Figure 17-23.  Synthetic and enha n ced vision systems.  A Synthetic Vision System (SVS) is an electronic means  to display a synthetic vision image of the external scene  topography to the flight crew.  [Figure 17-24]  It is not a  real-time image like that produced by an EFVS. Unlike  EFVS, SVS requires a terrain and obstacle database, a  precise navigation solution, and a display. The terrain  image is based on the use of data from a Digital Elevation  Model (DEM) that is stored within the SVS. With SVS, the  synthetic terrain/vision image is intended to enhance pilot  awareness of spatial position relative to important features  in all visibility conditions. This is particularly useful during  critical phases of flight, such as takeoff, approach, and  landing, where important features, such as terrain, obstacles,  runways, and landmarks, may be depicted on the SVS  display.  [Figure 17-25]  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); however, to date, SVS  has only been certified on head-down displays. Development  efforts to display a synthetic image on a HUD are currently  underway as are efforts that would combine SVS with a real- time sensor image produced by an EFVS. These systems are  known as Combined Vision Systems. While SVS is currently  certified as an aid to situation awareness only, the FAA  and aviation industry are working on defining operational  concepts and airworthiness criteria that would enable SVS  to be used for operational credit in certain low visibility  conditions. Other future enhancements to SVS displays could  include integrating ADS-B to display traffic information.  Enhanced Vision (EV) or Enhanced Flight Vision System  (EFVS) is an electronic means to provide a display of  Figure 17-24.  SVS system.  Figure 17-25.  Night time SVS system.  Synthetic Vision System  Enhanced Flight Vision System  17-28  Synthetic vision system  Enhanced vision system 