Section I. PRINCIPLES OF OPERATION

CHAPTER 4

D I R E C T AND GENERAL SUPPORT MAINTENANCE

Section I. PRINCIPLES OF OPERATION

4-1. General

"sum" and "difference" are obtained from the

same output.

The marker beacon lamp flashes a coded signal as

d. The mechanisms Ml, M2, M3, and M4 are

the aircraft passes over each of the three marker

similar in construction and operating principle.

beacon transmitters. These transmitters are located

Each mechanism is a permanent magnet moving-

at approximately 4.5 miles, 1.0 mile, and 200 feet

coil type microammeter, which consists essentially

from the end of the runway. The outer marker

of a moving coil mounted and pivoted in the field of

beacon is identified by a 2 dashes-per-second signal,

a permanent magnet (fig. 4-2). Current flowing

the middle marker beacon by a 6 dots-per-second

through the coil causes the coil to rotate, which

signal and the boundary marker beacon by an

moves the attached pointer across the scale. The

unkeyed signal. As these signals are received, the

deflection of the moving coil is proportional to the

pilot checks the distance to the runway and the

current flowing through the coil, and hence, to the

altitude at the time of crossing.

strength of the signal received from the aircraft

4-2. Block Diagram Analysis

receiver.

a. Figure 4-1 shows a simplified block diagram of

the Instrument Approach System. The signal is

split at the ground transmitter; one section is

modulated at 90 cycles per second, and the other is

modulated at 150 cycles per second. For con-

venience in flight, these sectors are designated as

blue and yellow sectors and the indicator face is

marked with corresponding blue and yellow sec-

tors. The blue sector is transmitted to the right of

the beam in respect to the landing aircraft and the

yellow sector is transmitted to the left of the landing

aircraft. The signals are received by the localizer

receiver and separated by frequency discriminators

or hand-pass filters, individually rectified and

recommended by differential connection of the

rectifier output to actuate the pointer. The glide

path transmitter and receiver operate in a similar

manner to establish the glide path intelligence.

Figure 4-1. Simplified block diagram of indicator in instrument

b. The vertical pointer supplies a visual in-

approach system.

dication of the lateral position of the aircraft with

rcspect to the on-course signal of the localizer beam.

The horizontal pointer shows the relationship of the

aircraft to the glide-path beam. When the aircraft is

properly aligned on the approach path, the pointers

are crossed at the center of the dial.

c. The dotted circuit of figure 4-1 shows the

connections of the flag mechanism in the output

circuit of the receivers. The warning flags operate

from the summation of the rectifier outputs while

the pointers operate from the difference of these

outputs. T h i s arrangement provides positive

"nonoperating" signals by virtue of the fact that the

Figure 4-2. Permanent magnet, moving-coil type mechanism.

4-1