Division Geometric parameters

Figures 2.7.j to 2.7.l show the source-receiver geometries when the observer O is (a) behind, (b) alongside and (c) ahead of the segment S₁S₂ where the flight direction is from S₁ to S₂ . In these diagrams

O

is the observer location

S₁ , S₂

are the start and end of the segment

S_p

is the point of perpendicular closest approach to the observer on the segment or its extension

d _1, d ₂

are the distances between start, end of segment and observer

d_s

is the shortest distance between observer and segment

d_p

is the perpendicular distance between observer and extended segment (minimum slant range)

λ

is the length of flight path segment

q

is the distance from S₁ to S_p (negative if the observer position is behind the segment)

The flight path segment is represented by a bold, solid line. The dotted line represents the flight path extension which stretches to infinity in both directions. For airborne segments, when the event metric is an exposure level L_E , the NPD distance parameter d is the distance d_p between S_p and the observer, called the minimum slant range (i.e. the perpendicular distance from the observer to the segment or its extension, in other words to the (hypothetical) infinite flight path of which the segment is considered to be part).

However, for exposure level metrics where observer locations are behind the ground segments during the takeoff roll and locations ahead of ground segments during the landing roll, the NPD distance parameter d becomes the distance d_s , the shortest distance from the observer to the segment (i.e. the same as for maximum level metrics).

For maximum level metrics, the NPD distance parameter d is d_s , the shortest distance from the observer to the segment.