| 1 | /*
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| 2 | *+
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| 3 | * Name:
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| 4 | * palRefco
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| 5 |
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| 6 | * Purpose:
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| 7 | * Determine constants in atmospheric refraction model
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| 8 |
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| 9 | * Language:
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| 10 | * Starlink ANSI C
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| 11 |
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| 12 | * Type of Module:
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| 13 | * Library routine
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| 14 |
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| 15 | * Invocation:
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| 16 | * void palRefco ( double hm, double tdk, double pmb, double rh,
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| 17 | * double wl, double phi, double tlr, double eps,
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| 18 | * double *refa, double *refb );
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| 19 |
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| 20 | * Arguments:
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| 21 | * hm = double (Given)
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| 22 | * Height of the observer above sea level (metre)
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| 23 | * tdk = double (Given)
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| 24 | * Ambient temperature at the observer (K)
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| 25 | * pmb = double (Given)
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| 26 | * Pressure at the observer (millibar)
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| 27 | * rh = double (Given)
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| 28 | * Relative humidity at the observer (range 0-1)
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| 29 | * wl = double (Given)
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| 30 | * Effective wavelength of the source (micrometre)
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| 31 | * phi = double (Given)
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| 32 | * Latitude of the observer (radian, astronomical)
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| 33 | * tlr = double (Given)
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| 34 | * Temperature lapse rate in the troposphere (K/metre)
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| 35 | * eps = double (Given)
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| 36 | * Precision required to terminate iteration (radian)
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| 37 | * refa = double * (Returned)
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| 38 | * tan Z coefficient (radian)
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| 39 | * refb = double * (Returned)
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| 40 | * tan**3 Z coefficient (radian)
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| 41 |
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| 42 | * Description:
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| 43 | * Determine the constants A and B in the atmospheric refraction
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| 44 | * model dZ = A tan Z + B tan**3 Z.
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| 45 | *
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| 46 | * Z is the "observed" zenith distance (i.e. affected by refraction)
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| 47 | * and dZ is what to add to Z to give the "topocentric" (i.e. in vacuo)
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| 48 | * zenith distance.
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| 49 |
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| 50 | * Authors:
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| 51 | * PTW: Patrick T. Wallace
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| 52 | * TIMJ: Tim Jenness (JAC, Hawaii)
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| 53 | * {enter_new_authors_here}
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| 54 |
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| 55 | * Notes:
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| 56 | * - Typical values for the TLR and EPS arguments might be 0.0065 and
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| 57 | * 1E-10 respectively.
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| 58 | *
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| 59 | * - The radio refraction is chosen by specifying WL > 100 micrometres.
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| 60 | *
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| 61 | * - The routine is a slower but more accurate alternative to the
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| 62 | * palRefcoq routine. The constants it produces give perfect
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| 63 | * agreement with palRefro at zenith distances arctan(1) (45 deg)
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| 64 | * and arctan(4) (about 76 deg). It achieves 0.5 arcsec accuracy
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| 65 | * for ZD < 80 deg, 0.01 arcsec accuracy for ZD < 60 deg, and
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| 66 | * 0.001 arcsec accuracy for ZD < 45 deg.
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| 67 |
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| 68 | * History:
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| 69 | * 2012-08-24 (TIMJ):
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| 70 | * Initial version. A direct copy of the Fortran SLA implementation.
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| 71 | * Adapted with permission from the Fortran SLALIB library.
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| 72 | * {enter_further_changes_here}
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| 73 |
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| 74 | * Copyright:
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| 75 | * Copyright (C) 2004 Rutherford Appleton Laboratory
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| 76 | * Copyright (C) 2012 Science and Technology Facilities Council.
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| 77 | * All Rights Reserved.
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| 78 |
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| 79 | * Licence:
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| 80 | * This program is free software; you can redistribute it and/or
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| 81 | * modify it under the terms of the GNU General Public License as
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| 82 | * published by the Free Software Foundation; either version 3 of
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| 83 | * the License, or (at your option) any later version.
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| 84 | *
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| 85 | * This program is distributed in the hope that it will be
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| 86 | * useful, but WITHOUT ANY WARRANTY; without even the implied
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| 87 | * warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
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| 88 | * PURPOSE. See the GNU General Public License for more details.
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| 89 | *
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| 90 | * You should have received a copy of the GNU General Public License
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| 91 | * along with this program; if not, write to the Free Software
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| 92 | * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
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| 93 | * MA 02110-1301, USA.
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| 94 |
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| 95 | * Bugs:
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| 96 | * {note_any_bugs_here}
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| 97 | *-
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| 98 | */
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| 99 |
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| 100 | #include "pal.h"
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| 101 |
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| 102 | void palRefco ( double hm, double tdk, double pmb, double rh,
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| 103 | double wl, double phi, double tlr, double eps,
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| 104 | double *refa, double *refb ) {
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| 105 |
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| 106 | double r1, r2;
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| 107 |
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| 108 | /* Sample zenith distances: arctan(1) and arctan(4) */
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| 109 | const double ATN1 = 0.7853981633974483;
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| 110 | const double ATN4 = 1.325817663668033;
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| 111 |
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| 112 | /* Determine refraction for the two sample zenith distances */
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| 113 | palRefro(ATN1,hm,tdk,pmb,rh,wl,phi,tlr,eps,&r1);
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| 114 | palRefro(ATN4,hm,tdk,pmb,rh,wl,phi,tlr,eps,&r2);
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| 115 |
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| 116 | /* Solve for refraction constants */
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| 117 | *refa = (64.0*r1-r2)/60.0;
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| 118 | *refb = (r2-4.0*r1)/60.0;
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| 119 |
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| 120 | }
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