source: trunk/MagicSoft/slalib/refco.c@ 9488

Last change on this file since 9488 was 731, checked in by tbretz, 24 years ago
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1#include "slalib.h"
2#include "slamac.h"
3void slaRefco ( double hm, double tdk, double pmb, double rh,
4 double wl, double phi, double tlr, double eps,
5 double *refa, double *refb )
6/*
7** - - - - - - - - -
8** s l a R e f c o
9** - - - - - - - - -
10**
11** Determine constants A and B in atmospheric refraction model
12** dz = A tan z + B tan^3 z.
13**
14** z is the "observed" zenith distance (i.e. affected by
15** refraction) and dz is what to add to z to give the "topocentric"
16** (i.e. in vacuo) zenith distance.
17**
18** Given:
19** hm double height of the observer above sea level (metre)
20** tdk double ambient temperature at the observer (deg k)
21** pmb double pressure at the observer (millibar)
22** rh double relative humidity at the observer (range 0-1)
23** wl double effective wavelength of the source (micrometre)
24** phi double latitude of the observer (radian, astronomical)
25** tlr double temperature lapse rate in the troposphere (degk/metre)
26** eps double precision required to terminate iteration (radian)
27**
28** Returned:
29** *refa double tan z coefficient (radian)
30** *refb double tan^3 z coefficient (radian)
31**
32** Called: slaRefro
33**
34** Notes:
35**
36** 1 Typical values for the tlr and eps arguments might be 0.0065 and
37** 1e-10 respectively.
38**
39** 2 The radio refraction is chosen by specifying wl > 100 micrometres.
40**
41** 3 The routine is a slower but more accurate alternative to the
42** slaRefcoq routine. The constants it produces give perfect
43** agreement with slaRefro at zenith distances arctan(1) (45 deg)
44** and arctan(4) (about 76 deg). It achieves 0.5 arcsec accuracy
45** for ZD < 80 deg, 0.01 arcsec accuracy for ZD < 60 deg, and
46** 0.001 arcsec accuracy for ZD < 45 deg.
47**
48** Last revision: 4 June 1997
49**
50** Copyright P.T.Wallace. All rights reserved.
51*/
52{
53 double r1,r2;
54
55/* Sample zenith distances: arctan(1) and arctan(4) */
56 static double atn1 = 0.7853981633974483;
57 static double atn4 = 1.325817663668033;
58
59/* Determine refraction for the two sample zenith distances. */
60 slaRefro ( atn1, hm, tdk, pmb, rh, wl, phi, tlr, eps, &r1 );
61 slaRefro ( atn4, hm, tdk, pmb, rh, wl, phi, tlr, eps, &r2 );
62
63/* Solve for refraction constants. */
64 *refa = ( 64.0 * r1 - r2 ) / 60.0;
65 *refb = ( r2 - 4.0 * r1 ) / 60.0;
66}
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