TRACING PAPER can help radar interpretation.

Recognition of distant landmarks is speeded
when their images are traced directly on the radarscope.
First appeared in Boating magazine • All rights reserved • © 2006 John Ellsworth

You are approaching unfamiliar shores after a lengthy passage at sea. It's still daylight. Navigation has been by dead reckoning until recent landfall. Because you have no GPS or your unit is not working, and you don't have a radio direction finder (RDF), you expect to slip into the harbor with the aid of visual bearings and radar . . . but the shores and harbor are blanketed by fog. To make things more difficult, the radarscope image does not look anything like the land outline on the chart! How do you identify the land forms and objects for a fix?

Your boat may have a low profile, therefore limiting your radar range. The higher your antenna is located, of course, the longer the range (see figure 1).

Figure 1. The tanker's radar exceeds the yacht's because its antenna is higher. Radar can pick up objects beyond the horizon if they're high enough—therefore the tanker sees more at a distance.

But because portions of the land may lie beneath the horizon, only the higher elevations will provide a radar reflection for the land silhouette. and this outline may not match the land configuration as it appears on the chart. (A mountain top, for example, may appear as an island as in figure 2).

Figure 2. The tanker's scope picks up most of Chipper Island for easy identification. A range and bearing may be had from the flashing buoy outside the harbor and possibly from the center of one of the North Islands. But the yacht's skipper, without aid of tracing paper, would be hard put to recognize the three blips on the radarscope. And the flashing buoy and North Islands are all below the yacht's radar horizon at extreme distance.

On a sheet of tracing paper, trace the land outline and mark the high altitude points from the chart. Place the tracing paper on the radarscope. Adjust the radar scale to match that of the chart while aligning the patterns on each. Make sure that the North directions correspond while matching the patterns. The high altitude marks will align with the "islands." This alignment enables easy scope interpretation. As you get closer to the harbor, the patterns will gradually resemble each other and you can discard the tracing paper when your objective is fully revealed.