"Standalone" or "Autonomous" GPS operation use radio signals from satellites alone. A-GPS additionally uses network resources to locate and utilize the satellites faster as well as better in poor signal conditions. In very poor signal conditions, for example in a city, these signals may suffer multipath propagation where signals bounce off buildings, or be weakened by passing through atmospheric conditions, walls or tree cover. When first turned on in these conditions, some standalone GPS navigation devices may not be able to work out a position due to the fragmentary signal, rendering them unable to function until a clear signal can be received continuously for up to 12.5 minutes (the time needed to download the GPS almanac and ephemeris).[2]