! DESCRIPTION:
! ************
!
! This is an extension of the original GRACE gravity models format (document GR-GFZ-FD-001 GRACE 327-732(v1.1) November 27,2003).
! New acronyms for the gravity coefficients have been introduced in order to enable a modelling of the coefficients
! more complex than a single bias or bias+drift.
!
! Existing acronyms:
! ------------------
! GRCOEF: bias, the date indicates the mid-point of data used for this coefficient
! GRCOF2: bias, the dates indicate the time span of data used for this coefficient
! GRDOTA: rate, the date indicates the epoch to which coefficients with rates are mapped
! (GRDOTA needs to be associated with either a GRCOEF or GRCOF2 coefficient)
!
! New acronyms:
! -------------
! G_BIAS, GDRIFT, GCOS..., GSIN... (see below)
!
! G_BIAS and GDRIFT:
! ******************
! They replace GRCOEF and GRDOTA; they have the same format as GRCOF2 (with 2 dates, t1 and t2),
! except that the dates have a different meaning.
!
! G_BIAS: is the bias at a reference epoch T_refep.
! The first date t1 has a double meaning:
! It is this reference epoch (t1 = T_refep), it is also the start of the validity range for this bias coefficient.
! The second date t2 is the end of the validity range for this bias coefficient.
! GDRIFT: is the rate /year. The dates t1 and t2 indicate the range of validity of this rate.
!
! If G_BIAS is not associated with a GDRIFT coefficient having the same range of validity, then the potential at any given
! date T is given by:
! coef(T) = G_BIAS(t1,t2) with T belonging to [t1,t2[
!
! If G_BIAS is associated with a GDRIFT coefficient having the same range of validity, then the potential at any given
! date T is given by:
! coef(T) = G_BIAS(t1,t2) + GDRIFT(t1,t2) * (T - t1) with T belonging to [t1,t2[
!
! It is therefore possible with G_BIAS and GDRIFT to create a modelling of the coefficients
! with successive bias and drift values, similar to the one used by IGS for the Earth orientation parameters.
! The resulting modelling of the coefficients can take the shape of a piece-wise-linear function, if the function at the
! end of an interval coincides with the function at the beginning of the following interval, but it can also be discontinuous
! in order to accomodate for earthquakes, for instance.
!
! GCOS1A, GSIN1A, GCOS2A, GSIN2A,...:
! ***********************************
! Periodic coefficients at the annual (1A), semi annual (2A), etc, periods.
!
! The periodic signal is computed at any given date T using the formula:
! coef_perio(T) = GCOS1A * cos( 2 PI * (T - 1st January) / year ) + GSIN1A * sin( 2 PI * (T - 1st January) / year ) + etc.
!
! GCOS1A: Amplitude of cosine term at the annual period
! GSIN1A: Amplitude of sine term at the annual period
! GCOS2A: Amplitude of cosine term at the semi annual period
! GSIN2A: Amplitude of sine term at the semi annual period
! etc...
! In all cases, the two dates indicate the range of validity of the periodic coefficient. It is possible
! therefore to have, for instance, a different set for each year.
!
! These periodic functions must be combined with a Bias (+ optionally Drift) function, in order to obtain the complete
! modelling of the time variations of the gravity field.
!
! In summary:
! ***********
! coefficients with one date: GRCOEF, GRDOTA
! coefficients with two dates: GRCOF2, G_BIAS, GDRIFT, GCOS... and GSIN...
! For these coefficients, the first date is inclusive (T >= first date), the second date is exlusive (T < second date).