PPI Range Information
Range information on a PPI scope is provided through the use of range marks. Range
marks show up as bright concentric circles on the scope. Their spacing takes various
values (1 mile, 5 miles, and so forth), although at any one time the spacing will be
uniform throughout the display.
The PPI scope may be expanded for short ranges. For example, a 5 mile range may
actually cover the same area on the face of the tube as a 30 mile range.
Off-Centered PPI Scope
When an aircraft makes RADAR approaches, you will often use an off-centered scope.
By moving the placement of the antenna sight (the "main bang") on the RADAR scope,
you will be able to see a greater area of airspace in one direction. At the same time, this
diminishes the airspace area you will be able to view in the opposite direction.
For example, suppose 30 miles is the greatest range on a range selector switch. It is
possible to extend the sweep out to 60 miles in any desired sector by off-centering the
main bang to the edge of the tube face. Whether targets could be seen out to 60 miles
depends on the RADAR system itself and the type of target. Figure 5-17 shows two PPI
scopes with the same range selected but with one off-centered to increase the area
covered to the southeast.