This is the description of the GRGS format, used for variable and static fields, including additionnal time-variable components. It is also called GINS format, GINS being the name of our orbit computation software. See « Body » and « Comments » for details about the time-variable components.

For a description of the extended GRACE format, see here.

Header

Header: 6 lines

  1. comment line (name of the solution)
  2. comment line
  3. format (4e20.14)
    • reference radius (meters)
    • 1/flatening
    • GM value (SI units)
    • mean earth rotation rate (rad/s)
  4. format (17x,f7.2) : reference date in year
  5. format (17x,i3) : maximum degree
  6. comment line

Body

Ordinary lines: format (2i3,a3,2e21.14,2e13.6,1x,i2)

  • 2I3 : degree l and order m of the coefficients
  • a3 : see among the 7 following cases
    • ‘ ‘: static term (reference date is 2004.0 for GL04S and 2005.0 for RL02MF)
    • ‘DOT’: secular drift term (in /year) (optionnal time-variable term)
    • ‘S1A’: annual sinus term (optionnal time-variable term)
    • ‘C1A’: annual cosinus term (optionnal time-variable term)
    • ‘S2A’: semi-annual sinus term (optionnal time-variable term)
    • ‘C2A’: semi-annual cosinus term (optionnal time-variable term)
    • ‘SUM’: pre-SUMATRA term (optionnal time-variable term)
  • 2e21.14 : see among the 2 following cases
    • normalized spherical harmonic coefficients : C and S for degree l and order m
    • amplitudes of optionnal C and S time-variable components (DOT,S1A,C1A,S2A,C2A,SUM…)
  • 2e13.6 : sigmas (C and S)
  • i2 : unused

Comments

When variable terms are present, the computation of the value of one coefficient C or S (l,m) at date t (in years) has to be done in the following way :

C or S (l,m) at date t =

  • STATIC term
  • + DOT term * (t-t_ref)
  • + S1A term * sin(2.*pi*(t-t_ref))
  • + C1A term * cos(2.*pi*(t-t_ref))
  • + S2A term * sin(4.*pi*(t-t_ref))
  • + C2A term * cos(4.*pi*(t-t_ref))
  • + SUM term if one works BEFORE December 24, 2004