1. A method for forming a set of cells for time difference measurements for a mobile terminal camped on a first cell of a cellular network and being in idle mode, the method comprising the steps of: receiving, at the mobile terminal, a first set of cell identifiers respectively associated with neighboring cells of the first cell, with each of the neighboring cells sending a radio signal; measuring, at the mobile terminal, received signal strengths of the neighboring cells having identifiers which are included in the first set of cell identifiers, with a number N of cells having a signal strength exceeding a predefined threshold constituting a set of available cells; reading, at the mobile terminal, a synchronization channel for the set of available cells, thereby measuring time differences for the set of available cells; selecting, at the mobile terminal, a second set of cells from the set of available cells using a predefined selection rule, the second set of cells including M<N cells, wherein the predefined selection rule causes a non-selection of a cell having a same cell identity as another cell in the set of available cells if it is probable that the cell which is not selected and the another cell belong to one sectorized base station; and reading, at the mobile terminal, a synchronization channel for the second set of cells, thereby measuring time differences for the second set of cells.