The same can happen as a satellite signal is restored. GPS receivers are designed to handle these obstructions seamlessly, but rare errors can be hundreds of miles. Altitude must be assumed, leading to horizontal errors which can exceed a mile. SA is described below, along with some suggested solutions. Reflection of GPS signals from buildings or cliffs can change signal timing, or garble GPS data, causing position errors.
Atmospheric refraction can change signal timing, causing position errors.
This is because the error when a location is marked can add to GPS errors while re-locating the mark. GPS altitude errors can be 1.5 times as large as horizontal errors. Most of these errors are either small or easily avoidable by knowing and operating your GPS receiver well. Some of the following can thus be skipped, but SA can still reappear - anytime, anywhere. SA is bothersome, but with a little care, uncorrected GPS is fine for most recreational uses.