Document title: Readme.txt for NDADS DE MAGB_GMS_62MS_V datatype
Project: DE
NDADS Datatype: MAGB_GMS_62MS_V
Super-EID: DOCUMENT                      
There may be other documents also identified by this super-EID.
NDADS filename: MAGB_GMS_README.TXT                       
TRF entry: B46546.txt in NSSDC's controlled digital document library, Feb. 1998.
Document text follows:
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DYNAMICS EXPLORER 2 (DE-2) MAGNETOMETER (MAG-B) HIGH RESOLUTION GMS DATA

This data set contains the MAG-B high resolution data in GMS coordinates with
the following orbit-attitude (OA) parameters added to the data set:  model
magnetic field, magnetic local time, invariant latitude, spacecraft altitude,
and geomagnetic longitude and latitude. The three components of the magnetic
field in this Cartesian system are B-Radial (positive outward), B-Theta
(positive southward) and B-Phi (positive eastward), all oriented relative to
the MAGSAT geomagnetic field model (Langel et al., GRL, 7, 793, 1980). 

As with most telemetered geophysical data, the vector components of the
magnetic field archived here suffer from occasional bad data points. These
spurious data entries were caused for the most part by noise introduced in the
satellite station telemetry link. Such spurious data can usually be recognized
by workers familiar with similar satellite magnetic field data sets. These are
usually single point excursions which show no geophysical correlation between
the three components of the field. 

The dominant source of error in the DE-2 magnetic field measurements is the
uncertainty in the attitude of the spacecraft. The DE-2 spacecraft was designed
to obtain a final attitude knowledge of +0.3 degrees, which it appears to have
attained most of the time. However, episodes of spacecraft nutation with a
period near 30 sec and an amplitude of several tenths of a degree were common
as well as occasional several minute errors in the horizon sensor output which
produced sudden shifts in attitude of several tenths of a degree. Note that as
a general rule, an error in spacecraft attitude of 0.1 deg will cause an error
in the measured field of about 100 nT per component when the field is
transferred from spacecraft to inertial coordinates. For this reason it is
common practice to make "baseline corrections" using various physical arguments
if higher accuracies are required (e.g., Gary et al., JGR, 99, 11417, 1994). 
The absolute accuracy of the DE-2 magnetometer can be evaluated after launch
only through comparison with precision vector/scalar geomagnetic observatories
located on the ground. On the basis of such cross-comparisons a 1-sigma error
in the total field magnitude over the course of the DE-2 mission of +28 nT has
been determined (Langel, Geological Soc. of India Mem., 24, 3, 1992). 

The data set consists of daily files from day 81227 to 83047.  Each file
contains all the available data for a given day.  If there were no magnetometer
data for a given time, the time record was left out.  If there were
magnetometer data but no OA data for a particular time record, fill value of
9999999.0 were used for the OA data. 

Each daily file contains a different number of records depending upon 
how much data were taken for a given day.  

Logical record: Fixed length, 60 byte records for binary files, VMS binary

Year	integer*4
Day	integer*4 Day of year, January 1 = day 1
Time	integer*4 Time of measurement in milliseconds from beginning of day
Spare	integer*4 Spare data field
BR	real*4    Radial-Component of Measured magnetic field in GMS coord., nT
BTH	real*4    Theta-Component of Measured magnetic field in GMS coord., nT
BPH	real*4    Phi-Component of Measured magnetic field in GMS coord., nT
MR	real*4    Radial-Component of Model magnetic field in GMS coord., nT
MTH	real*4    Theta-Component of Model magnetic field in GMS coord., nT
MPH	real*4    Phi-Component of Model magnetic field in GMS coordinates, nT
GALT	real*4    Satellite altitude, km
MLONG	real*4    Magnetic longitude, degrees, -90.0 to 90.0 deg.
MLAT	real*4    Magnetic latitude, degrees, -180.0 to 180.0 deg.
MLT	real*4    Magnetic local time, hours, see Note 1
ILAT	real*4    Invariant latitude, degrees, see Note 1

The files are unformatted so the files may be read using the following FORTRAN
statement:

       READ(UNIT) YEAR, DAY, TIME, SPARE, BR, BTH, BPH, MR, MTH, MPH, GALT,
     + MLONG, MLAT, MLT, ILAT

Remember to declare year, day, time, spare as integer*4.

================================================================
Note 1: The MLT and ILAT algorithms were supplied by M. Sugiura (PI for the
Magnetometer Investigation) prior to launch and used in the generation of the
Orbit-Attitude database.

SOFTWARE

Two IDL (Interactive Data Language from Research Systems, Inc.) procedures are
included with this data set.  The first IDL procedure reads the data and does
not average.  The second program reads and averages the data.  A brief
explanation of the IDL procedures follows: 

   1. Program DEHIRES_load loads the high resolution magnetometer data for
      a time interval no greater than one hour.

      dehires_load, filename, start, finish, br, bth, bph, mod_br, mod_bth,$
                    mod_bph, galt, mlt, ilat, mlong, mlat, time, flag
      Inputs:   Daily file name, start time, end time
      Outputs:  Br, Bth, Bph, Mr, Mth, Mph, altitude, magnetic local time,
                invariant latitude, magnetic longitude, magnetic latitude, 
                time, and a flag if there was no data in the requested interval.

   2. Program DELOAD_AVE loads and averages the magnetometer data.

      deload_ave, filename, start, finish, res, abr, abth, abph, amod_br,$
                   amod_bth, amod_bph, mid_alt, mid_mlt, mid_ilat, mid_mlat,$
                   mid_mlng, mid_time, flag
      Inputs:   Daily file name, start time, end time, averaging resolution
      Outputs:  Averaged Br, Bth, Bph, Mr, Mth, Mph, and midpoints of altitude,
                magnetic local time, invariant latitude, magnetic longitude,
                magnetic latitude, time, and a flag if there was no data in 
                the interval that was to be averaged.

   Note: For both procedures, the start and stop input times should be in
         milliseconds. The averaging resolution input to DELOAD_AVE should be
         "1/2", "1/4" or "1/8" (second).

Updated 1-20-95, J. Slavin
Updated, 1995 Aug 28; Robert.M.Candey.1@gsfc.nasa.gov