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  Craig B. Markwardt, NASA/GSFC Code 662, Greenbelt, MD 20770
  UPDATED VERSIONs can be found on my WEB PAGE:


  Read gravity model from disk file

Major Topics

  Physics, Gravity, Geodesy, Spacecraft Navigation

Calling Sequence



  GEOGREAD reads a gravity model from a disk file. The gravity
  model must have already been prepared. There are a number of
  freely available models.
  Each model must have a "description" file which describes, in IDL
  syntax, the name, content and format of the model file. The
  ROOTFILE parameter is the name of this description file. The
  description must provide the name of the model file (the FILENAME
  field), which must reside in the same directory.
  The format of the description file (and hence also the format of
  the structure returned in GEOGMOD), is as follows, an example
  modified from egm96.desc:
  { $
  name: 'EGM96', $ ;; Title of the model
  type: 'GRAVITY', $ ;; Type of model 'GRAVITY' or 'BFIELD'
  filename: 'EGM96.GEO', $ ;; Model coefficient file name (same dir)
  reference: 'Lemoine, ...' ;; Complete literature reference
  url: 'ftp://ftp.csr.utexas.edu/pub/grav/EGM96.GEO.Z', $ ;; Source URL
  nmax: 360L, $ ;; Maximum order (inclusive)
  mmax: 360L, $ ;; Maximum degree (inclusive)
  normalized: 1, $ ;; Coefficients are normalized (1=yes, 0=no)
  mu: 398600.44150D+09, $ ;; GM for central body [m^3/s^2]
  a: 6378136.30d, $ ;; Mean equatorial radius [m]
  tide: 'ZERO', $ ;; Tide system (ZERO, FREE, or MEAN)
  epoch: 1986.0d, $ ;; Epoch of model coefficients (Julian year)
  C21: -.1869876359548955D-09,$ ;; C21 coefficient (if not in Cnm table)
  S21: .1195280120306540D-08,$ ;; S21 coefficient (if not in Cnm table)
  C20_dot: 1.16275534D-11,$ ;; C20 rate (unitless; yr^-1)
  C21_dot: -0.32d-11, $ ;; C21 rate (unitless; yr^-1)
  S21_dot: +1.62d-11, $ ;; S21 rate (unitless; yr^-1)
  rowstart: 4L, $ ;; Coefficient starting row (first row = 0)
  nrows: 65338L, $ ;; Number of coefficient rows in file
  ncolrange: [6,8], $ ;; Column range for degree (first col = 0)
  mcolrange: [9,11], $ ;; " " " order
  Ccolrange: [12,30], $ ;; " " " C coefficients
  Scolrange: [31,49], $ ;; " " " S coefficients
  dCcolrange: [50,62], $ ;; " " " C std deviation
  dScolrange: [63,75] $ ;; " " " S std deviation
  The xCOLRANGE fields describe which character columns in the model
  file, inclusive, contain the quantity of interest. You can use a
  text editor which reports the column number to find these values.
  Exclude any character columns that contain field delimiters such
  as commas.
  Since the C21 and S21 coefficients are commonly not included in
  the table itself, their values are allowed to be specified in the
  description file. If the coefficients *are* in the table, then
  they must be set to zero in the description file to avoid double
  computations. The coefficient rates can be used to extrapolate to
  different epochs from the reference epoch (specified by EPOCH).


  ROOTFILE - scalar string, the name of the model description file.
  GEOGMOD - upon return, an IDL structure containing the model
            information. In addition to the fields listed above,
            other fields are appended which contain (pointers to)
            the coefficient data, etc.

Keyword Parameters

  STATUS - upon return, a status indicator. A value of 1 is OK, 0
            indicates an error condition.
  ERRMSG - upon return, an error message, if any. If no error
            occurred, then ERRMSG is set to ''.
  CEOFF_ERR - if set, then coefficient standard deviations are also
              read in.q


  GEOGREAD, 'egm96', egm96
  GEOGRAV, egm96, r, phi, a
  Read the gravity model "EGM96" and evaluate it at position "R" in
  body coordinates. The potential and acceleration are returned in
  PHI and A.

Modification History

  Documentation additions, CM, 26 Sep 2004

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