This paper deals with a possibility of detecting a non-level flight aircraft for a radar which is designed for measuring the height keeping accuracy of an aircraft in level flight. In the radar, the height of aircraft is estimated under the assumption of a level and straight flight using the non-linear least squares method. Herein, the following observation equation is proposed for detecting the vertical speed of the aircraft, V_z Ri=[(V_xt_i+X_0)^2+(V_zt_i+Z_0)^2/cos^2β_i]^<1/2>+e_i (i=1, 2, ..., n) Where R_i, β_i, t_i and e_i are the range, depression angle, observation time and observation error of the i-th observation, respectively. V_x is the X-component of aircraft speed, X_0 and Z_0 are the initial values of X and Z axes, respectively. The results of a Monte Carlo simulation show the feasibility of detecting the descending/climbing aircraft in a certain area of the radar coverage.
