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simgear/simgear/magvar/coremag.cxx
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curt 7c410d36e3 wim van hoydonck: 
Updated to World Magnetic Model 2005.
2006-10-28 22:12:48 +00:00

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// coremag.cxx -- compute local magnetic variation given position,
// altitude, and date
//
// This is an implementation of the NIMA (formerly DMA) WMM2000
//
// http://www.nima.mil/GandG/ngdc-wmm2000.html
//
// Copyright (C) 2000 Edward A Williams <Ed_Williams@compuserve.com>
//
// Adapted from Excel 3.0 version 3/27/94 EAW
// Recoded in C++ by Starry Chan
// WMM95 added and rearranged in ANSI-C EAW 7/9/95
// Put shell around program and made Borland & GCC compatible EAW 11/22/95
// IGRF95 added 2/96 EAW
// WMM2000 IGR2000 added 2/00 EAW
// Released under GPL 3/26/00 EAW
// Adaptions and modifications for the SimGear project 3/27/2000 CLO
//
// Removed all pow() calls and made static roots[][] arrays to
// save many sqrt() calls on subsequent invocations
// left old code as SGMagVarOrig() for testing purposes
// 3/28/2000 Norman Vine -- nhv@yahoo.com
//
// Put in some bullet-proofing to handle magnetic and geographic poles.
// 3/28/2000 EAW
//
// Updated coefficient arrays to use the current WMM2005 model,
// (valid between 2005.0 and 2010.0)
// Also removed unused variables and corrected earth radii constants
// to the values for WGS84 and WMM2005.
// Reference:
// McLean, S., S. Macmillan, S. Maus, V. Lesur, A.
// Thomson, and D. Dater, December 2004, The
// US/UK World Magnetic Model for 2005-2010,
// NOAA Technical Report NESDIS/NGDC-1.
//
// 25/10/2006 Wim Van Hoydonck -- wim.van.hoydonck@gmail.com
//
// The routine uses a spherical harmonic expansion of the magnetic
// potential up to twelfth order, together with its time variation, as
// described in Chapter 4 of "Geomagnetism, Vol 1, Ed. J.A.Jacobs,
// Academic Press (London 1987)". The program first converts geodetic
// coordinates (lat/long on elliptic earth and altitude) to spherical
// geocentric (spherical lat/long and radius) coordinates. Using this,
// the spherical (B_r, B_theta, B_phi) magnetic field components are
// computed from the model. These are finally referred to surface (X, Y,
// Z) coordinates.
//
// Fields are accurate to better than 200nT, variation and dip to
// better than 0.5 degrees, with the exception of the declination near
// the magnetic poles (where it is ill-defined) where the error may reach
// 4 degrees or more.
//
// Variation is undefined at both the geographic and
// magnetic poles, even though the field itself is well-behaved. To
// avoid the routine blowing up, latitude entries corresponding to
// the geographic poles are slightly offset. At the magnetic poles,
// the routine returns zero variation.
//
// This library is free software; you can redistribute it and/or
// modify it under the terms of the GNU Library General Public
// License as published by the Free Software Foundation; either
// version 2 of the License, or (at your option) any later version.
//
// This library 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
// Library 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., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
//
// $Id$
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <simgear/constants.h>
#include <simgear/sg_inlines.h>
#include "coremag.hxx"
static const double pi = 3.14159265358979;
static const double a = 6378.137; /* semi-major axis (equatorial radius) of WGS84 ellipsoid */
static const double b = 6356.7523142; /* semi-minor axis referenced to the WGS84 ellipsoid */
