But change is the only constant in life, and in aerial navigation, everything is in place to change to the satellite-based global positioning system - GPS.
With the GPS constellation of 24 satellites in place, the government is debating the schedule for phasing in the role GPS navigation will play, and what system, if any, will be its back-up. Precisely when the government will shut down terrestrial navigation systems isn't as important as the fact that they will be phased out eventually. In other words, now is the time to learn about GPS navigation.
The GPS satellite-based global navigation system is not only relatively simple to use, but very precise. Unlike VORs, it doesn't become less or more sensitive as you fly from or to a waypoint, nor does it have any angular relationship (signal "scalloping") when converging on your destination. It works as well on the ground as at altitude, and isn't subject to the same line-of-sight limitations as VOR.
GPS offers two levels of service, Standard Positioning Service (SPS) and Precise Positioning Service (PPS). Available to all users worldwide, SPS gives an accurate horizontal (latitude/longitude) position indication within 100 meters 95 percent of the time, and an accurate position within 300 meters 99.99 percent of the time. PPS is much more accurate, but it's available only to the military.
How Does GPS Work?
The U.S. Department of Defense developed the satellite-based Navstar GPS system to accurately navigate in three dimensions - latitude, longitude, and altitude. The system has three main components:
- The space component of GPS includes 24 satellites, three of which are active spares. The satellites orbit the earth at an altitude of approximately 11,000 miles. Each satellite circles the earth twice daily, which allows at least five satellites (needed for GPS/IFR approach navigation) generally to be in view 24 hours a day, anywhere on earth.
The satellite signal is resistant to interference from weather, earth-based radio signals, and electronic equipment. But GPS is a line-of-sight system and is subject to terrain masking. If the satellite angle is low, and the mountains are high (such as at Aspen, Colorado), you won't get a signal.
- The control component of GPS consists of a network of ground monitors. These stations continually monitor satellite performance in order to maintain accuracy of the system. Some of these stations also upload corrections and other data to the satellites.
- The GPS receiver, either handheld or panel-mounted, is the system's third component. It processes satellite data, determines position, displays database information, and provides various other functions depending on the manufacturer.
A GPS receiver locates itself using triangulation and ranging. The receiver needs to "see" at least three satellites to fix its two-dimensional position (lat/long), and at least four satellites to fix its three-dimensional position (lat/long, and altitude).
Each satellite broadcasts a coded signal that contains information about its unique location in space and the position of the entire constellation. The GPS receiver can calculate an approximate distance called "pseudo-range" by matching its own code with that of the satellites it is monitoring, and then timing how long it takes the satellite signals to reach the receiver. To obtain a precise fix, the receiver synchronizes its internal clock with that of the atomic clock (which is accurate to three nanoseconds, or three billionths of a second) in each satellite.
It does this by determining a range to each of the satellites it is monitoring. If the ranges don't agree - if they don't pass through the same point in space - the receiver adjusts its clock until the four lines of position agree. The precise point at which the lines of position intersect is the receiver's three-dimensional position. A computer in the receiver uses the position information, along with stored database information, to perform all of the calculations and functions that make a GPS unit so useful for navigation and flight planning.
Although a GPS receiver uses latitude/longitude coordinates to identify a position, it normally uses waypoint names, fix names, and database identifiers in place of latitude and longitude in the GPS display.
GPS receivers approved for IFR incorporate a feature that monitors the accuracy of the satellite signals, and alerts the pilot if the position information may be inaccurate. The feature is called receiver autonomous integrity monitoring (RAIM). For the receiver to perform this function at least one additional satellite, in addition to those needed for navigation, must be in view. In other words, the receiver must see five satellites to verify, using RAIM, a three-dimensional position and four satellites to RAIM-verify a two-dimensional position.
Because it checks the integrity of the signals it receives, and because atmospheric, satellite position, and timing errors are relatively small, GPS is more accurate than any other en route navigation system. Receivers approved for instrument approaches automatically set their display sensitivity to different levels throughout the approach.
IFR GPS
More than 4,852 GPS overlay and stand-alone non-precision IFR approaches are in existence today, and more are coming on line each month. Overlay approaches apply GPS coordinates to existing VOR/DME, RNAV, or ADF approaches, which means you can fly them with GPS or the applicable system by having the required equipment in your airplane and using the appropriate instrument approach chart. Stand-alone GPS approaches are just that, designed from scratch to be flown with an IFR approach-approved GPS receiver.
To date, no public-use precision GPS approaches exist that give lateral and vertical guidance as does an ILS. Without an additional signal to sharpen the GPS receiver's consistent accuracy to better than 100 meters, GPS just won't meet the minimums for a Category I precision approach - 200 feet vertically and a half-mile horizontally. But precision GPS approaches are on the horizon.
The FAA has successfully tested differential GPS systems that use ground-based equipment to correct the satellite signals and thus provide precision approach accuracy. The FAA plans to implement WAAS in three phases. When completed, the Wide Area Augmentation System will provide the accuracy, integrity, and availability for en route use and Category I precision approaches nationwide.
Meanwhile, the FAA is scheduled to test LAAS, the Local Area Augmentation System for Category II and III instrument approaches in 1998, with final testing in 1999. When these systems will be on line is tentative; the FAA schedule says they may be up and running by 2010.
GPS Receivers
VFR GPS receivers come in two basic forms, handheld and panel-mount. No IFR-approved handheld receivers exist because there are too many variables that affect accuracy and reliability such as power supply, antenna position, and electrical interference. The fact that handhelds don't have RAIM means they don't have the power or reliability necessary for instrument operations.
