The following paragraphs summarize the important points of this chapter.
RADAR is an electronic system that uses reflected electromagnetic energy to detect the presence and position of objects invisible to the eye.
TARGET POSITION is defined in reference to true north, the horizontal plane, and the vertical plane.
TRUE BEARING is the angle between true north and the line of sight to the target, measured in a clockwise direction in the horizontal plane.
ELEVATION ANGLE is the angle between the horizontal plane and the line of sight, measured in the vertical plane.
RANGE is the distance from the radar site to the target measured along the line of sight. The concepts are illustrated in the figure.
RANGE to any target can be calculated by measuring the time required for a pulse to travel to a target and return to the radar receiver and by dividing the elapsed time by 12.36 microseconds.
The MINIMUM RANGE of a radar system can be calculated from the formula:
PULSE-REPETITION TIME is the time between the beginning of one pulse and the beginning of the next pulse and is the reciprocal of prf.
AMBIGUOUS RETURNS are echoes from targets that exceed the prt of the radar system and result in false range readings. The maximum (unambiguous) range for a radar system can be determined by the formula:
The PEAK POWER of a radar system is the total energy contained in a pulse. Peak power is obtained by multiplying the maximum power level of a pulse by the pulse width.
Since most instruments are designed to measure AVERAGE POWER over a period of time, prt must be included in transmitter power measurements. The formula for average power is:
The product of pw and prf is called the DUTY CYCLE of a radar system and is the ratio of transmitter time on to time off.
The formula for the peak power (using average power) of a radar system is:
Antenna height and ROTATION SPEED affect radar range. Since high-frequency energy does not normally bend to follow the curvature of the earth, most radar systems cannot detect targets below the RADAR HORIZON. The distance to the horizon for a radar system can be determined by the formula:
The slower an antenna rotates, the larger the HITS PER SCAN value. The likelihood that a target will produce a usable echo is also increased.
The bearing to a target may be referenced to true north or to your own ship. Bearing referenced to true north is TRUE BEARING and bearing referenced to your ship is RELATIVE BEARING, as shown in the illustration. The bearing angle is obtained by moving the antenna to the point of maximum signal return.
Radar systems that detect only range and bearing are called TWO-DIMENSIONAL (2D) radars. Radars that detect height as well as range and bearing are called THREE-DIMENSIONAL (3D) RADARS.
The target RESOLUTION of a radar system is its ability to distinguish between targets that are very close together.
RANGE RESOLUTION is the ability to distinguish between two or more targets on the same bearing and is primarily dependent on the pulse width of the radar system. The formula for range resolution is: resolution = pw X 164 yards per microsecond
BEARING RESOLUTION is the ability of a radar to separate targets at the same range but different bearings. The degree of bearing resolution is dependent on beam width and range. The accuracy of radar is largely dependent on resolution.
ATMOSPHERIC CONDITIONS affect the speed and direction of travel of electromagnetic wavefronts traveling through the air. Under normal conditions, the wavefronts increase uniformly in speed as altitude increases which causes the travel path to curve downward. The downward curve extends the radar horizon as shown in the illustration. The density of the atmosphere, the presence of water vapor, and temperature changes also directly affect the travel of electromagnetic wavefronts.
The major components in a typical PULSE RADAR SYSTEM are shown in the illustration. The SYNCHRONIZER supplies the timing signals to coordinate the operation of the entire system. The TRANSMITTER generates electromagnetic energy in short, powerful pulses. The DUPLEXER allows the same antenna to be used to both transmit and receive. The RECEIVER detects and amplifies the return signals. The INDICATOR produces a visual indication of the range and bearing of the echo.
SCANNING is the systematic movement of a radar beam while searching for or tracking a target.
STATIONARY-LOBE SCANNING is the simplest type of scanning and is usually used in 2D search radar. Monopulse scanning, used in fire-control radars, employs four signal quantities to accurately track moving targets. The two basic methods of scanning are MECHANICAL and ELECTRONIC.
The CONTINUOUS WAVE (cw) method transmits a constant frequency and detects moving targets by detecting the change in frequency caused by electromagnetic energy reflecting from a moving target. This change in frequency is called the DOPPLER SHIFT or DOPPLER EFFECT.
In the FREQUENCY MODULATION (fm) method, a signal that constantly changes in frequency around a fixed reference is used to detect stationary objects.
The PULSE-MODULATION (pm) METHOD uses short pulses of energy and relatively long listening times to accurately determine target range. Since this method does not depend on signal frequency or target motion, it has an advantage over cw and fm methods. It is the most common type of radar.
Radar systems are also classified by function. SEARCH RADAR continuously scans a volume of space and provides initial detection of all targets. TRACK RADAR provides continuous range, bearing, and elevation data on one or more specific targets. Most radar systems are variations of these two types.