85
GEOSAT MEAN SEA SURFACE
ERM 1 TO 12
60
30
0
-30
-60
-110 -50 0 50 90
Figure 5 - Global Mean Sea Surface as computed from GEOSAT observations collected over the first
six months of the Exact Repeat Mission (ERM 1 to 12: November 1986 to May, 1987).
al., 1988; Marsh et al., 1989). The orbit error of the GEM-
T1-derived GEOSAT ephemerides is estimated to be at the
1-m level, while the respective accuracy of the GEM-T2
improved ephemerides is estimated to be at the 50 cm level
or better (Haines et al., 1990).
The algorithm portion of the operational system currently
being developed at CCS includes:
• the development of software to make an assessment of
the accuracies of the latest computed orbits of GEOSAT,
and to study the stability of sea surface solutions through
comparisons with the latest available high degree and order
geoidal solutions (e.g. OSU89); and
• the design of optimum methodologies for the
simultaneous crossover and collinear arc solutions, which
are necessary in order to remove various unmodelled
significant errors (e.g. orbit, ocean dynamic state errors
etc.) inherent in the altimetry observations.
Along with precise orbit information, an enhanced
processing of radar altimeter data allows the generation of
sea surface heights on a global or regular scale with high
spatial and temporal resolution. Figure 5 illustrates a
typical solution for the global mean sea surface using
roughly six months of data from the first twelve GEOSAT
17-day ERMs (November 8, 1986 to May 30, 1987).
Subsequent analyses with oceanographic and geodetic data
provides the additional capability to derive improved
models of the marine geoid and to estimate large scale
features of the sea surface topography.
The primary goal of the current studies is to provide a
homogeneous intermediate-wavelength (100-500 km) and
long-wavelength (>500 km) gravity database over the
global oceans, particularly for the support of: •
• mesoscale oceanographic tasks (e.g. detection of current
boundaries and mesoscale eddies);
• generating improved maps of geoid height, gravity
anomalies, and deflections of the vertical; and
• the definition of specific geophysical/geologic areas for
more detailed follow-up gravity surveys or resource
exploration studies.
Figure 6 illustrates typical world maps of altimetry-derived
Significant Wave Heights (SWH) computed over two
adjacent 5-day periods during ERM 1. They clearly portray
the early November rough seas in both the middle North
Atlantic and the Southern seas. The maps also reveal that
most of the oceans experience SWHs less than 4 metres. In
addition, like many other 5-day average maps we have
computed, they clearly slow that the sea state is
predominantly latitudinally structured and that is has long
wavelength nature. Christou (1990) in an extensive
treatment of GEOSAT data to study the space-time ocean
variability and its effects on the length of day has
demonstrated even more clearly the capability of satellite
altimetry to depict related space/time ocean surface current
variability. Figure 7 is just a "snap-shot" of zonally
averaged sea-level-slope time series within a longer
(2-year long) record of the mean surface ocean current
velocity fields (ibid). Each time series represents the zonal
average of current velocity fluctuations with respect to the
2-year long mean (Nov. '86 to Nov. '89) for every 1°
latitude band between 65° S and 65° N. The most obvious
observation that can be drawn from Figure 7 is that most of
the zonal velocity variability is confined within the north
and south tropical zones (i.e., between 23° S and 23° N),
while significant variability is observed in the northern
mid-latitudes (i.e., between 30° N and 50° N) and southern
higher latitudes (i.e., between 50° S and 60° S). The
northern mid-latitude zonal velocity fluctuations are likely
related to the large mid-latitude oceanic gyres (i.e. Gulf