Figure 5-16. Enhanced and synthetic vision displayed on primary  flight displays.  Figure 5-19. Installed flight bag.  Figure 5-18. Portable flight bag.  such as takeoff, approach, and landing where important  features such as terrain, obstacles, runways, and landmarks  may be depicted on the SVS display. [Figure 5-15] During  approach operations, the obvious advantages of SVS are  that the digital terrain image remains on the pilot’s display  regardless of how poor the visibility is outside. An SVS  image can be displayed on either a head-down display or  head-up display (HUD). Development efforts are currently  underway that would combine SVS with a real-time sensor  image produced by an EFVS. These systems will be known  as Combined Vision Systems (CVS).  SVGS is a combination of flight guidance display technology  and high precision position assurance monitors. The SVGS  flight instrument display provides a continuous, geo- spatially correct, database driven, computer-generated  synthetic depiction of the nearby topography, including  obstacles, and a display of the landing runway. The SVGS  display may be implemented on a head down Primary Flight  Display, and/or a Head-Up Display (HUD). SVGS includes  additional symbology, integrity and performance monitors  and annunciations that enable low visibility operations.  These additional monitors assure an accurate depiction of  the external scene. An SVGS differs from an EFVS in that it  does not produce a real-time image of the external scene.  SVGS may not be used in lieu of natural vision. SVGS is  intended to be used to increase situational awareness  on the straight-in final approach segment of published  instrument approaches and requires Special Authorization.  For an in-depth discussion regarding Enhanced Flight  Vision Systems, see Chapter 4 of this handbook as well as  AC 90-106 (current version).  The FAA’s NextGen program will transform the NAS to  accommodate a projected three-fold increase in air  operations in the coming decade. Technological and  systemic changes are being developed to significantly  increase the capacity, safety, efficiency, and security of air  operations in the NAS. The FAA will continue to evaluate,  standardize and regulate emerging and enhanced  technologies to ensure their safe and advantageous use  in the NAS. One key capability envisioned to achieve these  goals is the concept of equivalent visual operations (EVO),  where flight operations continue irrespective of the actual  weather conditions. One way EVO might be attained is by  using a combined vision system (CVS) which combines  real-time EFVS imagery with a database-derived synthetic  rendering of surrounding terrain, obstacles, and flight  environment, to provide a virtual visual flight depiction  for the pilot.  5-9  Enhanced Flight Vision System (EFVS)  Combined Vision System Technology  Synthetic Vision Guidance System (SVGS) 