Quantitative interpretation of time-domain airborne electromagnetic (AEM) data is hampered by uncertainty in altimetry, system geometry, transmitter waveform, data averaging, and timing. We present a simple calibration method that serves to define these issues by the use of a closed multiturn loop of known electrical and physical properties that is insulated from the ground beneath it. By predicting the secondary response of the AEM receiver and comparing it with the measured data, we have identified and quantified systematic errors mentioned above in several systems. In addition, we identify an alternative subprocess that uniquely calculates altimeter and geometry errors by measuring the current induced in a ground loop of known properties and comparing it with predictions. The ground-loop method is used best over resistive cover to minimize limitations caused by nonuniform conductive ground and is a calibration tool that makes AEM data consistent with quantitative models. Fluctuating geometric errors caused by bird swing limit the accuracy of applying the geometry corrections from one flyover to an entire survey.