Radar on a Cruising Sailboat

When we started cruising on Callipygia, she lacked radar. We had it installed, almost as an afterthought, right before we left on our shakedown cruise to Maine. At that point we really thought you only needed radar if you were going to be encountering fog, and we realized that in Maine we probably were. And we did. But little did we realize how much wider would become the uses we would have for our radar. We were happy that we had bought the best system we could afford - and we can safely say that our radar system (and knowing how to use it properly) saved our bacon on more than one occasion. We ran into several cruisers who had radar, but had only the most minimal level of understanding of how to interpret its display. This included two who suffered serious collisions, and others who wouldn't leave harbor in the dark--even though it was the optimal time.

These are the notes we compiled while studying the following resources: the Starpath Radar Trainer;   the instruction manual for our Raytheon radar system;  and the book "Radar Afloat" by Tim Bartlett.

Table of Contents for this Article
1.  Radar and GPS 2.  Tuning the Radar 3.  Basic Interpretation
4.  Collision Avoidance 5.  Relative Motion Diagram 6.  Storm Avoidance
7.  Radar Navigation 8.  Handy Tricks with Radar

9.  Glossary

10. Abbreviations    


1. Radar Compared to GPS

Radar has two basic uses underway: position fixing or confirmation which is called piloting,; and collision avoidance. You can with radar, for example, take the range and bearing to charted landmarks if they can be identified on the radar screen, and a range and bearing is a fix. The variable range marker (VRM) and electronic bearing line (EBL) make this very convenient. Collision avoidance can be used to avoid storms as well as ships or islands.

Of all coastal electronic navigation aids, radar is the most important. GPS can provide a more accurate position than radar can. However, in coastal navigation, radar can tell if you're in the middle of a narrow channel where you do not need to know your precise coordinates. Also, radar is in general a far more dependable means of navigation. At sea, whereas the GPS tells me my precise position it is not really needed in the middle of the ocean. The real value of GPS is its ability to tell us accurate course over ground and speed over ground. What the radar can do that the GPS cannot is warn of collision risk with moving targets. With good land mass targets, often available in dangerous situations, I can find from the radar everything that GPS tells me (only more slowly and less accurately). They are both important and every vessel should have and use both of these nav aids. These two, together with a depth sounder, are your main arsenal for safe navigation. GPS, especially interfaced to an electronic chart plotter, is the boss of the group when it comes to position navigation, because it is quickest, easiest, and most accurate. A key point is we need some means to confirm the GPS position, and in coastal or inland waters, radar is most often the best way.

An experience: On returning to the Chesapeake Bay (retracing our outward route) from our shakedown cruise to Maine in 2001, we ended up slogging our way down the New Jersey coast much slower than we expected. It wasn't until 1430 hours that we turned into Delaware Bay. We motor-sailed just outside the channel to avoid the busy traffic. There were no safe anchorages/harbors anywhere nearby. We consulted Reed's Nautical Almanac and the Coast Pilot to confirm that the Chesapeake and Delaware Canal was lighted. We decided to go through the canal in the dark hoping to stop at Schaeffer's Marina towards the Chesapeake Bay end of the Canal. We arrived at Reedy Point near midnight, very very confusing lights. Immensely thankful for our trusty radar, we were able to pick up the entrance to the Canal on the screen. We picked our way into the Canal mouth, and using radar along with canal lights safely make our way to Schaeffer's, where we gratefully docked alongside at 0313 hours. (See the Ship's Log for more.)

Points to remember when using radar:

2.  Tuning the Radar

Read the system manual, also read other system manuals for additional insight. Note that Standy reduces power consumption by, typically, about ½ or more. It also extends life of the radar unit.  

3.  Basic Interpretation of the Radar Screen

A few pointers to remember when you're looking at a radar screen:

4.  Collision Avoidance

This is the premiere function of radar, telling what traffic is out there and what it is doing. But this is not a simple matter of just looking at the radar screen. The analysis involves first and foremost determining whether or not the target poses a risk of collision. Next is determining what the circumstance is that leads to this risk. For targets closing in on a diagonal track, as opposed to coming from dead ahead, the analysis is a bit more involved. Develop standard simple plotting procedures that will let you know as quickly as possible what is taking place. Also, review the pertinent Navigation Rules and be familiar with them.

In order to comfortably use the radar in a potential collision situation, you must have practiced with EBLs, VRMs, DRMs, SRMs and CPAs ahead of time. Otherwise, the stress involved in a real life predicament will freeze your brain.

