LaRCsim/ls_gravity.c

   1 /***************************************************************************
   2
   3         TITLE:  ls_gravity
   4
   5 ----------------------------------------------------------------------------
   6
   7         FUNCTION:       Gravity model for LaRCsim
   8
   9 ----------------------------------------------------------------------------
  10
  11         MODULE STATUS:  developmental
  12
  13 ----------------------------------------------------------------------------
  14
  15         GENEALOGY:      Created by Bruce Jackson on September 25, 1992.
  16
  17 ----------------------------------------------------------------------------
  18
  19         DESIGNED BY:    Bruce Jackson
  20
  21         CODED BY:       Bruce Jackson
  22
  23         MAINTAINED BY:  Bruce Jackson
  24
  25 ----------------------------------------------------------------------------
  26
  27         MODIFICATION HISTORY:
  28
  29         DATE    PURPOSE                                                 BY
  30
  31         940111  Changed include files to "ls_types.h" and
  32                 "ls_constants.h" from "ls_eom.h"; also changed DATA types
  33                 to SCALAR types.                                        EBJ
  34
  35
  36 $Header$
  37 $Log$
  38 Revision 1.1  1997/05/29 00:09:56  curt
  39 Initial Flight Gear revision.
  40
  41  * Revision 1.2  1994/01/11  18:50:35  bjax
  42  * Corrected include files (was ls_eom.h) and DATA types changed
  43  * to SCALARs. EBJ
  44  *
  45  * Revision 1.1  1992/12/30  13:18:46  bjax
  46  * Initial revision
  47  *
  48
  49 ----------------------------------------------------------------------------
  50
  51                 REFERENCES:     Stevens, Brian L.; and Lewis, Frank L.: "Aircraft
  52                                 Control and Simulation", Wiley and Sons, 1992.
  53                                 ISBN 0-471-
  54
  55 ----------------------------------------------------------------------------
  56
  57                 CALLED BY:
  58
  59 ----------------------------------------------------------------------------
  60
  61                 CALLS TO:
  62
  63 ----------------------------------------------------------------------------
  64
  65                 INPUTS:
  66
  67 ----------------------------------------------------------------------------
  68
  69                 OUTPUTS:
  70
  71 --------------------------------------------------------------------------*/
  72 #include "ls_types.h"
  73 #include "ls_constants.h"
  74 #include <math.h>
  75
  76 #define GM      1.4076431E16
  77 #define J2      1.08263E-3
  78
  79 void ls_gravity( SCALAR radius, SCALAR lat, SCALAR *gravity )
  80 {
  81
  82         SCALAR  radius_ratio, rrsq, sinsqlat;
  83
  84         radius_ratio = radius/EQUATORIAL_RADIUS;
  85         rrsq = radius_ratio*radius_ratio;
  86         sinsqlat = sin(lat)*sin(lat);
  87         *gravity = (GM/(radius*radius))
  88                 *sqrt(2.25*rrsq*rrsq*J2*J2*(5*sinsqlat*sinsqlat -2*sinsqlat + 1)
  89                                 + 3*rrsq*J2*(1 - 3*sinsqlat) + 1);
  90
  91 }