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
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:
- The microwave emissions from radar are a potential risk to health. Do not expose crew or nearby boaters to waves from your radar. Therefore, turn off the radar if someone goes forward on the deck or you are close by other boats.
- Communicate radar adjustments and information among crew and watches.
- Generally use Heads Up mode. However, for piloting it may be handy to do North Up so you can match what you see on the radar screen with what you see on the chart. Practice the two modes.
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.
- It is always easier to tune with targets on the screen than with none in range. Practice with this where there is traffic to prepare for when there isn't. Watch for a case with traffic safely passing in a rain or snow squall to practice with, and see the effects of, rain and sea clutter.
- Do not "over tune." Some controls work against each other. As a general rule, keep all optional controls in the off or minimum settings. Set Gain to have a light background of speckles when set to the high ranges. Then use the other controls only as needed. With no targets and significant waves present, to look for close targets, first zero the rain and sea clutter, set range to high value, increase gain till a light speckled background, reduce to lower range, and then increase sea and sea clutter to break it up into speckled pattern of dots.
- For optimum resolution (i.e. to distinguish two close vessels, or identify a landmark) , use the lowest range scale that shows the target, and lower the gain to prevent distorting the display.
- When looking for targets at your maximum range, turn up the gain temporarily to a more continuous pattern of speckles... and watch the screen intently. When new targets first come into view, they may show only on every other sweep or maybe every tenth sweep.
- The size of the blip on the screen is not a measure of the size of the target--unless the object is big and close.
A few pointers to remember when you're looking at a radar screen:
- If a target moves down the screen at your speed, then it is a buoy, or else something anchored or dead in the water.
- If a blip does not move at all on the radar screen, it is doing precisely what you are -- same course and same speed.
- If a target moves only to the left or right, and not up or down on the screen, then its speed is identical to yours, but it is pointing higher or lower.
- If something moves straight up or down the screen, then it is pointing exactly the same direction as you but is going faster or slower respectively.
- If something moves diagonally from you, then both its course and speed are different from yours.
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:
- A main challenge underway is dealing with the tracks of targets on unstabilized radar. In typical small craft radar (ie unstabilized), the plot trails of radar targets are smeared out due to the yaw of the vessel in a seaway. As the heading of your vessel swings around in this seaway, the apparent location of a radar target moves with it. If you were on a heading of 200, for example, and you observed a target at 7.0 miles off bearing 40° on the starboard bow (its true bearing being 240), when the radar beam hit that target and plotted it on the radar screen it would show it on the radar screen at 7.0 miles off at 040 relative. Then if a wave sent you onto a heading of 204 for a moment, and the radar beam happened to hit that target at that moment, it would see and mark the target at 7.0 miles off at 034° relative -- even though the actual true bearing to the target is still 240. The wave caused the a rotation in the radar's reference line. Furthermore, if your heading slowly swung back and forth between these two limits as the radar marked the target positions, it would plot the single target as a smear between 034 and 040. At 7.0 miles off, this angular spread corresponds to an arc length or target width of about 0.7 miles. Note that the arc length is about 0.1 x time range per each 6° of angular width. As the target got closer in this same average seaway, then at 2 miles off, the effect of an average 6° heading swing would only lead to a 0.2 mile spread in target width.
- The Plot (wake, track) option is very useful and should be practiced with so you can track the progress of moving targets.
- Use a portable range scale (tongue depressor) or a ruler to estimate distance. Rings are the same distance apart even if you change the range.
- Read your manual to find out all of what is available.
- When you first see a target, set the EBL and the VRM. Put an X on the screen with a china marker or dry-erase marker, and beside it write the time. After 6 minutes, put another X on the screen with the time. If it moves inside the VRM it is getting closer. If it proceeds along the EBL you are on a collision course. Remember the 6-minute rule. When the time = exactly 6 minutes, Distance = Speed/10 or Speed = 10 x Distance. Therefore, set the plot to 6 minutes, then time in minutes between plots times 10 = knots of SRM (speed of relative motion). Note that in a seaway, the trails are often smeared out which makes a precise estimate more difficult.
