Information provided with POGO 2, 4, 6 CD-ROM Magnetometer Data Sets 65-081A-05I, 67-073A-06B, and 69-051A-21A _______________________________________________________________________________ This document was assembled in July, 1997 at NSSDC by H. K. Hills from: 1. Paper copy of information provided with the 10 POGO CD-ROMs (OGO 2, 4, 6); 2. Michael Purucker's July 10 e-mail announcement of "POGO magnetic field data availability"; and 3. Directory information from the CDs. The title was assigned, and this document was entered into NSSDC's Technical Reference File (TRF) as # B46054. This document is also archived in NSSDC's controlled digital document library. Updated August 5, 1997. _______________________________________________________________________________ The Geomagnetic Group at the Goddard Space Flight Center has completed a project in 1997 to merge the magnetics data from the Polar Orbiting Geophysical Observatory (OGO 2, 4, 6 otherwise known as POGO) with the ephemeris data. This important source of scalar data for geophysicists modeling the Earth's magnetic field has been previously available only in highly decimated form, or for specific local times or geographic regions. The low-altitude polar-orbiting OGOs collected data from all local times, in contrast to Magsat, which collected data from only two local times. However, Magsat carried vector magnetometers in addition to the scalar instrument. In the past, this data/ephemeris merging process was performed each time the data was accessed, and only a small sample of merged data was actually saved. With reconstructed programs, the entire set of Encyclopedia POGO data has finally been merged with the ephemeris data and is now available on CDROMs. However, the file sizes are very large, up to 255 MB. The Polar Orbiting Geophysical Observatories were large satellites with several other experiments on board. Measurements of the Earth's magnetic field were collected from October 1965 to April 1971. These scalar measurements were taken with a rubidium vapor magnetometer that was located at the end of a 6-meter boom. Some characteristics of the satellite orbits are listed below: NSSDC ID # Satellite Launch Date Inclination Perigee Apogee ----------- --------- ------------- ----------- ------- ------- 65-081A-05I OGO 2 Oct. 14, 1965 87.3 deg 410 km 1510 km 67-073A-06B OGO 4 Jul. 28, 1967 86.0 deg 410 km 910 km 69-051A-21A OGO 6 Jun. 5, 1969 82.0 deg 400 km 1100 km OGO 2 collected data until October 2, 1967. Due to an early failure in the attitude control system, the data is limited to dusk local times when the orbit was in full sunlight. OGO 4 collected data almost continuously until January 19, 1969. OGO 6 operated almost continuously until August 29, 1970 then sporadically until June 1971, although the latest data included are for April 26, 1971. OGO 2 and 4 sampled data at a rate of .5 seconds, and OGO 6 collected data every .288 seconds. The ephemeris data were collected on the ground by tracking stations then output from a program at once per minute. Both datasets are time ordered. The error associated with the POGO data from all known sources was determined to be 5.63 nT (Langel 1974). In order to include position information with each POGO data point, the magnetics data was merged with the ephemeris data. To account for the difference in the sampling rates, a Sterling five point interpolation was used. This routine, STIROB.F, uses five consecutive minutes of orbit data to obtain the X, Y, and Z position of the satellite for each point measured. The interpolated positions in celestial coordinates are then converted to latitude, longitude, and altitude with the routine SATPOS.F. This Fortran subroutine performs the transformation based on the rotation