a wideband passive receiver coupled to selectively receive an RF input signal spectrum or an RF calibration input signal, the receiver being configured to output digital signals representing the RF input signal spectrum or the RF calibration input signal, the RF input signal spectrum comprising signals from unknown signal sources and the receiver operating subject to environmental distortions;

an equalizing filter coupled to receive the digital signals from the receiver, the filter having a filter response controlled by time-varying filter control signals and being configured to output filtered digital signals, the equalizing filter being configured to provide adaptive equalization of transfer function characteristics for the wideband passive receiver;

a calibration signal generator configured to generate a wideband RF calibration signal to provide the RF calibration input signal for the receiver and configured to generate a desired output signal based upon the generated RF calibration input signal, the calibration signal generator being further configured to generate the wideband RF calibration signals and the desired output signals periodically over time to provide calibration dwells for the receiver system during its operation; and

equalizer control circuitry coupled to receive the filtered digital signals as feedback and coupled to receive the desired output signal from the calibration signal generator, the equalizer control circuitry being configured to generate the time-varying filter control signals to provide adaptive equalization of the transfer function characteristics for the wideband passive receiver, and the filter control signals being configured to correct distortions caused by operational variations in the transfer function characteristics of the receiver system operating subject to environmental distortions.