Plane & Pilot
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.
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Leg Sequencing
If you haven’t added any procedures into your flight plan, the unit will automatically sequence from one leg to the next right to your destination and will provide error information to drive the autopilot in NAV mode as well. Unless you have a GPS or use Sandel’s SN3308 “glass HSI” in the AutoSlave mode, you’ll have to change the OBS setting on your CDI to the new desired track at each waypoint. For course correction, many autopilots look at both the cross-track error from your active leg and the difference between the GPS desired track and OBS setting on your CDI or HSI.

If you push the OBS key (on the KLN94, and 430 or 530) or the Direct and OBS keys (on the CNX80), the next waypoint then behaves like a VOR. You dial in a Course-To with the OBS needle on your CDI and your flight-plan line on the map will pivot about the next waypoint to align with that course. With OBS selected, the leg sequencing is stopped. Using Direct-To on a course does the same thing, but doesn’t suspend leg sequencing. One example of flying a different course to your next waypoint is (after takeoff) to join a nearby airway that goes through that waypoint.

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.

Stay Tuned For Part Two
The addition of departures, arrivals, approaches and flying airways adds sufficient complication to a flight plan as to require a good understanding of the capabilities described here. In part two of this article, we’ll see how the Direct-To and Activate Leg tools become critical in manipulating IFR flight-plan legs so that your flight plan and ATC directions don’t conflict. We’ll also describe how procedures are added into a flight plan you create, and what problems that can cause you, often at busy and critical times.


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