Points to remember:

Two aspects of the Navigation Rules are very important here:

5.  Relative Motion Diagram

When targets are moving straight up or down the screen, you don't need to do this diagram. Simply add or subtract your speed to theirs to get SRM. However, it's a good idea to get in the habit of doing an RMD for other cases. Practice it until it comes easily to you.

Radar maneuvering problems can be solved by one of two methods: traditional graphic methods using plotting sheets (or rapid radar plotting with a china marker right on the radar screen) or they can also be solved directly with mathematical formulas solved with a calculator or computer. The latter method, if practiced, can (we are told) be quicker and more accurate. See Dutton's Navigation and Piloting for the formulae.

On Callipygia we mostly used the radar screen to plot the Relative Motion Diagram. You can also use a plotting sheet or Maneuvering Board, or even graph paper. Mark two positions of the target with their times (6 minutes apart), along their DRM as they appear on the radar. The Direction of Relative Motion is the line of the target's track--if you do an EBL parallel to the track, you will get the DRM. From the first target position, draw a vertical line down a distance equivalent to the speed your boat has moved between the two readings. Draw a line from this point to the second position of the target. The direction of this line is the target's true course (relative to your course). The length of this line is the true speed of the target. It's not hard, and it's not always obvious but it's really important so you know who should stand on and who should give way. It will tell you if you are overtaking, being overtaken, or if you're approaching head on or crossing it will tell you what the relationships are. You should record this information in the log.


6.  Storm Avoidance

The first order guess of the right course would be that course which is perpendicular to the course of the approaching storm target. This will certainly increase your CPA over doing nothing, but this is not optimum. The optimum course is forward of that perpendicular course by an amount alpha (α) which depends on your relative speeds. In the perpendicular course you are just "sliding off" as the storm approaches. In the optimum course, you run and slide. You don't move off its track as fast, but you get longer to progress away from its impact and end up with a bigger CPA. The amount to add to the relative perpendicular course is

α = arcsin ( u / v ) where u = your speed and v = target speed.

Remember, α gives you the angle forward of the perpendicular to the storm's true relative course, not forward of your first bearing to it.

Some rules of thumb for getting an α when you don't have a calculator or can't use the formula:

When you choose the optimum course, the CPA occurs when the storm center crosses your stern. If you just take the perpendicular course, the CPA occurs before it crosses your stern. If a storm is headed straight toward you, you could go either right or left of its path. In this case, however, there would be a preferred side to take -- you would go toward the so-called "navigable" side of the storm (left in the northern hemisphere) as opposed to the "dangerous" side (right in the NH).

The philosophy here is to turn away from the storm center and get it behind you, then keep turning until the DRM = 90°. That will maximize the CPA so that it will occur when the storm center crosses your stern.


7.  Radar Navigation

As a rule, your GPS will be the primary means of exact position location, but it will get you into trouble if you use it as the sole means of navigation, especially at night. In those circumstances the direct view of your position relative to land masses seen visually on the radar will be the preferred means of navigation. This is particularly the case in confined waters when there might not be time nor need to continually transfer GPS positions on to a chart. Various electronic plotting aids may be an option in these cases, but the radar is usually a more dependable solution. Even in cases where GPS and electronic or paper chart plotting are the main means in use, radar observations for confirmation are the hallmark of good navigation.

A normal position assessment might proceed by plotting the GPS position on the chart and then, from that position on the chart, note the range and bearing to some charted landmark that is likely to be a prominent radar target. Then go to the radar to check if that is true. At the same time, when in soundings, one should check that the depth is what it should be as well. On most electronic chart displays, the range and bearing to a landmark can be made with the mouse cursor in a matter of seconds. Without such things, we must plot the position on a chart using parallel rulers and dividers. This is a valuable way to use radar for position navigation whenever possible. It not only confirms your position, but also helps you identify radar targets (land masses) on the screen. Without this ongoing practice, it may be difficult to identify a headland or bay or islet, etc., when you do need it. It also builds simple confidence in your work. If you rely solely on the GPS you will be anxious about your work and you have a right to be. In coastal navigation, the process of going back and forth from radar to chart has the advantage of keeping you informed of the name of the headland or bay you are nearest–extremely useful in communicating with other vessels or the Coast Guard if you have an emergency.

