Several types of failures can occur in the GPS satellites that transmit the ranging signals to the users. Among them, a specific type of failure in the signal generation process aboard the satellite may result in an anomalous waveform being transmitted, called an ?evil waveform?. Evil waveforms are GPS signals that have a distorted PRN code modulation waveform. The main impact is a rupture of the symmetry of the cross- correlation peak inside the tracking channel, therefore inducing a different measurement error for two receivers that would not have the same architecture. As a consequence, there is a potential for evil waveforms to induce large tracking errors of differential systems if left undetected. The aim of this paper is to analyze the performance of the latest proposed ground Signal Quality Monitoring (SQM) techniques, and to present a summary of the final recommendations adopted by ICAO GNSSPanel. The paper starts with a description of the mathematical model of the evil waveform, and its predicted effect on the GPS receiver is presented. Next, we discuss the rationale for the methodology employed to design the currently proposed ground Signal Quality Monitoring (SQM) techniques, and describe one of these techniques. Some theoretical and collected statistics of the test thresholds including measurement noise and multipath level used by this technique are then presented. Then, we present simulation results showing the level of protection provided by this technique, depending on the airborne receiver architecture. Observed behaviors of real receivers tracking one of these evil waveforms is then shown. We finally conclude with the actual recommendations formulated by the GNSSP.