static const double r_0 = 6371.2; /* standard Earth magnetic reference radius */
static double gnm_wmm2005[13][13] =
{
{0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{-29556.8, -1671.7, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{-2340.6, 3046.9, 1657.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{1335.4, -2305.1, 1246.7, 674.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{919.8, 798.1, 211.3, -379.4, 100.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{-227.4, 354.6, 208.7, -136.5, -168.3, -14.1, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{73.2, 69.7, 76.7, -151.2, -14.9, 14.6, -86.3, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{80.1, -74.5, -1.4, 38.5, 12.4, 9.5, 5.7, 1.8, 0.0, 0.0, 0.0, 0.0, 0.0},
{24.9, 7.7, -11.6, -6.9, -18.2, 10.0, 9.2, -11.6, -5.2, 0.0, 0.0, 0.0, 0.0},
{5.6, 9.9, 3.5, -7.0, 5.1, -10.8, -1.3, 8.8, -6.7, -9.1, 0.0, 0.0, 0.0},
{-2.3, -6.3, 1.6, -2.6, 0.0, 3.1, 0.4, 2.1, 3.9, -0.1, -2.3, 0.0, 0.0},
{2.8, -1.6, -1.7, 1.7, -0.1, 0.1, -0.7, 0.7, 1.8, 0.0, 1.1, 4.1, 0.0},
{-2.4, -0.4, 0.2, 0.8, -0.3, 1.1, -0.5, 0.4, -0.3, -0.3, -0.1, -0.3, -0.1},
};
static double hnm_wmm2005[13][13]=
{
{0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{0.0, 5079.8, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{0.0, -2594.7, -516.7, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{0.0, -199.9, 269.3, -524.2, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{0.0, 281.5, -226.0, 145.8, -304.7, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{0.0, 42.4, 179.8, -123.0, -19.5, 103.6, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{0.0, -20.3, 54.7, 63.6, -63.4, -0.1, 50.4, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{0.0, -61.5, -22.4, 7.2, 25.4, 11.0, -26.4, -5.1, 0.0, 0.0, 0.0, 0.0, 0.0},
{0.0, 11.2, -21.0, 9.6, -19.8, 16.1, 7.7, -12.9, -0.2, 0.0, 0.0, 0.0, 0.0},
{0.0, -20.1, 12.9, 12.6, -6.7, -8.1, 8.0, 2.9, -7.9, 6.0, 0.0, 0.0, 0.0},
{0.0, 2.4, 0.2, 4.4, 4.8, -6.5, -1.1, -3.4, -0.8, -2.3, -7.9, 0.0, 0.0},
{0.0, 0.3, 1.2, -0.8, -2.5, 0.9, -0.6, -2.7, -0.9, -1.3, -2.0, -1.2, 0.0},
{0.0, -0.4, 0.3, 2.4, -2.6, 0.6, 0.3, 0.0, 0.0, 0.3, -0.9, -0.4, 0.8},
};
static double gtnm_wmm2005[13][13]=
{
{0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{8.0, 10.6, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{-15.1, -7.8, -0.8, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{0.4, -2.6, -1.2, -6.5, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{-2.5, 2.8, -7.0, 6.2, -3.8, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{-2.8, 0.7, -3.2, -1.1, 0.1, -0.8, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{-0.7, 0.4, -0.3, 2.3, -2.1, -0.6, 1.4, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{0.2, -0.1, -0.3, 1.1, 0.6, 0.5, -0.4, 0.6, 0.0, 0.0, 0.0, 0.0, 0.0},
{0.1, 0.3, -0.4, 0.3, -0.3, 0.2, 0.4, -0.7, 0.4, 0.0, 0.0, 0.0, 0.0},
{0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
};
static double htnm_wmm2005[13][13]=
{
{0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{0.0, -20.9, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{0.0, -23.2, -14.6, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{0.0, 5.0, -7.0, -0.6, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{0.0, 2.2, 1.6, 5.8, 0.1, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{0.0, 0.0, 1.7, 2.1, 4.8, -1.1, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{0.0, -0.6, -1.9, -0.4, -0.5, -0.3, 0.7, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{0.0, 0.6, 0.4, 0.2, 0.3, -0.8, -0.2, 0.1, 0.0, 0.0, 0.0, 0.0, 0.0},
{0.0, -0.2, 0.1, 0.3, 0.4, 0.1, -0.2, 0.4, 0.4, 0.0, 0.0, 0.0, 0.0},
{0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
