GEOGREAD Name
GEOGREAD
Author
Craig B. Markwardt, NASA/GSFC Code 662, Greenbelt, MD 20770
craigm@lheamail.gsfc.nasa.gov
UPDATED VERSIONs can be found on my WEB PAGE:
http://cow.physics.wisc.edu/~craigm/idl/idl.html
Purpose
Read gravity model from disk file Major Topics
Physics, Gravity, Geodesy, Spacecraft Navigation
Calling Sequence
GEOGREAD, ROOTFILE, MODEL [, STATUS=STATUS, ERRMSG=ERRMSG,
/COEFF_ERR]
Description
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.
FILE FORMAT:
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).
Inputs
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
Example
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