Earth’s rotation: observations and theory - Earth Dynamics: Deformations and Oscillations of the Rotating Earth - page 287

12 CONTINUE
C Add nodes at beginning and end by assuming first derivatives are constant
C in first two and last two subintervals.
DAY(1)=4.D0
C Find slope between second and third nodes.
SLOPE=(XWOB(3)-XWOB(2))/(DAY(3)-DAY(2))
XWOB(1)=SLOPE*(DAY(1)-DAY(2))+XWOB(2)
SLOPE=(YWOB(3)-YWOB(2))/(DAY(3)-DAY(2))
YWOB(1)=SLOPE*(DAY(1)-DAY(2))+YWOB(2)
DAY(3515)=30.25D0
C Find slope betwen third last and second last nodes.
SLOPE=(XWOB(3514)-XWOB(3513))/(DAY(3514)-DAY(3513))
XWOB(3515)=SLOPE*(DAY(3515)-DAY(3514))+XWOB(3514)
SLOPE=(YWOB(3514)-YWOB(3513))/(DAY(3514)-DAY(3513))
YWOB(3515)=SLOPE*(DAY(3515)-DAY(3514))+YWOB(3514)
C Set number of nodes.
N1=N+1
N2=2*N1-2
N3=N1-2
M1=4000
M2=8000
M3=4000
C Construct interpolation matrix C.
CALL SPMAT(N1,N2,N3,C,DAY,B,M1,M2,M3)
C Find equispaced time sequence.
C Find time from beginning of record.
TIME=4.D0-DT/2.D0
DO 13 I=1,M
TIME=TIME+DT
C Convert time to days from beginning of the record.
DAYS=365.25636D0*TIME
C Convert to Julian days.
DAYS=DAYS+44089.993750D0+2400000.D0
C Find calendar date.
IDAYS=INT(DAYS)
CALL CALDAT(IDAYS,MM,ID,IYYY)
C Interpolate co-ordinates.
CALL INTPL(TIME,XX,N1,C,DAY,XWOB,M1)
CALL INTPL(TIME,YY,N1,C,DAY,YWOB,M1)
GTX(I)=XX
GTY(I)=YY
C Write out equispaced time domain sequence.
WRITE(2,14)I,IYYY,MM,ID,TIME,GTX(I),GTY(I)

14 FORMAT(1X,4I5,F12.7,2F12.6)

13 CONTINUE

END

As an example of the interpolated VLBI pole path, we show that for the year
2004 in Figure 4.5. Pole positions, determined by the VLBI technique, are indi-
cated by crosses, and the interpolated path is shown as a continuous curve.
Of interest is the spectral density estimate based on the interpolated VLBI pole
path. The file ppintp.dat contains 3513 interpolated pole positions at equally spaced
time intervals of approximately 2.73 days. As illustrated in Figure 2.2, the first
sample is at half the sample interval from the beginning of the record segment
and the last sample is at half the sample interval from the end of the record seg-
ment. Once again, the record segment is of length 26.25 years and the spectral