{0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0},
};
static const int nmax = 12;
static double P[13][13];
static double DP[13][13];
static double gnm[13][13];
static double hnm[13][13];
static double sm[13];
static double cm[13];
static double root[13];
static double roots[13][13][2];
/* Convert date to Julian day 1950-2049 */
unsigned long int yymmdd_to_julian_days( int yy, int mm, int dd )
{
unsigned long jd;
yy = (yy < 50) ? (2000 + yy) : (1900 + yy);
jd = dd - 32075L + 1461L * (yy + 4800L + (mm - 14) / 12 ) / 4;
jd = jd + 367L * (mm - 2 - (mm - 14) / 12*12) / 12;
jd = jd - 3 * ((yy + 4900L + (mm - 14) / 12) / 100) / 4;
/* printf("julian date = %d\n", jd ); */
return jd;
}
/*
* return variation (in radians) given geodetic latitude (radians),
* longitude(radians), height (km) and (Julian) date
* N and E lat and long are positive, S and W negative
*/
double calc_magvar( double lat, double lon, double h, long dat, double* field )
{
/* output field B_r,B_th,B_phi,B_x,B_y,B_z */
int n,m;
/* reference date for current model is 1 januari 2005 */
long date0_wmm2005 = yymmdd_to_julian_days(5,1,1);
double yearfrac,sr,r,theta,c,s,psi,fn,fn_0,B_r,B_theta,B_phi,X,Y,Z;
double sinpsi, cospsi, inv_s;
static int been_here = 0;
double sinlat = sin(lat);
double coslat = cos(lat);
/* convert to geocentric coords: */
// sr = sqrt(pow(a*coslat,2.0)+pow(b*sinlat,2.0));
sr = sqrt(a*a*coslat*coslat + b*b*sinlat*sinlat);
/* sr is effective radius */
theta = atan2(coslat * (h*sr + a*a),
sinlat * (h*sr + b*b));
/* theta is geocentric co-latitude */
r = h*h + 2.0*h * sr +
(a*a*a*a - ( a*a*a*a - b*b*b*b ) * sinlat*sinlat ) /
(a*a - (a*a - b*b) * sinlat*sinlat );
r = sqrt(r);
/* r is geocentric radial distance */
c = cos(theta);
s = sin(theta);
/* protect against zero divide at geographic poles */
inv_s = 1.0 / (s + (s == 0.)*1.0e-8);
/* zero out arrays */
for ( n = 0; n <= nmax; n++ ) {
for ( m = 0; m <= n; m++ ) {
P[n][m] = 0;
DP[n][m] = 0;
}
}
/* diagonal elements */
P[0][0] = 1;
P[1][1] = s;
DP[0][0] = 0;
DP[1][1] = c;
P[1][0] = c ;
DP[1][0] = -s;
// these values will not change for subsequent function calls
if( !been_here ) {
for ( n = 2; n <= nmax; n++ ) {
root[n] = sqrt((2.0*n-1) / (2.0*n));
}
for ( m = 0; m <= nmax; m++ ) {
double mm = m*m;
for ( n = SG_MAX2(m + 1, 2); n <= nmax; n++ ) {
roots[m][n][0] = sqrt((n-1)*(n-1) - mm);
roots[m][n][1] = 1.0 / sqrt( n*n - mm);
}
}
been_here = 1;
}
for ( n=2; n <= nmax; n++ ) {
// double root = sqrt((2.0*n-1) / (2.0*n));
P[n][n] = P[n-1][n-1] * s * root[n];
DP[n][n] = (DP[n-1][n-1] * s + P[n-1][n-1] * c) *
root[n];
}
/* lower triangle */
for ( m = 0; m <= nmax; m++ ) {
// double mm = m*m;
for ( n = SG_MAX2(m + 1, 2); n <= nmax; n++ ) {
// double root1 = sqrt((n-1)*(n-1) - mm);
// double root2 = 1.0 / sqrt( n*n - mm);
P[n][m] = (P[n-1][m] * c * (2.0*n-1) -
P[n-2][m] * roots[m][n][0]) *
roots[m][n][1];
DP[n][m] = ((DP[n-1][m] * c - P[n-1][m] * s) *
(2.0*n-1) - DP[n-2][m] * roots[m][n][0]) *
roots[m][n][1];
}
}
/* compute Gauss coefficients gnm and hnm of degree n and order m for the desired time
achieved by adjusting the coefficients at time t0 for linear secular variation */
/* WMM2005 */
yearfrac = (dat - date0_wmm2005) / 365.25;
for ( n = 1; n <= nmax; n++ ) {
for ( m = 0; m <= nmax; m++ ) {
gnm[n][m] = gnm_wmm2005[n][m] + yearfrac * gtnm_wmm2005[n][m];
hnm[n][m] = hnm_wmm2005[n][m] + yearfrac * htnm_wmm2005[n][m];
}
}
/* compute sm (sin(m lon) and cm (cos(m lon)) */
for ( m = 0; m <= nmax; m++ ) {
sm[m] = sin(m * lon);
cm[m] = cos(m * lon);
}
/* compute B fields */
B_r = 0.0;
B_theta = 0.0;
B_phi = 0.0;
fn_0 = r_0/r;
fn = fn_0 * fn_0;
