! 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).