earth center to sea level at
33 * local vertical (surface normal) of C.G.
36 void fgGeocToGeod( double lat_geoc, double radius, double
37 *lat_geod, double *alt, double *sea_level_r )
39 double t_lat, x_alpha, mu_alpha, delt_mu, r_alpha, l_point, rho_alpha;
40 double sin_mu_a, denom,delt_lambda, lambda_sl, sin_lambda_sl;
42 if( ( (FG_PI_2 - lat_geoc) < ONE_SECOND ) /* near North pole */
43 || ( (FG_PI_2 + lat_geoc) < ONE_SECOND ) ) /* near South pole */
46 *sea_level_r = EQUATORIAL_RADIUS_KM*E;
47 *alt = radius - *sea_level_r;
49 t_lat = tan(lat_geoc);
50 x_alpha = E*EQUATORIAL_RADIUS_KM/sqrt(t_lat*t_lat + E*E);
51 mu_alpha = atan2(sqrt(RESQ_KM - x_alpha*x_alpha),E*x_alpha);
52 if (lat_geoc < 0) mu_alpha = - mu_alpha;
53 sin_mu_a = sin(mu_alpha);
54 delt_lambda = mu_alpha - lat_geoc;
55 r_alpha = x_alpha/cos(lat_geoc);
56 l_point = radius - r_alpha;
57 *alt = l_point*cos(delt_lambda);
58 denom = sqrt(1-EPS*EPS*sin_mu_a*sin_mu_a);
59 rho_alpha = EQUATORIAL_RADIUS_KM*(1-EPS)/
61 delt_mu = atan2(l_point*sin(delt_lambda),rho_alpha + *alt);
62 *lat_geod = mu_alpha - delt_mu;
63 lambda_sl = atan( E*E * tan(*lat_geod) ); /* SL geoc. latitude */
64 sin_lambda_sl = sin( lambda_sl );
66 sqrt(RESQ_KM / (1 + ((1/(E*E))-1)*sin_lambda_sl*sin_lambda_sl));
71 /* fgGeodToGeoc( lat_geod, alt, *sl_radius, *lat_geoc )
73 * lat_geod Geodetic latitude, radians, + = North
74 * alt C.G. altitude above mean sea level, ft
77 * sl_radius SEA LEVEL radius to earth center, ft (add Altitude to
78 * get true distance from earth center.
79 * lat_geoc Geocentric latitude, radians, + = North
83 void fgGeodToGeoc( double lat_geod, double alt, double *sl_radius,
86 double lambda_sl, sin_lambda_sl, cos_lambda_sl, sin_mu, cos_mu, px, py;
88 lambda_sl = atan( E*E * tan(lat_geod) ); /* sea level geocentric latitude */
89 sin_lambda_sl = sin( lambda_sl );
90 cos_lambda_sl = cos( lambda_sl );
91 sin_mu = sin(lat_geod); /* Geodetic (map makers') latitude */
92 cos_mu = cos(lat_geod);
94 sqrt(RESQ_KM / (1 + ((1/(E*E))-1)*sin_lambda_sl*sin_lambda_sl));
95 py = *sl_radius*sin_lambda_sl + alt*sin_mu;
96 px = *sl_radius*cos_lambda_sl + alt*cos_mu;
97 *lat_geoc = atan2( py, px );
101 /***************************************************************************
105 ----------------------------------------------------------------------------
107 FUNCTION: Converts geocentric coordinates to geodetic positions
109 ----------------------------------------------------------------------------
111 MODULE STATUS: developmental
113 ----------------------------------------------------------------------------
115 GENEALOGY: Written as part of LaRCSim project by E. B. Jackson
117 ----------------------------------------------------------------------------
119 DESIGNED BY: E. B. Jackson
121 CODED BY: E. B. Jackson
123 MAINTAINED BY: E. B. Jackson
125 ----------------------------------------------------------------------------
127 MODIFICATION HISTORY:
131 930208 Modified to avoid singularity near polar region. EBJ
132 930602 Moved backwards calcs here from ls_step. EBJ
133 931214 Changed erroneous Latitude and Altitude variables to
134 *lat_geod and *alt in routine ls_geoc_to_geod. EBJ
135 940111 Changed header files from old ls_eom.h style to ls_types,
136 and ls_constants. Also replaced old DATA type with new
143 Revision 1.1 1997/07/31 23:13:14 curt
146 Revision 1.1 1997/05/29 00:09:56 curt
147 Initial Flight Gear revision.
149 * Revision 1.5 1994/01/11 18:47:05 bjax
150 * Changed include files to use types and constants, not ls_eom.h
151 * Also changed DATA type to SCALAR type.
153 * Revision 1.4 1993/12/14 21:06:47 bjax
154 * Removed global variable references Altitude and Latitude. EBJ
156 * Revision 1.3 1993/06/02 15:03:40 bjax
157 * Made new subroutine for calculating geodetic to geocentric; changed name
158 * of forward conversion routine from ls_geodesy to ls_geoc_to_geod.
161 ----------------------------------------------------------------------------
165 [ 1] Stevens, Brian L.; and Lewis, Frank L.: "Aircraft
166 Control and Simulation", Wiley and Sons, 1992.
170 ----------------------------------------------------------------------------
174 ----------------------------------------------------------------------------
178 ----------------------------------------------------------------------------
181 lat_geoc Geocentric latitude, radians, + = North
182 radius C.G. radius to earth center, ft
184 ----------------------------------------------------------------------------
187 lat_geod Geodetic latitude, radians, + = North
188 alt C.G. altitude above mean sea level, ft
189 sea_level_r radius from earth center to sea level at
190 local vertical (surface normal) of C.G.
192 --------------------------------------------------------------------------*/
196 /* Revision 1.1 1997/07/31 23:13:14 curt