There are two separate aspects of chart navigation with radar.  One is the use of radar to locate or confirm an actual position on the chart.  The other is to use radar to guide you along a desired course without necessarily deriving the actual coordinates of your position along that course. You can, for example, use radar to maintain a specific distance off of a shoreline without caring so much exactly where you are along that shoreline.

In general, the key to chart navigation with radar is to coordinate it with other piloting aids, especially with GPS and depth sounder. With chart at hand, the most common procedure is to locate the GPS position on the chart, and from there figure the range and bearing to what you suspect might be a good radar target, and then look to the radar to confirm this observation.

If the distinction between the green and blue or white on your chart is not prominent, then it will pay to use a highlighter to outline the shoaling areas (blue) or foreshore (green). This forces you to go over each region you might pass through carefully and then the marking makes it stand out as a warning. Sometimes in the faint light of a wheel house or nav station it is difficult to see these crucial distinctions without this added highlighting.

Identification of specific landmarks from their radar image can be a challenge, hence the terminology of "a good radar target" versus something else. A good landmark target is one that is easily identified on the radar screen -- usually tall or steep along all its borders, with a unique shape, or a small but reasonably tall isolated islet. A drilling platform, for example, is a very good radar target. A RACON is an ideal radar target. A low spit of land can be a very poor radar target. How well a landmark shows up on the radar depends on its range and bearing, but a so called good target would be less sensitive to this. The key issue is the height of the land and the resolution of the radar. Resolution is how well two nearby objects are resolved (separated) on the radar screen.

Radar range is slightly farther than visual or geographic range due to refraction of microwaves. Maximum range = 1.2 x [sq.rt (ht of your radar) + sq rt (ht of target)]. If your antenna is 9 feet high and you are looking for a ship that is 81 feet high, then it will first faintly appear at about (3 + 9 or) 12 x 1.2 = about 14 miles. Hence even if you have a 24- or 36-mile radar, then you have to be looking for something higher than 81 feet or you won't see it from an antenna that is only 9 feet high. (The max. range scale specified on radar units has more to do with their power output, than how far you will see targets. If the target is beyond the "radar horizon" given above, you won't see it, no matter how much power you are broadcasting.) If you install the antenna much higher, say from a spreader at 16 feet, then you only gain 1 mile, and if you go on up to 25 feet, you still only gain another mile. On a small boat at sea, an antenna that is 25 feet high will be rocking so much with the waves that some of this elevation is wasted. Most small craft find that an antenna height of 9 to 12 feet (on a post in the quarter) is perfectly adequate and avoids extra weight aloft from the long heavy cable.

Radar resolution has two separate factors: bearing resolution and range resolution. The typical horizontal width of a small-craft radar beam is about 6°. This means that any two objects separated by less than 6 ° will be smeared together (unresolved) into a single target. The same pulse will hit both of them. As it turns out, the tangent of 6° is 1/10, so if two adjacent objects located a distance D away are to be resolved into separate targets on the radar screen they must be separated by a distance of at least D/10 from each other. Two vessels, for example, seen 3 miles off, must be 0.3 miles apart or they will appear as one. If the entrance to a bay is 0.4 miles across, we would not expect to see it as an opening (when headed straight toward it), until we were within some 4 miles of the entrance. It is a good idea to practice these things and make your own measurements with chart in hand to see how this works.

Range resolution is determined by the pulse length of the radar signal. A microwave travels at the speed of light, which is 186,000 miles per second. This can be converted to a speed of 328 yards per microsecond. If two objects in line (same bearing) are separated by less than one half a pulse length, then the nearest target would still be reflecting signals from the end of the pulse when the farther one starts to reflect signals from the front of the pulse. Therefore they would appear as one object. To be resolved, two objects at the same bearing must be separated by more than 164 yards per microsecond of pulse length.

Typical pulse lengths vary from 0.1 to 1 microsecond, and the one in use depends on the range. In some few units you can select pulse length, in most small craft units this is done automatically for you when you change ranges. In one unit, for example, on range 3 miles the pulse length is 0.3 microsec and on range 4 miles it is 0.8 microsec. Note that in this case, you could have two close vessels (tug and tow) that were separated by 100 yards at 2.8 miles off. On the 4 mile scale they would appear as one vessel (resolution 131 yards), but on the 3-mile scale they would show as two distinct close vessels (resolution 49 yards). Again, something to practice with using your own radar. You have to look up the pulse lengths used for the various range scales in the specifications section of your manual.

The following situations summarize the use of radar for navigation.