of the earth with respect to a Reference Day with the equation: DROT = DFRADY*6.3003894 + DELTDY*.01720214 + DSIDTM DFRADY Fraction of Day DELTDY Days since Reference Day DSIDTM Sidereal Time Both of these routines were also used to process data from the MAGnetic field SATellite (MAGSAT). Each CD includes source code for both of these routines, plus two other routines: POGOMRG.F, which merges POGO magnetics data with ephemeris data; and POGOCMP.F, which compares merged POGO data with previously merged data. Each CD also includes a README file, identifying the data and the original data sources. As duplicated by NSSDC, one CD also contains the August 5, 1997 copy of this present document. The POGO merged data are time ordered, in IEEE binary, and are in files as large as 255 MB. The record format is as follows: Variable Format Description -------- ------ --------------------------------------------- MJD INT Modified Julian Day (January 1, 1900 = 15020) MSEC INT Milliseconds of Day RLAT REAL Geocentric Latitude (degrees) RLON REAL Longitude (degrees) RALT REAL Altitude above Earth's surface (Km) B REAL Magnetic Field (nT) References: Langel, R. A., Processing of the Total Field Magnetometer Data from the OGO-2 Satellite, NASA/GSFC publication, X-612-67-272, 1967. [NSSDC Tech. Ref. File # B01414.] Langel, R. A., Near-Earth Magnetic Disturbance in Total Field at High Latitudes. 1. Summary of Data from OGO 2, 4, and 6, J. Geophys. Res., 79, 2363-2371, 1974. [NSSDC Tech. Ref. File # B18822.] Documentation for the OGO-F Experiment 21 Data Processing System, December 1970. [NSSDC Tech. Ref. File # B46097.] [This document will probably not be of interest to most users of the data sets, unless they want to know the details of the data processing operations. The document is comprised of five sections (all relating to the OGO-F Experiment 21 D.P.S.): 1. Description of formats (for 1st and 2nd job steps of the Encyclopedia Program OGOFENCY); 2. Program descriptions; 3. Common Area Descriptions; 4. User's Manual; 5. Generalized Routines - FTIO, DAIO, GENTRP, FMOVE, Orbit routines, Macros.] The two unpublished documents are available (in the form of scanned page-images) from NSSDC's controlled digital document library. ----------------------------------------------------------------------------- Data Inventory -------------- OGO 2 ----- 65-081A-05I 371.4 MB in 4 files. Volume Label = POGOMRG01 FILE NAME SIZE ------------ -------- EMRGOGO2.001 111.8 MB EMRGOGO2.002 106.7 MB EMRGOGO2.003 104.2 MB EMRGOGO2.004 48.7 MB ----------------------------------------------- OGO 4 ----- 67-073A-06B 1189.1 MB in 11 files. Volume Label = POGOMRG02 547.2 MB in 5 files. EMRGOGO4.001 116.1 MB EMRGOGO4.002 116.3 MB EMRGOGO4.003 115.8 MB EMRGOGO4.004 84 MB EMRGOGO4.005 115 MB Volume Label = POGOMRG03 313 MB in 3 files. EMRGOGO4.006 113.7 MB EMRGOGO4.007 110.3 MB EMRGOGO4.008 89 MB Volume Label = POGOMRG04 328.9 MB in 3 files. EMRGOGO4.009 101.9 MB EMRGOGO4.010 117.6 MB EMRGOGO4.012 109.4 MB Data dates fit between *4.007 and *4.008 ----------------------------------------------- OGO 6 ----- 69-051A-21A 2557.4 MB in 14 files. Volume Label = POGOMRG05 506.2 MB in 2 files. EMRGOGO6.001 250.8 MB EMRGOGO6.002 255.4 MB Volume Label = POGOMRG06 537.0 MB in 3 files. EMRGOGO6.003 251.1 MB EMRGOGO6.004 248.6 MB EMRGOGO6.005 37.3 MB Volume Label = POGOMRG07 493.7 MB in 2 files. EMRGOGO6.006 248.7 MB EMRGOGO6.007 245 MB Volume Label = POGOMRG08 529.2 MB in 3 files. EMRGOGO6.008 225.7 MB EMRGOGO6.009 83.9 MB EMRGOGO6.010 219.6 MB Volume Label = POGOMRG09 491.3 MB in 2 files. EMRGOGO6.011 246.6 MB EMRGOGO6.012 244.7 MB Volume Label = POGOMRG10 197.8 MB in 2 files. EMRGOGO6.013 146.7 MB EMRGOGO6.014 51.1 MB -----------------------------------------------