A protective system for a vehicle-borne tracking radar, designed to counter the effect of an off-target jammer whose interfering signals are picked up by a directive antenna together with echoes from an actual target, comprises a receiver with a sum channel and a difference channel. In the absence of a jammer, the sum channel is connected via a signal processor to telemetric circuitry in the radar associated with the vehicular guidance system which keeps the antenna axis trained upon the target as the vehicle homes in on same. When a strong jamming signal is detected, a switching device in the processor cuts off the sum channel from the telemetric circuitry and supplies the latter, instead, with rectified error signals from the difference channel, these error signals being also delivered prior to rectification to a tracking-control circuit which energizes an antenna rotator to hold its axis in line with the jammer. The presence of a jammer may be detected by a signal-level comparator with two circuit branches connected to the sum channel, these branches including respective integrators which are alternately supplied with incoming signals in a target window and in a jammer window occurring at different instants of a recurrence period. Another possible jammer-signal detector, which may be included in the processor, receives the rectified error signal from the difference channel in the target window and produces a "jammer present" signal whenever the accumulated error signals exceed a predetermined threshold.