The daily GDR data files, from both the Geodetic Mission (GM) and Exact Repeat Mission (ERM), are stored on a set of 10 CD-ROM disks as indicated in the table below:
| CD | Mission | Start Yr_Day | End Yr_Day | # GDRs |
|---|---|---|---|---|
| 1 | GM | 1985_090 | 1985_227 | 138 |
| 2 | GM | 1985_228 | 1985_365 | 138 |
| 3 | GM | 1986_001 | 1986_138 | 138 |
| 4 | GM | 1986_139 | 1986_273 | 135 |
| 5 | ERM | 1986_312 | 1987_082 | 136 |
| 6 | ERM | 1987_083 | 1987_235 | 153 |
| 7 | ERM | 1987_236 | 1988_040 | 163 |
| 8 | ERM | 1988_041 | 1988_210 | 170 |
| 9 | ERM | 1988_211 | 1989_031 | 187 |
| 10 | ERM | 1989_032 | 1989_364 | 298 |
Each GDR covers a period of approximately 24 hours and contains data for either 14 or 15 complete revolutions, beginning and ending near 72 N. The first CD (beginning of the GM) and fifth CD (beginning of the ERM) contain documentation, example source code, secondary correction files, figures, and HTML files which allow the user's Web browser to navigate the information on those disks. The remaining eight disks contain only GDR data files.
The HTML file, "index.htm", is at the root level of the CD along with the "read_me.txt" file, which gives a brief introduction to the data set and instructions for viewing the files on the CDs. Several directories are found at the root level of the CD:
/docs - text, HTML, and PostScript versions of this handbook, plus tabular information related to the overall mission (see 2b below).
/code - example programs allowing the user to make listings of the binary GDR files, and to convert between Unix and PC/VAX binary formats (see 2c below).
/figures - high-quality "PostScript" and bit-mapped "GIF" illustrations that are referred to by this handbook.
/cyc_maps - maps showing the global distribution of Geosat data for each cycle of the GM and ERM. The "gif " subdirectory contains images that can be viewed directly with a Web browser; the "ps" subdirectory contains PostScript versions of the cycle maps. CD#1 contains the 23-day cycle GM maps and CD#5 contains the 17-day cycle ERM maps. Note that these maps were generated from daily GDRs, starting from the beginning of the GM and ERM, so the "cycle" definitions don't exactly agree with the cycle definitions in the equator crossing tables.
/scnd_cxn - files to be used to make secondary corrections to the altimeter data, as discussed in Section 4 of this handbook.
/gdrs - daily GDR data files (on all 10 of the CDs).
Several files in the "docs" directory provide the user with additional tabular information. The format of the GDR files is contained in GDR_FMT.TXT and is also presented in Section 3 below.
The GM_EQC.TXT and ERM_EQC.TXT files provide lists of all Geosat equator crossings for the GM and ERM. These tables are useful for describing the collinear aspects of the two mission phases. Cycle periods are approximately 23 days for the GM (the near-repeat period) and 17 days for the ERM. A pass is defined as a half-revolution from pole-to-pole and pass # 1 in each cycle is defined as the ascending pass closest to the Greenwich meridian. All subsequent passes follow in time order. The ascending passes are therefore all odd numbered and the descending passes are even. Note that because of this arbitrary numbering system, the first cycle of both the GM and ERM do not begin with pass #1.
The relationship between equator crossing longitudes and pass numbers for all GM and ERM cycles is illustrated graphically in four plots: GM ascending, GM descending, ERM ascending, and ERM descending. In all four plots, the symbols are color-coded by cycle number. During the GM the slow drift of the orbit causes a rainbow effect as the equator crossing longitude moves westward for subsequent cycles. During the collinear ERM mission, the equator crossing longitudes are within +/- 2 km for all passes so that only the last cycle (in red) is displayed.
The ORB_EPO.TXT file documents the relationship between the (nominally 6-day) orbit ephemerides files generated at NASA/GSFC and the GDR data set. The start and stop times of each of the orbital solutions (arcs) is given, so the user will know when a "break" from one solution to the next occurs within the altimeter data. (Note there is no connection between the arc numbers given in this table and the pass numbers given in the equator crossing tables above.)
Two programs can be built from the contents of the "code" directory. Program GDR_SWAB.C is used to swap bytes in a GDR binary file to make the internal format compatible with PC, VAX, and other "Little-Endian" machines. The native format of the GDR files in this data set are compatible with "Big-Endian" machines, typical of most Unix workstations. The usage of this program (under Unix or DOS) is simply:
Program LIGDR.F is used to make text listings of the contents of the binary GDR files. The simplest usage is: "ligdr gdr". This lists a subset of the most important variables in the "gdr". With the "-a" option, the user gets a complete listing of all the variables in each data record (refer to Section 3 below). The user can restrict the listing to a certain sequence of records using "-#" and/or "+#" where "#" is a lower or upper limit on the data records to list. A complete listing between records 1000 and 2000 would be given by:
