get_fa_orbit returns orbit data as tplot variables (these are IDL data structures used
by the time series plotting routines distributed in the FAST IDL library).
This document describes
the values returned by the routine, as well as the
methods used to verify these return values. It does not describe how to call
the routine in detail, although a typical call would be:

`get_fa_orbit,t1,t2,/all,status=no_model,delta=1.,/definitive,/drag_prop`

where `t1`

and `t2`

are the start and stop times,

the `/all`

keyword means get all orbit parameters,

and `delta=1.`

means output the orbit data at 1 second time steps.

The last two key words define how to propagate the orbit forward in time.

For more details on calling the routine see the discussion in the get_fa_orbit procedure included in the FAST IDL software distribution.

The values returned by get_fa_orbit can be blocked into the following subsets:

origin: geographical position of the offset tilted dipole

z-axis: along dipole axis, positive to the north

y-axis: given by cross-product of earth spin axis (positive to the north) and dipole axis

x-axis: completes triad.

Note that the earth's spin axis lies in a plane parallel to the x-z plane of this coordinate system, and points to 180 degrees magnetic longitude.

Spacecraft position in geographic (spherical earth) coordinates is converted to magnetic coordinates by first translating to a system referenced to the geographic position of the offset tilted dipole, and then rotating to the coordinate system given above. This gives a modified radial distance (m_rad), as well as a magnetic latitude (m_lat) and magnetic longitude (m_lng).

For reference, the geographic coordinates of the offset tilted dipole used in get_fa_orbit are:

position: [-402.199, 287.504, 195.908] km

latitude: 79.3637 degrees

longitude: 288.454 degrees

`mag4wi.dat`

. Verified by direct comparison for
orbit 1761, using an independently coded IGRF model calculation.
NOTE: Any FAST software installation may contain more than one version of
`mag4wi.dat`

. For example, while a version of `mag4wi.dat`

that uses IGRF 1995 is
included in the "delivery" software, get_fa_orbit uses a version of the
file in `$FASTCONFIG/Fastorb/mag4wi.dat`

[`$FASTCONFIG`

is an environment variable
defined when running the FAST code]. This directory is meant to be used for local configuration files that over-ride the defaults,
but get_fa_orbit appears to *require* `$FASTCONFIG/Fastorb/mag4wi.dat`

be present.
Files in `$FASTCONFIG`

are not updated via "delivery."
When initially comparing models it was found that get_fa_orbit was using IGRF 1990. A
substantial improvement in the comparison between get_fa_orbit and the independently coded
IGRF model was found on replacing the
`$FASTCONFIG/Fastorb/mag4wi.dat`

file with the version in delivery.
The reference epoch of the `mag4wi.dat`

files installed can be checked by:

`grep " 199" `find /disks/fast/software/ -name "mag4wi.dat" -print``

where `/disks/fast/software`

should be replaced with the root directory for the site
specific FAST software installation.

The current version of the IGRF model used by FAST is the 1995 revision (also referred
to as the IGRF 7th generation model,
see IAGA Division V-MOD Geomagnetic Field Modeling: Minutes Sapporo 2003).
The file
`mag4wi.dat`

contains the coefficients
for this version of the IGRF model. You can use this file to update the
file in `$FASTCONFIG/Fastorb/mag4wi.dat`

, but this version will over-ride any future update in "delivery."

mean_earth_radius = 6371.2 ; mean radius of earth, in km

req = 6378.14 ; equatorial radius of earth, in km

rpo = 6356.76 ; polar radius of earth, in km

earth_flat_const = 1.006740 ; = 1 / (1 - f)^2, where f = 1/298.257 is the earth flattening factor

IGPP/UCLA Space Physics Center home page IGPP/UCLA FAST home page Bob Strangeway's home page

created by R. J. Strangeway.

email: strange@igpp.ucla.edu

Last modified: September 29, 2006.