Sidebilder
PDF
ePub
[graphic][subsumed][subsumed][merged small][subsumed][subsumed][subsumed][subsumed][subsumed]
[blocks in formation]

The objective of regulating the rate of travel of the electron beam in this manner is to establish a time base on the PPI which may be used for direct measurements of distances to targets without further need to take into account the fact that the transmitted pulse and its reflected echo make a round trip to and from the target. With the periphery of the PPI representing a distance of 20 miles from the center of the PPI at the 20-mile range scale setting, the time required for the electron beam to move radially from the center to the periphery is the same as the time required for the transmitted pulse to travel to a target at 20 miles and return to the antenna as a reflected echo or the time to travel 40 miles in this case. It follows that a point on the sweep or time base halfway between the center of the PPI and its periphery represents a distance of 10 miles from the center of the PPI. The foregoing assumes that the rate of travel of the electron beam is constant, which is the usual case in the design of indicators for navigational radar.

If the antenna is trained on a target at 10 miles while using the 20-mile range scale, the time for the 20-mile round trip of

seconds. At 123 microseconds, following the instant of triggering the transmitter and sending the timing trigger pulse to the indicator to deflect the electron beam radially, the electron beam will have moved a distance of 10 miles in its sweep or on the time base. On receiving the echo at 123 microseconds after the instant of the pulse, the receiver sends a video signal to the indicator which in turn acts to intensify or brighten the electron beam at the point in its sweep at 123 microseconds, i.e., at 10 miles on the time base. This brightening of the trace produced by the sweep at the point corresponding to the distance to the target in conjunction with the persistence of the PPI produces a visible image of the target. Because of the pulse repetition rate, this painting of an image on the PPI is repeated many times in a short period, resulting in a steady glow of the target image if the target is a reasonably good reflector.

In navigational and collision avoidance applications of radar, the antenna and the beam of r-f energy radiated from it are rotated at a constant rate, usually about 10 to 20 revolutions per minute in order to detect targets all round the observer's ship. In the preceding discussion of how a target image is painted on the PPI, reference is made only to radial deflection of the electron beam to produce the sweep or time base. If target images are to be painted at their relative bearings as well as distances from the center of the PPI, the sweep must be rotated in synchronization with the rotation of the antenna. Just as the electron beam may be deflected radially by electrostatic or electromagnetic means, the sweep may be rotated by the same means. The sweep is usually rotated electromagnetically in modern radars.

As the antenna is rotated past the ship's heading, the sweep, in synchronization with the antenna, is rotated past the 0° graduation on the relative bearing dial of the PPI. The image of

bearing and distance from the center of the PPI. As targets are detected in other directions, their images are painted on the PPI at their relative bearings and distances from the center of the PPI.

Up to this point the discussion of how target information is displayed on the PPI has been limited to how the target images are painted, virtually instantaneously, at their distances and relative bearings from the reference ship at the center of the PPI. It follows that through continuous display (continuous

repetition rate) of the positions of targets on the PPI, their motions relative to the motion of the reference ship are also displayed.

In summary the indicator of this basic radar system provides the means for measuring and displaying, in a useful form, the relative bearings and distances to targets from which reflected echoes may be received. In displaying the positions of the targets relative to the reference ship continuously, the motions of the targets relative to the motion of the reference ship are evident.

THE RADAR WAVE

Radar (radio) waves, emitted in pulses of electromagnetic energy in the radio-frequency band 3000 to 10,000 megahertz (MHz) used for shipborne navigational radar, have many characteristics similar to those of other waves. Like light waves of much higher frequency, radar waves tend to travel in straight lines or rays at speeds approximating that of light. Also, like light waves, radar waves are subject to refraction or bending in the atmosphere. Like waves in the sea, radar waves have energy, frequency, amplitude, wavelength, and rate of travel.

Whereas waves in the sea have mechanical energy, radar waves have electromagnetic energy, usually expressed in watt units of power. This electromagnetic energy has associated electric and magnetic fields, the directions of which are at right angles to each other. The orientation of the ELECTRIC AXIS in space establishes what is known as the POLARIZATION of the wave. Horizontal polarization is used normally with navigational radars, i.e., the direction of the electric axis is horizontal in space. Horizontal polarization has been found to be the most satisfactory type of polarization for navigational radars in that stronger echoes are received from the targets normally used with these radars when the electric axis is horizontal. See LINEAR AND CIRCULAR POLARIZATION.

Each pulse of energy transmitted during a few tenths of a microsecond or a few microseconds contains hundreds of complete oscillations. A CYCLE is one complete oscillation or one complete wave, i.e., that part of the wave motion passing zero in one direction until it next passes zero in the same direction (see figure 1-9). The FREQUENCY is the number of cycles completed

[blocks in formation]

If the radar waves actually traveled in straight lines or rays, the distance to the horizon grazed by these rays would be dependent only on the height of the antenna, assuming adequate power for the rays to reach this horizon. Without the effects of refraction, the distance to the RADAR HORIZON would be the same as that of the geometrical horizon for the antenna height. Like light rays, radar rays are subject to bending or refraction in the atmosphere resulting from travel through regions of different density. However, radar rays are refracted slightly more than light rays because of the frequencies used.

[blocks in formation]

Refraction in Standard Atmosphere

Where h is the height of the antenna in feet, the distance, d, to the radar horizon in nautical miles, assuming standard atmospheric conditions, may be found as follows:

d = 1.22√h

With the distances to the geometrical and optical horizons being 1.06 √ and 1.15 h, respectively, the radar horizon exceeds the geometrical horizon by 15% and the optical horizon by 6%. Thus, like light rays in the standard atmosphere, radar rays are bent or refracted slightly downwards approximating the curvature of the earth (see figure 1-10).

The distance to the radar horizon does not in itself limit the distance from which echoes may be received from targets. Assuming that adequate power is transmitted, echoes may be re

In the preceding discussion standard atmospheric conditions were assumed. The STANDARD ATMOSPHERE is a hypotheti cal vertical distribution of atmospheric temperature, pressure, and density which is taken to be representative of the atmos. phere for various purposes. While the atmospheric conditions at any one locality during a given season may differ considerably from standard atmospheric conditions, the slightly downward bending of the light and radar rays may be described as the typical case.

While the formula for the distance to the radar horizon (d = 1.22 h) is based upon a wavelength of 3 centimeters, this formula may be used in the computation of the distance to the radar horizon for other wavelengths used with navigational radar. The value so determined should be considered only as an approximate value because the mariner generally has no means of knowing what are the actual refraction conditions.

« ForrigeFortsett »