The Wide Area Augmentation System (WAAS) is being developed by the Federal Aviation Administration (FAA) to augment the Global Positioning System (GPS). The WAAS will aid GPS with the following services. First, it will broadcast spread-spectrum ranging signals from geo-stationary earth orbit communication satellite (GEO). The airborne WAAS receiver will add these new ranging signals to the GPS constellation measurements. By doing so the augmented position fix will be less sensitive to the failure of individual system components, thus improving time availability and continuity of service. Second, the WAAS will use a nationwide ground network to monitor the health of all satellites over airspace and flag situations that threaten flight safety. This data will be modulated on the WAAS ranging signals
and broadcast to the users, thereby guaranteeing the integrity of the airborne position fix. Third, the WAAS will use the ground network to develop corrections for the errors, which currently limit the accuracy of un-augmented GPS. This data will also be included on the WAAS broadcast and will improve position accuracy from approximately 30 meters to 8 meters.
When complete, the augmented system will provide an accurate position fix from satellites to an unlimited number of aircraft across the nation. It will be the primary navigation system for aircraft in oceanic routes, en route over our domestic airspace, in crowded metropolitan airspaces, and on airport approach. When GPS is used on final approach to airports, which demands the greatest safety and reliability. Air port applications will include: non-precision approach where GPS will be used solely for horizontal positioning, and precision approach where GPS will be used for both horizontal and vertical position fixing. To serve all these applications, GPS must be augmented to meet stringent requirements with respect to accuracy, integrity, continuity, and time availability.
Accuracy specifies the position error at the 95% level in the absence of system failures. With SA (Selective Availability) off, the accuracy of stand alone GPS is approximately 30 m in the horizontal and 50 m in the vertical.
This augmented accuracy is certainly adequate for precision approach operations, which requires 95% accuracies of a few meters or less.
Integrity and continuity are the main measures of flight safety - they address the ability of the navigation system to detect and repair system failures in a timely fashion. Integrity fails when the position error exceeds a certain protection limit and this error is not annunciated to the pilot or the aircraft guidance system within a specified time-to-alarm. In contrast, continuity fails when an aircraft operation must be aborted for any unscheduled reason. Continuity failures include obvious failures of the ground or airborne hardware, which are easily recognized. Clearly, they are not as dangerous as integrity failures, because the pilot or air traffic control has a much greater chance of successful intervention. Nonetheless, continuity requirements are still quite demanding.
Finally, time availability is the faction of time for which the system is operational - providing position fixes with the specified accuracy, integrity and continuity. In contrast to integrity and continuity, availability is not intended to measure flight safety. Rather it is a measure of the operational acceptability or economy of a navigation system. For en route flight down through nonprecision approach, the required availability is 99.999%, For precision approach it is between 99.9% and 99,99%.