by Richard T. Baldwin
The following section describes the processing involved with the UARS modeling project in 1994 at the Goddard Space Flight Center. All files referenced below can be found on CD: UARS001 in the Geomagnetics library.
The Upper Atmosphere Research Satellite (UARS) was launched in 1991 and collected magnetometer data through 1994. Data available at Code 921 span from 9/91 through 4/92 for selected quiet days in each month. The following notes discuss the acquisition, validation/processing, and modeling which has occurred to date.
UARS was launched into a circular orbit with an altitude of approximately 560 km and an inclination of 57 degrees on September 13, 1991. The triaxial fluxgate magnetometer associated with the Particle Environment Monitor (PEM) is mounted on a stick boom at the end of a 5 meter boom. The X component is measured along track while the Y and Z components are measure to the right and downward respectively. Once a month (approximately 36 days) the satellite rotates 180 degree around the Z axis (yaw maneuver); thus necessitating a sign change in the X and Y components. These changes are accounted for in the software that merges the ephemeris data with the magnetometer data.
The attitude solutions evolve from star cameras (with accuracy to 5-10 arc sec) and limb sensors. A system of angular momentum wheels and torquer bars are used to adjust the spacecraft orientation. The torquer bars are energized continuously in a varying way and are approximately 8 meters from the magnetometer. The torquer current fields are updated when magnetic field variations of greater than 1/5 arcsec occur. All 3 torquers change at the same time every 8 secs.
The attitude data is flagged if misaligned by more than 80 arcsec with the ephemeris information. The spacecraft origin is believed to be known to within 0.1 degrees; with dc offsets. Ephemeris information was provided by GPS tracking (cm accuracy).
There is a Braun permanent magnet located at the end of the 5 meter boom associated with the PEM. It is estimated that this contributes about 100-200 nT of noise. The resolution of the instrument is believed to be 2 nT.
Data processing at APL includes the follow procedures:
1) Binary counts reversed
2) Magnetometer counts, aspect magnetometer/torquer currents, AC channels (used for peak detectors : typical of auroral zone)
3) Noise rejection routine (computes 1st differences then sorts out adjacent difference signs and offsets)
4) Apply calibration matrix
5) Calculate torquer field as a time series of spacecraft coordinates
6) Subtract torquer field from magnetic field measurements at a high time resolution (0.1 sec) (data are in spacecraft coordinates, it assumed that magnetometer and spacecraft coordinates are the same).
7) Merge ephemeris (2 sec) with data and transform to Earth-centered inertial system from spacecraft.
The UARS magnetometer data is maintained by Brain Anderson of Johns Hopkin University Applied Physics Lab (JHU-APL). The data obtained by Code 921 were sent in daily files over the internet. Each file can be read with the UARS raw format (UARS).
An observation was recorded every 5 seconds. Selected quiet days from 9/91 to 4/92 are stored on /data/ in APL ASCII format. These data files are tarred as ``uarsvtuda.tar" (use the Unix command `tar -cvf tarfile' to explode the daily files). The number of quiet days selected is determined by the time of the yaw maneuvers and the disturbance level.
The daily UARS files were processed with /source/uars3.sc. This script accesses program /source/uarsform.f which reads the APL ASCII format for each daily file; computes positions (latitude and longitude) in geocentric coordinates; forms the coordinate transformation matrix; adds Dst, and computes geomagnetic latitude for optional 50 degree cutoff. The output file is in newfit binary format (BIN) and is stored on /data/ as a tar file ``uarsfitd.tar".
Plot files of the UARS data were generated with the script /source/datpg.sc which accesses the program /source/datapg.f. This routine reads newfit binary format yearly files; computes residuals for each pass along a continuous latitude scale (0 (equator); 90 N pole; 180 (equator); 270 S pole; 360 (equator)); optionally will compute torquer fields from torquer currents and modeled parameters; calculates ancillary information (dip equator, MLT, etc.), and writes a binary plot file with measured and residual X, Y, Z, and B values. These files are stored on /data/ in files designated ursYYDOY.plt where YY= year-1900 and DOY= day of year. The format is EPLT.
Much of the analysis of the UARS data was conducted as a training exercise (a precursor to Oersted) by Lars Toffner-Clausen of the Danish Meteorological Institute (DMI). The document `Initial Processing of UARS Magnetometer Data' by Toffner [Toffner-Clausen, 1996] contains many of the modeling procedures which were attempted. There exist a number of undocumented models which were also derived using various parameterizations.
Several utility routines have been written to aid in this research. The IDL routine /source/uarsplt1.pro plots UARS residuals and optionally can apply scale factors and plot multiple passes on each of several frames. It reads the binary plot file created by /source/dat1pg.sc or the scaled plot file generated during modeling. Program /source/torqest.sc uses source /source/torqest.f to compute the local torquer field as estimated from the APL torquer matrix.