- To get DRM (direction of relative motion), put EBL parallel to the target's plot. Use a rule through the track and figure out the CPA (closest point of approach). This is the perpendicular distance from the DRM to the center of the screen (you). Figure the time until the CPA by measuring how far from the target to the CPA, and dividing by its SRM.
- Figure out SRM for each target. If it is moving downscreen towards you at your speed, then it is dead in the water or a bouy. Otherwise it is a moving target.
- If it is moving downscreen at an SRM that is greater than yours, it is headed towards you at a speed equal to SRM minus your speed.
- If it is moving downscreen at an SRM less than yours, then you are overtaking it, and its speed is your speed minus SRM.
- If you slow down, the radar track for the target curves up the screen.
- Aspect is the angular perspective at which we see a vessel - ie the relative bearing of our vessel as seen from the other vessel. It is measured from 0° to 180° and labeled red when we are on the port side of the vessel or green when we are on the starboard side. To say we see a vessel with an aspect of 90° red, means he sees us on his port beam and we are looking square at his port side. We see his red running light and his mast head range lights are as open (separated) as possible. An aspect of 45° green, means he sees us broad on his starboard bow. He is headed at the moment 45° to the right of our line of sight to him. We would see his green starboard running light. A vessel with aspect 0° is headed straight toward us. Note that the big difference between visual observations and radar observations is the perception of a vessel's aspect. When a vessel turns we can usually detect a rotation of the hull or lights much more quickly by eye than we can on the radar. A radar observation in itself tells nothing of the aspect. This is partly why it is always important when a target is first detected on the radar to immediately go on deck with binoculars and start looking to see if you can discern its direction – towards us or away from us. Sometimes a glance at the radar might lead us to think we should be looking for a red aspect. Is it pointed to the right or left of us? The solution to the RMD (relative motion diagram) reminds us that the true aspect of a target headed towards us is always higher than it appears from the DRM. If we think we should be looking broad onto their bow from what we see in the radar, we are actually looking more toward their beam. The amount higher depends on how fast we are moving and on their relative bearing. True aspect is always aft of apparent. If aspect increases as the vessel approaches us, it will pass ahead of us. If it decreases, we will cross in front of it.
- For many practical applications, a quick plot on the radar screen will provide adequate information for safe efficient operation - at least as far as the basic collision avoidance operation is concerned, which is a solution of the RMD to find the course, speed, and aspect of the radar traffic. The more complex operations, such as choosing the change of course to make a specific CPA or related problems, are usually better done as transfer plotting on the maneuvering board. [Hint: We laminated a Maneuvering Board sheet, so we could draw on it with a china marker or dry-erase marker. Worked great.]
Two aspects of the Navigation Rules are very important here:
- Any action taken to avoid collision must be ovious and made early.
- However, it is often not possible to tell which side a target will pass if it is seems to be coming straight at you - it's directly ahead, or on a constant bearing. Therefore it is best not to alter course until that information is available to you. To gain time, reduce speed or just stop until you can see what's happening. If you alter course too early in this situation, you may actually bring yourself closer to the target.
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.
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:
- If your speed is the same (or more than) that of the storm, then alpha approaches 90 degrees and you run ahead of it.
- But if the storm is moving at all faster than you, then α gets smaller until when the storm is going twice as fast as you, then α is about 30.
- If the storm is going 3 times as fast as you, then α is about 20.
- If the storm is going 4 times as fast as you, then α is about 15, or five times then α is about 10.
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.
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.
- 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.
- Set EBL and VRM on this point and read off their values. Note the time and your heading.
- 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.
- 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.
- 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.
- Identify two or more good radar bearing landmarks on the radar that you can identify on the chart.
- Set EBL on these points and read off their values. Note the time and your heading.
- 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.
- If the bearing is of a tangent, then correct for one half of HBW as explained above.
- 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.
- 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.
- Identify a prominent landmark on the radar, such as a small island, that you can locate on the chart.
- 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.
- 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.
- 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.
