Subject: [IGSMAIL-0657] CODE AC Questionnaire
From: M. Rothacher
Date: Mon, 4 Jul 1994 3:45:00 PDT

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IGS Electronic Mail      Mon Jul  4  3:45:00 PDT 1994      Message Number 0657
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Author: M. Rothacher
Subject: CODE AC Questionnaire

Subject: CODE AC Questionnaire

             INTERNATIONAL GPS SERVICE FOR GEODYNAMICS

        	     Analysis Centre Questionnaire

-----------------------------------------------------------------------------
|                           GENERAL INFORMATION                             |
|---------------------------------------------------------------------------|
| Analysis Centre  | CODE (Center for Orbit Determination in Europe)        | 
|                  | Astronomical Institute, University of Berne,           |
|                  | Sidlerstrasse 5, CH - 3012 Berne, Switzerland          |
|                  | Phone: ++ 41 31 631 85 91 , Fax: ++ 41 31 631 38 69    |
|---------------------------------------------------------------------------|
| Contact Person(s)| Markus Rothacher  e-mail: [email-masked]      |
|                  |                   phone : ++ 41 31 631 85 92           |
|                  | Robert Weber      e-mail: [email-masked]       |
|                  |                   phone : ++ 41 31 631 85 92           |
|---------------------------------------------------------------------------| 
| Software Used    | Bernese GPS Software Version 3.5, developed at AIUB    |
|---------------------------------------------------------------------------|
| IGS Products     | CODwwwwn.EPH  GPS ephemeris/clock files in 7 daily     |
| generated for    |               files at 15 min intervals in SP3 format, |
| GPS week 'wwww'  |               including accuracy codes computed from   |
| day of week 'n'  |               the formal orbit errors of the solution  |
| (n=0,1,...,6)    | CODwwww7.ERP  ERP (pole, UT1-UTC) solutions for 1 week |
|                  | CODwwww7.SUM  Processing summary for 1 week            |
|                  |                                                        |
|                  | The files above contain the estimates for the middle   |
|                  | day of the 3-days solution                             |
|---------------------------------------------------------------------------|
| Preparation Date | June 30, 1994                                          |
-----------------------------------------------------------------------------
 
-----------------------------------------------------------------------------
|                           MEASUREMENT MODELS                              |
|---------------------------------------------------------------------------|
| Preprocessing    | Phase preprocessing in a baseline by baseline mode     |
|                  | using triple-differences. In most cases cycle slips    |
|                  | are fixed looking simultaneously at different linear   | 
|                  | combinations of L1 and L2. If a cycle slip cannot be   | 
|                  | fixed reliably, bad data points are removed or new     | 
|                  | ambiguities are set up.                                | 
|---------------------------------------------------------------------------| 
| Basic Observable | Carrier phase, code only used for receiver clock sync. |
|                  |--------------------------------------------------------|
|                  | Elevation angle cutoff :  20 degrees                   |
|                  | Sampling rate          :   2 minutes for 1-day solut.  |
|                  | Sampling rate          :   3 minutes for 3-days solut. |
|                  | Weighting              : 1.2 cm for double-diff. iono- |
|                  |                          sphere-free LC. Satellites    |
|                  |                          may be deweighted             |
|---------------------------------------------------------------------------| 
| Modelled         | Double-differences, ionosphere-free linear combination |
| observable       |                                                        |
|---------------------------------------------------------------------------| 
| RHC phase        | Phase polarisation effects applied                     |
| rotation corr.   |                                                        |
|---------------------------------------------------------------------------|
| Ground antenna   | Elevation-dependent phase center corrections are       |
| phase centre     | applied to model the differences between different     |
| calibrations     | antenna types (e.g. Rogue and Trimble)                 |
|---------------------------------------------------------------------------|
| Troposphere      | A priori model: Saastamoinen (standard atmosp. used) |
|                  |--------------------------------------------------------| 
|                  | Estimation      : zenith delays in 2 hour resp. 6 hour |
|                  |                   intervals for 1-day resp. 3-days sol.|
|                  | Mapping function: 1/cos(z), z:zenith angle             |
|---------------------------------------------------------------------------| 
| Ionosphere       | Not modelled (ionosphere eliminated by forming the     |
|                  | ionosphere-free linear combination of L1 and L2)       |
|---------------------------------------------------------------------------| 
| Plate motions    | ITRF92 station velocities fixed (see IGSMAIL#421)      |
|---------------------------------------------------------------------------| 
| Tidal            | Solid earth tidal displacement: IERS Standards 1992    | 
| displacements    |--------------------------------------------------------| 
|                  | Pole tide: not applied                                 |
|                  |--------------------------------------------------------| 
|                  | Ocean loading: not applied                             |
|---------------------------------------------------------------------------|
| Atmospheric load.| Not applied                                            |
|---------------------------------------------------------------------------| 
| Satellite center | Block I      x,y,z:  0.2100, 0.0000, 0.8540 m          |
| of mass          |--------------------------------------------------------|
| correction       | Block II/IIA x,y,z:  0.2794, 0.0000, 1.0259 m          |
|---------------------------------------------------------------------------| 
| Satellite phase  | Not applied                                            |
| centre calibrat. |                                                        |
-----------------------------------------------------------------------------