A: Range and Bearing fix with radar is the work horse for piloting -- at least so far as confirming the GPS position is concerned. The extreme and frequent value of this operation cannot be judged by how easy and short it is to explain it.

  1. Identify a landmark on the radar that you can identify on the chart. For optimum fix, this should be a well-defined radar target, whose bearing can be taken to an obvious center.
  2. Set EBL and VRM on this point and read off their values. Note the time and your heading.
  3. Convert the EBL bearing to a true bearing using your heading. If the landmark is at 128 R, for example, and you are on course 215 magnetic, then the EBL bearing is 215 + 128 = 343 magnetic.
  4. Then plot your line of position on the chart exactly as you would if you had taken a compass bearing to the landmark of 343 magnetic. That is, using the magnetic compass rose on the chart, draw a line emanating from the landmark in the direction of 343 - 180 = 163 magnetic.
  5. Your distance from the landmark is what you read on the VRM. Measure this off from the landmark on the chart and you have your position.

Notes: The key issues here are obvious: be sure you have the right landmark and carefully judge how to draw the bearing line and range circle on the chart relative to that landmark. Small, distinct, isolated targets are best for this method. If just using the method to confirm the GPS position, on the other hand, you have more flexibility in targets, but still, whenever in doubt, do range and bearing to several bodies.

If you have to use a tangent to a steep cliff or rock, be sure to correct it for half the horizontal beam width as explained in Lesson 6.4 -- if HBW is 6 °, then subtract 3 ° from right side tangents and add 3 ° to left side tangents, since you are seeing the targets smeared out by that amount. You have to judge with experience if a tangent is better than an estimate to a center for extended objects. Do not rely on buoy sightings for your own position location. Buoys may not be in the right spot, or you may be looking at an anchored vessel and not a buoy at all. The exceptions are RACON buoys which are about the best possible radar targets. Practice is the key factor for good work in this area.

A key role of radar is more often to check the GPS position than it is to actually establish your position from scratch. In this process, you plot your GPS position on the chart, then use parallel rulers and dividers to check the range and bearing to what might be good radar targets in range. Then look at the radar to confirm these observations. If in soundings, compare the depth as well.

B: Radar fixes from two or more bearings. These offer a quick method of radar piloting that is familiar to all navigators since it is directly analogous to compass bearing fixes. Unlike visual bearings, however, radar bearings taken from typical small craft radar are generally not as accurate as can be done carefully by eye using a high quality bearing compass. The problem is twofold, one the radar bearing must be corrected for the heading of the vessel when using Heads-up display, and in any display mode, the angular width of the radar beam tends to smear out the target size on the radar. Consequently, piloting with radar bearings is best done with small well defined targets whose center can be identified on the radar and on the chart.

If tangents must be used, then the measured bearing should be adjusted by one half of the horizontal beam (HBW) width for your radar. These vary from some 8 ° to about 2 °, meaning corrections of 1 to 4 °. HBW depends directly on the size of the antenna -- larger antennas have narrower beams -- and the precise values are listed with the radar specs. For tangent bearings to the right of an object, subtract one half of HBW and for tangents on the left of an object, add one half of the HBW.

  1. Identify two or more good radar bearing landmarks on the radar that you can identify on the chart.
  2. Set EBL on these points and read off their values. Note the time and your heading.
  3. Convert the EBL bearings to a true bearings using your heading. If the landmark is at 128 R, for example, and you are on course 215 magnetic, then the EBL bearing is 215 + 128 = 343 magnetic.
  4. If the bearing is of a tangent, then correct for one half of HBW as explained above.
  5. Then plot your lines of position on the chart exactly as you would if you had taken compass bearings to the landmarks. That is, using the compass rose on the chart, draw a line emanating from the landmarks in that direction.
  6. Where the lines of position cross on the chart is your position fix. Three bearings are much better than just two, since the size of the "cocked hat" intersection of the LOPs is some indication of the reliability of the fix.

C: Two bearings and a range. These can provide a good fix -- it's a standard procedure in routine piloting using a hand bearing compass. Two close bearings, however, such as two sides of a small island, are generally not a very good fix even using visual bearings. With radar, on the other hand, we can occasionally get a reliable fix from two tangents of some object by combining it with a range measurement to the object. This is effectively a way to do a Range and Bearing fix to an object that is too large to locate with a single bearing line.

