 of the GNSS. Therefore, the vendor can only state what the equipment is capable of, not what it will deliver in a specific installation. For a quick check to ensure the equipment is functioning okay (or the antenna cable length compensation value seems reasonable), it would be fine to use one receiver to check another. You could also compare the performance against a highly accurate and calibrated reference re- ceiver, but the limitations of such an approach are clear: the reference receiver needs to have much better performance than the DUT, which might not be possible. These pieces of equipment must be well calibrated and are rarely seen outside facilities devoted to time measurement. There are various techniques to accurately compare PRTCs remote from each other using GNSS receivers, but these techniques are applied mainly in specialized timing laboratories. An equivalent method is to measure the output of the PRTC against a time standard (atomic clock), given that this equipment is available to the engineer. Of course, the accuracy of that time standard is critical to the relevance of the test measurements. Most operators choose not to undertake this form of testing because it involves a considerable level of specialized equipment. To overcome the fact that the GNSS is part of the measured performance, an external device is needed to generate standardized signals that the receiver can be measured against. Figure 12-12 il- lustrates one approach by introducing a piece of equipment known as a GNSS simulator (the most common variety is predominantly aimed at the GPS receiver market, but others exist). In Figure 12-12, the GPS simulator accepts a 10-MHz reference input from a PRC/PRS for much the same reasons as a timer tester needs the signal. The GPS simulator produces GPS radio signals (most commonly for the L1 band), and the radio frequency (RF) output of the simulator is connec- ted to the antenna port of the GPS receiver. The simulator is configured to accept a position (latitude, longitude, and height) an