These routes are generally found on VFR terminal area  planning charts.  TRSAs are areas where participating pilots can receive  additional radar services. The purpose of the service is  to provide separation between all IFR operations and  participating VFR aircraft.  The primary airport(s) within the TRSA become(s) Class D  airspace. The remaining portion of the TRSA overlies other  controlled airspace, which is normally Class E airspace  beginning at 700 or 1,200 feet and established to transition to/  from the en route/terminal environment. TRSAs are depicted  on VFR sectional charts and terminal area charts with a solid  black line and altitudes for each segment. The Class D portion  is charted with a blue segmented line. Participation in TRSA  services is voluntary; however, pilots operating under VFR  are encouraged to contact the radar approach control and take  advantage of TRSA service.  NSAs consist of airspace of defined vertical and lateral  dimensions established at locations where there is a  requirement for increased security and safety of ground  facilities. Flight in NSAs may be temporarily prohibited by  regulation under the provisions of Title 14 of the Code of  Federal Regulations (14 CFR) part 99, and prohibitions are  disseminated via NOTAM. Pilots are requested to voluntarily  avoid flying through these depicted areas.  The primary purpose of the ATC system is to prevent a  collision between aircraft operating in the system and to  organize and expedite the flow of traffic. In addition to  its primary function, the ATC system has the capability to  provide (with certain limitations) additional services. The  ability to provide additional services is limited by many  factors, such as the volume of traffic, frequency congestion,  quality of radar, controller workload, higher priority duties,  and the pure physical inability to scan and detect those  situations that fall in this category. It is recognized that these  services cannot be provided in cases in which the provision  of services is precluded by the above factors.  Consistent with the aforementioned conditions, controllers  shall provide additional service procedures to the extent  permitted by higher priority duties and other circumstances.  The provision of additional services is not optional on the  part of the controller, but rather is required when the work  situation permits. Provide ATC service in accordance with  the procedures and minima in this order except when:  No pilot may operate an aircraft under basic VFR when the  flight visibility is less, or at a distance from clouds that is  less, than that prescribed for the corresponding altitude and  class of airspace.  [Figure 15-8]  Except as provided in 14 CFR  part 91, section 91.157, “Special VFR Weather Minimums,”  It is important that pilots be familiar with the operational  requirements for each of the various types or classes of  airspace. Subsequent sections cover each class in sufficient  detail to facilitate understanding regarding weather, type of  pilot certificate held, and equipment required.  If ATC issues control instructions to an aircraft through a  source other than another controller (e.g., Aeronautical Radio,  Incorporated (ARINC), FSS, another pilot), they ensure that  the necessary coordination has been accomplished with any  controllers listed above, whose area of jurisdiction is affected  by those instructions unless otherwise specified by a letter  of agreement or a facility directive.  ATC is responsible for ensuring that the necessary  coordination has been accomplished before allowing an  aircraft under their control to enter another controller’s area  of jurisdiction.  Before issuing control instructions directly or relaying  through another source to an aircraft that is within another  controller’s area of jurisdiction that will change that  aircraft’s heading, route, speed, or altitude, ATC ensures  that coordination has been accomplished with each of the  controllers listed below whose area of jurisdiction is affected  by those instructions unless otherwise specified by a letter  of agreement or a facility directive:  Terminal Radar Service Areas (TRSAs)  National Security Areas (NSAs)  Air Traffic Control and the National  Airspace System  Coordinating the Use of Airspace  Operating in the Various Types of Airspace  Basic VFR Weather Minimums  1.  The controller within whose area of jurisdiction the  control instructions are issued  2.  The controller receiving the transfer of control  3.  Any intervening controller(s) through whose area of  jurisdiction the aircraft will pass  15-7  1. A deviation is necessary to conform to ICAO  Documents, National Rules of the Air, or special  agreements where the United States provides ATC  service in airspace outside the country and its  possessions  2.  Other procedures/minima are prescribed in a letter of  agreement, FAA directive, or a military document  3.  A deviation is necessary to assist an aircraft when an  emergency has been declared 