INTERPRETING THE RCS-4B DISPLAY (cont)

TM 11-5820-917-13

Time

Relative

delay (ms)

time delay (ms)

Mode

Surface Wave

.27

1-Hop Es

.60

.33

1-Hop F

1.60

1.33

The relative time delay difference between the surface wave and Es modes

is not very great and diminishes as range increases. Figure 7-4 provides the

best solution to the dilemma: an ionogram showing both modes simultaneously.

7-25. Figure 7-7 shows another factor to consider in monitoring the RCS-4B

CRT. The bar graph shows received signal power vs frequency which need

not always be the received Chirp sounder signal. High atmospheric noise or

(as in the case of figure 7-7) the output of nearby transmitters can also affect

the bar graph. Strong interfering signals will cause the effects shown in

figure 7-7: (1) dots running vertically on the lower part of the display,

(2) momentary interruption of the received signal traces, and (3) associated

peaks in the signal level bar graph.

7-26. In summary, then, the following guidelines are offered in the use and inter-

pretation of the RCS-4B CRT display:

a. Before the RCS-4B is synchronized with any transmitter, set the INPUT

ATTENUATOR to 0 dB and run several sweeps to survey the local noise environ-

ment. If excessive noise appears on the display in the form of dots on the screen

and very high signal level indication, add 10 dB to the attenuator before attempt-

ing to operate the automatic synchronization feature (to lower the probability

that the auto-sync logic will wrongly interpret the dots as received signal).

b. After synchronization has occurred, adjust the INPUT ATTENUATOR to

the lowest value that, if possible, permits only the received Chirpsounder signal

to cause a display on the signal-level bar graph. For example, the top of figure

7-8 shows an ionogram recorded with the RCS-4B input attenuator set at 0. Note

the high noise level showing on the bar graph (the RCS-4B was situated near an

active machine shop). The bottom of figure 7-8 shows how the ionogram would

probably appear with the input attenuator set at 20 dB . Observe how much

simpler the lower display is to interpret.

c. From time-to-time, run received sweeps without signal to observe the

changing noise level. Remember that in determining the best frequency bands,

it is signal-to-noise ratio rather than signal power that is important. Check

also for any spikes in the bar graph display caused by interference signals.

Remember also that the bar graph is meaningless regarding propagation from

the transmitter if no trace is seen on the time-delay portion of the CRT display.

d. Know those frequencies where local interference may cause spurious out-

put on the signal level graph. Running periodic sweeps on a path where a Chirp-

sounder transmitter is not operating or running a received sweep in the RCS-4B's

MANUAL mode (on Path 1) will show the likely trouble spots.

7-11