GPS navigation: Only as safe as you make it

by Donald Talleur

A new generation of navigation equipment is finally here! We have a system that promises to allow the pilot to navigate direct to a destination without zig-zagging across the sky following VORs. GPS is indeed revolutionizing the way pilots think about navigating from one point to another, but it may also be revolutionizing the type of errors we can make while navigating.
Before using GPS, the pilot needs to ask him or herself if they know everything they should about the Global Positioning System and if they’re ready to trust it’s navigation guidance. Let’s look at some of the issues of GPS use.
GPS is a system of 24 satellites that form a constellation in orbit around the earth. They are spaced such that at least six satellites are usually in view at all times at any location on earth. This makes navigating to and from isolated areas that have few ground based navigation facilities possible. Without input from the aircraft, a GPS receiver requires at least four satellites to be in view in order to triangulate a reasonable aircraft position. Four satellites also provide adequate position fixing for navigation and instrument approaches, but there is a catch. A GPS satellite does not monitor it’s own signal "health" in the same manner as a VOR station. If the VOR senses a signal error it will shut down, but when a GPS senses a problem it takes a good deal of time for it to signal the ground control station.
As a result, if the pilot wishes to fly a GPS approach IFR, two extra satellites must be received so that the health of the other satellites can be monitored. This process is referred to as Reviver Autonomous Integrity Monitoring or RAIM. Since there is no way to be absolutely certain when RAIM will be lost (because less than six satellites are in view), other ground navigation facilities may be required to complete a flight in instrument weather conditions. As a result, it is not wise to trade in all your conventional navigation radios for a GPS receiver when you decide to upgrade. In fact, RAIM or no RAIM, it is recommended that all GPS approaches be backed up with navigation information from conventional navaids when possible.
Pilots planning to fly into the U.S. when an alternate airport is required because of low forecast weather conditions at the destination, should know that their alternate must have approaches other than GPS. This means you must have some navigation equipment other than GPS to make the flight legal.
Assuming you have RAIM and you don’t lose it during an instrument approach, the next consideration the pilot needs deals with proper GPS receiver setup. Unlike VOR receivers, GPS receivers lack industry wide standardization in display, modes, and pilot interface. This presents an avenue for error, especially when transitioning from one brand of GPS to the next. VORs present little challenge to properly setup. If you know the frequency to set in, and can find the ident knob, you’re pretty much good to go. The GPS receiver, on the other hand, is much like a mini computer and may have several pages of information to manipulate and several navigation modes that can be selected. One need not go beyond the manual to get a sense of the complexity of these receivers. The average GPS operator’s manual ranges from 150 to 200 pages. While understanding the basic features of any one GPS receiver is helpful when transitioning to a different one, it can be cumbersome to manipulate the new one in order to confirm that you’ve set it up correctly. Of course at least one commonality is that they all indicate that they have "gone active" somewhere near a final approach fix if the approach is properly loaded. But the indication of an active approach may not be in the same place from one receiver to the next, nor is the indication necessarily as salient from one to another. Some installations clearly violate the notion of proximity compatibility in that they locate the "active" indication in a location away from the guidance instrument that the pilot is likely to be focusing attention on. This is sometimes the result of retrofitting a GPS system into an aircraft that was not originally designed to have GPS. Most new aircraft manufacturers have used a modicum of good design principles and put these indications near the guidance instrument.
Clearly, some people are more at ease with the computer-like interface of the GPS receiver, but if you are one who is not, then please take ample time to study the system and seek instruction from someone who has experience with your brand of GPS. You will probably find the GPS easier to learn by using it during flight, but be careful not to become overly distracted when setting up the unit.
I learned that it is very easy to loose track of the outside world when trying to setup a GPS unit. One hour into a VFR flight, I realized I had spent the last 30 minutes fixating on the GPS with an occasional glance to the instruments to maintain control of the aircraft. What I had not done was look where I was going or look for traffic. That would not have been such a bad thing had I been with a safety pilot at the time. You can avoid the situation by always designating a safety pilot to watch for traffic while you play with the GPS.
The last issue deals with situational awareness. GPS allows us to navigate direct from one point to the next with no real restriction on the distance between those two points other than the range of the aircraft. Some see this as an advantage of the system. I see it as a potential for lost pilots. So far, I have overheard half a dozen pilots flying in or near the airspace where I usually work who where clearly uncertain of their location. Each had lost their GPS position or failed to realize that their direct route was taking them through airspace they had not been cleared through. I’ve used four different GPS units to date and all four give warning messages when approaching airspace that requires a through clearance. Why the message didn’t get through to these pilots, we’ll never know, but I can suspect at least one reason. From personal experience I’ve found that some GPS units give incessant amounts of warning messages. Most of my interaction with these units is pressing the message button twice to acknowledge and clear the message without actually reading it. After all, I know where I am, right? Unfortunately, this type of behavior can lead to a habit of simply ignoring the message regardless of where a pilot is flying. Habit may lead to "canceling" the message when it really needs to be attended to. You can avoid this potential pitfall by always reading the message before clearing it. I know it can be a hassle, but think how dangerous it can be to fly into controlled airspace without a clearance. Keep track of your location using the maps as well so when the GPS fails, you have a plan of action!
Although I may appear to have a bad impression of GPS, I happen to think it’s the way of the future. While the GPS system is relatively simple in design and theory, the use of this type of navigation system is far from easy. Some GPS units are easier than others to learn and some have neat features that make our flight less stressful, however, make sure you are up to speed using your GPS and don’t be lulled into a false sense of security.

Donald Anders Talleur is an assistant chief flight instructor at the University of Illinois, Institute of Aviation. He holds a joint appointment with the Pilot Training and Aviation Research Laboratory Departments. He has been flying since 1984 and in addition to flight instructing since 1990, has worked on numerous research contracts for the FAA, Air Force, Navy, NASA, and Army. He has authored or co-authored over 45 aviation related papers and articles and is also working on an M.S. degree in Engineering Psychology at the University of Illinois.