Article by Pat Schrawder
HELPFUL FEATURES FOR YOUR RADAR
The price, size, current drain and availability of radar has resulted in more and more boaters adding it to their marine electronics package. As its use has become more popular, the features that were once only found on very large, expensive sets are showing up in almost all units. Let’s take a look at two of these, the Automatic Radar Plotting Aid, ARPA (pronounced “r-pa”) and the RADAR overlay.
The ARPA (Automatic Radar Plotting Aid) functions for target tracking and risk analysis. Originally designed as an “add-on” to your radar, it is now already built into many radar units. It automatically tracks several acquired targets, calculates target bearing and range, speed and course, closest point of approach and time to closest point of approach. Each target is continually assessed and an audible alarm sounds if the target becomes dangerous (will be within a certain distance from your boat within a certain time) or is lost. Initial target acquisition is available only at certain ranges. ARPA should not be confused with the feature included in your radar known as “target tracking” or “target enhancement”. In these cases, target tracking simply puts a trail behind the target showing its direction of travel, but does not calculate true bearing, range, rate of travel, etc. ARPA is much better than a simple radar alarm zone in the information you receive. You can use it on an overnight trip to monitor activity around your boat while you’re sleeping. A dive boat will find this feature valuable for protection of its divers. If a vessel is on a collision course with you and you have to change course, you have time to get your divers out of the water first.
One advantage of ARPA is that you can track any vessel within your range. They do not have to have any corresponding equipment on their boat for this to happen. As stated before, there is nothing else you need to buy to use ARPA. Just check your user’s manual to make sure your model offers the function.
The other function is actually accomplished through the combined use of your radar and chartplotter. It is referred to as “Radar overlay” and is available with some radar units. You need to purchase a special rate compass in order to make this feature work. They run around $500 - $600. Once installed and connected, the radar does exactly what the phrase implies. You electronically “overlay” the radar display image over the chartplotter image. Now, you might wonder why you would want to do that but the answer is quite simple.
No matter how good your radar operates, it is difficult for it to show very low land mass and other stationary objects. With your radar image layered over your chart detail, you can see what is supposed to be there and what is new. For example, if your radar shows a small image that could be part of the coastline on your plotter, but that radar image is moving, then it is most likely a very small boat. Conversely, your radar might show an image that looks like the pathway is clear but your chartplotter indicates there is a small, low lying island right in the path of your travel. It is worth noting that, in most cases, the radar range scales in the overlay depend entirely on the range scales in the chart presentation, allowing you to look at the radar image on the chart information in whatever magnification level you want.
Aside from the obvious safety feature of radar overlay, it is interesting to compare the two images and see where they agree or differ. It gives you a lot of confidence in the accuracy of both as well as showing you their weaknesses and strengths. Check your radar and/or chartplotter manual to see if radar overlay is an option. If it is, the purchase of the additional compass and the installation are pretty simple and inexpensive ways to greatly enhance the use of your systems.
Both ARPA and Radar Overlay are certainly not essential features to have but they are both very useful. If you have ever used or seen them on a vessel, you will definitely think they are something you need to have.
Pat and her husband, Larry are owners of L&L Marine Electronics on Golf Course Road in West Ocean City, MD.
