/************************************************************************** * fg_geodesy.c -- routines to convert between geodetic and geocentric * coordinate systems. * * Copied and adapted directly from LaRCsim/ls_geodesy.c * * See below for the complete original LaRCsim comments. * * $Id$ * (Log is kept at end of this file) **************************************************************************/ #include #include "fg_geodesy.h" #include "../constants.h" /* ONE_SECOND is pi/180/60/60, or about 100 feet at earths' equator */ #define ONE_SECOND 4.848136811E-6 /* fgGeocToGeod(lat_geoc, radius, *lat_geod, *alt, *sea_level_r) * INPUTS: * lat_geoc Geocentric latitude, radians, + = North * radius C.G. radius to earth center, ft * * OUTPUTS: * lat_geod Geodetic latitude, radians, + = North * alt C.G. altitude above mean sea level, ft * sea_level_r radius from earth center to sea level at * local vertical (surface normal) of C.G. */ void fgGeocToGeod( double lat_geoc, double radius, double *lat_geod, double *alt, double *sea_level_r ) { double t_lat, x_alpha, mu_alpha, delt_mu, r_alpha, l_point, rho_alpha; double sin_mu_a, denom,delt_lambda, lambda_sl, sin_lambda_sl; if( ( (FG_PI_2 - lat_geoc) < ONE_SECOND ) /* near North pole */ || ( (FG_PI_2 + lat_geoc) < ONE_SECOND ) ) /* near South pole */ { *lat_geod = lat_geoc; *sea_level_r = EQUATORIAL_RADIUS_KM*E; *alt = radius - *sea_level_r; } else { t_lat = tan(lat_geoc); x_alpha = E*EQUATORIAL_RADIUS_KM/sqrt(t_lat*t_lat + E*E); mu_alpha = atan2(sqrt(RESQ_KM - x_alpha*x_alpha),E*x_alpha); if (lat_geoc < 0) mu_alpha = - mu_alpha; sin_mu_a = sin(mu_alpha); delt_lambda = mu_alpha - lat_geoc; r_alpha = x_alpha/cos(lat_geoc); l_point = radius - r_alpha; *alt = l_point*cos(delt_lambda); denom = sqrt(1-EPS*EPS*sin_mu_a*sin_mu_a); rho_alpha = EQUATORIAL_RADIUS_KM*(1-EPS)/ (denom*denom*denom); delt_mu = atan2(l_point*sin(delt_lambda),rho_alpha + *alt); *lat_geod = mu_alpha - delt_mu; lambda_sl = atan( E*E * tan(*lat_geod) ); /* SL geoc. latitude */ sin_lambda_sl = sin( lambda_sl ); *sea_level_r = sqrt(RESQ_KM / (1 + ((1/(E*E))-1)*sin_lambda_sl*sin_lambda_sl)); } } /* fgGeodToGeoc( lat_geod, alt, *sl_radius, *lat_geoc ) * INPUTS: * lat_geod Geodetic latitude, radians, + = North * alt C.G. altitude above mean sea level, ft * * OUTPUTS: * sl_radius SEA LEVEL radius to earth center, ft (add Altitude to * get true distance from earth center. * lat_geoc Geocentric latitude, radians, + = North * */ void fgGeodToGeoc( double lat_geod, double alt, double *sl_radius, double *lat_geoc ) { double lambda_sl, sin_lambda_sl, cos_lambda_sl, sin_mu, cos_mu, px, py; lambda_sl = atan( E*E * tan(lat_geod) ); /* sea level geocentric latitude */ sin_lambda_sl = sin( lambda_sl ); cos_lambda_sl = cos( lambda_sl ); sin_mu = sin(lat_geod); /* Geodetic (map makers') latitude */ cos_mu = cos(lat_geod); *sl_radius = sqrt(RESQ_KM / (1 + ((1/(E*E))-1)*sin_lambda_sl*sin_lambda_sl)); py = *sl_radius*sin_lambda_sl + alt*sin_mu; px = *sl_radius*cos_lambda_sl + alt*cos_mu; *lat_geoc = atan2( py, px ); } /*************************************************************************** TITLE: ls_geodesy ---------------------------------------------------------------------------- FUNCTION: Converts geocentric coordinates to geodetic positions ---------------------------------------------------------------------------- MODULE STATUS: developmental ---------------------------------------------------------------------------- GENEALOGY: Written as part of LaRCSim project by E. B. Jackson ---------------------------------------------------------------------------- DESIGNED BY: E. B. Jackson CODED BY: E. B. Jackson MAINTAINED BY: E. B. Jackson ---------------------------------------------------------------------------- MODIFICATION HISTORY: DATE PURPOSE BY 930208 Modified to avoid singularity near polar region. EBJ 930602 Moved backwards calcs here from ls_step. EBJ 931214 Changed erroneous Latitude and Altitude variables to *lat_geod and *alt in routine ls_geoc_to_geod. EBJ 940111 Changed header files from old ls_eom.h style to ls_types, and ls_constants. Also replaced old DATA type with new SCALAR type. EBJ CURRENT RCS HEADER: $Header$ $Log$ Revision 1.1 1997/07/31 23:13:14 curt Initial revision. Revision 1.1 1997/05/29 00:09:56 curt Initial Flight Gear revision. * Revision 1.5 1994/01/11 18:47:05 bjax * Changed include files to use types and constants, not ls_eom.h * Also changed DATA type to SCALAR type. * * Revision 1.4 1993/12/14 21:06:47 bjax * Removed global variable references Altitude and Latitude. EBJ * * Revision 1.3 1993/06/02 15:03:40 bjax * Made new subroutine for calculating geodetic to geocentric; changed name * of forward conversion routine from ls_geodesy to ls_geoc_to_geod. * ---------------------------------------------------------------------------- REFERENCES: [ 1] Stevens, Brian L.; and Lewis, Frank L.: "Aircraft Control and Simulation", Wiley and Sons, 1992. ISBN 0-471-61397-5 ---------------------------------------------------------------------------- CALLED BY: ls_aux ---------------------------------------------------------------------------- CALLS TO: ---------------------------------------------------------------------------- INPUTS: lat_geoc Geocentric latitude, radians, + = North radius C.G. radius to earth center, ft ---------------------------------------------------------------------------- OUTPUTS: lat_geod Geodetic latitude, radians, + = North alt C.G. altitude above mean sea level, ft sea_level_r radius from earth center to sea level at local vertical (surface normal) of C.G. --------------------------------------------------------------------------*/ /* $Log$ /* Revision 1.1 1997/07/31 23:13:14 curt /* Initial revision. /* */