We use GPS technology almost every day, instant granular positioning data has become so ingrained into our apps and devices that we sometimes get frustrated when a perfect GPS signal is not instantly available.
This highly informative post is an awesome technical write up on how the GPS satellite constellation works. It covers many factors and data points about what can make GPS (or technically any GNSS) less accurate, and the constant maintenance efforts required to keep these systems functional.
With GNSS data becoming a more regular meta-data component of security and surveillance systems, it can be very helpful to understand more about how these systems work, and how much you should, or should not, rely on their signals.