The different components of GPR equipment have particular characteristics that, when taken together as a whole, give the system the stability required for correct usage. Besides the information provided by manufacturers about parameters affecting the stability of GPR equipments, their evolution with use and ageing suggest that each GPR system employed in detailed qualitative studies should be subject to routine analysis. This type of analysis is especially important in novel systems and antennas to understand their real capabilities and limitations. In this work, several tests are carried out in order to evaluate the stability of a GPR system working with three different antennas with nominal frequencies of 500, 800, and 1000 MHz. Some tests published by other authors, together with other tests proposed here, are adapted to be the starting point to develop a methodology for calibrating GPR devices and to verify proper operation.