Error sources and limitations

There are two sources of error on head up orientation, heading flash alignment and range. If it is your own radar, these errors should be rectified in the set up. Pilots may not have this option.

Heading flash alignment

On passage, the heading flash alignment can be checked against an object ahead. Along side, the alignment can be checked against the relative bearing of a distant object or with a floating EBL, the angle of the line of structures or beacons relative to the berth alignment. If the heading flash alignment is in error, a centred Electronic Bearing Line (EBL) can be substituted for the heading flash.


Error in range caused by sweep origin can be identified from distortion of straight lines near the vessel (such as trails from passing beacons or the straight face of wharves or breakwaters.) Accommodating such errors in range is complex and depending on the magnitude, may render the radar unfit for navigation in confined waters.

If the distance of the sweep origin error is not too big and the Reference Point Distance is greater than the Turn Radius, a rough correction can be made by setting the VRM range at the Reference Point Distance corrected by the range error. If targets appear closer than they are, (passing beacons appear to be pulled in as they pass) set the VRM range at the Reference Point Distance minus the error amount. If targets appear further than they are (passing beacons appear to be pushed out) set the VRM range at the Reference Point Distance plus the error amount.


When using this technique, the performance limitations of the radar and the vessel must be borne in mind. In particular, awareness of the radar’s range accuracy (IMO’s minimum standard is 30 metres or 1% of the range scale in use) and that turn radii selected must be comfortably achievable by the vessel.