FAQ: What are all those D's doing on the satellite screen?

The D's indicate WAAS is turned on and the GPS V is receiving an extra ("differential") signal from special satellites that makes the V more accurate. WAAS is the US name for this system. In Europe it is called EGNOS, but the principle is the same. Normally you get about 5 meters or 15 feet accuracy at best, but with WAAS/EGNOS enabled you can get 2 meters or 7 feet accuracy. The satellite screen to the left is only showing a 6 meter accuracy, because it was inside and only having a view of half the sky at the moment the screen was captured. WAAS/EGNOS is based on special ground stations that send a signal to WAAS/EGNOS satellites. The signal tells the V how the atmosphere is making the GPS signal less accurate, so the V can invert the inaccuracy caused by the atmosphere and the reported coordinate is thus more accurate. You can tell which satellites are WAAS/EGNOS ones by the number below the signal strength bar: WAAS/EGNOS satellites have a number higher than 32. The European EGNOS satellite is number 33, the Asian one is number 44 and for instance number 35 is a US one. Note: you can also get a differential signal by attaching a special DPGS receiver to your V, which receives a same sort of signal from the coastguard. But if that were the case, you would know. =8-)