The lost track tool informs the controller that a correlated track that was monitored by the surveillance data processing system is no longer observed.
Modern automated ATS systems usually feature a coasting function that continues to display an aircraft's position in case no surveillance data is received for a short time, in order to achieve track continuity. The position is calculated based on the last available data and this is normally done for three consecutive missed plots (15 - 30 seconds). If no new data is received after this time has elapsed, the lost track tool is activated.
Representation on the situation display may very based on the system being used, but normally includes at least the aircraft callsign and the coordinates of the last observed position. Additional data, e.g. time of track loss, last observed level, SSR code, etc. may also be available. The data may be presented as a special symbol on the screen (placed where radar contact was lost) or in tabular form.
The images above show an example of a lost track tool. The first picture shows a correlated track (white) with information about callsign (BDD0606), actual level (FL 360) and ground speed (455 knots). The second picture shows the track in coasting mode, i.e. a new plot was not found as expected, therefore the position is extrapolated. The controller is notified by the X symbol that (a) the position is not derived from surveillance sensors and (b) there is a probability that the track will disappear. As mentioned above, the coasting symbol is normally shown three times at most. The third picture shows what a controller would see if a plot is not found for the fourth time. A lost track position indicator (red symbol and label) give visual indication of the position and the latest available data. The Lost track window shows additional data, such as the coordinates where the track was lost and the transponder code (1234).
Possible reasons for the activation of the lost track tool include:
- Aircraft leaving the area of surveillance sensor coverage
- Low flying aircraft being shadowed by terrain
- Aircraft landing, especially off-aerodrome (e.g. forced landing or ditching). Note that when an aircraft lands on an aerodrome (even in case of an emergency), the automated ATS system would normally recognize the situation as landing and will not trigger an alert when the track is lost.
- Lack of transponder signal combined with lack of primary surveillance. This can happen due to a variety of reasons, e.g. malfunction, inadvertent selection of standby mode, electrical system failure, etc.
- Collision (either with another aircraft mid-air or with terrain)
The benefits of the lost track tool include:
- Clear indication to the controller of an abnormal situation so that they can promptly investigate the reason.
- Search and rescue teams can be directed more accurately.
In order to reduce nuisance alerts, the lost track tool is only available for correlated tracks, and even then, filter out conditions may apply. For example, if the track was lost well outside the area of responsibility of the ATS unit, the alert may be suppressed.