1. important
   1. for
      1. plane
         1. finding
            1. land
      2. ship
         1. seeking
            1. assistance
      3. hiker
         1. disorientated
            1. by weather
      4. motorist
         1. find
            1. an address
      5. bussinesses
         1. track
            1. mobile
            2. assets
2. receivers
   1. used
      1. for
         1. positioning
         2. locating
         3. navigating
         4. timing
3. can
   1. determine
      1. position & time
         1. accurately
            1. position
            2. position to within 20m to approx. 1mm
            3. time(UTC)
            4. time to within 60ns to approx. 5ns
      2. speed & direction
      3. speed
         1. from
            1. Doppler shift measurement
4. types
   1. GPS
      1. civilian
         1. Standard Positioning System(SPS)
         2. free
      2. military
         1. Precise Positioning Services
         2. authorized
      3. constellation
         1. orbits
            1. inclined
            2. @55 degress
            3. equator
            4. 6 planes
            5. 4 satellites/plane
            6. plus spares satellites
            7. 4 atomic clocks per satellite
            8. at least 4 visible anytime anywhere
            9. Approx. 12 hrs long
            10. altitude= 20180km
         2. link: note: rotating earth