GPS Secondary areas

25.10.2004

Answer or Commentary:

We all recognize the above diagram from Pans-Ops dealing with the protection areas associated with GPS design of the initial and intermediate segments. However let us have a closer look at the area shown in red.

This area shows the secondary areas changing from intermediate (green) to initial (magenta). These areas are two different segments and can be looked at individually.

If an obstacle is inside the green intermediate secondary area, how is the applicable MOC calculated?

Case 1

Is the distance inside the secondary area calculated as shown with "d1" at right angles to the nominal track? If this is the case then zero MOC is reached at the red segment boundary line. Referring back to the first diagram this would mean that all obstacles whether in the initial or intermediate segments along the boundary line would have zero MOC. On the other hand the total width of the secondary area could continue to some unspecified point in space.

Case 2

The distance inside the secondary area is calculated as shown with "d2" at right angles to the inner edge of the secondary area. While the total width of the secondary area can also now be accurately determined. However this is in contradiction to the general ICAO principle of secondary areas

Any comments please I.W.