A method for characterising the density of Mode S interrogations and responses in the environment of at least one operating secondary radar (R1), said environment being defined by the area of the airspace covered by said radar (R1), said environment being traversed by Mode S targets, with a Mode S target being a target emitting a response to the Mode S interrogations emitted by a secondary radar, characterised in that it comprises: - a first step, in which said radar (R1): o detects and locates Mode S targets by means of their synchronous responses to the interrogations emitted by said radar (R1) or their positions transmitted in extended ADS-B squitters; o detects the asynchronous responses emitted by the same Mode S targets, therefore not requested by said radar (R1); o associates , for each located target, its asynchronous responses with the synchronous responses to said radar (R1) or the positions provided by said ADS-B squitters; - a second step in which said radar (R1): o determines , from said association, the rate of responses of each target by counting the number of synchronous and asynchronous responses received from said target for different given time periods; o determines , with said environment being divided into elementary space cells , the rate of responses per cells by counting the number of synchronous and asynchronous responses received by each target located in each cell, said rate characterizing the density of Mode S interrogations and responses per cell or per group of cells. - a first step, in which said radar (R1): o detects and locates Mode S targets by means of their synchronous responses to the interrogations emitted by said radar (R1) or their positions transmitted in extended ADS-B squitters; o detects the asynchronous responses emitted by the same Mode S targets, therefore not requested by said radar (R1); o associates , for each located target, its asynchronous responses with the synchronous responses to said radar (R1) or the positions provided by said ADS-B squitters; o detects and locates Mode S targets by means of their synchronous responses to the interrogations emitted by said radar (R1) or their positions transmitted in extended ADS-B squitters; o detects the asynchronous responses emitted by the same Mode S targets, therefore not requested by said radar (R1); o associates , for each located target, its asynchronous responses with the synchronous responses to said radar (R1) or the positions provided by said ADS-B squitters; - a second step in which said radar (R1): o determines , from said association, the rate of responses of each target by counting the number of synchronous and asynchronous responses received from said target for different given time periods; o determines , with said environment being divided into elementary space cells , the rate of responses per cells by counting the number of synchronous and asynchronous responses received by each target located in each cell, said rate characterizing the density of Mode S interrogations and responses per cell or per group of cells. o determines , from said association, the rate of responses of each target by counting the number of synchronous and asynchronous responses received from said target for different given time periods; o determines , with said environment being divided into elementary space cells , the rate of responses per cells by counting the number of synchronous and asynchronous responses received by each target located in each cell, said rate characterizing the density of Mode S interrogations and responses per cell or per group of cells. <EOS>