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 5-14] It is not a  real-time image like that produced by an enhanced flight  vision system (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,  developed. By showing airspeed, altitude, heading, and  aircraft attitude on the HUD glass, pilots were able to keep  their eyes outside of the flight deck rather than have to  continuously scan from outside to inside to view the flight  instruments. [Figure 5-12] Collimators make the image on  the glass appear to be far out in front of the aircraft so that  the pilot need not change eye focus to view the relatively  nearby HUD. Today’s head-up guidance systems (HGS)  use holographic displays. [Figure 5-13] Everything from  weapons status to approach information can be shown  on current military and civilian HGS displays.  5-8  Figure 5-15. An aircraft on an approach equipped with a SVS.  Figure 5-13. HGS using a holographic display.  Synthetic and Enhanced Vision Systems  Synthetic Vision System (SVS)  Figure 5-14. A synthetic vision system (SVS) is an electronic means to  display a synthetic vision image of the external scene topography to the  flight crew to assist during takeoffs, landings, and en route operations.  Figure 5-12. Head-up guidance system (HGS).  160  140  100  80  10  10  4 11  7 800 7 600 7 400 20 00 7 5  220 240