Final answer:
Selective availability could induce errors of up to a hundred meters in GPS signals, but this feature has been turned off since May 2000, resulting in more precise and accurate GPS data. Precision and accuracy are key in evaluating GPS effectiveness, with selective availability affecting precision rather than accuracy.
Step-by-step explanation:
With selective availability enabled, GPS signals can be intentionally degraded, leading to inaccuracies in the positioning data provided to users. When selective availability was active, the intentional error introduced could make GPS data inaccurate by as much as a hundred meters. However, since May 2000, the US government has turned off selective availability, thus improving the precision and accuracy of GPS systems. GPS relies on a constellation of 24 satellites which are equipped with precise atomic clocks. The accuracy of these clocks and the need to consider both special and general relativity are crucial for the system to function correctly. Without accounting for these relativistic effects, the GPS's ability to locate positions such as a restaurant would be compromised, leading to a loss of accuracy within minutes. Understanding the concepts of precision and accuracy is important when analyzing GPS data. If GPS measurements for a restaurant's location are consistently clustered close together but far away from the actual location, the system would have high precision but low accuracy. Conversely, if measurements are spread out but close to the real location, it would have low precision but high accuracy.