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Index Page recazl_c

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Procedure Abstract Required_Reading Keywords Brief_I/O Detailed_Input | Detailed_Output Parameters Exceptions Files Particulars Examples | Restrictions Literature_References Author_and_Institution Version Index_Entries

#### Procedure

       recazl_c ( Rectangular coordinates to AZ/EL )

       void recazl_c ( ConstSpiceDouble    rectan [3],
                       SpiceBoolean        azccw,
                       SpiceBoolean        elplsz,
                       SpiceDouble       * range,
                       SpiceDouble       * az,
                       SpiceDouble       * el         )

#### Abstract

       Convert rectangular coordinates of a point to range, azimuth and
       elevation.

#### Required_Reading

       None.

#### Keywords

       CONVERSION
       COORDINATES

#### Brief_I/O

       VARIABLE  I/O  DESCRIPTION
       --------  ---  --------------------------------------------------
       rectan     I   Rectangular coordinates of a point.
       azccw      I   Flag indicating how Azimuth is measured.
       elplsz     I   Flag indicating how Elevation is measured.
       range      O   Distance of the point from the origin.
       az         O   Azimuth in radians.
       el         O   Elevation in radians.

#### Detailed_Input

       rectan      are the rectangular coordinates of a point.

       azccw       is a flag indicating how azimuth is measured.

                   If `azccw' is SPICETRUE, azimuth increases in the
                   counterclockwise direction; otherwise it increases in
                   the clockwise direction.

       elplsz      is a flag indicating how elevation is measured.

                   If `elplsz' is SPICETRUE, elevation increases from
                   the XY plane toward +Z; otherwise toward -Z.

#### Detailed_Output

       range       is the distance of the point from the origin.

                   The units associated with `range' are those associated
                   with the input point.

       az          is the azimuth of the point. This is the angle between
                   the projection onto the XY plane of the vector from the
                   origin to the point and the +X axis of the reference
                   frame. `az' is zero at the +X axis.

                   The way azimuth is measured depends on the value of the
                   logical flag `azccw'. See the description of the argument
                   `azccw' for details.

                   `az' is output in radians. The range of `az' is [0, 2*pi].

       el          is the elevation of the point. This is the angle between
                   the vector from the origin to the point and the XY
                   plane. `el' is zero at the XY plane.

                   The way elevation is measured depends on the value of
                   the logical flag `elplsz'. See the description of the
                   argument `elplsz' for details.

                   `el' is output in radians. The range of `el' is [-pi/2,
                   pi/2].

#### Parameters

       None.

#### Exceptions

       Error free.

       1)  If the X and Y components of `rectan' are both zero, the
           azimuth is set to zero.

       2)  If `rectan' is the zero vector, azimuth and elevation
           are both set to zero.

#### Files

       None.

#### Particulars

       This routine returns the range, azimuth, and elevation of a point
       specified in rectangular coordinates.

       The output is defined by the distance from the center of the
       reference frame (range), the angle from a reference vector
       (azimuth), and the angle above the XY plane of the reference
       frame (elevation).

       The way azimuth and elevation are measured depends on the values
       given by the user to the `azccw' and `elplsz' logical flags. See the
       descriptions of these input arguments for details.

#### Examples

       The numerical results shown for these examples may differ across
       platforms. The results depend on the SPICE kernels used as
       input, the compiler and supporting libraries, and the machine
       specific arithmetic implementation.

       1) Create four tables showing a variety of rectangular
          coordinates and the corresponding range, azimuth and
          elevation, resulting from the different choices of the `azccw'
          and `elplsz' flags.

          Corresponding rectangular coordinates and azimuth, elevation
          and range are listed to three decimal places. Output angles
          are in degrees.

          Example code begins here.

