int main()
{
double lat_deg,lon_deg,h,var;
int yy,mm,dd;
lat_deg=45.0;
lon_deg=9.0;
h= 1; // altitude in km
mm= 10;
dd= 1;
yy= 11;
var=rad_to_deg(SGMagVar(deg_to_rad(lat_deg),deg_to_rad(lon_deg),h, yymmdd_to_julian_days(yy,mm,dd)));
fprintf(stdout,"var= %4.2f \n",var);
}
void SGMagVar(Mag_Real lat, Mag_Real lon, Mag_Real h,
Mag_Uint32 dat, float32* field)
{
int model = 9; // 9 is IGRF2005
int n,m,nmaxl;
Mag_Real yearfrac,sr,r,theta,c,s,psi,fn,fn_0,B_r,B_theta,B_phi,X,Y,Z;
Mag_Real sinpsi, cospsi, inv_s;
Mag_Real mm,c1_n,c2_n,c3_n,tmp;
static int been_here = 0;
Mag_Real sinlat;
Mag_Real coslat;
Mag_Real norm;
/* output field B_r,B_th,B_phi,B_x,B_y,B_z */
h = h/1000.0;
sinlat = sin(lat);
coslat = cos(lat);
/* convert to geocentric */
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++ ) {
mm = m*m;
for ( n = max_mag(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 = max_mag(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 gnm, hnm at dat */
nmaxl = 12; /* models except IGRF2005 */
switch(model) {
case 8: /* WMM2005 */
yearfrac = (dat - yymmdd_to_julian_days(5,1,1)) / 365.25;
for (n=1;n<=nmaxl;n++)
for (m = 0;m<=nmaxl;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];
}
break;
case 9: /* IGRF2005 */
yearfrac = (dat - yymmdd_to_julian_days(5,1,1)) / 365.25;
nmaxl = 13;
for (n=1;n<=nmaxl;n++)
for (m = 0;m<=nmaxl;m++) {
gnm[n][m] = gnm_igrf2005[n][m] + yearfrac * gtnm_igrf2005[n][m];
hnm[n][m] = hnm_igrf2005[n][m] + yearfrac * htnm_igrf2005[n][m];
}
break;
}
/* compute sm (sin(m lon) and cm (cos(m lon)) */
for (m = 0;m<=nmaxl;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 <= nmaxl; n++ ) {
c1_n=0;
c2_n=0;
c3_n=0;
for ( m = 0; m <= n; m++ ) {
tmp = (gnm[n][m] * cm[m] + hnm[n][m] * sm[m]);
c1_n += tmp * P[n][m];
c2_n += tmp * DP[n][m];
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 += (n + 1) * c1_n * fn;
B_theta -= c2_n * fn;
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;
norm = sqrt(X*X + Y*Y + Z*Z);
field[0]=X/norm;
field[1]=Y/norm;
field[2]=Z/norm; /* output fields */
/* find variation in radians */
/* return zero variation at magnetic pole X=Y=0. */
/* E is positive */
return;
}
