Terrain Referenced Navigation

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Terrain Referenced Navigation (TRN) is the terrain data-aided navigation with instruments. It has nothing to do with flying by sight.

Description of the TRN procedure

This is a navigation method in which the current position of the aircraft is determined by means of a section of the terrain profile of the area overflown, the flight route , using a digital terrain model and made available for navigation.

There are various sensory options for determining and recording a terrain profile during the flight:

is rarely used.
Advantage: exact height above the current position in a narrow point
Disadvantage: cannot be used in clouds, fog, etc.
is used almost exclusively.
Advantages: no problems with clouds, rain, fog etc.
Disadvantages: inaccurate height determination, as a large area is scanned

A piece of the terrain profile with a minimum length is first preprocessed before this data can be provided for the actual position determination algorithm.

The positioning algorithm uses the transferred piece of terrain profile and compares it with the saved terrain data of its digital terrain model. The profile is shifted in a matrix in several steps evenly in east and west, as well as in north and south directions from an assumed position . For each of the offset flight profiles, a comparison calculation is made with the terrain altitude data available in the digital terrain model, with one value being calculated for each comparison in the matrix field.

After all values ​​have been determined in this comparison matrix, there is a high probability that the position of the aircraft is where the values ​​in the matrix converge in the form of a funnel to the smallest value. This means that the aircraft is there where the profile of the terrain heights recorded by sensors corresponds best with the height values ​​of the digital terrain model.

Different statistical algorithms can be used for the comparison calculation between the measured terrain heights and the digital terrain data in the comparison matrix :

  • MAD - Mean Absolute Distance
  • MSE - Mean Square Error
  • Etc.

The integrity and accuracy of this value is just as important as the actual result of the position determination in the TRN method, which supplies the geographic coordinates of the position . For this u. a. the shape of the funnel in the comparison matrix can be analyzed and evaluated. In addition, the result can be used with other position-supplying systems, such as B. Laser - INS , GPS etc. evaluated and a common result integrated be.

Aircraft in which a TRN system has been / will be integrated

Possible uses of the TRN system

  • Use "only" to evaluate the other position-determining systems ( INS , GPS )
  • Standalone Positioning System (TRN)

Problems with the TRN system

  • At the terrain
    • The terrain does not contain enough slopes or the terrain is too flat.
    • Flights over water do not contain any altitude information. It is therefore very helpful for the assessment to know what type of object the aircraft is currently flying over.
    • Ambiguities in the terrain / in the terrain data. It is possible that the site has several similar / identical funnel-shaped position solutions.
  • With the digital terrain model
    • Insufficient resolution of the data material used / available
    • Insufficient accuracy of the data used / available
    • Missing data of the overflown area
    • Smaller missing data areas with certain data sources (e.g. with SRTM data)
  • Radar altimeter
    • Radar altimeter exit angle too great. The larger these are, the less precise the results will be.
    • Excessive lateral angle aircraft movements. If these are too big, no terrain profile can be recorded.
    • Too high altitude. If the flight altitude is too high, the illuminated area of ​​the radar altimeter becomes too large, which makes the radar altitude value less accurate.
  • Requirements
    • Too small area of ​​the search area
    • Too large area of ​​search area; here the response times of the system are no longer tolerable.
    • Absence of an accepted position, e.g. B. by INS or GPS
    • The real position is outside the search area. It cannot be found and leads to incorrect results.