          /.
             Program recazl_ex1
          ./
          #include <stdio.h>
          #include <string.h>
          #include "SpiceUsr.h"

          int main( )
          {

             /.
             Local parameters.
             ./
             #define NREC         11

             /.
             Local variables.
             ./
             SpiceChar            msg    [31];

             SpiceDouble          az;
             SpiceDouble          el;
             SpiceDouble          range;
             SpiceInt             i;
             SpiceInt             j;
             SpiceInt             n;

             /.
             Define the input rectangular coordinates and the
             different choices of the `azccw' and `elplsz' flags.
             ./
             SpiceDouble          rectan [NREC][3] = { {0.0,0.0,0.0},
                                                       {1.0,0.0,0.0},
                                                       {0.0,1.0,0.0},
                                                       {0.0,0.0,1.0},
                                                       {-1.0,0.0,0.0},
                                                       {0.0,-1.0,0.0},
                                                       {0.0,0.0,-1.0},
                                                       {1.0,1.0,0.0},
                                                       {1.0,0.0,1.0},
                                                       {0.0,1.0,1.0},
                                                       {1.0,1.0,1.0} };

             SpiceBoolean         azccw  [2] = { SPICEFALSE,  SPICETRUE };
             SpiceBoolean         elplsz [2] = { SPICEFALSE,  SPICETRUE };

             /.
             Create a table for each combination of `azccw' and `elplsz'.
             ./
             for ( i = 0; i < 2; i++ )
             {
                for ( j = 0; j < 2; j++ )
                {

                   /.
                   Display the flag settings.
                   ./
                   strncpy( msg, "AZCCW = #; ELPLSZ = #", 22 );
                   repml_c ( msg, "#", azccw[i], 'C', 31, msg );
                   repml_c ( msg, "#", elplsz[j], 'C', 31, msg );

                   printf( "\n" );
                   printf( "%s\n", msg );

                   /.
                   Print the banner.
                   ./
                   printf( "\n" );
                   printf( "  rect[0]  rect[1]  rect[2]   range      az   "
                           "    el\n" );
                   printf( "  -------  -------  -------  -------  ------- "
                           " -------\n" );

                   /.
                   Do the conversion. Output angles in degrees.
                   ./
                   for ( n = 0; n < NREC; n++ )
                   {
                      recazl_c ( rectan[n], azccw[i], elplsz[j],
                                 &range,    &az,      &el );

                      printf( "%9.3f %8.3f %8.3f %8.3f %8.3f %8.3f\n",
                              rectan[n][0], rectan[n][1], rectan[n][2],
                              range,        az * dpr_c(), el * dpr_c() );
                   }
                }
             }

             return ( 0 );
          }

          When this program was executed on a Mac/Intel/cc/64-bit
          platform, the output was:

          AZCCW = False; ELPLSZ = False

            rect[0]  rect[1]  rect[2]   range      az       el
            -------  -------  -------  -------  -------  -------
              0.000    0.000    0.000    0.000    0.000    0.000
              1.000    0.000    0.000    1.000    0.000    0.000
              0.000    1.000    0.000    1.000  270.000    0.000
              0.000    0.000    1.000    1.000    0.000  -90.000
             -1.000    0.000    0.000    1.000  180.000    0.000
              0.000   -1.000    0.000    1.000   90.000    0.000
              0.000    0.000   -1.000    1.000    0.000   90.000
              1.000    1.000    0.000    1.414  315.000    0.000
              1.000    0.000    1.000    1.414    0.000  -45.000
              0.000    1.000    1.000    1.414  270.000  -45.000
              1.000    1.000    1.000    1.732  315.000  -35.264

