Each pattern being associated with a reception channel, over a given time period, the unsolicited asynchronous replies, of long ADS-B squitters type, transmitted by targets present in the airborne environment of the radar, are detected, each of the squitters containing position information on the target which transmits it; for each detection, the long ADS-B squitter is decoded to check that the detected target is located in accordance with the position information contained in the squitter, the non-conforming detections being rejected; for each detection retained, the time of the detection, the value of the azimuth of the main beam of the antenna and the received power value on each of the reception channels is associated with the detection, the position information contained in the squitters giving the elevation segment wherein the detection is situated; the values obtained over the period being stored, the measured patterns being sampled, by elevation segment, from the stored values.