Introduction
The U.S. Navy Geosat Altimeter Mission lasted for nearly 5 years, from
April 1985 to January 1990. Geophysical data records (GDRs) from the final
3 years, when the satellite was performing the exact repeat mission (ERM),
were produced by NOAA's National Ocean Service and have been widely
distributed. The intense interest in these data is indicated by the many
articles published in the March and October 1990 issues of the Journal of
Geophysical Research (Oceans) dedicated to the Geosat mission. In contrast,
GDRs from Geosat's first 18 months (April 1985 to September 1986), known as
the Geodetic Mission (GM), are classified because of military applications
related to knowledge of the marine geoid. Only the altimetric heights are
considered secret, however, and unclassified subsets of these data have
been released. For example, global Geosat wind speed and wave height data
from the GM are available through NOAA as are the GM GDRs south of 30S.
Another unclassified data type is the crossover difference. A crossover is
defined as the intersection of the satellite ground track with itself. At
this location, the two crossing passes (one ascending and one descending)
provide independent sea level measurements at the same place but at different
times. The difference of these two sea level measurements (a crossover
difference) is not considered classified because the geo-potential component
of sea level is the same at the intersection of the two tracks and thus
cancels. Crossover differences contain information about uncertainties in
the satellite ephemeris and therefore enable correction of radial orbit
error. Orbit-corrected crossover differences form the basis for studies
of sea level variability, both in a statistical sense and for computation of
sea level time series. Tide model studies represent another type of
crossover difference application.
CD-ROM Contents
The Geosat Altimeter Crossover Difference records (XDRs) contained on this
CD-ROM are documented in a NOAA handbook. Although only the 18-month GM
data are classified, XDRs have been constructed for the first 2.5 years of
the Geosat mission, that is, data from the GM plus the first year of the
ERM. This was done to minimize the effect of the 5-week data gap
(October 1 to November 8, 1986) between the two missions when the altimeter
was off during orbit maneuvers. By computing crossovers for the entire
2.5-year period, continuous sea level time series can be derived. It is
important to understand that sea level time series and other oceanographic
information can only be derived from XDRs which have been adjusted to
remove radial orbit error (approximately 1 m amplitude). The XDRs contain
all information necessary to perform such adjustments, but no orbit
corrections are provided. Removal of orbit error is essentially a filtering
problem, and different oceanographic applications require different
approaches. The National Oceanographic Data Center is responsible for
disseminating Geosat data to the research community. Four introductory
files are included on each disc to provide information for working with the
Geosat Crossovers. Also included on disk 8 of 8 are two global XDR sets,
described in Section III of the READ_ME file found on each CD. The Geosat
XDRs on each CD-ROM are stored in files, each of which covers a particular
ocean region. The naming convention used for the files is: REG_# where #
is the number of the region as shown by figure 6 in the XDR handbook. If
the region had to be split into two parts to fit onto CDs, a or b is added.
For example, REG_06a is the first part of region 6, and REG_06b is the
second part of region 6. Although regions 4 and 7 are exceptions, note
that in general the regions are arranged in latitude bands as follows:
Regions and Latitude Band
| Regions | Latitude Bands |
| 1, 2, 3, 4: | 35N to 64N |
| 5, 6: | 6N to 35N |
| 7, 8, 9, 10: | 35S to 6N |
| 11 - 18: | 65S to 35S |
(A world map showing the boundaries for each regions is available on
request from the NODC)
Data in these files are in binary Hewlett-Packard (HP) form, similar to the
Geosat Exact Repeat Mission "T2" GDRs distributed on CD-ROM. The format
description is discussed in the XDR handbook and a tabular summary is given
in file 2 of each disc. The files can be manipulated using standard MS-DOS
commands, transferred over a network to a host computer, or accessed directly
on the CD-ROM. The data are in "Hewlett-Packard" binary format. Each disk
contains a Fortran program file to convert from HP binary to VAX binary.
Records are 72 bytes in length.
How to Get More Information:
Technical questions about reading the CD-ROM, the content
of the XDRs, or scientific applications should be addressed to:
John Lillibridge
NOAA Laboratory for Satellite Altimetry
NOAA/NESDIS
SSMC3, Room 3853
1315 East West Highway
Silver Spring, MD 20910-3232
Phone: 301-713-2857
Fax: 301-713-4598
E-mail:
John.Lillibridge@noaa.gov
Please note: Due to restrictions placed on NODC by the U.S.
Navy, non-US requests must be submitted via the Science Office of your
nation's Embassy in the United States. Please contact your Embassy and
have them request the data on your behalf.
How to Order:
