TM 1-1510-224-10 depressing    L5    will    cause    the scratchpadded to be displayed to the  right  of  the  text  at  L5.    Valid year  data  is  I  or  2  digits  in  the range 0 through 99. (3) GPS    quick    start-up    mode.          The QUICK   start-up   mode   is   the   mode   which   is   normally automatically    selected    by    the    ASE/ACS.        When    the QUICK  start-up   mode   is   complete,   the   QUICK   legend below the  NAV INIT  text  is  removed  to  cue  the  crew  that the  GPS  is  ready  for  normal  navigation.    The  transition from  QUICK   start   to   navigate   mode   occurs   when   3   or more satellites are being tracked and the figure of merit is less than 9. Figure of merit  (FOM)  is  a  single  digit  from  1  to  9.    It represents   estimated   position   error    (EPE)    in    feet    as follows: FOM = 1, then EPE < 82 FOM = 2, then EPE < 164 FOM = 3, then EPE < 246 FOM = 4, then EPE < 328 FOM = S, then EPE < 642 FOM = 6, then EPE < 1640 FOM = 7, then EPE < 3280 FOM = 8, then EPE < 16404 FOM = 9, then EPE > 16404 (4) GPS  normal  start-up  mode.    If  there is  a  large  difference  between  the  initial  GPS  position  and the   INS   alignment   position   (approximately   54   NM   or larger),    the    ASE/ACS    selects    the    NORMAL    start-up mode.  When autoselected, the legend  NORMAL appears below the text  NAV INIT  and  the  ASE/ACS  will  place  the GPS in the   INIT  mode;  send  the  INS  latitude,  longitude, and   datum   to   the   GPS;   and   reselect   the   NAV   mode. When   the    NORMAL    start-up    mode    is    complete,    the NORMAL legend below the  NAV INIT  text  is  removed  to cue the crew that the GPS is ready for normal navigation. The   transition   from    NORMAL   start   to   navigate   mode occurs  when  3  or  more  satellites  are  being  tracked  and the figure of merit is less than 9. (5) GPS cold start-up mode.    If  the  GPS almanac   data   is   more   than   5000   hours   old,   the   GPS initiates a  COLD start-up mode.  When this happens, the legend  COLD  will  appear  below  the  text   NAV  INIT  and the   GPS   will   initiate   a   search   for   a   good   satellite   to download  almanac  data  (this  process  can  take  up  to  13 minutes).  The total time to complete the  COLD  start-up is totally dependent on satellite visibility.  If  COLD start-up is initiated  and  satellite  number  14  (the  first  in  the  search sequence)    is    visible    and    good,    the    whole    almanac downloading  and  start-up  sequence  should  be  completed in less than 20 minutes.  When the  COLD  start-up  mode is complete, the COLD legend below the  NAV INIT text is removed to cue the crew that the GPS is ready for normal navigation.    The  transition  from   COLD  start  to  navigate mode occurs when 3 or more satellites are being tracked and the figure of merit is less than 9. (6) Map  datum  entry.    With  a  local  map datum  entered  in  the  scratchpad  and  GPS  in  the   INIT mode,  depressing  R2  will  cause  the  scratchpadded  data to be displayed to the left of the text at R2.  A valid datum is any number from I through 47 except 20.  Datum cross reference  is  shown  in  table  3-11.    Map  datum  default  is 47. 3-29.  CHALS USE OF GPS AND INS. a. CHALS Concept.  CHALS (Communications High Accuracy    Airborne    Location    System),    is    an    emitter location    system    that    provides    timely,    high    accuracy locations   required   for   targeting   and   to   support   emitter associations and battlefield situation assessment. CHALS provides this capability through coherent processing of differential doppler (DD) and time difference of arrival (TDOA) information received at a ground facility from the aircraft.  CHALS receivers aboard the aircraft will receive   and   digitize   emitter   signals.      The   data   will   be transmitted   over   the   data   link   to   the   GR/CS   integrated processing  facility  (IPF).    There,  CHALS  processors  will perform  the  required  computations  to  produce  accurate emitter   locations.      The   precise   navigation   required   is provided  by  the  inertial  navigation  system  (INS)  and  the global  positioning  system  (GPS).    GPS  also  provides  the primary     means     of     time     synchronizing     the     CHALS receivers (signal conditioners or SC's) aboard the aircraft. A  backup  for  the  GPS  is  provided  by  the  data  link.    The resultant emitter reports will be sent to GR/CS by CHALS. b. GPS  (and  INS)  Involvement.    The  accurate  and timely   navigation   (position   and   velocity)   is   provided   by integrating   an   INS   with   a   GPS,   and   integrating   both (through a series of intermediaries) with a CHALS ground based navigation processor (NP).  The SC, data link, and CHALS   HSSP   (high   speed   signal   processor)   form   the communication  link.    The  critical  airborne  interfaces  for CHALS  navigation  and  time  synchronization  include  the following: (1) INS to GPS and CHALS: Acceleration Velocity Position Altitude (2) GPS to CHALS: 3-78