NERS forecast message consists of three parts:
a) declaratins
b) dimenstions
3) data
Declarations define the format of the NERS forecast message, identifiers
of the used models, last epochs of used EOP and AAM series.
Dimensions define the number of knots for B-spline expansion of
final and forecast EOP, thee number of harmonics and cross-harmonic
consituents.
Data contain the arrays of arguments for B-spline, harmonic and cross-harmonic
expansions and arrays of coefficients of B-spline, harminc and cross-harmonic
expansions.
Internally, NERS uses SI units. That means radians (rad) for angles and \
seconds (s) since 2000.01.01_00:00:00.0 TAI for time.
The Earth orientation parameters are defined as petrubations to Euler angles
around axis 1, 2 and 3. Euler angles 1, 2 describe the so-call polar motion.
Euler angle 3 describes axial rotation of the Earth.
Archaic notation pole coordinates X, Y and UT1 is still commonly used.
Euler angle E1 = Y_pole
Euler angle E2 = X_pole
Euler angle E3 = k*(UT1 - TAI), where k = -1.00273781191135448D0
Traditionally, UT1 - TAI is experssed in units of time: seconds.
1s = pi/43200 rad. UT1 - TAI is not a substraction of two quantities, it
is one quantity. Contrary to its name, UT1 is NOT time. UT1 - TAI is angle.
Change of E1 and E2 with time is called polar motion. Change if E3 with
time is called axial rotation.
Some techiniqies uses UTC time tag for recording events. Contrary its name,
UTC is not time, but a nondifferntial function of time. In the context of
Earth sciences TAI is used as time. UTC time tag can be converted to time
using B-spline of the 0th degree expansion of function UTC - TAI (UTC).
The knots of the arguments and the coefficents of the expansion are present
in the NERS forecast message