- Using a drafting compass or beam compass plot the VRM range from the landmark.
- 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.
- 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).
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
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.
- Radar and natural ranges. As with visual navigation, any use of a natural range for monitoring course is especially valuable. When sailing toward or away from any two stationary radar targets in range, you have a quick and accurate means of determining if you are being set off course.
- Locating a channel entrance. Occasionally on approaching a coast there can be numerous small targets near the entrance. When looking for a buoy channel, read from the chart what the buoy spacing is along with the compass bearing of the channel. Then you can identify the buoys from the radar by measuring the spacing and confirming the bearing. Mark the candidates on the screen, and use a portable range scale to check separation. Then set EBL parallel to the lay of these targets and confirm its bearing.
- Choosing an anchorage site. In some circumstances, radar is useful for choosing a place to anchor within a crowded anchorage and then later used to confirm or check for anchor drag. Also, invaluable if you have to anchor in the dark.
- Plot trails from stationary targets. In some circumstances, with a prominent landmark or well identified buoy on the radar screen, you can use the length of its plot trail as a measure of your distance run for solving the relative motion diagram and thus save or confirm this simple computation.
- Squalls. You can use radar and the relative motion diagram to analyze squall motions. Once you confirm the motion of one or two, you can guess that subsequent ones during the night will move in the same way. Most squalls in the Northern Hemisphere, tend to move in a direction that is veered from that of the surface wind direction by about 20° or so, at typical speeds of about 15 knots.
- Landmark identification. Don't forget that you can measure the dimensions of landmarks with the radar. This will often help identify it, i.e. if this is that islet, it should be 0.43 miles across. Is it? Or if that indentation is the entrance, it should be 1.2 miles wide, etc. Set the optimum range and then use a portable range scale to check it. The very latest models of radar include a "Floating EBL" option that lets you make these measurements directly from the screen.
- Radar and atmospheric visibility. Radar is very often the best way to determine atmospheric visibility which is in turn needed to anticipate first views of land or vessel traffic. At twilight, measured values of the visibility can then be used to predict the visible range of lights that you will use later in the evening and night.
This can be quite confusing. Figure out how these apply on your own system.
- Gain is the amount of signal. This is the major control used in radar tuning and must usually be adjusted when making large changes in the range. Normal settings of this are done with the rain and sea clutter full off and on a high or maximum range scale. Then increase the Gain until you see a faint coverage of white specks on the background over the full radar screen. Note that full gain can turn the screen white and zero gain turn it black. Most operators prefer just a faint coverage of white specs in the background. If the Gain is too high, you will lose resolution and if too low you will miss targets. You must also experiment with the Brilliance control as it affects Gain. Gain must usually be reduced when large close targets are present or it will smear across the entire screen and block out all other targets. Gain may have to be increased when looking for small targets or when using rain clutter. Sometimes better range and bearing resolution can be achieved by reducing the gain and sometimes reducing the gain will help reduce clutter from rain or snow. Remember to always replace the gain to its normal settings if it has been changed for some reasons.
- Sea clutter. This is a control that should be kept at minimum or off unless needed. If set too high, it can block out close targets. Generally it does nothing for ranges farther than about 4 miles or so. In calm sea conditions, this should be kept off. In rough seas, the entire close in region of the screen on the lower ranges will be nearly solid white from wave reflections. In these cases, this control should be increased until this smear is broken up into a pattern of small dots. This is easy to optimize if you have close small targets present. Just increase the sea clutter till they stand out prominently. Without such targets, you have to just estimate this. It is important to not run this filter too high or you will lose small or even medium sized close targets. Always leave some clutter showing. If you are heeled over, or for any reason there is more clutter to windward compared to leeward, you can be reasonably confident that the sea clutter is right by turning it up till you see this distinction clearly on the radar screen, but still leaving some clutter on the weaker side.
