Getting a Fix on Radar

— By David Anderson

Radar is an invaluable tool. There are many times when having an eye that pierces darkness and fog makes all the difference, provided you have a good radar set and the skills to use it to its best advantage. The process of learning to use radar involves several steps. Besides understanding, in basic terms, how radar works, you need to learn how to use radar for simple piloting or chart navigation and then incorporate that knowledge into your routine navigation. Arguably the most important aspect of radar use is accurate interpretation of the images you see on your radar screen, especially the moving “targets.” This allows you to evaluate the risk of collision and maneuver safely according to navigation rules. Continuous use and practice will achieve decisive interpretation.

Radar works by sending out microwave pulses and detecting signals reflected back from “targets” around your boat. What it’s not is a television camera. On the radar screen, the user sees only blips or echoes of the targets, not realistic representations. Consequently, it takes practice to read a radar screen and to interpret what is really out there. You can teach yourself by practicing in good visibility. Compare how nature, the appropriate chart and the radar image fit together. You will find that there is usually quite a lot missing in the radar image owing to the one-sided illumination of the surroundings of your boat at the center of the display.

Targets and Antennas
The maximum range scale specified for a radar unit has more to do with power output than how far it can see targets. If the target is over the “radar horizon,” you won’t see it, no matter how much power you are broadcasting. Radar range is slightly farther than visual or geographic range due to the refraction of microwaves. It can be calculated as follows: Maximum radar range (NM) equals 1.2 x [÷ h (ft) + ÷ H (ft)], where h is the height of the antenna in feet and H is the height in feet of the land mass or target boat.

For example, if your antenna is mounted at a height of 12' above the water and you are looking for a vessel that is say 60' high, then it will faintly appear at a distance of about 13.5 nautical miles. Thus even if you have a 24- or 36-mile radar, you need to be looking for something higher than 60' or you won’t see it from your antenna. If you mount the antenna on a spreader, say 20' above the water, you gain an extra mile. If you opt for mast mounting, at a height of say 30', you gain another mile. However on a small boat at sea, an antenna of this height will be rocking so much from wave action that much of this elevation is wasted. For most small craft pole mounting an antenna at a height of 9' to 12' is perfectly adequate and avoids carrying extra weight aloft from the long heavy cable.

Minimum Range
Minimum range is a more subtle computation, having to do with the pulse length and processing of the microwave signal but there is also a geometric element, which arises from the shadowed region that lies below the beam pulse.

The vertical width of a typical radar beam is about +/- 15° from the horizontal. If the antenna is mounted at height h, the beam first strikes the water at distance of h / tan (15°) feet. For a 30' antenna, this is 30/0.268, 112' or about 37 yards from the antenna. With a 12' antenna this distance is reduced to 44' or 15 yards from the antenna. So on a typical small craft, even one with a high-mounted antenna, this is not really a limitation.

The electrical limitation on minimum range is 164 yards for each microsecond of pulse length. Most radars switch to shorter pulse lengths at lower ranges, with something in the order of 0.12 microseconds being typical for ranges less than 1 mile. This translates to 0.12 x 164 or about 20 yards from the antenna but enhanced signal processing usually doubles this electronic limitation.

The lowest range scale on many radars is 0.25 miles or, 0.125 miles or 220 yards. Often the last 50 yards or so is filled with so much noise that these pulse length and height considerations are not the actual practical limitation to minimum range.

Radar Resolution
Resolution is a measure of how well two nearby objects are resolved or separated on a radar screen and comprises two separate factors: bearing resolution and range resolution.

The typical horizontal width of a 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. For example, two boats seen 5 miles off, must be 0.5 miles apart or they will appear as one. Similarly, if the entrance to a harbor is 0.2 miles across, it will not be seen as an opening (when headed straight toward it) until you are within some 2 miles of it. It is a good idea to become familiar with bearing resolution and these relationships by making your own measurements with a chart in hand to see how it works with your radar.

The pulse length of a radar signal determines range resolution. A microwave travels at the speed of light, that is 186,000 miles per second or 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 will still be reflecting signals from the end of the pulse when the farther one starts to reflect signals from the front of the pulse and they will 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. You can select pulse length in some units but, in most small craft units, it’s done automatically when you change ranges.

In one unit, for example, on a 3-mile range, the pulse length is 0.3 microseconds and on a 4-mile range it is 0.8 microseconds. Consider the case of two close vessels (say a tug and tow) separated by 100 yards at a distance of 2.8 miles. On the 4-mile scale they will appear as one vessel (resolution 131 yards), but on the 3-mile scale they will show as two distinct close vessels (resolution 49 yards). This is something to practice using your radar unit. You will need to look up the pulse lengths used for the various range scales in the specification section of your radar manual. Needless to say, you must also tune your radar for best resolution. For example, if the gain is too high it will smear out the targets.

Decoding Echoes
How well a landmark shows on radar depends on its range and bearing. The key issue is the height of the land and the resolution of the radar. Isolated targets like other vessels, buoys, small islands or drilling-rigs are easier to interpret than large irregular landmasses. At longer distances, isolated targets all appear as simple dots or small line segments. As they get closer, the target size increases but, unless the object is big and fairly close, the “size” of the echo on the screen is not a measure of the actual size of the target.

The shape of the target also influences acquisition or contact distance. Round and pointed bodies reflect only a small part of the incoming energy back to the scanner. The same applies to surfaces inclined towards the horizontal, such as the windscreens of some motor yachts.

When you are moving, the motion of any targets on the screen is relative motion, not true motion. If you are moving towards a stationary buoy at 5 knots, it appears on your radar screen as the buoy is moving towards you at a speed of 5 knots. The only stationary target on a radar screen is one that is moving in exactly the same direction and at the same speed as you are.

Preventative Maneuvers
You must first decide whether or not a target poses a risk of collision and then you must determine what the circumstance is that leads to this risk. For example, it’s fairly easy to determine a target moving straight down your ship’s heading line on a collision course, but is this a vessel you are going to run into from astern or a target headed full steam right for your bow? For targets closing in on a diagonal track the analysis is a bit more involved. Finally, you must decide what needs to be done to avoid a collision.

Underway, radar has two basic uses, position fixing or position confirmation and collision avoidance. The variable range marker (VRM) and the electronic bearing line (EBL) are the tools that enable these to be tasks to be undertaken with confidence. The EBL provides the bearing to a target, while the VRM indicates range to the target at that particular point in time. As time goes by, it is easy to see if you are gaining or losing range and bearing to the target. Are you on a collision course with another vessel if you maintain the same bearing but continue to close range? Is the current or tide sweeping you to the wrong side of a channel buoy even though your compass heading implies that you are heading for the correct side of the buoy? The EBL and VRM will provide you with the answers.