Radar Explained
Most chartplotter systems today are MFDs. That is Multi Function Displays. This means that you can add radar and sonar to them. The buying decision usually starts with the display. Stand alone radar units are still available, and have the inherent advantage that comes with dedicated devices.
A radar transmits a pulse, then listens for an echo. Based on the time it takes for the pulse to travel to the target and back, the radar knows the range. As the radar spins, it transmits approximately 1000 pulses per revolution, so every one third of one degree of rotation has a transmitted pulse, a pause and a listening for returns.The radar spends most of its time listening for returns. So for simplicity, it transmits for half a micro second, waits 2 micro seconds, then listens for 300 micro seconds. It repeats this process about 1000 times for each revolution of the radar, so it gets 1000 sets of reflections back. These are processed by a computer and displayed on the screen.
Radar stands for Range and Direction Finder. As the radar sweeps past a target, it receives echoes from that direction and that time. As it rotates, it displays targets on the screen as blobs. All those blobs on the screen look like just so much mumbo jumbo until a person interprets their meaning. That big blob on the left may be land, but the little blob that beeps steadily progressing across my screen is a boat. Those three blobs that are not moving may be buoys, but it is all based on human interpretation. The four controls of the radar are: TUNE, GAIN, SEA and RAIN. In the new radars, these are automated.Tune is the frequency or channel that you listen to. Gain is how “Hard” you listen or how sensitive the receiver is set.
Sea and Rain are filters. These two controls remove stray clutter from the screen. Rain slightly desensitizes the entire screen, where as Sea acts radically near the boat and less and less out to 3 miles and then not at all. Leave your rain clutter setting “off” unless you are in rain.
Sea clutter is the most commonly misused control. Today, with Auto Tune, Auto Gain, and Auto Sea, you would think everything would be idiot proof, but the radar still must be told if it is in the harbor, running the coast, or in open water off shore.Other radar functions included VRM, or Variable Range Marker. and EBL or Electronic Bearing Line. As the names imply, these are variable measuring lines that show on the radar screen and can be used to measure the distance and bearing of a radar target.
A guard zone can be set up around your boat using the guard feature of radar. If the targets change inside the zone that you set up, an alarm sounds.
ARPA, or Automatic Radar Plotter is now included in many radars, most notable, again is Furuno. What used to be a $500.00 option is now included in their new NavNet 3D line up of UHD (Ultra High Definition) radars. ARPA allows you to lock onto a target or “acquire” a target, and track its course and speed. The end game on ARPA to CPA or closest point of approach.
Raymarine, Garmin, and Simrad (including Northstar and Lowrance) all have new Digital radar antennas. JRC makes many of them. Furuno has once again maintained its’ position as the best, but we are nearing the era of “good enough” radar.
