Definition:
~ Radioaltimeter indicates an aircraft's elevation with regard to terrain it is flying over
~ it assists navigation in the third dimension (altitude) while allowing latitude and longitude updates,
~ when combined with inertial navigation through so-called "terrain contour matching" algorithms
~ Despite its "slowness" (due to slow barometric variations), barometric altitude is also a very useful navigation aid due to its "smooth" nature (slow drift)
~ A radioaltimeter is a radar which measures "air draft" under plane
~ se readings are distorted by sudden changes in relief and radio interference
~ Barometric altitude, which indicates elevation above sea level, is a lot less subject to distortion,
~ at least at high frequencies, but changes in atmospheric pressure due to
~ weather changes render readings subject to drift in the long term and likely to require “zero” corrections
~ two readings are therefore complementary but a bridge is required between them as
~ "air draft" provided by radioaltimeter only becomes an elevation reading if altitude of terrain being flown over is added
~ This can only be provided by a digital map of terrain, provided latitude and longitude position is known
~ This shows how three-dimensional navigation parameters and digital mapping, are inextricably linked
~ However, this overlap does not stop here:
~ * A vertical accelerometer in an inertial unit can further enhance accuracy of vertical velocity and altitude calculations
~ * Flying over a specifically selected section of terrain can provide both a vertical and horizontal navigation fix
~ A suitably programmed computer can then compare profile of terrain being flown over
~ (change in air draft provided by radioaltimeter) to expected profile according to path
~ indicated by navigation system in that particular moment
~ If there is a strong correlation (match) between the two, this means horizontal navigation has moderate errors
~ If there is a discrepancy, system can look for the best path correction to produce the best correlation and
~ discrepancy will be considered a horizontal navigation error
~ algorithms which enable this are described as terrain contour matching algorithms
~ Using relatively simple sensors (radioaltimeter, baro-altimeter and vertical accelerometer),
~ as well as algorithms, decametric" quality three-dimensional navigation is achieved, temporarily outstripping even GPS
~ Only an additional navigation aid based on optronic sensors is capable of possibly doing better

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