What you need to know about marine radar
Knowing the basics about marine radar systems will help you to make an informed purchase decision.
If you’ve ever yelled into a canyon to create an echo, you understand the basic principle of how radar operates. However, a little deeper understanding of radar theory can help you identify the system that’s right for you and operate it correctly.
Radar measurement of range, or distance, is possible because of radiated electromagnetic energy properties and solving a version of the 60D = ST formula taught in our Marine Navigation Course.
Transmitted radar (and radio) energy travels through the air in a straight line at approximately the speed of light (about 186,300 statute miles or 161,890 nautical miles per second). The number varies slightly because of atmospheric and weather conditions.
Reflection of electromagnetic energy
Transmitted electromagnetic waves reflect back toward the radar after striking a reflective surface—an obstacle that lies in the direction the antenna was pointed.
The radar energy paints the target with a microwave signal. This signal is picked up by the antenna and passed to the radar’s receiver for processing. This is called a radar echo or return.
Radar echoes appear on the radar display. A display typically shows a rotating strobe with the radar (your vessel) at the center. The rotating beam indicates the direction the antenna is pointing and the bearing toward a target.
As mentioned earlier, radar operates similarly to sound-wave reflection, like shouting in a canyon to produce an echo. If you know the speed of sound in air (about 1,100 feet per second), you can estimate the object’s distance from the time it takes an echo to return.
Similarly, radar transmits electromagnetic energy to an object and receives a small portion of the reflected energy back. Radar sets use this echo and other data (timing, antenna direction, etc.) to determine the direction and distance of the reflecting object. Then it displays the result on the screen.
Distance is determined by comparing the difference between the time the signal is transmitted and the time the echo is returned. The round-trip time is divided by two to obtain the time the wave took to reach the target. This is converted to distance using 60D = ST.
The antenna’s directivity, or ability to concentrate transmitted energy in a particular direction, determines the target’s bearing. The narrower the beam, the better the bearing accuracy (or resolution). The bearing angle from the radar to the target is determined by measuring the direction in which the antenna is pointing when the echo is received.
The true bearing of a radar target is the angle between true north and a line pointed directly at the target. The bearing angle to the radar target may also be measured in a clockwise direction from your boat’s centerline. This is referred to as the relative bearing.
Most radar system antennas radiate energy in a one-directional beam that can be moved in bearing by rotating the antenna. Because of the beam’s shape, the echo signal strength varies in intensity as the beam moves across the target. The point of maximum echo occurs when the beam points directly at the target. Depending on antenna design, most marine radars produce a beam width between 1.8 and 5.5 degrees. The narrower beam is usually found in larger open-array antennas. While the wider beam width is found in round radome antennas.
Keep in mind these features when using, researching or purchasing a radar system: maximum and minimum ranges and range and bearing resolutions (target resolution).
A radar unit’s maximum and minimum ranges (Rmax and Rmin) are factors of the width of the radar’s transmitted pulse and the number of pulses transmitted per second. Today’s marine radars provide an Rmax between 24 and 72 nautical miles and an Rmin as small as 20 meters, well within our requirements.
A radar’s target resolution is its ability to distinguish between targets close in range or bearing. Radar range resolution is a radar’s ability to distinguish between two or more targets on the same bearing but at different ranges. The degree of range resolution depends mostly on the width of the transmitted pulse and the target type and size. Pulse width is the primary factor in range resolution, with a shorter pulse providing better range resolution.
Bearing resolution is the minimum angular separation between two equal targets at the same range needed to display them as two distinct targets. Bearing resolution characteristics are determined by the radar antenna’s beam width. The narrower the beam width, the better the radar’s bearing resolution. Today’s marine radars are usually capable of distinguishing between targets a hundred yards apart in range or bearing.
Now that you know how radar systems work and what features to look for, you’re better prepared to make the best choice for your needs.
Learn more by taking our Radar for Boaters course. –Ron Davidson
Originally published in The Pirates Log, the newsletter of Poverty Bay Sail & Power Squadron/16.