for ( n = 1; n <= nmax; n++ ) {
double c1_n=0;
double c2_n=0;
double c3_n=0;
for ( m = 0; m <= n; m++ ) {
double tmp = (gnm[n][m] * cm[m] + hnm[n][m] * sm[m]);
c1_n=c1_n + tmp * P[n][m];
c2_n=c2_n + tmp * DP[n][m];
c3_n=c3_n + m * (gnm[n][m] * sm[m] - hnm[n][m] * cm[m]) * P[n][m];
}
// fn=pow(r_0/r,n+2.0);
fn *= fn_0;
B_r = B_r + (n + 1) * c1_n * fn;
B_theta = B_theta - c2_n * fn;
B_phi = B_phi + c3_n * fn * inv_s;
}
/* Find geodetic field components: */
psi = theta - ((pi / 2.0) - lat);
sinpsi = sin(psi);
cospsi = cos(psi);
X = -B_theta * cospsi - B_r * sinpsi;
Y = B_phi;
Z = B_theta * sinpsi - B_r * cospsi;
field[0]=B_r;
field[1]=B_theta;
field[2]=B_phi;
field[3]=X;
field[4]=Y;
field[5]=Z; /* output fields */
/* find variation in radians */
/* return zero variation at magnetic pole X=Y=0. */
/* E is positive */
return (X != 0. || Y != 0.) ? atan2(Y, X) : (double) 0.;
}
#ifdef TEST_NHV_HACKS
double SGMagVarOrig( double lat, double lon, double h, long dat, double* field )
{
/* output field B_r,B_th,B_phi,B_x,B_y,B_z */
int n,m;
/* reference dates */
long date0_wmm2005 = yymmdd_to_julian_days(5,1,1);
double yearfrac,sr,r,theta,c,s,psi,fn,B_r,B_theta,B_phi,X,Y,Z;
/* convert to geocentric coords: */
sr = sqrt(pow(a*cos(lat),2.0)+pow(b*sin(lat),2.0));
/* sr is effective radius */
theta = atan2(cos(lat) * (h * sr + a * a),
sin(lat) * (h * sr + b * b));
/* theta is geocentric co-latitude */
r = h * h + 2.0*h * sr +
(pow(a,4.0) - (pow(a,4.0) - pow(b,4.0)) * pow(sin(lat),2.0)) /
(a * a - (a * a - b * b) * pow(sin(lat),2.0));
r = sqrt(r);
/* r is geocentric radial distance */
c = cos(theta);
s = sin(theta);
/* zero out arrays */
for ( n = 0; n <= nmax; n++ ) {
for ( m = 0; m <= n; m++ ) {
P[n][m] = 0;
DP[n][m] = 0;
}
}
/* diagonal elements */
P[0][0] = 1;
P[1][1] = s;
DP[0][0] = 0;
DP[1][1] = c;
P[1][0] = c ;
DP[1][0] = -s;
for ( n = 2; n <= nmax; n++ ) {
P[n][n] = P[n-1][n-1] * s * sqrt((2.0*n-1) / (2.0*n));
DP[n][n] = (DP[n-1][n-1] * s + P[n-1][n-1] * c) *
sqrt((2.0*n-1) / (2.0*n));
}
/* lower triangle */
for ( m = 0; m <= nmax; m++ ) {
for ( n = SG_MAX2(m + 1, 2); n <= nmax; n++ ) {
P[n][m] = (P[n-1][m] * c * (2.0*n-1) - P[n-2][m] *
sqrt(1.0*(n-1)*(n-1) - m * m)) /
sqrt(1.0* n * n - m * m);
DP[n][m] = ((DP[n-1][m] * c - P[n-1][m] * s) *
(2.0*n-1) - DP[n-2][m] *
sqrt(1.0*(n-1) * (n-1) - m * m)) /
sqrt(1.0* n * n - m * m);
}
}
/* compute gnm, hnm at dat */
/* WMM2005 */
yearfrac = (dat - date0_wmm2005) / 365.25;
for ( n = 1; n <= nmax; n++ ) {
for ( m = 0; m <= nmax; m++ ) {
gnm[n][m] = gnm_wmm2005[n][m] + yearfrac * gtnm_wmm2005[n][m];
hnm[n][m] = hnm_wmm2005[n][m] + yearfrac * htnm_wmm2005[n][m];
}
}
/* compute sm (sin(m lon) and cm (cos(m lon)) */
for ( m = 0; m <= nmax; m++ ) {
sm[m] = sin(m * lon);
cm[m] = cos(m * lon);
}
/* compute B fields */
B_r = 0.0;
B_theta = 0.0;
B_phi = 0.0;
for ( n = 1; n <= nmax; n++ ) {
double c1_n=0;
double c2_n=0;
double c3_n=0;
for ( m = 0; m <= n; m++ ) {
c1_n=c1_n + (gnm[n][m] * cm[m] + hnm[n][m] * sm[m]) * P[n][m];
c2_n=c2_n + (gnm[n][m] * cm[m] + hnm[n][m] * sm[m]) * DP[n][m];
c3_n=c3_n + m * (gnm[n][m] * sm[m] - hnm[n][m] * cm[m]) * P[n][m];
}
fn=pow(r_0/r,n+2.0);
B_r = B_r + (n + 1) * c1_n * fn;
B_theta = B_theta - c2_n * fn;
B_phi = B_phi + c3_n * fn / s;
}
/* Find geodetic field components: */
psi = theta - (pi / 2.0 - lat);
X = -B_theta * cos(psi) - B_r * sin(psi);
Y = B_phi;
Z = B_theta * sin(psi) - B_r * cos(psi);
field[0]=B_r;
field[1]=B_theta;
field[2]=B_phi;
field[3]=X;
field[4]=Y;
field[5]=Z; /* output fields */
/* find variation, leave in radians! */
return atan2(Y, X); /* E is positive */
}
#endif // TEST_NHV_HACKS
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