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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. 

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## Synthetic and Enhanced Vision Systems Synthetic Vision System (SVS) 

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, 

