We stated earlier that radar is used to determine the distance and direction to and the height of distant objects. These three pieces of information are known, respectively, by the standard terms range, bearing, and altitude. The use of these standard terms allows anyone interested in a specific target to establish its position quickly and accurately. Radar operators determine a target's range, bearing, and altitude by interpreting its position displayed on a specially designed cathode-ray tube (CRT) installed in a unit known as a plan position indicator (PPI).

While most radars are used to detect targets, some types are used to guide missiles to targets and to direct the firing of gun systems; other types provide long-distance surveillance and navigation information. Range and bearing (and in the case of aircraft, altitude) are necessary to determine target movement. To be a successful radar operator, you must understand the capabilities and limitations of your radar system in determining range, bearing, and altitude.

Range

The radar measurement of range (or distance) is possible due to the properties of radiated electromagnetic energy. This energy normally travels through space in a straight line, at a constant speed, and varies only slightly due to atmospheric and weather conditions. The frequency of the radiated energy causes the radar system to have both a minimum effective range and a maximum effective range.

MINIMUM RANGE.-Radar duplexers alternately switch the antenna between the transmitter and the receiver so that one antenna can be used for both functions. The timing of this switching is critical to the operation of the radar and directly affects the minimum range of the radar system. A reflected pulse will not be received during the transmit pulse and subsequent receiver recovery time. The minimum range of a radar, therefore, is the minimum distance between the radar's antenna and a target at which a radar pulse can be transmitted, reflected from the target, and received by the radar receiver. If the antenna is closer to the target than the radar's minimum range, any pulse reflected from the target will return before the receiver is connected to the antenna and will not be detected.

MAXIMUM RANGE.-The maximum range of a pulse-radar system depends on carrier frequency; peak power of the transmitted pulse; pulse-repetition frequency (PRF) or pulse-repetition rate (PRR) (PRF and PRR are synonymous terms.); and receiver sensitivity, with PRF/PRR as the primary limiting factor.

The peak power of a pulse determines how far the pulse can travel to a target and still return a usable echo. A usable echo is the weakest signal that a receiver can detect, process, and present on a display.

The PRR determines the rate at which the range indicator is reset to zero. As the leading edge of each pulse is transmitted, the indicator time base used to measure the returned echo is reset, and a new sweep appears on the screen.

RANGE ACCURACY.-The shape and width of the radio-frequency (RF) pulse influences minimum range, range accuracy, and maximum range. The ideal pulse shape is a square wave that has vertical leading and trailing edges. The vertical edge provides a definite point from which to measure elapsed time on the indicator time base. A sloping trailing edge lengthens the pulsewidth. A sloping leading edge provides no definite point from which to measure elapsed time on the indicator time base.

Other factors affecting range are the antenna's height, beamwidth, and rotation rate. A higher antenna will create a longer radar horizon, allowing a greater range of detection. An antenna with a narrow beamwidth, provides a greater range capability, since it provides more concentrated beam with a higher energy density per unit area. A slower antenna rotation rate, providing more transmitted pulses during the sweep, allows the energy beam to strike each target more times, providing stronger echo returns and a greater detection range.

From the range information, the operator knows the distance to an object. He now needs bearing information to determine where the target is in reference to the ship.

Bearing

Radar bearing is determined by the echo's signal strength as the radiated energy lobe moves past the target. Since search radar antennas move continuously, the point of maximum echo return is determined either by the detection circuitry as the beam passes the target or visually by the operator. Weapons control and guidance radar antennas are positioned to the point of maximum signal return and are maintained at that position either manually or by automatic tracking circuits.

You need to be familiar with two types of bearing: true and relative.

TRUE BEARING.-True bearing is the angle between true north and a line pointed directly at the target. This angle is measured in the horizontal plane and in a clockwise direction from true north.

RELATIVE BEARING.-Relative bearing is the angle between the centerline of the ship and a line pointed directly at the target. This angle is measured in a clockwise direction from the bow. Most surface-search radars provide only range and bearing information. Both true and relative bearing angles are illustrated in figure 1-2.

Altitude

Altitude or height-finding radars use a very narrow beam in the vertical plane. This beam is scanned in elevation, either mechanically or electronically, to pinpoint targets. Tracking and weapons-control radar systems in current use scan the beam by moving the antenna mechanically or the radiation source electronically.

Most air-search radars use electronic elevation scanning techniques. Some older air-search radar systems use a mechanical elevation scanning device; but these are being replaced by electronically scanning radar systems.