lower availability than that actually encountered in flight to provide protection for the lowest end receiver models. Because RAIM operates autonomously, that is without the assistance of external signals, it requires redundant pseudorange measurements. To obtain a 3D position solution, at least four measurements are required. To detect a fault, at least 5 measurements are required, and to isolate and exclude a fault, at least six measurements are required, however often more measurements are needed depending on the satellite geometry. Typically there are seven to 12 satellites in view. The test statistic used is a function of the pseudorange measurement residual (the difference between the expected measurement and the observed measurement) and the amount of redundancy. The test statistic is compared with a threshold value, which is determined based on the requirements for the probability of false alarm (Pfa) and the expected measurement noise. In aviation systems, the Pfa is fixed at 1/15000. The horizontal integrity limit (HIL) or horizontal protection level (HPL) is a figure which represents the radius of a circle which is centered on the GPS position solution and is guaranteed to contain the true position of the receiver to within the specifications of the RAIM scheme (i.e. which meets the Pfa and Pmd). The HPL is calculated as a function of the RAIM threshold and the satellite geometry at the time of the measurements. The HPL is compared with the horizontal alarm limit (HAL) to determine if RAIM is available. RAIM prediction websites To enable pilots to quickly determine whether en route or approach level RAIM will be available, the FAA and EUROCONTROL have created "dispatch level" websites that predict RAIM status to meet pre-flight check requirements. The FAA's RAIM prediction website "AC 90–100" covers US territories in graphical map format (showing green for RAIM available and red for RAIM not available) EUROCONTROL provides international coverage for most waypoints in the worldwide aviation waypoint database and displays results in a "timeline" bar showing predictions of whether baro-aided or non-baro-aided RAIM will be available. EUROCONTROL puts a disclaimer on its data (stating that USCG data takes precedence), whereas the FAA certifies its website as meeting regulatory requirements. As of 1 July 2012 AUGUR coverage has been limited to ECAC airspace only. Since 2006, the N-RAIM Prediction Service, hosted by NAVBLUE, offers worldwide coverage for all PBN applications including RNP 10, RNAV 5, RNAV 2, RNAV 1, RNP