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Figure 9: Position accuracy of uncorrected GPS
With good DGPS corrections, your 958 should provide better than four-meter accuracy (repeatable and absolute) 95 percent of the time. SOG readings should be accurate to 0.1 knot.
You can connect your 958 to your loran receiver (as long as the loran has the appropriate NMEA 0183 output capability), and display and navigate with real-time loran TDs. With this configuration, you’ll keep the repeatable accuracy of your loran, but will add all of the navigation features of the 958, including the worldwide absolute accuracy of GPS.
Introducing radar
The name RADAR is derived from the phrase “Radio detection and ranging.”
How does radar work?
958 Operator’s Manual, Rev. A
Radar is easy to understand if you take this example: Suppose you shout towards a cliff, and in a few seconds you hear the echo. Knowing the speed of sound is constant (approximately
1,100 feet per second), you can therefore start a stop-watch at the time you yell (moment of transmission) and stop the watch when the echo (reflective sound) is heard. Let’s say that time is
6 seconds. You can now calculate your distance from the cliff by multiplying the speed of sound per second, times the number of seconds, divided by 2. (Don’t forget: the total time
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is for the sound to travel to the cliff and back; you want the distance one way only.)
The calculation is: 1,100 x 6 = 6,600/2 = 3,300 feet.
Radio waves transmitted with high power and very high frequency bounce back from hard objects (called “targets”) exactly the same way your voice is bounced from the cliff. The speed of radio waves is constant (162,000 nautical miles per second), so a computer inside the radar receiver acts as a stop-watch to measure the time. The result is then displayed on a radar screen. The technical name for a radar screen is
Plan Position Indicator (PPI).
How does the radar screen work?
Think of a radar operator as looking down from a helicopter flying above the boat. The helicopter is always above the boat and in the center of the display. The ship’s heading is indicated by the heading marker, an electronic line that extends from the center of the screen to the edge of the display. It points towards the bow of the boat.
Range rings are concentric rings that represent distance from your vessel. They are changed any time the radar range is adjusted; therefore, the operator must be careful to note what range the display is on. An object three rings away on a 0.5 nm per range ring is 1.5 nm from the boat. However, on a 5 nm per range ring, the object is 15 nm from the boat.
Detectability of radar
In standard atmospheric conditions, there are three basic horizons: geometric, optical, and radar. Geometric is the straight line between the sighting point and the horizon. The optical horizon is caused by the normal bending of light waves and this increases the geometric horizon by about 6 percent.
The radar horizon is created by the even greater curvature of radio waves and extends the horizon by about 15 percent.
Radio waves generally travel in straight lines. Hence the detectable range of radar increases in accordance with the height of the radar antenna and the target. For this reason, and when set to a long range, a coast line may not be seen yet the high mountains inland further away may be detected.
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Another important factor in the detectability of a radar echo is the strength of the reflected wave. This strength will vary depending on the type of material, size, shape, and height of the target. Among the strong reflectors are bluffs facing the ship, buildings, breakwaters, rock walls, and mountains. Some of the weaker targets are wooden boats, sandy beaches, and rubber lifeboats.
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