[IGSMAIL-1853] EOP improvements due to the ITRF96 change

[Jim]

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IGS Electronic Mail      Thu Apr  2 14:22:37 PST 1998      Message Number 1853
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Author: Jim Ray
Subject: EOP improvements due to the ITRF96 change

Early results indicate that the change in terrestrial frame realization
made by the IGS on 01 March has improved the IGS Rapid polar motion results
significantly, to about the level of the previous Final results.  In other
words, the IGS Rapid polar motion coordinates are now precise at roughly the
0.1 mas level (3 mm rotation on the Earth's surface) while the Finals are
even better and also improved due to the frame change.  It is difficult to
objectively evaluate the accuracy of these series because there is no other
independent series of comparable quality.

The IGS and its many collaborators are to be congratulated on achieving
this new level of performance.  The change, which was accepted at the IGS
Analysis Center Workshop in Darmstadt in February, replaces the previous
coordinates/velocities of 13 ITRF94 sites with a much more robust set of
47 ITRF96 sites; see IGS Mail #1838.  (Further changes are planned to
maintain and improve this realization in the future.)

This step involved the considerable efforts of many groups and individuals
including: the analysis centers for all space geodetic techniques in
providing global sets of station coordinates and velocities; the IERS
Terrestrial Reference Frame section at IGN (Paris) for evaluating and
combining the data sets to form ITRF96; the IGS Analysis Coordinator for
carefully testing the new coordinate sets and preparing detailed
recommendations for its use; and the IGS ACs for quickly and accurately
implementing the new strategy.  For maximum usefulness, the IGS results
(especially the Rapid orbit and Earth orientation parameters) depend
critically on the prompt and reliable delivery of observational data from
the tracking network via the Data Centers.

The new performance has important implications for geophysical studies as
well as practical applications.  Among other things, the IERS Bulletin A
Rapid Service and Prediction values for polar motion are clearly better as
a result of the improved, more reliable Rapid IGS results.

The effect of the frame change is illustrated in the table below which
compares IGS Rapid products to the Finals for 3 weeks before and after
the change.  It can be seen that the bias and RMS differences between
Rapid and Final polar motion determinations have been greatly reduced.
The Rapid orbit ephemerides have also improved.  (Some transition
problems did occur during the first 10 days of March which may obscure
the full measure of improvement that might be obtained.)

       IGS RAPID EOP, ORBIT, CLOCK COMPARED TO IGS FINAL PRODUCTS
     =============================================================
          |  Xpole  Ypole   Xrate  Yrate   |    ORBIT    |  CLOCK
          |          (BIAS / RMS)          |  RMS  WRMS  |   RMS
     WEEK |       (10**-5 arcseconds)      |   (meters)  |  (ns)
     -------------------------------------------------------------
     0944 |     18    -43     26      9    |  .11   .09  |   1.0
          |     31     32     43     50    |             |
          |                                |             |
     0945 |     -3    -30      6     -6    |  .14   .06  |    .4
          |     12     11     32     21    |             |
          |                                |             |
     0946 |     14    -31     12     -3    |  .06   .05  |    .3
          |     20     16     24     19    |             |

     ----- change from 13 ITRF94 to 47 ITRF96 fiducial sites -----

     0947 |     -3    -17    -10    -27    |  .05   .04  |    .5
          |     11     10      8     33    |             |
          |                                |             |
     0948 |      1     -8     -5     -6    |  .06   .05  |    .3
          |      9     19     23     23    |             |
          |                                |             |
     0949 |     -3     -3      7      3    |  .05   .04  |   1.2*
          |      7      5     12     40    |             |
     -------------------------------------------------------------
     * IGR clock drift changes occurred on days 4 and 5.

While the IGS realization of the ITRF has been the limiting error in the
Rapid polar motion determinations, and probably a significant error in the 
Finals, that is most likely no longer the case.  Instead, orbit modeling
errors probably now dominate the errors of both series.  (Orbit modeling
was and continues to be the primary error in length-of-day determinations.)
However, it is reassuring that the current errors are now so very small.

All components of the IGS deserve credit for this remarkable achievement.

--Jim Ray
  USNO/IERS Sub-bureau for Rapid Service & Predictions


[Mailed From: Jim Ray (USNO 202-762-1444) <jimr at Maia.usno.navy.mil>]