The value of the GPS SPS PS goes far beyond the specific parameters that it contains. It has a great deal of information regarding how GPS operates and ideally should be read in full (including the appendices) by everyone who works with GPS at the measurement level. This column reviews the information in the latest SPS PS and shows how it improves the performance of GPS applications. Since this information is used in real time to generate error bounds at high probabilities (“protection levels”) for applications with demanding integrity and continuity requirements, it also discusses the origin of these parameters and the level of reliance that can be placed on them.