spread-spectrum   rf   signals,   1575.42   MHz   (LI-RF) and 1227.60 MHz (L2-RF).  Each signal is modulated with  a  unique  code  sequence  and  a  navigation  data message.   The   code   sequence   allows   the   navigation sets  to  identify  the  satellite,  and  the  data  message provides  the  navigation  sets  information  about  the operation of the satellite. An observer on the ground will observe the same satellite ground track twice each day, but the satellite will  become  visible  4  minutes  earlier  each  day because of a 4 minute per day difference between the rotation of the earth and the satellite orbit time. The satellites  are  positioned  so  a  minimum  of  four satellites are always observable to a user anywhere on earth. Satellite Signal Structure The satellites transmit their signals using spread spectrum   techniques.   Two   types   of   techniques   are used:  course  acquisition  (C/A)  code  and  precise  (P) code.   The   C/A   code   is   available   to   military   and civilian  GPS  users.  The  P  code  is  available  only  to U.S.  military,  NATO  military  and  other  users  as determined  by  the  DOD. Since only the P code is on both frequencies, the military users can make a dual-frequency comparison to  compensate  for  ionospheric  propagation  delay. The   C/A   code-only   users   must   use   an   ionospheric model,  which  results  in  lesser  navigation  accuracy. Superimposed  on  both  codes  is  the  NAVIGATION- message  (NAV-msg),  containing  satellite  ephemeris data,  atmospheric  propagation  correction  data,  and satellite  clock-bias  information. Satellite  Ranging GPS  navigation  is  based  on  the  principle  of satellite ranging. Satellite ranging involves measuring the time it takes the satellite signal to travel from the satellite  to  the  navigation  set.  By  dividing  the  travel time  by  the  speed  of  light,  the  distance  between  the satellite and the navigation set is known. By ranging three  satellites,  a  three-dimensional  picture,  such  as the  one  shown  in  figure  1-12,  can  be  developed.  The distance  measurement  to  each  satellite  results  in  a sphere  representing  the  distance  from  the  navigation set to the satellite. The point where the three spheres intersect  (X)  is  the  position  of  the  navigation  set, This   explanation   does   not   account   for   errors.   For satellite ranging to provide accurate position data, the following three sources of error must be compensated for: Satellite  position  and  clock  error Atmospheric  delay  of  satellite  signals Navigation  set  clock  error With  these  errors  compensated  for,  the  GPS  can determine  position  fixes  within  50  feet  or  less  and  is accurate  to  within  a  tenth  of  a  meter-per-second  for velocity and 100 nanoseconds for time. This accuracy, however,  requires  inputs  from  four  satellites. USER   EQUIPMENT User equipment is installed in ships, aircraft, and motorized  vehicles.  The  vehicle  version  can  also  be carried  by  personnel  (particularly  SEAL  teams  and other  special  forces  units)  as  a  manpack.  The  most common  manpack  version  is  the  AN/PSN-8(  ).  The most   common   shipboard   GPS   receiver   is   the AN/WRN-6.  These  GPS  receivers  will  be  described later  in  this  chapter. Signal  Acquisition During  operation,  navigation  sets  collect  and store  satellite  almanac  data  in  critical  memory.  The almanac  data  is  normally  available  when  the navigation   set   is   first   turned   on   and   provides information  on  satellite  locations.  Operators  may input  information  about  the  navigation  set  position, time,  and  velocity  to  enhance  the  information  in critical memory. With this information, the navigation set  determines  which  satellites  are  available  and searches  for  the  code  sequences  that  identify  those particular  satellites.  When  the  C/A  code  of  an available   satellite   is   identified,   the   navigation   set switches  to  the  more  accurate  P  code,  collects  the navigation   data   message,   and   updates   critical memory. 1-15