Re: Re[2]: New ground proximity warning.

Date:         06 Jan 97 01:47:44 
From:         "Mark A. Brown" <mark@optima.nal.go.jp>
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Re terrain matching methods, there is another method which can be
employed in addition to simple linear profile matching to improve
accuracy, and that is terrain type matching. The strength of the
returned radar signal depends on the type of surface over which you're
flying -- woods, open fields, concrete jungle, water etc. This
information can also be used for navigation to supplement terprom,
provided that you have suitable information in the database.

Of course, cruise missiles don't just use terrain matching but also
have inertial guidance systems. IRSs tend to drift after a while,
however, so a terrain referenced system can be used to improve the nav
system accuracy. It doesn't matter if your terrain profile is slightly
innacurate, as the system can "dead-reckon" on IRS until it finds a
reasonable match.

For CFIT, I suggest that the terrain model doesn't have to be all that
accurate, so long as it is a "convex hull" for the real terrain
(i.e. the bounding polygon surface encloses the real surface
completely). After all, you're not using it for high-accuracy
navigation but merely to avoid flying into the ground. Surface
features like gullies and ravines, which are largely irrelevant, can
be smoothed out of the model safely, giving great benefits for
storage.

Mark.