NAVCALC -- Calculates satellite subpoints and orbit times
   NAVCALC navdataset option <keywords>
   NAVCALC imgdataset option <keywords>
Parameters:
   navdataset | name of ADDE dataset containing system navigation files
                for a satellite; specify as group/descriptor or alias
   imgdataset | name and position of ADDE dataset containing the image
                to use as a navigation source; specify as alias.position
                or group/descriptor.position; you must also specify TYPE=I
                to indicate this is an image dataset (def=0 for position)
   option  | calculation to perform; choose from LOS, VIEW, SP, HGT, FWD,
             or INV (no def)
             LOS  - compute time ranges when a polar orbiting satellite
                    is visible (in line of sight) from a surface point
             VIEW - compute times that a polar orbiting satellite and
                    sensor view a surface point
             SP   - compute satellite subpoints for a time (geostationary)
                    or range of times (polar orbiting)
             HGT  - estimate height of a feature from Earth location
                    and image coordinates
             FWD  - compute Earth location of an image coordinate
             INV  - compute image coordinates of an Earth location
Keywords:
   ANTENNA= | minimum angle, in degrees, above horizon at which satellite
              signal is readable; valid only with LOS option (def=5)
   DAY=bday eday | beginning and ending day; see Remarks (def=current, or
                   from imgdataset)
   ELEINC=inc num | increment between elements and number of element
                    values for multiple calculations using the FWD option
   HEIGHT= | height in km of Earth location; valid only with FWD, INV,
             LOS, and VIEW options (def=0 if LATLON keyword is used to
             specify location; station height if STATION keyword is used)
   HTOL=   | height tolerance in km for HGT option (def=0.5)
   LATINC=inc num | increment between latitudes and number of latitude
                    values for multiple calculations using the INV option
   LATLON=lat lon | Earth location of point; valid only with HGT, INV,
                    LOS, and VIEW options (no def)
   LINELE=lin ele | image coordinates of point; valid only with FWD and
                    HGT options (no def)
   LININC=inc num | increment between lines and number of line values
                    for multiple calculations using the FWD option
   LONINC=inc num | increment between longitudes and number of longitude
                    values for multiple calculations using the INV option
   SENSOR= | sensor type; valid POES types are AVHRR (def), HIRS, or MSU;
             valid DMSP types are OLS (def), SSM/I, SSM/T, or SSM/T2;
             valid GOES and GVAR types are IMAGER (def) and SOUNDER;
             keyword is valid with all options but SP
   STATION= | station ID of surface point; valid only with HGT, INV, LOS,
              and VIEW options
   TIME=btime etime inc | beginning and ending time (def=0 23:59:59, or
                          from imgdataset);'inc' specifies interval between
                          subpoint calculations (def=etime minus btime)
   TYPE=S | use system navigation files specified in 'navdataset' (def)
       =I | use navigation of image specfied in 'imgdataset'; overrides
            SENSOR keyword
Remarks:
   Use both the DAY and TIME keywords to specify the time period for
   navigation calculations. For example, DAY=182 183 TIME=21 3 specifies
   a range from 21 UTC on day 182 through 03 UTC on day 183. Specify
   'inc' with the TIME keyword only when using the SP option with polar
   orbiting satellites. NAVCALC does not list day and/or time ranges of
   geostationary satellite navigation; only the data closest to the
   specified time on the beginning day is listed.
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