Contact: [e-mail] ESA/CNES - 15 Years progress in Satellite Altimetry, Venice, Italy, 2006,

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The DNSC05-DNSC06 Global Marine Gravity Field.
Ole B. Andersen, and Per Knudsen, Danish National
Space Center. R. Trimmer and S. Kenyon, National
Geospatial Mapping Agency, St Louis, Mo, USA P.
Berry, De Montford University, Leiceister, UK
Lakes
Introduction. The latest version (DNSC05) of
the formerly KMS global marine gravity fields is
presented, This gravity field has a resolution of
1 minute by 1 minute and covers all marine
regions of the world including the Arctic Ocean
up to the North Pole. By starting out from the
original waveform data and retracking the entire
ERS-1 GM mission using a highly advanced expert
based system of multiple retrackers the return
time from both the open sea surface and from all
ice-covered regions within the coverage of the
ERS-1 can be derived with higher accuracy that
presently available. This presentation describes
the combined effort in improving the ERS-1 GM
dataset through retracking and regression to 2 Hz
(3 km) and its effect on gravity field modeling
close to the coast Extensive comparisons carried
out at the National Geospatial-Intelligence
Agency is also presented to document the
findings. The DNSC05 will not be released. The
DNSC06 will be released later this summer in
connection with the release of the global
geopotential model EGM2006.
The figure to the right shows a zoom over lakes
in central Africa. Using the EAPRS expert
retracking system data over the following lakes
have been recovered Lake Victoria, Lake
Tanganiyka Lake Rukwa and and Lake Malawi Even
smaller lakes close to Lake Victoria are seen to
be captures by the EAPRS data Below are
the 2-Hz data over Lake Victoria from ERS-GM
shown to the left and the derived Gravity
anomalies are shown to the right for the Victoria
Lake which is big enough to derive gravity
anomalies without the need for closeby land
gravity stations.
Retracking
ERS-GM have been retracked using the EAPRS
rule-based Expert System (P. Berry - De Montford
University). The EAPRS expert system contains
11 retrackers tuned for land,ocean, and ice
retracking. Retracker bias have been solved.
Additional data have been recovered from WAP
products. The retrack-inversion uses logaritmic
noise transformation. Lakes, Ice evertying is
retracked Problem only 88 of data available.
The Sea Ice retracker picks up many data in i.e.
polar regions that is not ocean retracket. This
retracker also picks up data near coast and in
currents (correct echos are similar in shape)
Large distribution of patch waveforms (44)
within 5 km of the coastline, and around 10 from
about 25 km from the coast can be seen. At
about 50 km from coastline only 60 of waveforms
in this region are ocean retracked.
Polar Regions
Inclusion of ICESAT data from the first two 3
month periods have enable the derivation and
extension of the Gravity field from 82N up until
86N so only a very small region is not covered
with satellite altimetry anymore. For this region
gravity anomalies from the ArcGP gravity field
project is inserted into the DNSC05 gravity field
to make its coverage global.
Unadjusted altimetric geoid height observations
in the ice-covered regions east of Greenland. The
left figure shows the 1Hz data in the Radar
Altimeter Database System (RADS), which uses the
data delivered by the ESA retracking system, and
the figure to the right shows the available 1 Hz
retracked altimetric data.
Comparison with Marine Data in theGulf Stream
Region.
The Effect of the Mean Dynamic Topography.
The Mean Dynamic Topography contriburtes to the
mean sea level halt N MDT This will have
an effect on the derived gravity field (roughly
1/100 of the total gravity field).
/- 300 mGal /- 3 mGal.
Sea level trend over 9 years (1993-2001) from the
TOPEX/POSEIDON Pathfinder data
Sea surface temperature trend over 9 years
(1993-2001) from the Reynolds AVHRR data The
colour scale has been tuned so that Greyish
colours correspond to no change in sea surface
temperature or sea level. Yellow and Red colours
correspond to an increase, and Blue and Green
colours correspond to a decrease. The colour
scale ranges between /- 0.3 degree per year for
the sea surface temperature changes and /- 2
centimetres per year for the sea level changes.