Module 18—Radar Principles
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Index-1 to 3
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.
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.
Radar systems are often
divided into operational categories based on energy transmission methods—continuous wave (CW), frequency
modulation (FM), and pulse modulation (PM).
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
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.
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.
ANSWERS TO QUESTIONS Q1. AND Q44.
A1. Horizontal plane.
A3. Approximately the speed of light (162,000 nautical miles
A4. 12.36 microseconds.
A5. Pulse width.
A8. Average power.
A9. Duty cycle.
A10. Relative bearing.
A12. Frequency or phase.
A13. Target resolution.
A14. Beam width and range.
A16. Temperature inversion.
A18. High-voltage pulse from the modulator.
A20. Single lobe.
A21. The reflected signals decrease in strength.
A22. Mechanical and electronic.
A26. Fast-moving targets.
A28. Travel time.
A30. Pulse modulation.
A32. Track radar.
A33. Frequency modulated
A34. 360 degrees.
A35. Radar horizon.
A36. Wide vertically, narrow horizontally.
A37. 2D and 3D.
A38. Range and bearing.
A39. Increased maximum range.
A41. A narrow circular beam.
A43. Very narrow.
A44. Capture beam.
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