A transponder, able to equip a cooperative target facing a Doppler radar, includes at least one receiving antenna able to receive a signal transmitted by said radar and a transmitting antenna able to retransmit a signal. The signal received by the receiving antenna is amplitude-modulated before being retransmitted by the transmitting antenna to produce a variation of the radar cross-section of the target, the variation triggering a frequency shift between the signal transmitted and the signal received by the radar comparable to a Doppler echo. The transponder applies notably to the field of radars, more particularly for collaborative systems also operating at low velocity or nil velocity. It applies for example to assisted take-off, landing and deck-landing of drones, in particular rotary-wing drones, as well as manned helicopters.