| 1 | #include "slalib.h"
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| 2 | #include "slamac.h"
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| 3 | void slaE2h ( float ha, float dec, float phi, float *az, float *el )
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| 4 | /*
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| 5 | ** - - - - - - -
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| 6 | ** s l a E 2 h
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| 7 | ** - - - - - - -
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| 8 | **
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| 9 | ** Equatorial to horizon coordinates: HA,Dec to Az,El
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| 10 | **
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| 11 | ** (single precision)
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| 12 | **
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| 13 | ** Given:
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| 14 | ** ha float hour angle
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| 15 | ** dec float declination
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| 16 | ** phi float observatory latitude
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| 17 | **
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| 18 | ** Returned:
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| 19 | ** *az float azimuth
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| 20 | ** *el float elevation
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| 21 | **
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| 22 | ** Notes:
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| 23 | **
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| 24 | ** 1) All the arguments are angles in radians.
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| 25 | **
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| 26 | ** 2) Azimuth is returned in the range 0-2pi; north is zero,
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| 27 | ** and east is +pi/2. Elevation is returned in the range
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| 28 | ** +/-pi/2.
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| 29 | **
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| 30 | ** 3) The latitude must be geodetic. In critical applications,
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| 31 | ** corrections for polar motion should be applied.
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| 32 | **
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| 33 | ** 4) In some applications it will be important to specify the
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| 34 | ** correct type of hour angle and declination in order to
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| 35 | ** produce the required type of azimuth and elevation. In
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| 36 | ** particular, it may be important to distinguish between
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| 37 | ** elevation as affected by refraction, which would
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| 38 | ** require the "observed" HA,Dec, and the elevation
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| 39 | ** in vacuo, which would require the "topocentric" HA,Dec.
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| 40 | ** If the effects of diurnal aberration can be neglected, the
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| 41 | ** "apparent" HA,Dec may be used instead of the topocentric
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| 42 | ** HA,Dec.
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| 43 | **
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| 44 | ** 5) No range checking of arguments is carried out.
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| 45 | **
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| 46 | ** 6) In applications which involve many such calculations, rather
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| 47 | ** than calling the present routine it will be more efficient to
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| 48 | ** use inline code, having previously computed fixed terms such
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| 49 | ** as sine and cosine of latitude, and (for tracking a star)
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| 50 | ** sine and cosine of declination.
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| 51 | **
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| 52 | ** Defined in slamac.h: D2PI
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| 53 | **
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| 54 | ** Last revision: 10 July 1994
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| 55 | **
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| 56 | ** Copyright P.T.Wallace. All rights reserved.
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| 57 | */
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| 58 | {
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| 59 | float sh, ch, sd, cd, sp, cp, x, y, z, r, a;
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| 60 |
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| 61 | /* Useful trig functions */
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| 62 | sh = (float) sin ( ha );
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| 63 | ch = (float) cos ( ha );
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| 64 | sd = (float) sin ( dec );
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| 65 | cd = (float) cos ( dec );
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| 66 | sp = (float) sin ( phi );
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| 67 | cp = (float) cos ( phi );
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| 68 |
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| 69 | /* Az,El as x,y,z */
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| 70 | x = - ch * cd * sp + sd * cp;
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| 71 | y = - sh * cd;
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| 72 | z = ch * cd * cp + sd * sp;
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| 73 |
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| 74 | /* To spherical */
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| 75 | r = (float) sqrt ( x * x + y * y );
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| 76 | a = ( r == 0.0f ) ? 0.0f : (float) atan2 ( y, x ) ;
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| 77 | *az = ( a < 0.0f ) ? (float) ( (double) a + D2PI ) : a;
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| 78 | *el = (float) atan2 ( z, r );
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| 79 | }
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