An RNP level or type is applicable to a selected airspace,  route, or procedure. As defined in the Pilot/Controller  Glossary, the RNP level or type is a value typically expressed  as a distance in nautical miles from the intended centerline  of a procedure, route, or path. RNP applications also  account for potential errors at some multiple of RNP level  (e.g., twice the RNP level).  United States standard values supporting typical RNP  airspace are shown in Figure 2-56. Other RNP levels as  identified by ICAO, other states, and the FAA may also be  used.  United States standard levels of RNP typically used for  various routes and procedures supporting RNAV operations  may be based on use of a specific navigational system  or sensor, such as GPS, or on multi-sensor RNAV systems  having suitable performance.  Note: The performance of navigation in RNP refers not only  to the level of accuracy of a particular sensor or aircraft  navigation system, but also to the degree of precision  with which the aircraft is flown. Specific required flight  procedures may vary for different RNP levels.  Minimum En Route Altitudes (MEAs), Minimum Reception  Altitudes (MRAs), Maximum Authorized Altitudes (MAAs),  Minimum Obstacle Clearance Altitudes (MOCAs), Minimum  Turning Altitudes (MTAs) and Minimum Crossing Altitudes  (MCAs) are established by the FAA for instrument flight  along Federal airways, as well as some off-airway routes.  The altitudes are established after it has been determined  that the NAVAIDs to be used are adequate and so oriented  on the airways or routes that signal coverage is acceptable,  and that flight can be maintained within prescribed route  widths.  For IFR operations, regulations require that pilots operate  their aircraft at or above minimum altitudes. Except when  necessary for takeoff or landing, pilots may not operate an  aircraft under IFR below applicable minimum altitudes, or  if no applicable minimum altitude is prescribed, in the case  of operations over an area designated as mountainous, an  altitude of 2,000 feet above the highest obstacle within a  horizontal distance of 4 NM from the course to be flown. In  any other case, an altitude of 1,000 feet above the highest  obstacle within a horizontal distance of 4 NM from the  course to be flown must be maintained as a minimum  altitude. If both a MEA and a MOCA are prescribed for a  particular route or route segment, pilots may operate an  aircraft below the MEA down to, but not below, the MOCA,  only when within 22 NM of the VOR. When climbing to  a higher minimum IFR altitude (MIA), pilots must begin  climbing immediately after passing the point beyond  which that minimum altitude applies, except when ground  obstructions intervene, the point beyond which that higher  minimum altitude applies must be crossed at or above the  applicable MCA for the VOR.  If on an IFR flight plan, but cleared by ATC to maintain VFR  conditions on top, pilots may not fly below minimum en  route IFR altitudes. Minimum altitude rules are designed to  ensure safe vertical separation between the aircraft and the  terrain. These minimum altitude rules apply to all IFR flights,  whether in IFR or VFR weather conditions, and whether  assigned a specific altitude or VFR conditions on top.  The MEA is the lowest published altitude between radio  fixes that assures acceptable navigational signal coverage  and meets obstacle clearance requirements between those  fixes. The MEA prescribed for a Federal airway or segment,  RNAV low or high route, or other direct route applies to the  entire width of the airway, segment, or route between the  radio fixes defining the airway, segment, or route. MEAs  for routes wholly contained within controlled airspace  normally provide a buffer above the floor of controlled  airspace consisting of at least 300 feet within transition  areas and 500 feet within control areas. MEAs are established  based upon obstacle clearance over terrain and manmade  objects, adequacy of navigation facility performance, and  communications requirements.  RNAV MEAs are depicted on some IFR en route low altitude  charts, allowing both RNAV and non-RNAV pilots to use the  same chart for instrument navigation.  MRAs are determined by FAA flight inspection traversing  an entire route of flight to establish the minimum altitude  the navigation signal can be received for the route and for  off-course NAVAID facilities that determine a fix. When the  MRA at the fix is higher than the MEA, an MRA is established  for the fix and is the lowest altitude at which an intersection  can be determined.  An MAA is a published altitude representing the maximum  usable altitude or flight level for an airspace structure  2-35  Maximum Authorized Altitude (MAA)  Minimum Reception Altitude (MRA)  RNAV Minimum En Route Altitude  IFR En Route Altitudes  Minimum En Route Altitude (MEA)  Application of Standard RNP Levels  Standard RNP Levels  RNP Levels 