Terrain-contour adjustment

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Basic sketch of the terrain-contour comparison

The terrain contour comparison ( English Ter rain Co ntour M atching , short TERCOM ) is a procedure in the navigation . With this method, cruise missiles are mainly guided into a target area today .

background

With the terrain-contour comparison , the navigation system has a surface contour map which shows the spatial profile of the terrain overflown and the target area. At the same time, the navigation system uses radar to record a surface map of the area currently being flown over. The navigation system determines its current position in space by comparing the surface between the saved and currently measured terrain contour. The increased accuracy enables a missile equipped with TERCOM to fly closer to obstacles and generally at lower altitudes.

This flight profile makes detection with ground penetrating radar and thus fighting it more difficult. When intercepting such missiles with combat aircraft , a radar with look-down / shoot-down capability is usually required.

A common method for this surface comparison is the Iterative Closest Point Algorithm .

technology

TERCOM “maps” consist of a series of strips of terrain that the missile will fly over as a series of coded heights. Since a radar altimeter measures the distance, i.e. the height above the ground and not an absolute height, the maps are usually coded with the change in height and not with the absolute height. In addition, the land strips are saved on both sides of the expected flight corridor. These map series are usually created with data from radar satellites ( radar mapping ). When flying over water, the contour maps are replaced by magnetic field maps.

In flight, the radar altimeter stores measurements in a smaller buffer. Here measurements are regularly saved over a period of time and mean values ​​of the individual measured values ​​are generated. The series of individual values ​​are held in the buffer and thus generate a series of measurements similar to those recorded in the “maps”. These two series of measurements are compared with one another. The row of the buffer is overlaid with the known map. The position and direction can be determined from this information. The navigation system then uses this data to correct the flight path.

During the flight to the target, the accuracy of the system only needs to be precise enough to avoid collisions with the terrain. This enables the system to work with cards with relatively low resolutions during the flight phase. Only the part of the map for the final approach has a higher resolution.

Due to the limited size of the available memory in mass storage devices of the 1960s and 1970s and their slow access times, the amount of data for the available memory was too large to store the entire flight path in this way. Instead, individual small sections of terrain were saved and paired with a conventional inertial navigation system (INS). Such systems are also referred to as TAINS (TERCOM-Aided Inertial Navigation System).

TERCOM systems have the advantage that the accuracy does not depend on the duration of the flight, as is the case with inertial navigation systems, which experience a "drift" over a longer flight duration and are therefore less accurate for longer distances. TERCOM systems experience constant corrections during the flight. The accuracy is therefore only dependent on the accuracy of the radar mapping information, which is usually in the range of meters, and the ability of the processor to match the data.

This restricted the first generation of TERCOM to targets in the order of several hundred meters and thus the use of nuclear warheads. The use of conventional warheads requires significantly greater accuracy, which in turn requires additional systems for the final approach.

Another disadvantage of the early TERCOM systems was the fact that the entire route including the starting point had to be planned. If the missile is launched from an unknown location or moves too far from the planned route, the missile is lost. In this regard, the INS can help the missile to fly the first part of the planned and thus programmed path, but cannot correct major errors. As a result, TERCOM systems are far less flexible than more modern systems with Global Positioning System (GPS), which are able to attack from any location and do not depend on information recorded in advance. However, TERCOM's ability to interfere is much lower than with GPS.

Similar usage

The principle of terrain-contour comparison is also used in medical navigation to record the position and orientation of a patient on an operating table .

See also

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