This article was originally published on SailNet in November 2000.
If youíve read my previous articles, you might recall my declaration: "If I could have only one aid to navigation, it would be a radar." Of course I have considerable experience in radar navigation and feel very comfortable using it, but thatís not to say that you need years and years of experience in order to be able to navigate using radar. A few hours spent reviewing the principles and getting familiar with the operation of your radar system, coupled with a few days of coastal navigation, will allow you to use radar with confidence.
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.
Fixes with Range and Bearing to One Object When only a single navigation aid is available, a bearing and range from it will allow you to obtain a reasonable fix. A good time to take this type of radar fix is when approaching a harbor whose entrance is marked with a single, prominent light such as the Chesapeake Light at the entrance of the Chesapeake Bay. Even though you are beyond the visual range of the light itself, the radar return from the lighthouse can provide bearings and ranges. Remember that a typical radar bearing is only accurate to within plus or minus five degrees of the true bearing and the farther away the return is, the greater the azimuth error. Therefore, you must carefully evaluate the resulting position and check it with the sounding from the depthmeter. If a visual bearing is available from the object, use that bearing instead of the radar bearing when laying down the fix. Prior to using this single object method, you must ensure that you have correctly identified the object from which the bearing and range are to be taken. Quite obviously any misidentification can lead to disaster.
Fixes with Bearings to Two or More Objects
|"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."|
Use this method to plot a position quickly on the chart for evaluating radar targets for range measurements. Use two or more objects, such as points of land or small isolated islands that can be positively identified. Make sure that they are about 90 degrees apart for two bearings and 130 degrees apart for three objects. Take the bearings and plot them on the chart. Now, look at the chart and identify objects that should be easily identified on the radar. Note the relationship of these objects to each other and the coastline or any nearby islands. Then look at the scope and identify the returns that match up to this relationship from the chart. Do not look at the scope first and then try to match up scope returns to chart features. This method can result in gross misidentification errors since you can usually find a matchup that seems reasonable. Always go from the chart to the scope. Identify chart features first, noting their relationship to each and only then look at the scope.
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.Using Racons
Racons, also called radar beacons, radar responders, or radar transponder beaconsóare receiver/transmitter (R/T) transponder devices used as a navigation aid that displays on the radarscope. This display is much stronger than a normal radar return. A racon responds to a received radar pulse by transmitting an identifiable mark back to the radar set. The displayed response has a length on the radar display corresponding to a few nautical miles, encoded as a Morse character for identification. The inherent delay in the racon causes the displayed response to appear behind the echo from the structure on which the racon is mounted. Measure the range to the racon from the first dot or dash nearest the center. Racons and their identifying Morse mark are normally indicated on marine 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.
Collision Avoidance and Weather
|"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."|
Your radar will also identify other vessels and show weather returns. Another ship will provide a fairly strong return, especially if it's made of steel or has a radar reflector up. A wooden or fiberglass vessel without a reflector will result in a very weak return, and in some cases no return. 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. At other times turning the gain down will lessen the weather returns while showing the return from the vessel. If you have an antenna tilt control, you can use it in conjunction with the gain to "see through" the weather. Decreasing the antenna's tilt while reducing the gain will allow you to see under the weather and show strong returns from nearby vessels and land. Severe weather contains lots of moisture and provides strong returns. Fog and mist contain less water and increasing the gain will allow you to "burn through" while thunderstorms have lots of moisture and you must turn the gain down in order to see returns from other vessels.
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.
Radar Basics by Jim Sexton
Radar Proximity Warning Systems by Jim Sexton
Radar Safety and Microwave Exposure by Jim Sexton
SailNet Store Section: Radar Displays