Radar image overlay on an ECDIS system-an overview

An ECDIS system is a real-time integrated marine navigation system which combines the data from several information sources onto one display. The data includes an electronic navigational chart, positioning information and information from devices such as a depth sounder, gyrocompass, ARPA radar and radar. When a radar image is overlaid on top of the chart image, uncharted hazards such as other vessels are shown relative to the own ship position, on the same display as charted hazards such as shoals. This combination of information can be used by the mariner to choose the best course. By observing how closely the radar echo of a shoreline matches the chart, possible positioning system errors or chart errors are indicated. So radar data adds extra information to the system which can be used in several ways. This paper focuses on the radar image overlay in an ECDIS system. A description is given of how the analog radar data is ingested-which signals are used, how they are obtained, and how they are processed then digitized in order to make them available for software processing. The functions performed by the software in a digital signal processing module are outlined, including how data, which arrives as arrays of amplitude variation corresponding to changing scanner positions, are transformed to a two-dimensional "image". Methods used and possible future development regarding combining the radar data with the electronic chart display are described. Screen captures from systems on board vessels operating in the St. Lawrence seaway are presented and discussed to show what radar overlay adds to the ECDIS system in terms of the information it offers to the mariner.