[IGSMAIL-1384] CODE Changes of June 30, 1996

Thu Jul 11 23:55:04 PDT 1996

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IGS Electronic Mail      Thu Jul 11 23:55:04 PDT 1996      Message Number 1384
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Author: Markus Rothacher, Tim Springer
Subject: CODE Changes of June 30, 1996

Dear Colleagues,

starting with day 182, 1996 (June 30, 1996, start of GPS week 860) the CODE 
Analysis Center of the IGS has changed its processing strategy in several ways.
The changes produced in the CODE results by some of these modifications
(e.g. the change from ITRF93 to ITRF94) could be obtained
by recomputing the daily solutions (transformations of the normal equations),
whereas the impact of other changes (e.g. the orbit model changes and subdaily
ERP model) cannot be assessed so easily and a reprocessing going back to the 
observations would be necessary.
Below we list the changes adopted and the effects they will have on the 
CODE products:

 1) The coordinate system has been changed from ITRF93 to ITRF94. According 
    to the IGS processing standards again 13 sites are fixed in the routine 
    analysis to define the reference frame. All the solutions since July 19, 
    1993 (day of year 200, 1993) were recomputed using the ITRF94 reference 
    frame to provide as long a time series of ERPs as possible to the IERS. 
    The comparison of these new solutions with the solutions referring to the
    ITRF93 shows

                    Offset (1993.0)          Drift (1993.0)

       X-pole      0.27 +- 0.01 mas       0.20 +- 0.003 mas/year
       Y-pole      0.68 +- 0.01 mas       0.25 +- 0.010 mas/year

    For the epoch 1996.5 the offsets between the two series are 0.97 mas and
    1.55 mas in the x- and y-component, respectively.
    Some more details are given below. 

 2) Several modifications concern the orbit modeling:
      - Change of the Love Number k2 from 0.285 to 0.030
      - Change from GEMT3 to the JGM3 earth potential field
      - Improved model for the solid earth tides (earth potential, see IERS
        Standards 1995, draft of chapter 6)
      - General relativity term introduced (see IERS Standards 1992, p.123)

    These changes have an influence mainly on the orbits and the UT1-UTC drift.
    The UT1-UTC drift in our series will change by about 0.010 to 
    0.015 msec/day starting June 30, 1996, in the sense that the
    difference between our UT1-UTC series and the IERS products will be 
    smaller.

 3) Subdaily pole model: the model by Ray for the correction of the subdaily
    pole and UT1-UTC variations has been adopted. It changes the daily
    ERP values by up to about 0.1 mas. The UT1-UTC drift is also slightly 
    changed. The ERP values we supply to the IERS and to the global data 
    centers do not contain the contribution of the subdaily model (i.e. the
    ERP values submitted are linear function of time over one day and do not
    vary with diurnal and semi-diurnal periods). 

 4) A new set of antenna phase center variations is in use starting
    June 30, 1996 (GPS week 860, day of year 181). Because the Dorne Margolin
    antennas -- making up most of the antennas in the global IGS network -- 
    were left unchanged the effect of this change on the ERP series is 
    expected to be marginal. First tests didn't show any significant 
    rotations or any significant scaling of the coordinate system. The sites
    equipped with Trimble antennas change by a few millimeters in
    horizontal position and about 1 cm in height (compared to the results
    with the phase center corrections we were using for almost 4 years).

The AC questionnaire will be updated according to the new strategy and sent
to the IGSCB as soon as possible.

We hope to serve you with this information and remain

                        with best regards,

                                                 Markus Rothacher
                                                 Tim Springer

                                                 CODE Analysis Center

PS: Below you find some additional information about the changes in our 
    products (orbits, coordinates and ERPs) based on the reprocessing of two 
    days with our OLD and NEW strategy (day 181 and 182 of the year 1996):

    CODE product changes due to ALL changes (not only ITRF93-94 switch)      
    ===================================================================      

                        DX    DY    DZ     RX    RY    RZ   SCALE   RMS      
                       (mm)  (mm)  (mm)  (mas) (mas) (mas)  (ppb)   (mm)     
    ORB 181 NEW-OLD      5     1     1   -1.57 -1.07 -0.89   0.2    14.8     
    ORB 182 NEW-OLD      7     1     1   -1.47 -1.01 -0.89   0.2    17.8     

    CRD 181 NEW-OLD     13     2     5   -1.4  -1.0  -0.7   -0.1     8.4     
    CRD 182 NEW-OLD     16    -0     6   -1.4  -0.9  -0.8    0.0     9.1     

    ERP 181 NEW-OLD                       1.26  1.62   -                     
    ERP 182 NEW-OLD                       0.95  1.49   -                     

--------------------------------------------------------------------------------
Dr. Markus Rothacher        Phone  : ++41-31-631-8591          
Astronomical Institute      Fax    : ++41-31-631-3869
University of Berne         E-mail : rothacher at aiub.unibe.ch 
Sidlerstrasse 5             AFTP   : ftp ubeclu.unibe.ch (130.92.6.11)
CH-3012 Bern                         cd aiub$ftp after login         
Switzerland                 WWW    : http://ubeclu.unibe.ch/aiub/            

[Mailed From: MARKUS ROTHACHER <ROTHACHER at aiub.unibe.ch>]