Figure 3-18.  Analog display (top) and digital display (bottom) from  a Cessna 172.  selected navigation facility or fix. They also provide pilotage  information so the aircraft can be maneuvered to keep it on  a predetermined path. The pilotage information can be in  either two or three dimensions relative to the ground-based or  space-based navigation information.  [Figures 3-21  and  3-22]  GPS is a satellite-based navigation system composed of a  network of satellites placed into orbit by the United States  Department of Defense (DOD). GPS was originally intended  for military applications, but in the 1980s the government  made the system available for civilian use. GPS works in  all weather conditions, anywhere in the world, 24 hours a  day. A GPS receiver must be locked onto the signal of at  least three satellites to calculate a two-dimensional position  (latitude and longitude) and track movement. With four or  more satellites in view, the receiver can determine the user’s  three-dimensional position (latitude, longitude, and altitude).  Other satellites must also be in view to offset signal loss  and signal ambiguity. The use of the GPS is discussed in  more detail in Chapter 17, Navigation. Additionally, GPS is  discussed in the Aeronautical Information Manual (AIM).  This chapter provides an overview of aircraft structures.  A more in-depth understanding of aircraft structures and  controls can be gained through the use of flight simulation  software or interactive programs available online through  aviation organizations, such as the Aircraft Owners and Pilots  Association (AOPA). Pilots are also encouraged to subscribe  to or review the various aviation periodicals that contain  valuable flying information. As discussed in Chapter 1, the  National Aeronautics and Space Administration (NASA) and  the FAA also offer free information for pilots.  The control instruments display immediate attitude and power  changes and are calibrated to permit adjustments in precise  increments.  [Figure 3-20]  The instrument for attitude display  is the attitude indicator. The control instruments do not  indicate aircraft speed or altitude. In order to determine these  variables and others, a pilot must reference the performance  instruments.  The navigation instruments indicate the position of the  aircraft in relation to a selected navigation facility or fix.  This group of instruments includes various types of course  indicators, range indicators, glideslope indicators, and  bearing pointers. Newer aircraft with more technologically  advanced instrumentation provide blended information,  giving the pilot more accurate positional information.  Navigation instruments are comprised of indicators that  display GPS, very high frequency (VHF) omni-directional  radio range (VOR), nondirectional beacon (NDB),  and instrument landing system (ILS) information. The  instruments indicate the position of the aircraft relative to a  Chapter Summary  Control Instruments  Navigation Instruments  3-13  Global Positioning System (GPS) 