          AZCCW = False; ELPLSZ = True

            rect[0]  rect[1]  rect[2]   range      az       el
            -------  -------  -------  -------  -------  -------
              0.000    0.000    0.000    0.000    0.000    0.000
              1.000    0.000    0.000    1.000    0.000    0.000
              0.000    1.000    0.000    1.000  270.000    0.000
              0.000    0.000    1.000    1.000    0.000   90.000
             -1.000    0.000    0.000    1.000  180.000    0.000
              0.000   -1.000    0.000    1.000   90.000    0.000
              0.000    0.000   -1.000    1.000    0.000  -90.000
              1.000    1.000    0.000    1.414  315.000    0.000
              1.000    0.000    1.000    1.414    0.000   45.000
              0.000    1.000    1.000    1.414  270.000   45.000
              1.000    1.000    1.000    1.732  315.000   35.264

          AZCCW = True; ELPLSZ = False

            rect[0]  rect[1]  rect[2]   range      az       el
            -------  -------  -------  -------  -------  -------
              0.000    0.000    0.000    0.000    0.000    0.000
              1.000    0.000    0.000    1.000    0.000    0.000
              0.000    1.000    0.000    1.000   90.000    0.000
              0.000    0.000    1.000    1.000    0.000  -90.000
             -1.000    0.000    0.000    1.000  180.000    0.000
              0.000   -1.000    0.000    1.000  270.000    0.000
              0.000    0.000   -1.000    1.000    0.000   90.000
              1.000    1.000    0.000    1.414   45.000    0.000
              1.000    0.000    1.000    1.414    0.000  -45.000
              0.000    1.000    1.000    1.414   90.000  -45.000
              1.000    1.000    1.000    1.732   45.000  -35.264

          AZCCW = True; ELPLSZ = True

            rect[0]  rect[1]  rect[2]   range      az       el
            -------  -------  -------  -------  -------  -------
              0.000    0.000    0.000    0.000    0.000    0.000
              1.000    0.000    0.000    1.000    0.000    0.000
              0.000    1.000    0.000    1.000   90.000    0.000
              0.000    0.000    1.000    1.000    0.000   90.000
             -1.000    0.000    0.000    1.000  180.000    0.000
              0.000   -1.000    0.000    1.000  270.000    0.000
              0.000    0.000   -1.000    1.000    0.000  -90.000
              1.000    1.000    0.000    1.414   45.000    0.000
              1.000    0.000    1.000    1.414    0.000   45.000
              0.000    1.000    1.000    1.414   90.000   45.000
              1.000    1.000    1.000    1.732   45.000   35.264

       2) Compute the apparent azimuth and elevation of Venus as seen
          from the DSS-14 station.

          Task Description
          ================

          In this example, we will obtain the apparent position of
          Venus as seen from the DSS-14 station in the DSS-14 topocentric
          reference frame. We will use a station frames kernel and
          transform the resulting rectangular coordinates to azimuth,
          elevation and range using azlrec_c.

          In order to introduce the usage of the logical flags `azccw'
          and `elplsz', we will request the azimuth to be measured
          clockwise and the elevation positive towards the +Z
          axis of the DSS-14_TOPO reference frame.

          Kernels
          =======

          Use the meta-kernel shown below to load the required SPICE
          kernels.

             KPL/MK

             File name: recazl_ex2.tm

             This meta-kernel is intended to support operation of SPICE
             example programs. The kernels shown here should not be
             assumed to contain adequate or correct versions of data
             required by SPICE-based user applications.

             In order for an application to use this meta-kernel, the
             kernels referenced here must be present in the user's
             current working directory.

             The names and contents of the kernels referenced
             by this meta-kernel are as follows:

                File name                        Contents
                ---------                        --------
                de430.bsp                        Planetary ephemeris
                naif0011.tls                     Leapseconds
                earth_720101_070426.bpc          Earth historical
                                                 binary PCK
                earthstns_itrf93_050714.bsp      DSN station SPK
                earth_topo_050714.tf             DSN station FK

             \begindata

             KERNELS_TO_LOAD = ( 'de430.bsp',
                                 'naif0011.tls',
                                 'earth_720101_070426.bpc',
                                 'earthstns_itrf93_050714.bsp',
                                 'earth_topo_050714.tf'         )

             \begintext

             End of meta-kernel.

