Signal Quality Monitoring (SQM) is about measuring the distortion of a navigation signal due to the payload, without having the measurement polluted by propagation error and noise. The focus is the signal distortions that could cause a tracking error at the user level. A well known example is the Evil Waveform (EWF) that affected GPS SVN-19 in 1993, but there are also smaller scale deformation on every GNSS satellite. These errors can be derived from the knowledge of the correlation function with the required accuracy. The goal of this work is to measure the correlation function distortion that would lead to tracking error as low as 2 cm (or even less), that is 1/15000 of a chip for the CIA code or 1/5000 for the BOC(I,I). This imply that the correlation function has to be determined with such a small accuracy relative to its peak amplitude. This will allow to characterise signal anomaly generated at the payload level. In order to obtain the correlation function with as little noise as possible, the technique of long coherent integration is used, as well as a high gain antenna so as to keep the noise down below the required threshold. To do this a signal sample is digitized and recorded using a bit-grabber connected to the high gain antenna, then the signal characteristics (code delay, carrier phase and the navigation data) are determined in post-processing, in order to generate a replica that will in a second step, be correlated with the recorded signal over a very long integration time to obtain the "long coherent integration" (LCI). The correlation integration was done over duration reaching up to 120 seconds. The bit-grabber has a 125 MHz sampling rate, so the full correlation triangle can be sampled at 245 independent points for the CIA code (1.023Mchip/s). The required antenna size and integration time to achieve the accuracy will be discussed according to the navigation signal type. Actually, the sharper the signal correlation peak, the easier it is to determine the tracking error induced by signal deformation and the required antenna gain and size is reasonable. Multipath is of course an issue when trying to determine the correlation function with such an accuracy. The threshold below which multi path must be will be determined.From the correlation function, the tracking error for the most common types of correlators, such as narrow correlator and the double delta correlator family can be determined as a function of the correlator spacing for every operational satellite and for every signal. In addition, the desired signal spectra can be determined using a fourrier transform of the correlation function, from which the satellite payload transfer function can be derived With the possibility of determining signal deformation effect with a reasonably sized antenna, CNES aims at making periodic measurements on every GNSS satellites, including the new IOV GALILEO satellite in order to better know the existence and evolution of signal distortions.