There seems to be a popular misconception that GPS satellites are geostationary. Unlike TV sats (DirecTV, Dish, etc.) GPS sats are not geostationary, or "hovering" over the same spot on earth, they move around the planet. As they orbit, the number of birds in view at any spot on the planet naturally fluctuates with time. This is exacerbated because the planet is also rotating under the sats. As of last year, there had been 68 GPS birds launched compared to the original 24, so coverage is getting better. If you take a reference, and do nothing but wait an hour and then take another reference, you will normally indicate a different number of birds. This is why I see some frustration in these forums. Some claim a "fix" because they took a reference, did something, and then their number went up, while others are puzzled, because they did the same thing and their numbers went down. Elapsed time is generally the culprit. Day-to-day measurements in one spot will see variations in the number of sats available. Now throw weather, sunspots, RFI, multipath reflections, and a lot of other variables into the mix. In a perfect scenario, you would always get enough sats to fly, but very few of us have a horizon-to-horizon field of view. This gif indicates a "normal" fluctuation over time at 45° Latitude. Watch the blue spot on the globe as it goes through the day; you will see a variation between 6 and 12 sats as a norm. However, where you are, and your surroundings have a huge impact. OR: "Your mileage may vary".