This chapter focuses on the current procedures pilots and  air traffic control (ATC) use for instrument flight rule (IFR)  arrivals in the National Airspace System (NAS). The objective  is to provide pilots with an understanding of ATC arrival  procedures and pilot responsibilities as they relate to the  transition between the en route and approach phases of  flight. This chapter emphasizes standard terminal arrival  routes (STARs), descent clearances, descent planning, and  ATC procedures, while the scope of coverage focuses on  transitioning from the en route phase of flight, typically  the origination point of a STAR to the STAR termination fix.  Optimum IFR arrival options include flying directly from the  en route structure to an approach gate or initial approach  fix (IAF), a visual arrival, STARs, and radar vectors. Within  controlled airspace, ATC routinely uses radar vectors for  separation purposes, noise abatement considerations  when it is an operational advantage, or when requested by  pilots. Vectors outside of controlled airspace are provided  only on pilot request. The controller tells the pilot the  purpose of the vector when the vector is controller-initiated  and takes the aircraft off a previously assigned non-radar  route. Typically, when operating on area navigation  (RNAV) routes, pilots are allowed to remain on their own  navigation.  The most significant and demanding navigational  requirement is the need to safely separate aircraft. In a  non-radar environment, ATC does not have an independent  means to separate air traffic and must depend entirely on  information relayed from flight crews to determine the  actual geographic position and altitude. In this situation,  precise navigation is critical to ATC’s ability to provide  separation.  Even in a radar environment, precise navigation and  position reports, when required, are still a primary means  of providing separation. In most situations, ATC does not  have the capability or the responsibility for navigating  an aircraft. Because they rely on precise navigation by  the flight crew, flight safety in all IFR operations depends  directly on the pilot’s ability to achieve and maintain  certain levels of navigational performance. ATC uses radar  to monitor navigational performance, detect possible  navigational errors, and expedite traffic flow. In a non- radar environment, ATC has no independent knowledge  of the actual position of the aircraft or its relationship to  other aircraft in adjacent airspace. Therefore, ATC’s ability  to detect a navigational error and resolve collision hazards  is seriously degraded when a deviation from a clearance  occurs.  The concept of navigation performance, previously  discussed in this book, involves the precision that must  be maintained for both the assigned route and altitude.  Required levels of navigation performance vary from area  to area depending on traffic density and complexity of  the routes flown. The level of navigation performance  must be more precise in domestic airspace than in oceanic  and remote land areas since air traffic density in domestic  airspace is much greater. For example, there are three  million flight operations conducted within Chicago Center’s  airspace each year. The minimum lateral distance permitted  between co-altitude aircraft in Chicago Center’s airspace  is eight nautical miles (NM) (3 NM when radar is used).  The route ATC assigns an aircraft has protected airspace  on both sides of the centerline, equal to one-half of the  lateral separation minimum standard. For example, the  overall level of lateral navigation performance necessary for  flight safety must be better than 4 NM in Center airspace.  When STARs are reviewed subsequently in this chapter, it is  demonstrated how the navigational requirements become  more restrictive in the arrival phase of flight where air traffic  density increases and procedural design and obstacle  clearance become more limiting.  The concept of navigational performance is fundamental to  the code of federal regulations and is best defined in Title  14 of the Code of Federal Regulations (14 CFR) Part 121, §  121.103 and 121.121, which state that each aircraft must  be navigated to the degree of accuracy required for ATC.  The requirements of 14 CFR Part 91, § 91.123 related to  compliance with ATC clearances and instructions also  reflect this fundamental concept. Commercial operators  must comply with their Operations Specifications  (OpSpecs) and understand the categories of navigational  operations and be able to navigate to the degree of  accuracy required for the control of air traffic.  In the broad concept of air navigation, there are two major  categories of navigational operations consisting of Class  I navigation and Class II navigation. Class I navigation is  any en route flight operation conducted in controlled or  uncontrolled airspace that is entirely within operational  service volumes of International Civil Aviation Organization  (ICAO) standard navigational aids (NAVAIDs) (very high  frequency (VHF) omnidirectional radio range (VOR), VOR/  distance measuring equipment (DME), non-directional  beacon (NDB), etc.).  Class II navigation is any en route operation that is not  categorized as Class I navigation and includes any operation  or portion of an operation that takes place outside the  operational service volumes of ICAO standard NAVAIDs.  For example, aircraft equipped only with VORs conducts  Navigation in the Arrival Environment  3-2 