A dozen or so handheld receivers are on the market, ranging in price from $500 to $1,200. Some of them are lightweight, palm-sized units, and others are larger and incorporate electronic moving maps. The latest handhelds can receive and process signals from up to a dozen satellites. Most handhelds use a cigarette lighter adapter or AA alkaline batteries capable of 8 to 15 hours service, depending on the unit. Some units, both handheld and panel-mount, also have a communication transceiver.
A variety of features are available in various handheld and panel-mount GPS receivers, depending on price. Among these features are databases of airports, airport diagrams and frequencies, navaids, airway intersections; moving maps, navigation indicators such as an HSI; and a "nearest airport" function, which is a handy safety feature. If you need to land immediately, you can punch a button or buttons and the GPS unit will give you bearing and distance to the nearest suitable field along with runway and frequency information.
One great safety-enhancing feature of GPS, and especially those with moving-map displays, is alerting the pilot to the proximity of special-use airspace including military operating areas, alert areas, restricted areas, prohibited areas, and Class C and B airspace. Yes, the pilot should always be aware of the airplane's position, but in busy, congested airspace it's a revelation to look at an electronic screen and see a planview of the airplane's geographic position and the boundaries of special-use airspace.
GPS units also allow you to program your own waypoints and perform a number of E6B computations such as winds aloft, fuel planning, and descent guidance. Most units also have a Flight Plan function that enables you to load a number of waypoints into the flight plan. When you reach one waypoint, the GPS automatically switches to the next. Some panel-mount GPS units can be (and IFR GPS must be) wired to drive the "navigation indicator" (the airplane's HSI or CDI).
If you haven't used your handheld GPS for several months it will take longer to acquire satellites and obtain a fix. The same thing happens if you turn the unit off and then turn it on again several hundred miles away.
Most receivers "remember" their last position and look for satellites in specific places. If they aren't there, the receiver has to look for them all over again. Panel-mount receivers have to do the same thing if the airplane is moved a significant distance while the unit is turned off. A handheld's portable antenna may be shielded from certain satellites by part of the aircraft structure. Moving the antenna to a more open portion of the windshield, or turning the airplane, usually restores the signal.
Handheld GPS receivers aren't required to have a current database. This is also true of panel-mount VFR receivers because the pilot is responsible for accurate VFR pilotage. However, relying on an out-of-date GPS database is like flying without current VFR charts.
It's a different story for IFR-approved GPS receivers. If you're going to fly IFR, you must have a current database. Generally, annual VFR database subscriptions run $150, and IFR subscriptions are $599.
Remember, you cannot legally use a VFR GPS as your primary navigation system, and you cannot use it to file and fly IFR, or to fly an instrument approach. For this, you need an IFR en route and approach-approved GPS with a current database containing the approach and departure waypoints, proper switches and annunciators, and an external navigation display.
Most VFR GPS receivers offer similar functions and modes of operations. Panel-mount units may use knobs, buttons, or a combination of the two to input and access information and "pages" or screen displays. Handhelds generally use buttons.
To select a waypoint, you press the Go To or Direct button to set up the basic navigation display, retrieve a waypoint identifier (typically an airport or navaid) from the unit's database (or enter the identifier manually), and press the Enter button. Then you can page through the screens to the display of your choice such as a moving map or electronic CDI. In addition to pointing your way, most handheld receivers display your ground speed, estimated time en route, and whatever else you want to display on the customizable screens.
Precise navigation and incredibly extensive databases aren't the only virtues of aviation GPS units. The flight planning functions they offer have brought a new level of sophistication and capability to light aircraft. The only limitations to the flight planning features that can be built into a GPS unit are the imagination of the designers and the processing power and electronic memory of the GPS computer.
GPS navigation truly is a marvelous system, and compared to the systems it will eventually replace, it's revolutionary, not evolutionary. But like any aircraft system, using GPS efficiently, properly - and safely - takes training.
Students, instructors, and flight schools shouldn't wait to become proficient with GPS navigation, and a number of the manufacturers offer training books, videos, and, in at least one case, a computer-based training program. In addition, training courses and/or materials are available from AOPA Air Safety Foundation (800/638-3101); Jeppesen (800/621-JEPP); King Schools (800/854-1001); and Sporty's Pilot Shop (800/LIFTOFF).
Regardless of the navigation system used, no pilot should forgo staying proficient in aviation's fail-safe method of navigation - pilotage and dead reckoning. As good as GPS is, it has an Achilles heel, the same one that affects any electrical component on an aircraft. If the electricity goes away, so does the GPS.
GPS Manufacturers
For information on the latest models of handheld and panel-mounted GPS receivers, contact the manufacturers.
AlliedSignal (Bendix/King)
400 North Rogers Road
Olathe, KS 66062
913/782-0400
Arnav
P.O. Box 73730
Puyallup, WA 98373
253/848-6060
Garmin International
1200 East 151st St.
Olathe, KS 66062
913/397-8200
Lowrance Avionics
12000 East Skelly Drive
Tulsa, OK 74128-1703
800/324-4740
Magellan
960 Overland Court
San Dimas, CA 91772
800/707-5221
Narco
270 Commerce Drive
Fort Washington, PA 19034
800/223-3636
Northstar
30 Sudbury Road
Acton, MA 01720
800/628-4487
Skyforce
14100 Parke Long Court,
Suite F
Chantilly, VA 20151
703/502-7820
Trimble
2105 Donley Drive
Austin, TX 78758
800/487-4662
II Morrow
P.O. Box 13549
Salem, OR 97309
800/742-0077
Related Articles
