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 Get RINEX navigation files from SOPAC or CDDIS archives  
  
 Usage: sh_get_nav -archive <cddis/sopac> -yr <yr> -doy <doy> -ndays <num> -rxobs -allnav -ftp_prog <ftp/ncftp>
  
        <archive>  Either sopac or cddis  [Default sopac] 
        <yr> 4 char year of nav data requested  [Required] 
        <doy> 3 char day of year of nav data requested [Required] 
        <num> Number of consecutive days of data to retreive [Default 1] 
        <rxobs> Retreive nav files from RINEX observation day directory 
                archives rather than RINEX navigation file archives. [Default no] 
        <allnav> Retreive all nav files, NOT only combined brdc or auto files. [Default no]
        <ftp/ncftp> select ftp program to use [default is ftp]
  
 Examples: sh_get_nav -archive sopac -yr 1998 -doy 235 -ndays 1 -ftp_prog ncftp
           sh_get_nav -archive cddis -yr 1998 -doy 235 -ndays 7 
           sh_get_nav -archive sopac -yr 1998 -doy 235 -ndays 7 -rxobs 
  
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