This article was originally published on SailNet in November 2000.
Using radar for navigation in all kinds of situations does require practice and experienceómore so than most of the other electronic aids to navigation. Keep in mind that after a few weeks of practice, you will be able to read the scope just as easily as you might a chart. And at night, or in restricted visibility conditions, the radar will allow you to navigate and avoid other traffic just like you would normally do in daylight and perfect weather conditions.
Because of some inherent errors and inaccuracies in alignment, radar ranges are more precise than radar bearings. When taking a radar fix, itís important to know that the best fix will result from using two or more distances from objects that you can easily identify on both the radar and the chart. The next most accurate fix is one using both a bearing and distance from the same return. The least accurate fix is from two or more radar bearings. Of course when visibility permits you can always combine a radar range with a visual bearing on the same object, such as an offshore buoy or lighthouse. When you plot a radar fix, remember to label it with the time and the word "RADAR" horizontally near the fix symbol.
Fixes with Simultaneous Ranges Itís important to measure objects directly ahead or astern first, and measure objects closest to the beam last. This procedure is the opposite to that recommended for taking visual bearings, where objects closest to the beam are measured first; however, both recommendations rest on the same principle. When measuring objects to determine a line of position, measure first those which have the greatest rate of change in the quantity being measured; measure last those which have the least rate of change in that quantity. This minimizes delay errors. Record the ranges to the navigation aids used and lay the resulting range arcs down on the chart. Theoretically, these lines of position should intersect at a point coincident with the shipís position at the time of the fix. However, the inherent inaccuracy of the radar coupled with the relatively large scale of most piloting charts usually precludes a pinpoint fix. In this case, the navigator must carefully interpret the resulting fix. Check the echo sounder with the charted depth where the fix lies. If both soundings consistently correlate, that is an indication that the fixes are accurate. If there is disparity in the sounding data, then that is an indication that either the radar ranges were inaccurate or they were mis-plotted. This practice of checking sounding data with each fix cannot be overemphasized. Remember to apply the tidal height for the time of the fix.
|"Use two or more objects, such as points of land or small isolated islands that can be positively identified, both on the chart and by the radar."|
The radar is as important for collision avoidance as it is for navigation. Therefore, you must rotate between these two functions. Determining the amount of time spent on each requires good judgement. If the day is clear and the traffic heavy, you may want to use the radar mostly for collision avoidance. If the weather worsens and starts to obscure visual NAVAIDS, the importance of radar for safe navigation increases.
When switching charts, make sure that you have a fix or DR position plotted that is common to both charts. Transfer the last fix or DR from the old chart to the new one as rapidly as possible. In some cases you can simply record the latitude and longitude of the position and plot them on the new chart. In other cases it may be just as fast to replot the bearings and or ranges on the new chart, assuming that the objects used are common to both charts.
All racons operate over the frequency range 9300-9500 MHz (X-band) and most also operate in the 2900-3100 MHz (S-band). A racon's line-of-sight range is over 15 nautical miles, but actual range depends upon a number of factors, including mounting height, atmospheric conditions, and the racon receiver's sensitivity setting.
In order to conserve battery power, racons installed on buoys in the US are programmed to operate 50 percent of the time. These racons are normally active for 20 seconds and then off for the next 20 seconds. Racons installed on shore, where battery life is not a factor, are normally programmed to operate 75 percent of the time. Racons are usually not programmed with a duty cycle greater than 75 percent to ensure that the return never masks another radar return.
The anti-clutter rain control on a radar could mask a racon return and may need to be shut off. The anti-clutter sea control on certain radars could also degrade a racon response in some situations. The clutter-rejection circuitry on some radar equipment may also suppress a racon response in some instances.
|"Weather returns on the scope can sometimes hide a return from another vessel. Increasing the gain can "burn" through the weather and allow you to see the return from the vessel."|
Advanced Radar Systems Some newer radars have a target-tracking mode that allows you to identify returns from other vessels and display a continuous readout of range, bearing and CPA (Closest Point of Approach) for up to 10 targets. If any tracked vessels enter a preset range, the alarm will sound. When these radars are interfaced with your GPS you can display route waypoints on the radar screen along with your course and speed. Several of the more sophisticated electronic charting programs will allow the radar display to be superimposed on the electronic chart. If you have difficulty interpreting the radarscope, this expensive feature may prove to be very useful.
SailNet Store Section: Radar Displays
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