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OBSERVATIONS OF A COASTAL CURRENT USING ERS-1 SAR 
PIERRE LAROUCHE 
Institut Maurice-Lamontagne, Ministére des Péches et des Océans 
B.P. 1000, Mont-Joli, Québec 
Canada G5H 3Z4 
Commission II, Group 4 
KEY WORDS: SAR, ERS-1, Currents, Circulation, Oceanography 
EXTENDED ABSTRACT 
Coastal currents are important dynamical phenomena's in the marine ecosystem. Currents such as the 
Gulf Stream and the Kurushio are major forces transporting heat from the equatorial regions to the 
poles regulating the heat budget of the Earth. Regionally, currents also transport pollutants and fish 
larvae between the different coastal ecosystems. It thus appear important to evaluate the long-term 
variability of these currents in the context of global change. Unfortunately, coastal currents often 
exhibit a strong spatial variability making it difficult to monitor their behavior using in-situ 
measurements. Only remote sensing can provide the large scale vision necessary to observe these 
variations. Thermal infrared remote sensing has often been the primary tool used to observe currents 
such as the Gulf Stream. The presence of clouds is the most often encountered problem with the use 
of such sensors and alternate means of observing these currents would be welcomed. For large-scale 
currents, radar altimetry is one of the available tools. It is however hardly usable for smaller scale 
coastal currents of a few kilometers width. Theoretical work showed that current shears should be 
detected by space borne synthetic aperture radars (SAR). To be detected, such shears should be of 
the order of 10-4 s-1. The Gaspé current located in the St. Lawrence Gulf (Canada) is a coastal 
current that meets this criteria. The goal of this project was thus to verify the possibility for a space 
borne instrument to detect the location of coastal currents by comparing active microwave C-band 
SAR data with concurrent in-situ measurements of current shears. 
Two ERS-1 MLD SAR images (September 25 1991 and May 19 1992) were used for the first phase 
ofthe project. In-situ measurements consisted of a series of transects perpendicular to the mean 
current direction made during the two satellite overpass using an acoustic Doppler current profiler 
(ADCP). Salinity and temperature profiles were also gathered during the transects to determine the 
density structure of the current. 
Image processing first consisted of locating the transects location and extracting sub-images 
corresponding to these areas. An adaptive Frost filter was then applied on these sub-images to 
remove speckle while preserving the high frequency features present. Grey level values were then 
extracted from the images along a line corresponding to the ship track and smoothed using a running 
mean of size 10. 
The analysis of the data from the three transects indicates that the ERS-1 SAR detected the location of 
the Gaspé current boundary when environmental conditions were favorable (winds between 1 and 5 
m s- 1) thus opening the possibility of using this tool to monitor coastal currents. It was thus shown 
that shears of 1 x 10-4 s-1 were indeed detectable as predicted by theory. The observations in this 
experiment were however made only with a current flowing roughly in the SAR range direction. 
Further observations should thus be made with a broader set of currents (including azimuth traveling 
Cents) to further evaluate the capacity of a space borne SAR to detect this type of dynamical 
atures. 
OBSERVATIONS D'UN COURANT COTIER AU MOYEN DU RADAR A 
SYNTHÈSE D'OUVERTURE DU ERS-1 
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