cspice_dvnorm

 Abstract I/O Examples Particulars Required Reading Version Index_Entries
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#### Abstract

```
CSPICE_DVNORM returns the derivative of the vector norm of a 3-vector.

For important details concerning this module's function, please refer to
the CSPICE routine dvnorm_c.

```

#### I/O

```
Given:

state   a double precision 6-vector, the second three components
of the vector being the derivatives of the first three
with respect to some scalar.

state =  ( x, dx )
--
ds

A common form for 'state' would contain position and
velocity.

the call:

dvnorm = cspice_dvnorm(state)

returns:

dvnorm   the double precision value of d||x|| corresponding
------
ds
to 'state'.

1/2         2    2    2  1/2
Where ||x|| = < x, x >    =  ( x1 + x2 + x3 )

v = ( dx1, dx2, dx3 )
---  ---  ---
ds   ds   ds

d||x||   < x, v >
------ =   ------    =  < xhat, v >
ds             1/2
< x, x >

```

#### Examples

```
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.

;;
;; Create several 6-vectors (6x1 arrays) with the structure
;;
;;   s = |  x  |
;;       |     |
;;       |  dx |
;;       |  -- |
;;       |  ds |
;;
;; where 'x' is a 3-vector (3x1 array).
;;

;;
;; Create 's' with 'x' of varying magnitudes. Use 'x'
;; and '-x' to define the derivative as parallel and
;; anti-parallel.
;;
mag = [ -4.D, 4, 12 ]

x   = [ 1.D, sqrt(2.D), sqrt(3.D) ]

s1  = [ x * 10.D^mag,  x]
s2  = [ x * 10.D^mag, -x]
s3  = [ dblarr(3), x * 10.D^mag ]

;;
;; Calculate the derivative of the vector norms with respect
;; to 's'.
;;
dvnorm1 = cspice_dvnorm( s1 )
dvnorm2 = cspice_dvnorm( s2 )
dvnorm3 = cspice_dvnorm( s3 )

print, 'Parallel x, dx/ds         : ', dvnorm1
print, 'Anti-parallel x, dx/ds    : ', dvnorm2
print, 'Zero vector x, large dx/ds: ', dvnorm3

IDL outputs:

Parallel x, dx/ds         :        2.4494897
Anti-parallel x, dx/ds    :       -2.4494897
Zero vector x, large dx/ds:        0.0000000

```

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None.

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```
ICY.REQ

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#### Version

```
-Icy Version 1.0.0, 10-MAY-2010, EDW (JPL)

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#### Index_Entries

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derivative of 3-vector norm

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`Wed Apr  5 17:58:00 2017`