The GEOSAT Files:

This directory includes two files with geoid height and gravity
anomaly profiles along the ground track of the Geosat Exact Repeat
Mission. The data are the average of 44 repeat cycles as described
in the article "High-Accuracy, High-Resolution Gravity Profiles
From 2 Years of the Geosat Exact Repeat Mission" by D. T. Sandwell
and D. C. McAdoo (Journ. Geophys. Res., v. 95, p. 3049-3060, 1990).
Averaging many repeat cycles improves the accuracy (approx. 1 mgal)
and resolution (approx. 10 km) of the data as well as the coverage
in areas of seasonal sea ice. (For a description of the original,
unaveraged data see: Cheney, R.E., B.C. Douglas, R.W. Agreen, L.
Miller, and D. L. Porter, Geosat Altimeter Geophysical Data Record
(GDR) User Handbook, NOAA, Tech. Memo., NOS-NGS-46, 32 pp., Natl.
Ocean Serv., Rockville, Md., 1987).

     Along track gravity anomalies were computed from along track
vertical deflection by Fourier transformation of the profiles,
exchange of the real and imaginary components and inverse Fourier
transformation. For a more complete description of the method see:
Haxby, W.F., G.D. Karner, J.L. Labrecque and J.K. Weissel, Digital
Images of Combined Oceanic and Continental Data Sets and Their Use
in Tectonic Studies, EOS Trans. AGU, v. 64, p. 995-1004, 1983.
Since Geosat coverage is not dense in 2-dimensions, it was assumed
that the gravity anomalies are 1-dimensional and are lineated
perpendicular to each profile. Thus, these gravity anomaly profiles
will not necessarily agree with shipboard measurements of gravity
along the same trackline. Nevertheless, they reveal many previously
unknown details in the marine gravity field.

File Format

     This directory contains two files:

     geo44asc.bin (987755 records) contains the ascending profiles
which run southeast to northwest between 72S and 72N.

     geo44des.bin (991313 records) contains all of the descending
profiles.

The fields within each record are as follows:
     time_offset         2 byte integer giving the offset from the
                         beginning of each pass in units of 0.489
                         seconds.

     latitude            4 byte integer giving degrees north *
1000.

     longitude           4 byte integer giving degrees east of
                         Greenwich meridian (0 to 360) * 1000.

     geoid height        2 byte integer giving meters above
reference                          ellipsoid * 100.

     gravity anomaly          2 byte integer in milligal * 100.

     uncertainty in      2 byte integer in milligal * 100.
     gravity anomaly

The data are grouped by passes which consist of continuous profiles
having no tome gaps greater than 5 seconds (approx. 33 km).  The
first record of each pass is slightly different from the other
records; the time_offset field is 0, the gravity anomaly field
contains the number of records in the pass (as a 2 byte int), and
the uncertainty field contains 1000 (as a 2 byte int). The program
seehead.c reads the headers and skips the data.