Indicator Unit Height Tube Disply

The second or smaller tube on the Indicator is the Height Tube.

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The electrical signal that produces the brightness variation and hence the map display on the PPI tube, is also sent to the height tube that has a single vertical scan line. Variations in the signal level due to noise or echoes, deflect the line to the RIGHT of the display.

The different signal levels received as the scanner rotates, appear as bulges on the display with the range marker appearing as a permanent line jutting out to the RIGHT of the display.

Signals close to the aircraft appear at the bottom of the trace and those at a greater distance at the top. When a transmitted pulse leaves the aircraft no echoes occur from the space between the aircraft and the ground and a gap can be seen in the scatter signal.

The first echo is from the ground immediately below the aircraft and appears as a sudden increase in the scatter level. A height marker can be seen projecting to the LEFT of the display and by altering the height knob on the 207 Switch Unit, this marker can be set just at the beginning of the ground echo. The height of the aircraft can then be read off on a calibrated scale attached to the height knob.

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