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: ---------------------- 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