Table of contents
CSPICE_DVHAT calculates the unit vector corresponding to a state and
the time derivative of the unit vector.
Given:
s1 any double precision state.
help, s1
DOUBLE = Array[6]
If the position component of the state is the zero vector, this
routine will detect it and will not attempt to divide by zero.
the call:
cspice_dvhat, s1, sout
returns:
sout a state containing the unit vector pointing in the direction of
position component of `s1' and the derivative of the unit vector
with respect to time.
help, sout
DOUBLE = Array[6]
`sout' may overwrite `s1'.
None.
Any 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.
1) Suppose that `state' gives the apparent state of a body with
respect to an observer. This routine can be used to compute the
instantaneous angular rate of the object across the sky as seen
from the observers vantage.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File name: dvhat_ex1.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
--------- --------
de421.bsp Planetary ephemeris
pck00008.tpc Planet orientation and
radii
naif0009.tls Leapseconds
\begindata
KERNELS_TO_LOAD = ( 'de421.bsp',
'pck00008.tpc',
'naif0009.tls' )
\begintext
End of meta-kernel
Example code begins here.
PRO dvhat_ex1
;;
;; Load SPK, PCK, and LSK kernels, use a meta kernel for
;; convenience.
;;
cspice_furnsh, 'dvhat_ex1.tm'
;;
;; Define an arbitrary epoch, convert the epoch to ephemeris time.
;;
EPOCH = 'Jan 1 2009'
cspice_str2et, EPOCH, et
;;
;; Calculate the state of the moon with respect to the earth-moon
;; barycenter in J2000, corrected for light time and stellar
;; aberration at `et'.
;;
target = 'MOON'
frame = 'J2000'
abcorr = 'LT+S'
observer = 'EARTH BARYCENTER'
cspice_spkezr, target, et, frame, abcorr, observer, state, ltime
;;
;; Calculate the unit vector of `state' and the derivative of the
;; unit vector.
;;
cspice_dvhat, state, ustate
;;
;; Calculate the instantaneous angular velocity from the magnitude
;; of the derivative of the unit vector.
;;
;; v = r x omega
;;
;; ||omega|| = ||v|| for r . v = 0
;; -----
;; ||r||
;;
;; ||omega|| = ||v|| for ||r|| = 1
;;
omega = cspice_vnorm( ustate[3:5] )
print, 'Instantaneous angular velocity, rad/sec', omega
;;
;; It's always good form to unload kernels after use,
;; particularly in IDL due to data persistence.
;;
cspice_kclear
END
When this program was executed on a Mac/Intel/IDL8.x/64-bit
platform, the output was:
Instantaneous angular velocity, rad/sec 2.4810666e-06
Let `s1' be a state vector with position and velocity components P
and V respectively. From these components one can compute the
unit vector parallel to P, call it U and the derivative of U
with respect to time, DU. This pair (U,DU) is the state returned
by this routine in `sout'.
1) If `s1' represents the zero vector, then the position
component of `sout' will also be the zero vector. The
velocity component will be the velocity component
of `s1'.
2) If the input argument `s1' is undefined, an error is signaled
by the IDL error handling system.
3) If the input argument `s1' is not of the expected type, or it
does not have the expected dimensions and size, an error is
signaled by the Icy interface.
4) If the output argument `sout' is not a named variable, an
error is signaled by the Icy interface.
None.
None.
ICY.REQ
None.
J. Diaz del Rio (ODC Space)
E.D. Wright (JPL)
-Icy Version 1.0.6, 10-AUG-2021 (JDR)
Added -Parameters, -Exceptions, -Files, -Restrictions,
-Literature_References and -Author_and_Institution sections. Edited the
header to comply with NAIF standard.
Added meta-kernel to the example. Added -Particulars section.
Removed reference to the routine's corresponding CSPICE header from
-Abstract section.
Added arguments' type and size information in the -I/O section.
-Icy Version 1.0.5, 09-MAY-2016 (EDW)
Eliminated typo in example code; no change to functionality.
-Icy Version 1.0.4, 04-MAY-2010 (EDW)
Added complete example code.
-Icy Version 1.0.3, 03-DEC-2009 (EDW)
Edited header, improved/clarified -I/O descriptions.
-Icy Version 1.0.2, 09-DEC-2005 (EDW)
Added -Examples section.
-Icy Version 1.0.1, 10-OCT-2005 (EDW)
Corrected notation |r| to ||r||; edited header.
-Icy Version 1.0.0, 16-JUN-2003 (EDW)
State of a unit vector parallel to a state vector
|