User Manual

Theory of Operation (Cont.)

12 RDR-1600 Pilot’s Guide TM106101(8/01) TM106101(8/01) RDR-1600 Pilot’s Guide

5.3WEATHER RADAR PRINCIPLES

Airborne weather avoidance radar, as its name implies, is for avoiding

severe weather – not for penetrating it. Whether to fly into an area of radar

echoes depends on echo intensity, spacing between the echoes, and the

capabilities of both pilot and aircraft. Remember that weather radar detects

only precipitation drops; it does not detect minute cloud droplets. Therefore,

the radar scope provides no assurance of avoiding instrument weather in

clouds and fog. Your indicator may be clear between intense echoes; this

clear area does not necessarily mean you can fly between the storms and

maintain visual sighting of them.

The geometry of the weather radar radiated beam precludes its use for

reliable proximity warning or anti-collision protection. The beam is charac

terized as a cone-shaped pencil beam. It is much like that of a flashlight or

spotlight beam. It would be an event of chance, not of certainty, that such a

beam would come upon another aircraft in flight.

WARNING

Weather radar is not practical as a pilot operable collision

avoidance system. Weather analysis and avoidance are the

primary functions of the radar system.

5.2RADAR PRINCIPLES

Radar is fundamentally a distance measuring system using the principle of

radio echoing. The term RADAR is an acronym for Radio Detecting And

Ranging. It is a method for locating targets by using radio waves. The trans-

mitter generates microwave energy in the form of pulses. These

pulses are then transferred to the antenna where they are focused into a

beam by the antenna. The radar beam is much like the beam of a flashlight.

The energy is focused and radiated by the antenna in such a way that it is

most intense in the center of the beam with decreasing intensity near the

edge. The same antenna is used for both transmitting and receiving. When

a pulse intercepts a target, the energy is reflected as an echo, or return

signal, back to the antenna. From the antenna, the returned signal is trans-

ferred to the receiver and processing circuits located in the receiver

transmitter unit. The echoes or returned signals are displayed on an

indicator.

Radio waves travel at the speed of 300 million meters per second and thus

yield nearly instantaneous information when echoing back. Radar ranging

is a two-way process that requires 12.36 micro-seconds for the radio wave

to travel out and back for each nautical mile of target range. As shown in

the distance illustration in Figure 5.2-1, it takes 123.6 micro-seconds for a

transmitted pulse of radar energy to travel out and back from an area of

precipitation 10 nautical miles away.

Figure 5.2-1.Radar Transmit-Receive Timing