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#include <stdio.h>
#include <math.h>
#include "P_.h"
#include "astro.h"
/* correct the apparent altitude, aa, for refraction to the true altitude, ta,
* each in radians, given the local atmospheric pressure, pr, in mbars, and
* the temperature, tr, in degrees C.
*/
void
unrefract (pr, tr, aa, ta)
double pr, tr;
double aa;
double *ta;
{
double r; /* refraction correction*/
if (aa >= degrad(15.)) {
/* model for altitudes at least 15 degrees above horizon */
r = 7.888888e-5*pr/((273+tr)*tan(aa));
} else {
/* better model for altitudes below 15 degrees */
double a, b, aadeg = raddeg(aa);
a = ((2e-5*aadeg+1.96e-2)*aadeg+1.594e-1)*pr;
b = (273+tr)*((8.45e-2*aadeg+5.05e-1)*aadeg+1);
r = degrad(a/b);
}
*ta = aa - r;
}
/* correct the true altitude, ta, for refraction to the apparent altitude, aa,
* each in radians, given the local atmospheric pressure, pr, in mbars, and
* the temperature, tr, in degrees C.
*/
void
refract (pr, tr, ta, aa)
double pr, tr;
double ta;
double *aa;
{
#define MAXRERR degrad(1./3600.) /* desired accuracy, rads */
double d, t, t0, a;
/* first guess of error is to go backwards.
* make use that we know delta-apparent is always < delta-true.
*/
unrefract (pr, tr, ta, &t);
d = 0.8*(ta - t);
t0 = t;
a = ta;
/* use secant method to discover a value that unrefracts to ta.
* max=7 ave=2.4 loops in hundreds of test cases.
*/
do {
a += d;
unrefract (pr, tr, a, &t);
d *= -(ta - t)/(t0 - t);
t0 = t;
} while (fabs(ta-t) > MAXRERR);
*aa = a;
#undef MAXRERR
}
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