Improved HF Radar sensing of Ocean Surface Currents

Oceanographic HF radars use the Bragg resonant echoes from the ocean returning to the radar to estimate radial surface currents. More than 400 oceanographic HF radars worldwide generate these ocean surface currents maps hourly with 1 to 5 km range resolution.

The boundaries of the first order Bragg peaks in the echo Doppler spectra -known as "FOL boundaries" have to be set in order to recognize the first-order radar backscattered data, useful and necessary for mapping currents, while excluding second order data that lead to erroneous ocean current estimates. These Bragg peaks contain Doppler-shifted echoes from a variety of radial current within a semi-circular range cell. From existing observations of both nominal and anomalous ocean current scenarios we note how the conventional estimation algorithms fail under challenging circumstances, e.g. ocean conditions influenced by tides, strong off-shore currents, around islands...

We propose an improvement for the estimation of the FOL boundaries, allowing maximum coverage of surface currents. Our approach uses and keeps track of the spatial and temporal history of data, applying Bayesian estimation techniques to the observed radar backscatter spectra to detect and filter out erroneous observations. The application of a Kalman filter to a sequence of Doppler spectra from nearby to distant ranges and in a few different locations has already resulted in significant improvement in the results and hence in identifying data that lead to accurate ocean current estimates.

Figure 1 illustrates the application of our method to estimate the FOL boundaries surrounding ­the positive Doppler Bragg peak –light blue to red colors in the figure. The conventional estimates, pink stars, sometimes do not truly bound the first-order peak as described below. On the other hand, the filtered results, white line, display the improved FOL estimates.

Note how the white estimates more accurately bound the region in Doppler-range space containing the first-order Bragg echoes. Notice the especially improved FOL boundary estimates in the region between 25 km range and 55 km range from the radar. Most of the erroneous estimates lay in the middle of the Bragg peak, which presents lower SNR because of the presence of an island in front of the field of view of the ASBI radar site - in the Sunda Strait in Indonesia.