The basic ability of GPS is to locate us in space, and express position accurately in latitude and longitude, and less accurately in altitude. A unit receives signals and calculates the distances from several satellites simultaneously. In brief, knowing the distance from one satellite limits possible aircraft position to the surface of a sphere. Two satellite distances, and position is limited to a circle where two spheres intersect. Three satellites typically provide two potential positions. So four satellites are typically the minimum needed to define accurate position.
However, satellites used must be above the horizon, and the geometry between them--and the aircraft receiving the signals--must fall within acceptable limits. Thus, there are times when satellites and an aircraft are not in position to afford the accuracy that we would wish.
Most GPS receivers warn when the accuracy of position has degraded--called dilution of position--and may indicate the amount of error at that instance. Units also warn when they are receiving signals from an insufficient number of satellites. Some units--particular those used for IFR--can predict whether there will be adequate accuracy upon arrival at a destination.
GPS units are very dependable, but no more dependable then their power supply. What will you do when batteries give out, or the electrical system of your aircraft fails? It is wise to use back-up navigation at all times. This may be as simple as keeping your finger on a paper sectional chart, or sequentially tuning VOR frequencies.
It is very easy to fly using GPS and not really know where you are. If all you know is that your destination is at 312 degree magnetic and 137 nautical miles, you may have problems finding a close-by airport if a diversion is needed. Of course, if your GPS has a moving map, this helps. Many GPSs have an emergency search function for the closest airports (typically the "NRST" function). However, the way to find these airports is not standardized, and on some receivers the key combinations are not at all intuitive--this could be detrimental during an emergency. And even if you are not fumbling with the buttons, the closest airport may not be adequate for your aircraft. What is the minimum runway length that you need? Will you accept a grass strip? Is night lighting available? There might be a set-up function that can define what you consider minimum requirements, so the GPS knows what you need in an emergency.
Continual change
As we fly, the GPS notes continual change in position and uses this data to calculate the flight vector--the direction of flight and speed relative to the Earth. Unfortunately, this provides only indirect information as to true airspeed and aircraft performance. A GPS cannot tell a pilot how close the airplane is to a stall, or whether the airplane is moving faster through air at one altitude compared with another. Only groundspeed is calculated, and that is by comparing positions in rapid sequence. The result is the velocity at that moment.
While this is an accurate momentary determination, it is probably not the rate at which you are approaching a destination. For example, you could be heading east at 110 knots with a destination due north. In that direction you will never reach your destination, yet your GPS may say that you will be there in 42 minutes based upon calculated instantaneous groundspeed. If now turning north, a stiff headwind may lower groundspeed to 90 kt, and now your time to destination may be 51 minutes. So here is an anomalous situation where GPS says it will take longer heading directly to a destination than to fly in a direction that will never take you there.
The time-in-flight error is more insidious along a route with several doglegs. Groundspeed along each leg is unknown to the GPS computer, so the time to each waypoint and the time to the final destination are incorrectly calculated using current groundspeed. It's not likely that your groundspeed will be the same for each leg of the route.
GPS can calculate groundspeeds, but it cannot predict groundspeeds. While time to waypoint is most accurate on a direct course, GPS cannot predict changing winds aloft along a route. The longer the flight, the less likely time to destination is correct. So, do not bet your last gallon of fuel that the estimate is correct. Winds aloft shift and vary.
These inherent errors are typically small compared with pilot errors in operating a GPS. Most significant are data entry mistakes. While most modern GPS receivers no longer require you to enter a waypoint by plugging in latitude and longitude, there have been infamous navigation errors from a reversal of just two numbers, or the mistaken entry of the wrong quadrant of the globe. (In the United States all coordinates are northwest--north of the equator, and west of the Prime Meridian through Greenwich, England.)
Most modern GPS receivers have built-in databases, and waypoints are entered using alphanumerics. This method is less error-prone than a long string of lat/lon numbers, but it still leaves plenty of opportunity for error. Different places may have the same name--think of Las Vegas, Nevada (LAS), and Las Vegas, New Mexico (LVS). There are waypoints with similar-sounding names; a small spelling mistake will be readily accepted by the GPS as a valid entry, and yet create a very large navigation error. The GPS cannot error check if you enter MIA (Miami, Florida) instead of MIE (Muncie, Indiana).
I made a mistake because I thought I knew how to spell GIPER intersection without looking at the chart. However, my clearance was to GIPPR! It sounded the same and took some time to sort things out. Such an error is difficult to identify perusing an alphanumeric display. An electronic map depiction of waypoints can help, and if possible the route should always be visually checked for consistency.
Waypoint sequencing
GPS units can accept a route containing several waypoints and automatically sequence from one to the next. It is always worthwhile to add more waypoints to the route. If you are flight planning using airways, add most if not all of the VORs and intersections. "You can never have too many waypoints on an assigned route," is good advice especially when flying in instrument meteorological conditions. Waypoints are often the key to negotiating shortcuts with air traffic controllers, and they and greatly assist in positional awareness. ATC knows the location of every waypoint--they are all depicted on controllers' radar screens. Controllers will refer to them, especially in terminal airspace, and flying will be easier if the same fixes are readily available as points on your route.
