cspice_subpt |
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## AbstractCSPICE_SUBPT determines the coordinates of the sub-observer point on a target body at a particular epoch, optionally corrected for planetary (light time) and stellar aberration. The call also returns the observer's altitude above the target body. Deprecated: This routine has been superseded by the routine cspice_subpnt. This routine is supported for purposes of backward compatibility only. ## I/OGiven: method a string providing parameters defining the computation method to use. [1,c1] = size(method); char = class(method) or [1,1] = size(method); cell = class(method) The choices are: 'Near point' The sub-observer point is defined as the nearest point on the target relative to the observer. 'Intercept' The sub-observer point is defined as the target surface intercept of the line containing the observer and the target's center. In both cases, the intercept computation treats the surface of the target body as a triaxial ellipsoid. The ellipsoid's radii must be available in the kernel pool. Neither case nor white space are significant in 'method'. For example, the string ' NEARPOINT' is valid. target the name of the observed target body. 'target' is case-insensitive, and leading and trailing blanks in 'target' are not significant. Optionally, you may supply a string containing the integer ID code for the object. For example both 'MOON' and '301' are legitimate strings that indicate the moon is the target body. [1,c2] = size(target); char = class(target) or [1,1] = size(target); cell = class(target) This routine assumes that the target body is modeled by a tri-axial ellipsoid, and that a PCK file containing its radii has been loaded into the kernel pool via cspice_furnsh. et the epoch(s), expressed as seconds past J2000 TDB, of the observer: 'et' is the epoch at which the observer's state is computed. [1,n] = size(et); double = class(et) When aberration corrections are not used, 'et' is also the epoch at which the position and orientation of the target body are computed. When aberration corrections are used, 'et' is the epoch at which the observer's state relative to the solar system barycenter is computed; in this case the position and orientation of the target body are computed at et-lt or et+lt, where 'lt' is the one-way light time between the sub-observer point and the observer, and the sign applied to 'lt' depends on the selected correction. See the description of 'abcorr' below for details. abcorr the aberration correction to apply when computing the observer-target state and the orientation of the target body. [1,c3] = size(abcorr); char = class(abcorr) or [1,1] = size(abcorr); cell = class(abcorr) For remote sensing applications, where the apparent sub-observer point seen by the observer is desired, normally either of the corrections 'LT+S' 'CN+S' should be used. These and the other supported options are described below. 'abcorr' may be any of the following: 'NONE' Apply no correction. Return the geometric sub-observer point on the target body. Let 'lt' represent the one-way light time between the observer and the sub-observer point (note: NOT between the observer and the target body's center). The following values of 'abcorr' apply to the "reception" case in which photons depart from the sub-observer point's location at the light-time corrected epoch et-lt and *arrive* at the observer's location at 'et': 'LT' Correct for one-way light time (also called "planetary aberration") using a Newtonian formulation. This correction yields the location of sub-observer point at the moment it emitted photons arriving at the observer at 'et'. The light time correction uses an iterative solution of the light time equation. The solution invoked by the 'LT' option uses one iteration. Both the target position as seen by the observer, and rotation of the target body, are corrected for light time. 'LT+S' Correct for one-way light time and stellar aberration using a Newtonian formulation. This option modifies the state obtained with the 'LT' option to account for the observer's velocity relative to the solar system barycenter. The result is the apparent sub-observer point as seen by the observer. 'CN' Converged Newtonian light time correction. In solving the light time equation, the 'CN' correction iterates until the solution converges. Both the position and rotation of the target body are corrected for light time. 'CN+S' Converged Newtonian light time and stellar aberration corrections. This option produces a solution that is at least as accurate at that obtainable with the 'LT+S' option. Whether the 'CN+S' solution is substantially more accurate depends on the geometry of the participating objects and on the accuracy of the input data. In all cases this routine will execute more slowly when a converged solution is computed. The following values of 'abcorr' apply to the "transmission" case in which photons *depart* from the observer's location at 'et' and arrive at the sub-observer point at the light-time corrected epoch et+lt: 'XLT' "Transmission" case: correct for one-way light time using a Newtonian formulation. This correction yields the sub-observer location at the moment it receives photons emitted from the observer's location at 'et'. The light time correction uses an iterative solution of the light time equation. The solution invoked by the 'LT' option uses one iteration. Both the target position as seen by the observer, and rotation of the target body, are corrected for light time. 'XLT+S' "Transmission" case: correct for one-way light time and stellar aberration using a Newtonian formulation This option modifies the sub-observer point obtained with the 'XLT' option to account for the observer's velocity relative to the solar system barycenter. 'XCN' Converged Newtonian light time correction. This is the same as XLT correction but with further iterations to a converged Newtonian light time solution. 'XCN+S' "Transmission" case: converged Newtonian light time and stellar aberration corrections. obsrvr the scalar string name of the observing body. The observing body is an ephemeris object: it typically is a spacecraft, the earth, or a surface point on the earth. 'obsrvr' is case-insensitive, and leading and 'obsrvr' are not significant. Optionally, you may trailing blanks in supply a string containing the integer ID code for the object. For example both 'MOON' and '301' are legitimate strings that indicate the Moon is the observer. [1,c4] = size(obsrvr); char = class(obsrvr) or [1,1] = size(obsrvr); cell = class(obsrvr) the call: [spoint, alt] = ## ExamplesAny numerical results shown for this example may differ between platforms as the results depend on the SPICE kernels used as input and the machine specific arithmetic implementation. Example(1): % % Find the sub point position of the moon on the earth at % epoch JAN 1, 2006 using the "near point" then the "intercept" % options. Apply light time correction to return apparent position. % % % Load the meta kernel listing the needed SPK, PCK, LSK % kernels. % cspice_furnsh( 'standard.tm' ) % % Calculate the location of the sub point of the moon % on the earth . et = cspice_str2et( 'JAN 1, 2006' ); % % First use option 'Near Point' % [point1,alt1] = ## ParticularsA sister version of this routine exists named mice_subpt that returns the output arguments as fields in a single structure. ## Required ReadingFor important details concerning this module's function, please refer to the CSPICE routine subpt_c. MICE.REQ FRAMES.REQ PCK.REQ SPK.REQ TIME.REQ ## Version-Mice Version 1.0.3, 23-JUN-2014, NJB (JPL) Updated description of converged Newtonian light time correction. Replaced double quotes with single quotes in constant strings appearing in comments. -Mice Version 1.0.2, 18-MAY-2010, BVS (JPL) Index line now states that this routine is deprecated. -Mice Version 1.0.1, 11-NOV-2008, EDW (JPL) Edits to header; Abstract now states that this routine is deprecated. -Mice Version 1.0.0, 22-NOV-2005, EDW (JPL) ## Index_EntriesDEPRECATED sub-observer point |

Wed Apr 5 18:00:35 2017