Indian air controllers also complained that the Kazakh pilots sometimes confused their calculations because they are accustomed to using metre altitudes and kilometre distances, while most other countries use feet and nautical miles respectively [REF].
Furthermore, Indira Gandhi International Airport did not have secondary surveillance radar, which provides extra information, such as the aircraft's identity and altitude, by reading transponder signals; instead the airport had primary radar, which produces readings of distance and bearing, but not altitude.