Earth’s rotation: observations and theory - Earth Dynamics: Deformations and Oscillations of the Rotating Earth - page 278

4.2.1 The VLBI pole path
The pole path determined by the very long baseline interferometry (VLBI) tech-
nique is listed on the Goddard Space Flight Center website (http:

// gemini.gsfc.

nasa.gov 2009a). The downloadable file 2009a.eops contains 4443 pole

position co-ordinates together with their standard errors. The time tags for each
pole position are given in Julian days minus 2,400,000. The programme
PP2009A extracts from the file 2009a.eops the time in days from the beginning
of the record, the pole co-ordinates, and their standard errors.

/ solutions /

C PROGRAMME PP2009A.FOR

C
C PP2009A.FOR extracts pole co-ordinates and their standard errors
C from the file 2009a.eops on the Goddard Space Flight Center website.
C

IMPLICIT DOUBLE PRECISION(A-H,O-Z)
OPEN(UNIT=1,FILE='2009a.eops',STATUS='OLD')
OPEN(UNIT=2,FILE='pp2009a.dat',STATUS='UNKNOWN')
C Read in extracted data.
DO 10 I=1,4443
READ(1,11)DAY,XWOB,YWOB,UT1MC,PSI,EPS,EXWOB,EYWOB,
1 EUT1MC,SEPSI,SEEPS,WMS,C1,C2,C3,C4,N,SC,SD,RX,RY,
2 DLOD,F1,F2,UX,UY,UL,Q1,Q2,P1,P2
11 FORMAT(1X,F12.6,2F9.6,F11.7,2F9.3,2F9.6,F10.7,2F8.3,
1 F8.2,4F7.4,I7,A7,F6.2,2F10.6,F11.7,2A3,2F10.6,F11.7,2A3,2A33)
C Set time origin to zero.
DAY=DAY-44089.993750D0
C Write out extracted data.
WRITE(2,12)I,DAY,XWOB,EXWOB,YWOB,EYWOB

12 FORMAT(1X,I4,F16.6,4F10.6)

10 CONTINUE

END

Tests reveal that there are 346 null values in the extracted file pp2009a.dat, leav-
ing 4097 pole positions and their standard errors. Of these, 537 pairs were found
to have identical time tags, or time tags di ering by less than one day. These

were averaged along with their time tags, leaving a total of 3560 pole position
co-ordinates and their standard errors in the file pp2009a.dat.
Traditionally, a left-handed co-ordinate system is used, with the x 2 co-ordinate
toward 90 ◦ W. Values are given in seconds of arc subtended at the geocentre. We
convert to a right-handed co-ordinate system with the x 1 co-ordinate toward Green-
wich and the x 2 co-ordinate toward 90 ◦ E. We also convert the pole co-ordinates to
centiseconds of arc subtended at the geocentre. Each pole position is represented by
the complex quantity x 1

ff

ix 2 . The time base is converted to years from the begin-
ning of the record by dividing by 365.25636, the number of mean solar days in one
year (Mueller, 1969). It is also useful to have the pole position dates in calendar
days. The conversion from Julian days to calendar days is accomplished by the sub-
routine CALDAT(JULIAN,MM,ID,IYYY) adapted from Press et al. (1992, p. 16).

+