Wide area augmentation system (WAAS) of the standard GPS  furnishes additional error correction information, allowing  Category I precision approaches (similar to basic instrument  landing system (ILS) minimums) to units equipped to receive  and integrate the data. Most general aviation pilots learn  to work with an FMS unit primarily using GPS signals,  possibly with WAAS and LORAN‑C options. Older RNAV  units made use of VOR and DME information to compute  positions within range of the navaids. Newer units contain  databases that allow route programming with automatic  sequencing through the selected navigation points. Therefore,  flight management system (FMS) is the best descriptor of the  current GPS units integrating VOR (and DME, optionally)  to allow point-to-point navigation outside established flight  routes. You will learn to use the FMS data entry controls to  program a flight route, review the planned route, track and  make modifications to the planned route while en route, plan  and execute a descent, and fly an approach procedure that is  based solely on RNAV signals. You should remember that  FMS/RNAV units requiring external signals for navigation  are usually restricted to line‑of‑sight reception (LORAN‑C  being somewhat of an exception). Therefore, navigation  information in valleys and canyons that could block satellite  signals may be severely restricted. Users in those areas  should pay particular attention to the altitude or elevations  of the satellites when depending on space‑based signals and  plan flight altitudes to ensure line-of-sight signal reception.  Review the GPS unit’s documentation sufficiently to  determine if WAAS is installed and how WAAS corrections  are indicated. the present position of the aircraft. Using this positional  information, the equipment can calculate the bearing and  distance to or from any waypoint and permit navigation  directly between any two waypoints. In this way, RNAV  overcomes a fundamental limitation of conventional  navaid point‑to‑point navigation techniques, which require  navigating between electronic navigation transmitters on the  ground. The following examples illustrate this limitation. An aircraft equipped with conventional VOR receivers is  positioned at Point A as shown in the diagram at the top  of  Figure 3-1,  and the pilot wishes to navigate directly to  Point B. Although there appear to be a few VOR stations in  the vicinity of the aircraft, it is not clear whether reception  is possible from the aircraft’s present position. If the VOR  stations are within reception range, the pilot has two choices:  (1) fly to intercept the closest airway, then track it to the  intersection; or (2) fly to intercept an extension of the radial  that defines Point B (assuming reception is possible). Neither  alternative provides the pilot with a means of flying directly  to the intersection. Suppose the same aircraft is positioned at Point A as shown  at the bottom of  Figure 3-1  and the pilot wishes to navigate  directly to Point C, which is neither a VOR station nor airway  intersection. This pilot has an even more difficult situation.  Assuming the VOR stations are within reception range, the  pilot needs to create two makeshift airways using a navigation  plotter and chart, fly to intercept one of them, then track  to Point C (which the pilot has defined as the intersection  between the two courses). Flying a direct course to Point C  with any degree of accuracy is not possible. Since RNAV  systems are not bound by these limitations, the entire airspace  is available for navigational use. The national airspace system  can thus accommodate more aircraft. However, when the  pilot leaves the established airways, he or she also leaves the  guaranteed obstruction clearances designed into the airway  system. Always plan flights above the maximum elevation  figure (MEF) displayed on sectional charts when flying  off airways, and be aware that manmade obstructions such  as towers may not be added to charts for some time after  construction. If flying a new routing, allow for construction,  which may not be published yet. You will learn how the FMS can automatically perform many  of the flight planning calculations that were traditionally  performed by hand, and the importance of keeping flight  planning skills fresh. You will also discover how the FMS can  help you detect and correct errors made in the flight planning  process, how the complexities of the FMS make some new  kinds of errors possible, and techniques to help avoid them. Last, you will see how advanced cockpit systems can be used  to navigate using ground‑based navigation facilities such as  VOR and DME. Maintaining pilot skills using ground‑based  navigation facilities is a simple matter of occasionally using  them as the primary means of navigation, and as a backup to  verify position and progress when RNAV is used. RNAV is possible through use of a variety of navigation  facilities and installed aircraft equipment operated in the  U.S. National Airspace System. This handbook focuses  on the more common GPS RNAV, a satellite‑based radio  navigation system available to aircraft equipped with a GPS  receiver. In addition to its ability to receive signals from  GPS satellites, a GPS receiver also contains a computer  processor and a navigation database that includes much of the  Area Navigation (RNAV) is a navigation technique that  allows pilots to navigate directly between any two points  on the globe. Using RNAV, any location on the map can be  defined in terms of latitude and longitude and characterized  as a waypoint. Onboard RNAV equipment can determine  FMS/RNAV Computer Area Navigation (RNAV) Basics RNAV Concept 3-2