The Global Positioning System employs 21 Government satellites plus 3 spares circling the earth every 12 hours. Three satellites are required for 2D positioning and 4 satellites are required for 3D positioning. Typical reception for a GPS receiver is 5-7 satellites. 3D. But many conditions such as too few satellites, poor satellite geometry or poor reception can decrease accuracy.
Forest cover can totally block reception.
Therefore, in practice, accuracy is usually within 40-75 meters. Using 4 satellites is better than using 3. More than 4 does not make substantial improvement. Using satellites that are high in the sky will result in better accuracy. Although light canopy cover is usually inconsequential, some inaccuracies can occur with crown closure and some leaf types.