Radar Overlay Alignment Troubleshooting

Radar Overlay is a very useful feature in TimeZero because it allows radar returns to be displayed directly on top of the nautical chart. When everything is correctly configured, radar echoes such as coastlines, buoys, and other vessels should appear in the right position and match the chart accurately.

If the radar overlay does not align properly with the chart, the cause is almost always related to one of two things: either the radar antenna itself is not correctly aligned with the bow of the vessel, or the heading sensor being used to rotate the radar image is not properly adjusted. In order to solve the issue, it is important to check these two points in the correct order.
 

Verify Radar Antenna Alignment First

The first step in troubleshooting should always be to confirm that the radar antenna is correctly aligned with the bow of the vessel. Even a small mounting error can cause the radar picture to appear rotated, which will later create an offset when radar overlay is enabled.

To check this, it is recommended to work in the Radar WorkSpace using the radar display by itself, not the overlay. Switch the radar to Head Up mode, so that the top of the radar screen represents the direction the bow is pointing.

On a calm day, with little or no wind or current, steer the vessel directly toward a fixed object that is clearly visible on the radar screen and easy to line up with the bow. This could be a lighthouse, buoy, pier, or a coastline feature.

In correct conditions, the object should appear at the very top of the radar screen, perfectly aligned with the Head Up marker. If the target appears slightly to the left or right instead of centered, then the radar antenna is not aligned correctly.

The Radar will usually provide an "Antenna Heading Align" setting which allows you to correct this mounting offset. Adjust this value until the radar image is properly centered and the target appears exactly at the top of the display.

Once the antenna itself has been aligned, the radar picture is now correctly oriented relative to the vessel.

Check the Heading Sensor if Overlay Offset Remains

After correcting the antenna alignment, Radar Overlay should normally match the chart. If there is still an offset visible only when overlay mode is enabled, then the remaining error comes from the heading sensor.

TIPS: It is also important to keep in mind that radar overlay alignment depends not only on the radar and heading sensor, but also on the quality of the chart being used. In some areas, especially when using very large-scale charts that are not intended for precise navigation, the chart itself may not be perfectly geo-referenced. In these situations, even a correctly aligned radar overlay may appear slightly offset simply because the chart background is not fully accurate. For this reason, whenever possible, it is best to use an AIS target as a reference when evaluating radar overlay alignment. AIS-equipped vessels transmit their own position directly from their onboard GPS, which generally provides a much more reliable and precise source of alignment than charted coastline details. Comparing radar echoes with an AIS target is often the most trustworthy way to confirm correct radar overlay positioning.

The heading sensor is essential because it is used to rotate the radar image so that it matches the chart orientation. If the heading information is slightly incorrect, the radar overlay will appear shifted or rotated even though the radar antenna is properly aligned.

The best way to verify heading accuracy is to compare the vessel’s Course Over Ground (COG) with its Heading (HDG). You can do this by adding both COG and HDG data boxes in TimeZero in the NavData sidebar.

Under calm conditions, with low wind, no current, and minimal wave action, steer the boat in a straight line at a steady speed of about 6 to 10 knots. In these conditions, the direction the vessel is traveling (COG) should match the direction the vessel is pointing (HDG). The two values should normally agree within plus or minus one degree.

To confirm the result, repeat the test while steering in different directions, such as north, south, east, and west. If you notice a difference of more than two or three degrees between COG and heading, then the heading sensor needs calibration. This is a common issue with magnetic heading sensors, which are often affected by local magnetic deviation or installation environment.

Note: If the vessel is equipped with a satellite compass, heading calibration offsets are much less common. Satellite compasses are generally not sensitive to local magnetic interference. In this case, if an overlay misalignment remains, it usually means that the satellite compass itself is not physically mounted perfectly in line with the bow of the vessel. When this happens, an offset correction must be introduced in the heading sensor settings to compensate for the mounting orientation.

Conclusion

Radar overlay alignment issues are almost always caused either by radar antenna misalignment or by heading sensor inaccuracy. The correct troubleshooting process is to first ensure that the radar antenna is aligned with the bow using the Antenna Heading Align adjustment. Once that is correct, any remaining overlay offset should be addressed by verifying and calibrating the heading sensor using a comparison between heading and course over ground.

Finally, always remember that chart accuracy can also play a role. When possible, AIS targets provide the most reliable reference for confirming correct overlay alignment.