There is no standard altitude for cruising. Generally, it is around 35,000 feet, but that can vary considerably depending on length of flight, weather conditions, air turbulence and the location of other planes in the sky. Cruising speeds are at a constant mach number, about 82 percent of the speed of sound. This translates to a groundspeed of about 550 miles per hour, although that too can vary considerably with headwinds, tailwinds and other factors. During flight, pilots normally follow designated airways, or highways in the sky, that are marked on flight maps and are defined by their relationship to radio navigation beacons, whose signals are picked up by the aircraft.
Some jets also have inertial navigation systems onboard to help pilots find their way.
These computer-based systems calculate the plane's position from its point of departure, by closely tracking its heading, speed and other factors after it leaves the gate. Some aircraft also are capable of using signals from a constellation of satellites to pinpoint their position. This is known as the Global Positioning System. Commercial aircraft are increasingly using it. GPS enables aircraft to operate, with the permission of Air Traffic Control, to operate safely off predetermined airways.