Topic Area

Identifying Ocean Fronts and Eddies with Satellite Altimetry

Project Team Member(s)

LCDR Keith Williams, USN

 


Major Findings

 

- Sea surface height is measured from satellite using radar.  Because the time versus position (ephemeris) of the satellite is known for each measurement, atmospheric corrections can be made and the altitude of the Earth¹s surface (ocean or otherwise) can be calculated.  

 

- Since currents are detectable as slopes in the sea surface, the worlds ocean currents can be detected and monitored. Small scale features are visible as well, like eddies, which are generated by the large scale currents (by the Gulf Stream, for example). Altimeter data is also used for tide modelling.1

 

- Radar Altimeter - The Radar Altimeter is a purely nadir looking instrument with a footprint of a few kilometers. It sends radar signals to the earth and ocean surface and collects the return pulse. The returned power as a function of travel time is called the wave form. Processing of the waveform provides information on: the wave height and wind speed (over oceans), the surface backscatter, and the height of the satellite above the surface. Together with a precisely computed orbital altitude, the latter gives the height of the surface above a well-defined geocentric reference frame. This provides the possibility to monitor the global ocean circulation, regional current systems, and to study the marine gravity field.2

 

- The U.S. Navy GEOdetic SATellite, GEOSAT, was launched on 12 March 1985.  Its altimeter was accurate to about 5 cm. After a classified mission for the Navy, GEOSAT's scientific Exact Repeat Mission (ERM) began on 8 November 1986 after it was maneuvered into a seventeen-day repeat orbit.  This orbit offers the opportunity to determine the mean sea levels at these points (information on local gravity can be inferred from this data), and to study sea level changes (and thus ocean variability). When the ERM ended in January 1990, due to failure of the two on board tape recorders, more than three years of precise altimeter data were available. The studies made on GEOSAT data are numerous and the GEOSAT data set is regarded as a milestone in both satellite oceanography and satellite geodesy.3

 

-TOPEX/Poseidon was launched on 10 August 1992 into a 10-day repeat orbit with a high altitude (1300 km, high for an altimeter satellite) and relatively low inclination (66 deg). Due to the low inclination data coverage is limited. Equipped with two experimental altimeters, one French and one US-made, TOPEX/Poseidon is dedicated to the altimetry mission. When using the US-made altimeter, this satellite measures the sea surface with an approximate accuracy of 2 cm.4  

 

- TOPEX/Poseidon sea level anomalies are computed as the ocean topography deviations from a four-year mean sea surface. Tides have been removed using the UTCSR Model 3.0. The mean sea surface is computed as a plane in each 0.5 degree bin along the satellite's ground track. Removing this estimated mean sea surface removes the majority of geoid error, geoid gradient error, and geographically correlated orbit error from the sea level anomaly measurement.  The accuracy of the resultant maps is 3 to 4 cm.5

 

- TOPEX/Poseidon Research includes:

            - Oceanic circulation including details on the movement of Rossby and Kelvin waves.

            - Oceanic and coastal tides.

            - El Niño, La Niña, and the Pacific Decadal Oscillation.

            - El Niño-like circulation in the Atlantic Ocean.

            - Oceanic seasons in the Mediterranean.

            - Ocean floor topography from surface data used to refine the geoid model.6

 

 

 

- Jason-1 is the first follow-on to the highly successful TOPEX/Poseidon mission.  It was launched on Friday, 07DEC01.  Its objectives are:

            - Extend ocean surface topography into the 21st century.

            - Provide a 5-year view of global ocean surface topography.
            - Increase understanding of ocean circulation and seasonal changes.
            - Improve forecasting of climate events like El Niño.
            - Measure global sea-level change.
            - Improve open ocean tide models.
            - Provide estimates of significant wave height and wind speeds over the ocean.7

 

- The First and Second European Remote-Sensing Satellites (ERS-1 and ERS-2) were developed by the European Space Agency as a family of multi-disciplinary Earth Observation Satellites. They orbit the Earth in about 100 minutes and in 35 days have covered nearly every corner of the globe at least once. Both satellites are still in good health and provide a wealth of observations through their excellent suite of instruments. 2

 

AN EXAMPLE OF FEATURES DISCERNIBLE USING SATELLITE ALTIMETRY

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Applications of Satellite Altimetry Data: 

            - Derived Products: Wind Speed; Wave Height; Dynamic Sea Surface Height

            - Model Assimilation: Altimetry data is converted to a temperature profile and used for the Analysis Fields

               in the Parallel Ocean Program (POP) model; Some work being done to accomplish direct simulation.

            - Naval Oceanographic Office: Ocean Bogus (Fronts and Eddies); Digitized Fronts ­ used for overlays of

               altimeter tracks to infer positions of fronts and eddies; Used for comparison with IR satellite imagery to

               deduce the existence of weak fronts at depth.


References:

 

http://www.deos.tudelft.nl/altim/altim.html

http://www.deos.tudelft.nl/altim/gulfstream/info.shtml

http://www.deos.tudelft.nl/general/topics/geosat.shtml

http://www.deos.tudelft.nl/general/topics/topex.shtml

http://ftp.csr.utexas.edu/sst/sla.html

http://topex-www.jpl.nasa.gov/mission/mission.html

http://topex-www.jpl.nasa.gov/mission/jason-fact-sheet.html

http://www.deos.tudelft.nl/altim/gulfstream/#07

http://www.deos.tudelft.nl/ers/instruments.html