1 /***************************************************************************
5 ----------------------------------------------------------------------------
7 FUNCTION: Converts geocentric coordinates to geodetic positions
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11 MODULE STATUS: developmental
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15 GENEALOGY: Written as part of LaRCSim project by E. B. Jackson
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19 DESIGNED BY: E. B. Jackson
21 CODED BY: E. B. Jackson
23 MAINTAINED BY: E. B. Jackson
25 ----------------------------------------------------------------------------
31 930208 Modified to avoid singularity near polar region. EBJ
32 930602 Moved backwards calcs here from ls_step. EBJ
33 931214 Changed erroneous Latitude and Altitude variables to
34 *lat_geod and *alt in routine ls_geoc_to_geod. EBJ
35 940111 Changed header files from old ls_eom.h style to ls_types,
36 and ls_constants. Also replaced old DATA type with new
43 Revision 1.1 1997/05/29 00:09:56 curt
44 Initial Flight Gear revision.
46 * Revision 1.5 1994/01/11 18:47:05 bjax
47 * Changed include files to use types and constants, not ls_eom.h
48 * Also changed DATA type to SCALAR type.
50 * Revision 1.4 1993/12/14 21:06:47 bjax
51 * Removed global variable references Altitude and Latitude. EBJ
53 * Revision 1.3 1993/06/02 15:03:40 bjax
54 * Made new subroutine for calculating geodetic to geocentric; changed name
55 * of forward conversion routine from ls_geodesy to ls_geoc_to_geod.
58 ----------------------------------------------------------------------------
62 [ 1] Stevens, Brian L.; and Lewis, Frank L.: "Aircraft
63 Control and Simulation", Wiley and Sons, 1992.
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71 ----------------------------------------------------------------------------
75 ----------------------------------------------------------------------------
78 lat_geoc Geocentric latitude, radians, + = North
79 radius C.G. radius to earth center, ft
81 ----------------------------------------------------------------------------
84 lat_geod Geodetic latitude, radians, + = North
85 alt C.G. altitude above mean sea level, ft
86 sea_level_r radius from earth center to sea level at
87 local vertical (surface normal) of C.G.
89 --------------------------------------------------------------------------*/
92 #include "ls_constants.h"
95 /* ONE_SECOND is pi/180/60/60, or about 100 feet at earths' equator */
96 #define ONE_SECOND 4.848136811E-6
97 #define HALF_PI 0.5*PI
100 void ls_geoc_to_geod( lat_geoc, radius, lat_geod, alt, sea_level_r )
107 SCALAR t_lat, x_alpha, mu_alpha, delt_mu, r_alpha, l_point, rho_alpha;
108 SCALAR sin_mu_a, denom,delt_lambda, lambda_sl, sin_lambda_sl;
110 if( ( (HALF_PI - lat_geoc) < ONE_SECOND ) /* near North pole */
111 || ( (HALF_PI + lat_geoc) < ONE_SECOND ) ) /* near South pole */
113 *lat_geod = lat_geoc;
114 *sea_level_r = EQUATORIAL_RADIUS*E;
115 *alt = radius - *sea_level_r;
119 t_lat = tan(lat_geoc);
120 x_alpha = E*EQUATORIAL_RADIUS/sqrt(t_lat*t_lat + E*E);
121 mu_alpha = atan2(sqrt(RESQ - x_alpha*x_alpha),E*x_alpha);
122 if (lat_geoc < 0) mu_alpha = - mu_alpha;
123 sin_mu_a = sin(mu_alpha);
124 delt_lambda = mu_alpha - lat_geoc;
125 r_alpha = x_alpha/cos(lat_geoc);
126 l_point = radius - r_alpha;
127 *alt = l_point*cos(delt_lambda);
128 denom = sqrt(1-EPS*EPS*sin_mu_a*sin_mu_a);
129 rho_alpha = EQUATORIAL_RADIUS*(1-EPS)/
131 delt_mu = atan2(l_point*sin(delt_lambda),rho_alpha + *alt);
132 *lat_geod = mu_alpha - delt_mu;
133 lambda_sl = atan( E*E * tan(*lat_geod) ); /* SL geoc. latitude */
134 sin_lambda_sl = sin( lambda_sl );
135 *sea_level_r = sqrt(RESQ
136 /(1 + ((1/(E*E))-1)*sin_lambda_sl*sin_lambda_sl));
140 void ls_geod_to_geoc( lat_geod, alt, sl_radius, lat_geoc )
146 SCALAR lambda_sl, sin_lambda_sl, cos_lambda_sl, sin_mu, cos_mu, px, py;
148 lambda_sl = atan( E*E * tan(lat_geod) ); /* sea level geocentric latitude */
149 sin_lambda_sl = sin( lambda_sl );
150 cos_lambda_sl = cos( lambda_sl );
151 sin_mu = sin(lat_geod); /* Geodetic (map makers') latitude */
152 cos_mu = cos(lat_geod);
153 *sl_radius = sqrt(RESQ
154 /(1 + ((1/(E*E))-1)*sin_lambda_sl*sin_lambda_sl));
155 py = *sl_radius*sin_lambda_sl + alt*sin_mu;
156 px = *sl_radius*cos_lambda_sl + alt*cos_mu;
157 *lat_geoc = atan2( py, px );