About Radar

If you have ever been suddenly engulfed by fog, caught in a blinding rain storm, or disoriented by an array of confusing navigation lights against the backdrop of a light cluttered shoreline, then you will appreciate how radar's electronic vision will enhance your ability to navigate safely. Although once considered a luxury, radar's increased popularity can be attributed to newer lightweight and compact designs (along with lower prices), and its ability to communicate and share information with other navigation instruments, called interfacing.

How Radar Works

Radar "sees" objects by reflected radio waves instead of light waves, but the principle is the same. For example, if you shout toward a cliff from a distance, after a six second delay you hear your own sound waves reflect back to you as an echo. Since sound waves travel at a constant speed (about 1,100 feet per second), you can time the duration of the delay between the shout and the echo and calculate your distance from the cliff (divide by 2 to arrive at a one-way distance). Thus, 1,100 x 6 = 6,600/2 = 3,300 feet to the cliff.

Likewise, a radar transmitter sends out a constant stream of short pulse radio waves (at 162,000 nautical miles per second) through a revolving antenna (or scanner), and a computer inside the radar receiver measures the time delay and calculates the distance. The rotating scanner simultaneously determines the bearing from which the echo came. The results are then converted and displayed on the radar screen.

Navigating with Radar

Boaters assisted with radar can more confidently enlarge their cruising grounds without feeling completely at the mercy of the weather and darkness. Most users operate radar in the heading-up mode because it's the easiest to interpret at a glance. This means that the top of the display screen is always oriented with the bow of the boat, and targets on the screen are always relative to your own heading.

A north-up mode is oriented so the top of the screen is always pointing north, making it possible to observe how navigation aids move on the screen as the boat remains on its intended track. Radars with a north-up mode must be interfaced with an electronic compass.

The latest and most versatile option is the course-up mode, which receives navigation input from a GPS or Loran and/or an autopilot (optional). Here the top of the screen is relative to the rhumb line course of a selected waypoint. If the heading line of your boat drifts off the rhumb line from either pilot error or current, the resulting movement will show the XTE (cross track error), which can then be easily corrected. This mode is also ideal for avoiding collisions.

Whether you interface with other navigation instruments or not, radar is a powerful navigation device that can be used in any situation. A BoatUS Member in Millersville, MD, said that for practical navigation, whether just spotting navigation markers or avoiding collisions, his radar was more useful than any other navigation device he had, including GPS.

Helpful Terminology

LCD vs. CRT Displays
Boat size and radar location will greatly influence whether you choose a lightweight LCD display or a larger, heavier, high-resolution CRT display. LCDs are most common in cockpits and on flybridges because they show up well in sunlight, are easily disconnected and stored below, and some are waterproof. Monochrome CRTs will show better definition out of direct sunlight, but are best permanently mounted below or in an enclosed cabinet on the flybridge.

Transmit power determines the strength of the signal sent by the antenna and the corresponding echo back--the stronger the transmitter, the more likely soft or distant targets will show up on the screen. Most users operate their radar within a 4-mile range 95% of the time, where high power is not critical. Antenna height is also a factor because radar is a line-of-sight system.

Open or closed array antenna?
As a rule, the larger the antenna, the better its chances of distinguishing separate objects that are close together in congested areas. Open arrays are generally larger and found on powerboats. Closed arrays, called radomes, are usually reserved for sailboats where protection from sails and rigging is a concern.