Even if you are flying VFR, the ability to see more waypoints will assist in positional awareness. Talking to air traffic control, saying you are "at DIXIE intersection" or "over the Crazy Woman VOR" pinpoints your location more efficiently than a lengthier, "I am about 20 miles northeast of Greenwood Lake." An added benefit is that the controllers will notice your advanced knowledge of the airspace system, and may treat you accordingly.
Adding many waypoints may uncover something that you never knew about your GPS. There may be a maximum number of waypoints you can add to a flight plan. There may be 12, 30, or more slots available--you should know that number for your GPS. If you are pushing the limit, you may not be able to add waypoints when you need them.
If a flight plan is very full, you may want to erase some waypoints already passed. Or you can divide the flight plan in two halves, and switch during a quiet time while en route. If there are a few redundant waypoints at the end of the first half and beginning of the second half, the time of switching is less critical. Do not add waypoints that will lead you away from your route. If you wish to have some stray waypoints available, use a non-active flight plan for storage.
Adding waypoints does involve some risk. A GPS cannot check a route for consistency. For example, the order of two waypoints may be reversed. A route that should be A to B to C to D may be erroneously entered as A to C to B to D. After you enter a route, check that the route makes sense. This is easiest done if the route can be displayed pictorially--looking at that depiction, double-backs and off-route waypoints are easily detected. However, even with a map display, consider the following before flying any route: Is the distance to destination approximately correct? Are the course headings of each leg similar? Have I inserted all the waypoints required?
The ability to call up a previously entered flight plan is a time-saver. However, there are potential risks here, too. If you haven't flown the route in some time, you may find that some navaid identifiers or even locations have changed. It is tempting to access a stored route containing some waypoints you need, and expect to correct and add to the route. However, this can be the origin of inadvertent double-backs or superfluous waypoints if you are interrupted and do not complete the entries and corrections.
Turning topics
Many GPS units have a turn anticipation function. Your course line will begin to turn before you reach a waypoint, so you won't overshoot the next leg. When the course is followed, the airplane does not overfly the waypoint at the apex of a turn, but cuts the corner. If the angle between legs is sharp, the airplane may turn well before reaching the waypoint.
Several years ago I learned this the hard way. I entered my destination airport, and also entered a subsequent airport almost in the opposite direction for my second planned landing. I had my head down in the cockpit while the autopilot was dutifully following the GPS and initiated an early turn away from my first destination toward my second destination. When I looked up, I was completely disoriented, and headed away from my intended destination.
The primary function of a GPS receiver is to navigate directly to an entered waypoint. A needle indicates deviation to the right or left of course, with each dot right or left usually equaling one nautical mile (while en route)--be sure you're aware of what the deviation equals, however, because this setting can be customized in many units, and the pilot who rented the airplane ahead of you may have changed it.
However, navigating directly to a waypoint may seduce a pilot into neglecting where the GPS is taking the airplane. A straight route may take an airplane over inhospitable terrain, mountains too high to cross, or water beyond safe gliding distance. Restricted or prohibited areas can be violated, and you can even find yourself unexpectedly in Mexico, Canada--or a temporary flight restriction (TFR)! In other words, the direct-to route suggested by GPS may be unwise, unsafe, or illegal.
For instrument flight, another mode besides "Direct To" is required, and is offered by most panel-mounted GPS units--remember, however, that only IFR-approved receivers can be used for navigation under IFR, and that no handheld GPSs are approved for IFR use. The mode permits choice of a course to or from an active waypoint. Unfortunately, the name for this mode is not standardized, and it may be labeled "OBS mode," "Suspend," or "Hold." Whatever name the manufacturer uses, when in this mode automatic sequencing to the next waypoint is suspended (assuming there is a subsequent waypoint in the active route), and the GPS allows the course line to be altered. You can think of this as converting the GPS to VOR-like function, though course deviation is indicated in miles and not angle as in VOR navigation.
The function is required when entering an instrument hold or procedure turn, as the same waypoint must be crossed twice or more. It is also used en route when joining and tracking a specified radial. However, if the "Hold" mode is inadvertently left active, the route will not be followed, and you may find yourself navigating blindly along a radial to nowhere.
The mode can be helpful for VFR navigation, too. Placing a waypoint on an airport, an extended runway line can be drawn. You can navigate to and join that line, and be perfectly lined up on final approach to the runway of your choice.
The ease and accuracy of GPS should not seduce a pilot into considering it infallible. Assure that you've entered the correct waypoints and selected the correct mode. Distances and courses should be consistent with what you calculated during your flight planning, and they should make sense when you compare them with other electronic displays, charts, or visual landmarks. After all, you, the pilot, are the final authority as to navigation--not the GPS.
Dr. Ian Blair Fries is a CFI, senior aviation medical examiner, and ATP, and holds a Lear 35 type rating. He serves on the AOPA Air Safety Foundation Board of Visitors and is cochairman of the AOPA Board of Medical Advisors.
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