Abstract: Characteristics of a target are measured by a radar or sonar. Pulses (101, 102, 103) are transmitted and in between (X) the transmissions of pulses a signal is received which depends on the transmitted pulses and on the distribution of the characteristics measured at different ranges. The distribution at different ranges of the characteristics measured is determined by representing it by means of a substantially linear system of equations in which the variables are the values of the characteristics measured at desired ranges, and by solving the system of equations for the variables. The transmitted pulses form a cyclically repeated pulse code or a continuously changing pulse train.