transmission to the satellite. Just before satellite time- of-rise,  the  injection  station’s  antenna  is  pointed  to acquire,  lock  on,  and  track  the  satellite  through  the pass.  The  receive  equipment  receives  and  locks  on  to the satellite signals and the injection station transmits the  orbital  data  and  appropriate  commands  to  the satellite. Transmission to the satellite is at a high bit rate,  so  injection  is  completed  in  about  15  seconds. The   message   transmitted   by   the   satellite immediately  after  an  injection  contains  a  mix  of  old and   new   data.   The   injection   station   compares   a readback  of  the  newly  injected  data  with  data  the satellite should be transmitting as a check for errors. If no errors are detected, injection is complete, If one or  more  errors  are  detected,  injection  is  repeated  at two-minute intervals   (updating   the   variable parameters as necessary) until satellite transmission is verified  as  being  correct. DOPPLER    PRINCIPLES Look  at  figure  1-7.  Stable  oscillator  frequencies radiating  from  a  satellite  coming  toward  the  receiver are  first  received  (T1)  at  a  higher  frequency  than transmitted,   because   of   the   velocity   of   the approaching satellite. The satellite’s velocity produces accordion-like   compression   effects   that   squeeze   the radio signals as the intervening distance shortens. As the satellite nears its closest point of approach, these compression   effects   lessen   rapidly,   until,   at   the moment  of  closest  approach  (T2),  the  cycle  count  of the  received  frequencies  exactly  matches  those  which are  generated.  As  the  satellite  passes  beyond  this point  and  travels  away  from  the  receiver  (T3), expansion  effects  cause  the  received  frequencies  to drop  below  the  generated  frequencies  proportionally to the widening distance and the speed of the receding satellite. FACTORS  AFFECTING  ACCURACY Measurement  of  Doppler  shift  is  complicated  by the fact that satellite transmissions must pass through the earth’s upper atmosphere on their way from space to  the  receiver.  Electrically  charged  particals  in  the ionospheric   layer   cause   refraction   of   these transmissions. To solve this problem, the satellites are designed  to  broadcast  on  two  frequencies  (150  and 400  MHz).  The  receiver  measures  the  difference  in refraction  between  the  two  signals  and  supplies  this measurement to the computer. The computer uses this refraction  measurement  as  part  of  its  computation  to obtain   accurate   fixes.   The   most   serious   problem affecting  accuracy  is  the  effect  of  uncertainty  in  the vessel’s   velocity   on   the   determination   of   position. Velocity  computation  problems  are  inherent  in  the system.  Position  error  resulting  from  an  error  in velocity  measurement  is  somewhat  dependent  on  the geometry  of  the  satellite  pass.  You  can  expect  about a  0.2  mile  error  for  every  one-knot  error  in  the vessel’s  velocity.  Knowing  this,  you  can  see  that precision  navigation  of  a  moving  vessel  requires  an accurate  measurement  of  the  velocity  of  the  moving vessel,   such   as   is   provided   by   a   good   inertial navigation  system  (See  the  section  on  Ship’s  Inertial Navigation   System.).   In   general,   intermittent precision  navigation  fixes  would  not  be  of  extreme value for a moving vessel unless it had some means of interpolating  between  these  precision  fixes.  A  good inertial navigation system provides such a means, and simultaneously    provides the   accurate   velocity measurements  required  to  permit  position  fixes  with the  NNSS. In  summary,  precision  navigation  for  moving vessels   can’t   be   provided   by   the   Navy   Navigation Satellite System alone, but can be provided by the use of  this  system  in  conjunction  with  a  good  inertial system. Given the orbital parameters of a satellite, the Doppler   shift   of   the   signal   transmitted   from   that satellite,  and  the  velocity  of  the  vessel,  it  is  possible to obtain a navigational fix if the satellite is within los of the navigation set and has a maximum elevation at the  time  of  closest  approach  (TCA)   of  between  10 and  70  degrees.  Satellite  passes  suitable  for  use  in obtaining  a  navigational  fix  will  usually  occur  at  no more  than  2-hour  intervals  (depending  on  user latitude   and   configuration   of   the   satellite   cons- tellation). It is a matter of your viewpoint whether you consider   the   inertial   system   as   a   means   of interpolating between the satellite navigation fixes or consider  the  satellite  fixes  as  a  means  for  correcting the inevitable long term drills (see the paragraphs on basic  components  of  an  inertial  navigation  system)  of even the best inertial navigation systems. 1-8