ChampToGeo
Contents:
Overview
Potsdam's ISDC (Information System and Data Center)
provides Champ's magnetic field data in CDF format, but the latter does not
have its attributes (e.g. UNITS, FRAME, etc) compatbile with QSAS. The Champ's CDF file contains the following important variables:
GEO_LAT, GEO_LON, GEO_ALT, QUALITY and VEC. The altitude, GEO_ALT, is given in km above a reference sphere. This plug-in
combines the first three variables into a xyz vector in GEOC (geographic) frame.
Note that the helper file, CH_VEC_B_TOTAL.qat, must be located in the same folder as the Champ's
CDF file. This
file contains some meta-data for above variables so that, for example, VEC is recognized as a vector and can be plotted
easily with QSAS.
Input data
The plug-in requires the following inputs:
- the altitude in km and latitude and longitude in degrees. These variables
are given directly in the CDF file and they are used to produce a time series of vectors in geographic (GEOC) frame.
- an optional measured magnetic field vector in the NEC (North-East-Centroid) frame. The origin of this
frame is moving with the satellite. Conversion of a magnetic field vector from the NEC frame to any other
geocentric frame requires the knowledge of the latitude and longitude of the satellite. This plug-in will
convert the vector from the NEC frame to the geographic frame and then to the desired another frame. If this
conversion is not needed, then any non-matrix variable (e.g. value "0") can be given as input.
- The last parameter specifies the frame of output vectors. By default it is GEOC (geographic) frame and no
addition conversion is done. If another frame is specified, then the CoordRotation plug-in
is automatically used to produce the vector in the desired frame.
Note: the conversion to the geographic frame is just a matter of sin and cos trigonometric
functions. Any further transformation relies on the knowledge of the position of the Sun and/or the orientation of
the geomagnetic or magnetic dipole axis (i.e. on the magnetic field model). The eccentricity of the new frame might
also introduce additional round-off and systematic errors.
Outputs
The output is a time series of position vectors of the Champ satellite in the specified reference frame.
If a measurement vector in the NEC frame was given, then it is also converted to the desired reference frame.
Acknowledgement
We aknowledge the DFG VO 855/3 grant: "Multi-spacecraft analysis toolkit for Swarm".
Eugen Sorbalo, Jacobs University, 21 May 2012