Under ideal conditions, the GPS may determine your position within 50 feet or less. In addition to accuracy, the biggest advantage to using satellites instead of ground based stations is that there is no need to be within a certain distance or in a line-of-sight position to a station on the ground, so that navigation information is available almost anywhere in the world with a view of the sky. While the satellite signal is not affected by clouds or other weather phenomenon, satellites low on the horizon may be blocked by mountains or other terrain.
In addition to the ability to calculate position, a GPS receiver typically has a range of other functionality which allows it to calculate information such as the direction of movement and speed. By incorporating a database of airports and other ground locations called waypoints a GPS can also provide navigational information relative to these points. Some GPS receivers also include terrain maps and other information which can be overlaid with the position, course of the aircraft and selected waypoints.
Different types of GPS receivers are available for aviation use, ranging from simple hand-held devices to panel mounted units with large map display screens. GPS receiver installations may also be approved for IFR use or limited to use as an aid to navigation and situational awareness under VFR. WAAS GPS receivers include additional capabilities to allow for precise horizontal vertical guidance during IFR precision approach procedures.
IFR approved GPS receivers include the ability to automatically monitor system availability and reliability and to alert the pilot when the navigation information may not be reliable. This feature is called RAIM, or Receiver Autonomous Integrity Monitoring. VFR only GPS receivers may not include this capability – it is important to know whether it is a feature of the GPS you are using, and to cross-check the navigation information provided by the GPS for VFR use with other sources, such as VOR stations and landmarks on the ground.
Another consideration is to insure that the GPS database is up to date. Most GPS receivers display the database expiration date as part of the power up process. Airspace, airport and waypoint information change on a regular basis and most aviation GPS receivers can be kept current with subscription database updates. If the database is not current, information on special airspace and other important navigation information may not be correct, leading the pilot to possibly enter restricted airspace or to navigate incorrectly. If an out-of-date GPS database is in use, it is imperative that the pilot cross check the information provided by the GPS with other sources such as current sectional charts.
Each make and model of GPS receiver has different functionality and different means of accessing its features. So it is essential to have an operating manual available for the specific unit you will be using, particularly if you plan to use it for navigation purposes on a cross-country flight.