          Example code begins here.

          /.
             Program recazl_ex2
          ./
          #include <stdio.h>
          #include "SpiceUsr.h"

          int main( )
          {

             /.
             Local parameters
             ./
             #define META         "recazl_ex2.tm"

             /.
             Local variables
             ./
             SpiceChar          * abcorr;
             SpiceChar          * obs;
             SpiceChar          * obstim;
             SpiceChar          * ref;
             SpiceChar          * target;

             SpiceDouble          az;
             SpiceDouble          el;
             SpiceDouble          et;
             SpiceDouble          lt;
             SpiceDouble          ptarg  [3];
             SpiceDouble          r;

             SpiceBoolean         azccw;
             SpiceBoolean         elplsz;

             /.
             Load SPICE kernels.
             ./
             furnsh_c ( META );

             /.
             Convert the observation time to seconds past J2000 TDB.
             ./
             obstim = "2003 OCT 13 06:00:00.000000 UTC";

             str2et_c ( obstim, &et );

             /.
             Set the target, observer, observer frame, and
             aberration corrections.
             ./
             target = "VENUS";
             obs    = "DSS-14";
             ref    = "DSS-14_TOPO";
             abcorr = "CN+S";

             /.
             Compute the observer-target position.
             ./
             spkpos_c ( target, et, ref, abcorr, obs, ptarg, &lt );

             /.
             Compute azimuth, elevation and range of Venus
             as seen from DSS-14, with azimuth increasing
             clockwise and elevation positive towards +Z
             axis of the DSS-14_TOPO reference frame
             ./
             azccw  = SPICEFALSE;
             elplsz = SPICETRUE;

             recazl_c ( ptarg, azccw, elplsz, &r, &az, &el );

             /.
             Express both angles in degrees.
             ./
             el =   el * dpr_c();
             az =   az * dpr_c();

             /.
             Display the computed position, the range and
             the angles.
             ./
             printf( "\n" );
             printf( "Target:                %s\n", target );
             printf( "Observation time:      %s\n", obstim );
             printf( "Observer center:       %s\n", obs );
             printf( "Observer frame:        %s\n", ref );
             printf( "Aberration correction: %s\n", abcorr );
             printf( "\n" );
             printf( "Observer-target position (km):\n" );
             printf( "%21.8f %20.8f %20.8f\n", ptarg[0], ptarg[1], ptarg[2] );
             printf( "Light time (s):        %19.8f\n", lt );
             printf( "\n" );
             printf( "Target azimuth          (deg):  %19.8f\n", az );
             printf( "Target elevation        (deg):  %19.8f\n", el );
             printf( "Observer-target distance (km):  %19.8f\n", r );
             printf( "\n" );

             return ( 0 );
          }

          When this program was executed on a Mac/Intel/cc/64-bit
          platform, the output was:

          Target:                VENUS
          Observation time:      2003 OCT 13 06:00:00.000000 UTC
          Observer center:       DSS-14
          Observer frame:        DSS-14_TOPO
          Aberration correction: CN+S

          Observer-target position (km):
              66886767.37916669   146868551.77222887  -185296611.10841593
          Light time (s):               819.63862811

          Target azimuth          (deg):         294.48543372
          Target elevation        (deg):         -48.94609726
          Observer-target distance (km):   245721478.99272084

#### Restrictions

       None.

#### Literature_References

       None.

#### Author_and_Institution

       J. Diaz del Rio     (ODC Space)

#### Version

       -CSPICE Version 1.0.0, 01-NOV-2021 (JDR)

#### Index_Entries

       rectangular coordinates to range, az and el
       rectangular to range, azimuth and elevation
       convert rectangular coordinates to range, az and el
       convert rectangular to range, azimuth and elevation

#### Link to routine recazl_c source file recazl_c.c

    Fri Dec 31 18:41:11 2021