However, it is not so simple. With significant advantages, there are significant disadvantages. In the event of failure, you must be able to identify errors, which is not possible without an understanding of the system and the availability of special funds. These tools include mode of blending  radar image (RI) on the chart - ie 'radar overlay.' In other words, overlapping of  the radar on the map background. This mode can be used as a diagnostic error-free operation of navigation equipment.

 

There are two types of connection ECDIS with radar: 'digital NMEA- pairing' (if there is a radar automatic target tracking, in which is possible to enter into the ECDIS data of tracking targets) connection through the radar processing unit (converts the radar signals into digital form). As a result, we obtain the imposition of radar images on the chart.

What changes will occur upon application of radar data on the chart:

- Instead of a point, which indicates the location of the buoy or milestones there will be an extended radar signal (RS) on the map, which is much higher than the real size of the object. And person should understand it correctly, because the true position is strictly in the middle of the front edge of the RS.

- There is an expansion of the signal when it reflects from the coastline at acute angles. The reflected signal comes from the coast to the area of water under observation at greater distances, but as you get closer, it returns to its true position. The most reliable estimate of the coastline at a 'perpendicular' towards the coastline radar reflection during  the vessel motion along the shore. There are the same effect on the radar, but not so clearly visible.

- Radar data of the coastline does not always correspond to the real coastline (and hence its cartographic image). This is due to the fact that the sloping bank badly reflects radar signals and often not visible on the radar, and the effective reflection occurs on the steep slopes or large onshore objects, which are over the coastline.

 

Which errors it can produce?

1) mapping, ie you should never completely trust the data that are presented on the map. In the radar overlay these discrepancies and errors can be seen through the disparity of map and overlay coastlines. But you must take into account all the features of the radar presentation from the banks described above.

2) error of positioning system. There are also possible errors and malfunctions in the determination of the coordinates using GPS (or GLONASS) which may be hundreds of meters. Using GNSS with differential mode of operation, the analysis of sonar data and their comparison with the depth map, periodic determination of the vessel known conventional methods can help to detect these errors. But the most effective method is the mode of the radar overlay. It is important that the radar detects the coastline, and if it does not meet the mapping and is mixing in the same direction by a fixed amount, so any errors in GNSS.

3) displacement of the radar signal as a result of uncertainty regarding the conversion of cartographical local ellipsoids. In this case, the radar presentation of the area will have a constant offset in the same direction.

 

4) heading sensor malfunction will be a result of  incorrect data on the ship's course, incorrect assessment of the bearings on the observed target, incorrect assessment of navigational hazards and faulty decision dismissing the purpose. Radar overlay will help to identify these mistakes. Initially radar  determines the relative direction of the ship and then recalculates course angles of the radar objects into bearing, by adding them to the current value of the course. The resulting RI overlaid on the map. If there will be a mistake in the ship's course , on the magnitude of this error all radar images will be distorted. It will turn around the ship's position on the value of the error rate and it will be easy to identify the characteristic mismatch and radar mapping of the coastline.

 

5) data from the AIS can also be checked using radar overlay. When everything is correct  the mark of the AIS target coincides in accordance with angle and distance with the mark on the radar. The same principle applies to control the information about the objectives coming from ARPA.

 

You can say exactly that the radar overlay mode is very useful, because you can quickly control the operation of the system and the data that comes from other systems.