IGRF Calculation

NAME
    gigrf  -  Select IGRF model by generation number and specify time epoch
    sdgrf / spgrf / sigrf - Select DGRF / PGRF / Tentative IGRF  model
                             and specify time epoch
    tyear  -  Change only time epoch of calculation, without changing the IGRF
               model and its reference year as already selected
    igrfc  -  Specify position and get total force IGRF value
    igrfm  -  Get full components of IGRF vector

SYNOPSIS

      call gigrf(ngen, year)
      call sdgrf(year)
      call spgrf(year)
      call sigrf(year)
      call tyear(year)
  year   [float]  Time epoch (in year) of calculation
  ngen   [int]    Generation number of IGRF model

      call igrfc(fi, fk, h, f)
  fi, fk [float]  Lat. and Lon. (in degrees) of the point of calculation
  h      [float]  Altitude (in m) of the point of calculation
  f      [float]  Calculated results.
                  Total force (in nT) value of the model is given.

      call igrfm(fm)
  fm     [float]  Array in which vector values (6 elements) of the model are
                  given.  3 orthogonal components North(X) / East(Y) / Downward
                  (Z) (in nT), and Horizontal(H) component (in nT), 
                  Inclination (in degrees) and Declination (in degrees) are
                  put into the array in sequence.

USAGE
      First specify the ModelGeneration (IGRF-n) or ModelType (IGRF/DGRF/PGRF)
          with the Year of Calc.  by  `call gigrf(ngen, year)`,
          `call sigrf(year)`, `call spgrf(year)`, or `call sdgrf(year)`.
      Then, `call igrfc(FI, FK, H, F)` gives TotalForce (F) of that model
          at the point of Lat.=FI, Long.=FK, Alt.=H
      If other components are desired, `call igrfm(FM)` .
          Here FM is an array with 6 elements, which correspond to
            North(X), East(Y), Downward(Z), Horizontal(H) components,
            Inclination(I) and Declination(D).