around a fix are called “radius-to-fix legs (RF legs).”These  turns, which are encoded into the navigation database,  allow the aircraft to avoid critical areas of terrain or  conflicting airspace while preserving positional accuracy  by maintaining precise, positive course guidance along  the curved track. The introduction of RF legs into the  design of terminal RNAV procedures results in improved  use of airspace and allows procedures to be developed to  and from runways that are otherwise limited to traditional  linear flight paths or, in some cases, not served by an IFR  procedure at all. Navigation systems with RF capability  are a prerequisite to flying a procedure that includes an  RF leg. Refer to the notes box of the pilot briefing portion  of the approach chart in Figure 4-15.  In the United States, operators who seek to take advantage  of RNP approach procedures must meet the special  RNP requirements outlined in FAA AC 90-101, Approval  Guidance for RNP Procedures with Authorization Required  (AR). Currently, most new transport category airplanes  receive an airworthiness approval for RNP operations.  However, differences can exist in the level of precision that  each system is qualified to meet. Each individual operator  is responsible for obtaining the necessary approval and  authorization to use these instrument flight procedures  with navigation databases.  Like any other authorization given to air carriers and Part 91  operators, the authorization to use VNAV on a conventional  non-precision approach, RNAV approaches, or LNAV/VNAV  approaches is found in that operator’s OpSpecs, AFM, or  other FAA-approved documents. There are many different  levels of authorizations when it comes to the use of RNAV  approach systems. The type of equipment installed in the  aircraft, the redundancy of that equipment, its operational  status, the level of flight crew training, and the level of the  operator’s FAA authorization are all factors that can affect  a pilot’s ability to use VNAV information on an approach.  Because most Part 121, 125, 135, and 91 flight departments  include RNAV approach information in their pilot training  programs, a flight crew considering an approach to  North Platte, Nebraska, using the RNAV (GPS) RWY 30  approach shown in Figure 4-16, would already know which  minimums they were authorized to use. The company’s  OpSpecs, FOM, and the AFM for the pilot’s aircraft would  dictate the specific operational conditions and procedures  by which this type of approach could be flown.  There are several items of note that are specific to this type  of approach that should be considered and briefed. One  is the terminal arrival area (TAA) that is displayed in the  To attain the benefits of RNP approach procedures, a key  component is curved flight tracks. Constant radius turns  The operational advantages of RNP include accuracy,  onboard performance monitoring and alerting which  provide increased navigation precision and lower  minimums than conventional RNAV. RNP DAs can be  as low as 250 feet with visibilities as low as 3/4 SM.  Besides lower minimums, the benefits of RNP include  improved obstacle clearance limits, as well as reduced  pilot workload. When RNP capable aircraft fly an accurate,  repeatable path, ATC can be confident that these aircraft  are at a specific position, thus maximizing safety and  increasing capacity.  The GBAS equipment in the aircraft uses the corrections  provided on position, velocity, and time to guide the  aircraft safely to the runway. This signal provides ILS look  alike guidance as low as 200 feet above touchdown.  GBAS will eventually support landings all the way to the  runway surface. Figure 4-14 is an example of a GBAS (LAAS)  approach into Newark, New Jersey.  The GBAS augments the GPS to improve aircraft safety  during airport approaches and landings. It is expected  that the end state configuration will pinpoint the aircraft’s  position to within one meter or less with a significant  improvement in service flexibility and user operating costs.  GBAS is comprised of ground equipment and avionics.  The ground equipment includes four reference receivers, a  GBAS ground facility, and a VHF data broadcast transmitter.  This ground equipment is complemented by GBAS avionics  installed on the aircraft. Signals from GPS satellites are  received by the GBAS GPS reference receivers (four  receivers for each GBAS) at the GBAS equipped airport.  The reference receivers calculate their position using GPS.  The GPS reference receivers and GBAS ground facility work  together to measure errors in GPS provided position.  The GBAS ground facility produces a GBAS correction  message based on the difference between actual and GPS  calculated position. Included in this message is suitable  integrity parameters and approach path information.  This GBAS correction message is then sent to a VHF data  broadcast (VDB) transmitter. The VDB broadcasts the GBAS  signal throughout the GBAS coverage area to avionics in  GBAS equipped aircraft. GBAS provides its service to a  local area (approximately a 20–30 mile radius). The signal  coverage is designed support the aircraft’s transition from  en route airspace into and throughout the terminal area  airspace.  4-30  Required Navigation Performance (RNP)  RNAV Approach Authorization 