  1. Identify a prominent landmark on the radar, such as a small island, that you can locate on the chart.
  2. Set EBL on the left and right tangents to the landmark, and read off their values. Note the time and your heading. At the same time, read and record the VRM range to the portion of the same landmark which is closest to you.
  3. Convert the EBL bearings to true bearings using your heading. Then correct each bearing for one half of the Horizontal Beam Width (HBW) as explained in Pub 1310. In practice you will use the value given in the specifications of your own radar unit. For plotting exercises in Radar Trainer assume an HBW of 4 °, which means you will add 2 ° to left-hand tangent and subtract 2 ° from the right-hand tangent.
  4. Then plot your lines of position on the chart exactly as you would if you had taken compass bearings to the landmarks. That is, using the compass rose on the chart, draw a line emanating from the landmarks in that direction.
  5. Using a drafting compass or beam compass plot the VRM range from the landmark.
  6. Your position fix is halfway between the two bearings, on the range circle plotted from the VRM.

D: Fix by Two or More Ranges. To take the best advantage of radar for chart navigation, you need some form of drafting compass for drawing circles. Most dividers have an optional lead to replace a point, but for doing much of this a dedicated drafting compass would be useful. An alternative is just to tie or rubber-band a pencil onto your dividers and use that. Or just use the dividers and mark positions along the arc with a pencil. This method relies on ranges alone (without bearings) which can in principle offer a more accurate fix than the quicker range and bearing to a single object. Also when using 3 or more targets (for ranges or bearings) you get a "cocked hat" of intersections which is some measure of the reliability of the fix. If the intersections are t00 large, then take a 4th target to help identify the bad one.

The disadvantage of any method using more than one target, however, is that your own motion -- if any -- must be taken into account for an accurate fix. In other words, all multi-body fixes are to some extent running fixes. Remember that objects ahead or astern change range more rapidly than objects abeam, so it is generally better to measure the ranges on the beam before those on the bow or stern. If you want to carry out a proper running fix, then the general procedure is to advance the point of reference and then draw the range circle.

  1. Identify two or more landmarks on the radar that you can locate on the chart. Confirm that these are good radar range targets, meaning sharp steep edges as opposed to low, gently rising edges. (Later you will confirm that the edges you are seeing on the chart are indeed above the horizon and what you are looking at on the radar).
  2. Set VRM and EBL on the chosen targets nearest the beam. Read and record the values. Then do the same with the second or third targets.
  3. Use the bearing lines measured to identify the point on the landmark whose range was measured. From that point, use a drafting compass or beam-compass, to draw in the range curve through your approximate position. Do the same with the second observation.
  4. Where the lines of position cross on the chart is your position fix. Again, three ranges are better than two, and these will be best when they are some 120 ° apart.

E: VRM as Piloting Aid. There are many creative ways to use the VRM circle for navigation. Here are a few suggestions. Others will undoubtedly occur to you to meet specific navigation problems.

  1. Sailing parallel to a coastline within radar range, you can set the VRM circle to just touch the coastline. Then as you proceed along the coast, just a quick look at radar screen tells if you are getting set in toward or away from the coast, or if you have wandered off course for any reason.
  2. Approaching a headland or rocks in view on the radar, you can decide how close you dare get in based on the chart, then add some safety factor, and set the VRM to that distance. Then as you approach, you can tell without further reckoning when you are at the minimum distance off.
  3. Some combination of (1) and (2) can often be useful such as crossing a large bay or entrance. Set the VRM to the distance off that you were following the coast up to the entrance and then leave it set as the coast falls away into the opening. The VRM will now not be touching any land, but you can see the lay of the coastline lower on the screen. Use a parallel line (parallel to ship's heading line) to project the tangent to the VRM backwards to see if your circle is penetrating into the entrance or slipping away from it -- i.e., getting set into it or out of it.
  4. You can navigate to a particular point on the chart in an easy manner if it happens to be equal distant from two distinct radar targets separated by at least half the distance off you care to achieve. Set the VRM to the particular distance, then drive in and adjust course as needed until both targets touch the VRM circle. This will put you at a unique place on the chart.

8. Handy Tricks with Radar

Again, as with the VRM methods, there are numerous uses of the EBL line for navigation, and other general techniques that can help with navigation in some form. A few useful ones are listed here.

9.  Glossary


This can be quite confusing. Figure out how these apply on your own system.


10.  Abbreviations

Using radar involves a veritable alphabet soup of abbreviations. Here are the ones you need to know:


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Bill Dillon (KG4QFM)
Pat Watt (KG4QFQ)
This page was last modified on August 9, 2009

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