Sunday, August 1, 2004
Sunday, August 1, 2004
Mastering The Panel-Mounted GPS Part 1: VFR Use
Bendix/King, Garmin, Chelton? At first glance, they all seem so different, but are they really? It turns out they have a lot in common.
|Learning to use even one of the modern IFR-approved GPS maps, let alone several of them, is challenging. Understanding the capabilities of a device requires as much class time as learning how to operate it. The how can be very different from unit to unit, but the what is surprisingly similar.|
|GPS Is Changing |
If you think the Global Positioning System is great now, wait until you see what’s coming. Within the next decade, GPS is scheduled for a major upgrade. Areas of weak reception, like indoors and inside parking garages, will begin to disappear, and accuracy is planned to improve tenfold. Civilian GPS units will be able to pinpoint locations within one meter, and for the military, this enhanced precision can be measured within centimeters. For pilots, that means that the new GPS system could allow aircraft to land in zero/zero conditions, and for the military, Navy pilots can put a fighter down on the deck of a pitching, heaving aircraft carrier—even when they can’t see it.
Currently, GPS units listen to any number of the 24 geosynchronous satellites. The constellation of satellites transmits data toward the Earth at about 500 watts. Traveling the 12,000 miles to the Earth’s surface causes the signals to arrive with a power density of only 10 (-13) watts per meter squared. To give that some real-world comparison, the television signal you receive at home is about one billion times stronger. On its way toward your receiver, the signal is often bent as it passes through the charged particles in the ionosphere. Thus, civilian GPS accuracy can vary by as much as 30 feet. In 2005, new signals, much less susceptible to ionospheric disturbances, will begin, and by 2008, civilian signals four times as powerful as today’s will enter service.
The FAA’s Wide Area Augmentation System (WAAS) began operation last year, with horizontal accuracy of one to two meters, and two to three meters vertically. WAAS approaches to an airport are limited, however, ending about 300 feet AGL. From that point on down to the surface, this new generation of GPS signals will enable a Local Area Augmentation System (LAAS). The result? Civilian aircraft will be able to use LAAS all the way to touchdown, without visual information from outside the cockpit.