- Rain clutter (FTC). When in or near a rain or snow squall, the radar screen becomes cluttered with reflections from the precipitation itself. This can be so severe that it can mask the presence of any target in a nearby squall--or if you are in the squall, mask the presence of traffic approaching even though they are not in the squall. Reflections from precipitation are usually easy to identify from their "wool like" appearance. A rain squall on the radar screen looks rather like a cloud drawn raggedly with charcoal. Often the exact boundaries of a squall, or at least the part with rain content, can be clearly seen on the radar and even maneuvered around if necessary. The rain clutter filter breaks up the continuous display of precipitation echoes into a speckled pattern. These filters generally work quite well in rain and snow and will reveal targets which might not otherwise be seen. In heavy snow or hail the radar may be effectively blocked out by this interference and these controls may not adequately solve the problem. Once the precipitation goes away, the filter should be shut off. Note that this control might also be used in fair weather in crowded or confined harbors that present much radar clutter, or regions with bright land areas, to sharpen the picture since it does reduce the sensitivity in a manner that is qualitatively different from reducing the gain. Note that unlike the Sea Clutter control that works close in and progressively less at larger ranges, the Rain Clutter control works uniformly over the full range of the display.
- Echo stretch. This is a radar option that enhances the sizes of targets. It can be useful when looking for or following a small target. Turn it on, and all targets get larger. It lengthens then along the arc about the center of the screen. This option should normally be run in the off mode.
- Interference rejection. Your radar unit will pick up noise from other vessels' radars which will appear as either a background of dots or dotted arcs that shoot across the screen. This can present problems in crowded harbors. Turning on this function will eliminate the interference background. The shooting dotted arcs are easily identified. They are transient and usually do not appear in the same place twice. This option can be left in the on mode with no deterioration of performance. It should be tried periodically in congested waters to see the effect and if it helps. As an aside, if you detect these while at sea, it is a sign of the presence somewhere of another vessel with its radar on, even though you do not see it on the screen or visually. The source of this interference can be well over your visible horizon and may not appear at all.
- Zoom and offset (shift) These functions appear on some modern radars, although their function and operation may differ with the models. Offset or shift relocates the center of the display away from your own position so you can concentrate on a specific region. Generally you set a cursor to the new center and press a button to shift to it. Zoom allows users to expand the range about the new center, also sometimes using the cursor position to determine the extent of the zoom. These can be very useful options for watching specific circumstances, but they do leave the radar set in an unusual display. This could lead to confusion in some cases, so it is important to convey to all in use of the radar about how it is set and to return it to normal when done with that observation.
- Guard sectors and alarms, and watch mode. This allows you to define a safety range ring using the VRM, and then set an alarm that will sound whenever a target is detected within that ring. Some equipment allows for two rings to define more complex alarms, or even allow for using the EBL to convert the rings into sectors. Test such arrangements extensively before relying on them. Read the manual carefully on their use, as the gain and other options must be set properly. Some radars also offer a power saving option that allows you to program the radar to remain in stand-by mode but still automatically come on every 5 or 10 minutes to make a few radar sweeps to look for traffic. This option combined with guard rings and alarms might offer some level of warning for short handed operations. Needless to say, however, a proper watch is not kept by such arrangements. There is no electronic device that can be relied upon completely to detect and warn you of approaching traffic with risk of collision. Serious collisions have occurred involving vessels depending on such a system.
- Tuning is synchronization of sent and received pulses. To use manual tuning, the best procedure is to tune on an isolated clean target a mile or two off and adjust until the image is sharp, with Gain set about 50% full range and Sea clutter and Rain clutter turned off. Your manual will be the best guide to this. Watch the tuning bar as well if one is there. If the sharpest image does not correlate with the fullest tuning bar, then again see a technician.
Using radar involves a veritable alphabet soup of abbreviations. Here are the ones you need to know:
- DRM - Direction of Relative Motion
- SRM - Speed of Relative Motion
- RMD - Relative Motion Diagram
- VRM - Variable Range Marker
- EBL - Electronic Bearing Line
- SHL - Ship's Heading Line
- CPA - Closest Point of Approach
- TCPA - Time to CPA
- MCPA - Minutes to CPA
- BCPA - Bearing to CPA
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