If unable to comply with the requirements of an RNAV  or RNP procedure, pilots need to advise ATC as soon as  possible. For example, “N1234, failure of GPS system,  unable RNAV, request amended clearance.” Pilots are not  authorized to fly a published RNAV or RNP procedure unless  it is retrievable by the procedure name from the navigation  database and conforms to the charted procedure. No other  modification of database waypoints or creation of user-  defined waypoints on published RNAV or RNP procedures  is permitted, except to change altitude and/or airspeed  waypoint constraints to comply with an ATC clearance/  instruction, or to insert a waypoint along the published  route to assist in complying with an ATC instruction. For  example, “Climb via the WILIT departure except cross 30  north of CHUCK at/ or above FL 210.” This is limited only  to systems that allow along track waypoint construction.  Pilots of aircraft utilizing DME/DME for primary navigation  updating should ensure any required DME stations are in  service as determined by NOTAM, ATIS, or ATC advisory.  DME/DME navigation system updating may require specific  DME facilities to meet performance standards. Based on  DME availability evaluations at the time of publication,  current DME coverage is not sufficient to support DME/DME  RNAV operations everywhere without IRU augmentation or  use of GPS. [Figure 1-28] DP chart notes may also include  operational information for certain types of equipment,  systems, and performance requirements, in addition to  the type of RNAV departure procedure.  While operating on RNAV segments, pilots are encouraged  to use the flight director in lateral navigation mode. RNAV  terminal procedures may be amended by ATC issuing radar  vectors and/or clearances direct to a waypoint. Pilots should  avoid premature manual deletion of waypoints from their  active “legs” page to allow for rejoining procedures. While  operating on RNAV segments, pilots operating /R aircraft  should adhere to any flight manual limitation or operating  procedure required to maintain the RNP value specified for  the procedure. In 2008, the FAA implemented the use of  en route host automation ICAO flight plan (FP) processing  for requesting assignment of RNAV SID, Standard Terminal  Arrivals (STARs), or RNAV routes U.S. domestic airspace.  This is part of a risk reduction strategy for introduction of  the En Route Automation Modernization (ERAM) system in  October 2008. ERAM also will use ICAO FP processing and as  a result aircrews should be aware that as the FAA updates to  ERAM the standard FAA flight plan equipment suffix codes  will change to the ICAO flight plan equipment suffix codes.  For procedures requiring GPS and/or aircraft approvals  requiring GPS, if the navigation system does not  automatically alert the flight crew of a loss of GPS, aircraft  operators must develop procedures to verify correct GPS  1-33  RNP is RNAV with on-board monitoring and alerting; RNP is  also a statement of navigation performance necessary for  operation within defined airspace. RNP 1 (in-lieu-of RNAV  1) is used when a DP that contains a constant radius to a  fix (RF) leg or when surveillance (radar) monitoring is not  desired for when DME/DME/IRU is used. These procedures  are annotated with a standard note: “RNP 1.”  All public RNAV SIDs and graphic ODPs are RNAV 1. These  procedures generally start with an initial RNAV or heading  leg near the departure end runway. In addition, these  procedures require system performance currently met  by GPS, DME/DME/Inertial Reference Unit (IRU) RNAV  systems that satisfy the criteria discussed in AC 90-100, U.S.  Terminal and En Route Area Navigation (RNAV) Operations.  RNAV departures are identifiable by the inclusion of the  term RNAV in the title of the departure. From a required  navigation performance (RNP) standpoint, RNAV departure  routes are designed with 1 or 2 NM performance standards  as listed below. This means you as the pilot and your aircraft  equipment must be able to maintain the aircraft within 1  or 2 NM either side of route centerline. [Figure 1-27]  When new RNAV departure procedures are designed,  they will require minimal vectoring and communications  between pilots and ATC. Usually, each departure procedure  includes position, time, and altitude, which increase  the ability to predict what the pilot will actually do.  RNAV departure procedures have the ability to increase  the capacity of terminal airspace by increasing on- time departures, airspace utilization, and improved  predictability.  interdependent, capacity diminishing routes. RNAV is a  method of navigation that permits aircraft operation on  any desired flight path within the coverage of ground- or  spaced- based NAVAIDs or within the limits of the capability  of self-contained aids or a combination of these. In the  future, there will be an increased dependence on the  use of RNAV in lieu of routes defined by ground-based  NAVAIDs. As a part of the evolving RNAV structure, the  FAA has developed departure procedures for pilots flying  aircraft equipped with some type of RNAV technology.  RNAV allows for the creation of new departure routes  that are independent of present fixes and NAVAIDs. RNAV  routing is part of the National Airspace Redesign (NAR) and  is expected to reduce complexity and increase efficiency  of terminal airspace.  • RNAV 1 procedures require that the aircraft’s total  system error remain bounded by ±1 NM for 95  percent of the total flight time.  • RNAV 2 requires a total system error of not more  than 2 NM for 95 percent of the total flight time. 