Shipborne Measurement of Surface Current Fields by HF Radar
Presented at the OCEANS'94, Session OM5, Water Currents, Floaters and Drifters
University of Hamburg, Institute of Oceanography,
Tropolwitzstrasse 7, D-22529 Hamburg, Germany.
Tel: +49-40-42838-5742, Fax: +49-40-42838-5713,
A full paper has been published in the conference
HF radar is a remote sensing technique for measuring surface currents
and ocean wave directional spectra
by using the process of backscattering of electromagnetic decameter waves
from the rough sea surface.
The landbased HF radar system CODAR (COastal raDAR) has originally been
introduced by Barrick in 1977. In Germany, the CODAR has been extended
for shipborne operation.
The system uses electromagnetic waves of about 10~m length (29.85~MHz).
There are several difficulties to be resolved for successful measurements of
surface current fields from a slowly sailing ship.
First of all, the ship's speed has to be less than the phase velocity
of the coupled ocean waves (half the electromagnetic wave length).
Practically, the speed should be about 1 to 2 knots. Speed and course
have to be as constant as possible. Variations greater than 0.75 knots
and 20 degrees per minute will significantly decrease the systems accuracy.
Another problem is the distortion of the receiving antenna characteristics
by the ship's metal environment. Before starting a measurement campaign,
the receiving antenna system as installed on the ship has to be calibrated.
A special direction finding algorithm is then used to gain sufficient
azimuthal resolution and accuracy.
The third difficulty in analysing the backscattered sea echos is
the ship's pitch and roll movement causing a superposed
phase modulation. The amplitude of this distortion
depends on the strength of the ship's movement and so is
a function of the sea state too. A special filter algorithm
has to be used to suppress this modulation.
At least, the measured surface current fields include the superposed speed
of the ship. The ship's velocity on the open ocean has to be measured
as exactly as possible (sigma ~1 cm/s). The Global Positioning
System (GPS) is used. After degradance of the GPS by Selective Availability
(SA), the accuracy is no longer sufficient
(sigma ~7 cm/s for 18-minute means)
and a P-code receiver or differential GPS has to be used.
The shipborne CODAR has been used during several cruises to the Arctic Front
above Mohn's Ridge near Svalboard. The measured surface current fields have
been compared to geostrophically calculated currents, derived from
CTD (Conductivity Temperature Depth) and
XBT (eXpendable Bathy Thermograph)
measurements. When subtracting the winddriven part from the CODAR current
fields, a good agreement can be found within the calculated accuracy.
This work has been funded by the German Research Foundation within the
last update 21-Jan-1998