----------------------------------------------------------------------------- 
|                             ORBIT MODELS                                  |
|---------------------------------------------------------------------------| 
| Geopotential     | GEMT3 + C21+S21 model up to degree and order 8         |
|                  |--------------------------------------------------------| 
|                  | GM = 398600.4415 km**3/sec**2                          |
|                  |--------------------------------------------------------| 
|                  | AE =   6378.137  km                                    |
|---------------------------------------------------------------------------| 
| Third-body       | Sun and Moon as point masses                           |
|                  |--------------------------------------------------------| 
|                  | Ephemeris: JPL DE200                                   |
|                  |--------------------------------------------------------| 
|                  | GMsun  = 132712500000 km**3/sec**2                     |
|                  |--------------------------------------------------------| 
|                  | GMmoon =    4902.7890 km**3/sec**2                     |
|---------------------------------------------------------------------------| 
| Solar radiation  | Direct radiation: ROCK4 and ROCK42 approximations (T10 |
| pressure         |                   and T20) for Block I and II satell.  |
|                  |                                                        |
|                  | Satellite masses used:                                 |
|                  |  PRN 02    878.2 kg    PRN 16-19 883.2 kg              |
|                  |      12    519.8           20    887.4                 |
|                  |      14    887.4       PRN 21    883.9 kg              |
|                  |      15    885.9           23    972.9                 |
|                  |  all other satellites            975.0 kg              |
|                  |--------------------------------------------------------| 
|                  | One scale factor and the y-bias estimated per arc      |
|                  |--------------------------------------------------------| 
|                  | Earth shadow model includes: cylindric shadow          |
|                  |--------------------------------------------------------| 
|                  | Reflection radiation: not included                     |
|                  |--------------------------------------------------------|
|                  | New GPS satellite attitude model: not applied          |
|---------------------------------------------------------------------------| 
| Tidal forces     | Solid earth tides: frequency independent Love's        |
|                  | number K2= 0.285                                       |
|                  |--------------------------------------------------------| 
|                  | Ocean tides: not applied                               |
|---------------------------------------------------------------------------| 
| Relativity       | Not applied                                            |
|---------------------------------------------------------------------------| 
| Numerical        | Integration alghorithms developed at AIUB by Gerhard   | 
| Integration      | Beutler (see references below). Representation of the  |
|                  | the orbit by a polynomial of degree 10 for 1 hour      |
|                  |--------------------------------------------------------|
|                  | Integration step: 1 hour                               |
|                  |--------------------------------------------------------| 
|                  | Starter procedure: no special starter procedure needed |
|                  |--------------------------------------------------------| 
|                  | Arc length: 72 hours                                   |
----------------------------------------------------------------------------- 

----------------------------------------------------------------------------- 
|               ESTIMATED PARAMETERS (APRIORI VALUES & SIGMAS)              |
|---------------------------------------------------------------------------| 
| Adjustment       | Weighted least-squares algorithms                      |
|---------------------------------------------------------------------------| 
| Rejection Criter.| No rejection during parameter estimation procedure.    |
|                  | Outliers are marked during pre-processing step         |
|---------------------------------------------------------------------------|
| Station          | 13 stations heavily constrained (0.1 mm) to the ITRF92 |
| coordinates      | positions as given in IGSMAIL#430, the remaining       |
|                  | stations estimated. The ITRF92 velocities are used for |
|                  | monthly coordinates updates.                           |
|                  | A priori station constraints are removed to compute    |
|                  | multi-day coordinate solutions of all sites.           |
|---------------------------------------------------------------------------| 
| Orbital          | 6 Keplerian elements at start of arc; solar radiaton   |
| parameters       | scale factor and y-bias estimated as constants for one | 
|                  | arc. No a priori sigmas used.                          |
|                  | A priori orbits predicted from the 3-days solution of  |
|                  | the previous day.                                      |
|                  | Pseudo-stochastic orbit parameters (small velocity     |
|                  | changes, constrained to 1.E-6 m/sec) are estimated     |
|                  | once per revolution for the eclipsing satellites.      |
|---------------------------------------------------------------------------| 
| Troposphere      | Zenith delays estimated once per 6 hours for each      |
|                  | station. Corrections to a priori model constrained to  |
|                  | 10 cm.                                                 |
|---------------------------------------------------------------------------| 
| Ionospheric      | Not estimated                                          |
| correction       |                                                        |
|---------------------------------------------------------------------------| 
| Ambiguity        | Estimated as real values with no a priori constraints  |
|---------------------------------------------------------------------------| 
| ERP              | X- and Y-pole coordinates, and UT1-UTC estimated as    |
|                  | a polynomial of degree 1. UT1-UTC fixed to the a       |
|                  | priori values at the beginning of the first day.       |
|                  | No a priori sigmas used. A priori values taken from    |
|		   | IERS Bulletin A.                                       |
|---------------------------------------------------------------------------| 
| Satellite clock  | Satellite clock biases are not estimated but elimi-    |
| bias             | nated by forming double-differences.                   |
|---------------------------------------------------------------------------| 
| Receiver clock   | Receiver clock corrections are estimated during the    |
| bias             | pre-processing using code measurements.                |
|---------------------------------------------------------------------------|
| Other            | 1) The difference between the Block I and Block II     |
| parameters       |    satellite antenna phase center offsets are estima-  |
|                  |    ted for each 3-days solution. The a priori con-     |
|                  |    straints in a satellite-fixed frame are 1.E-6,      |
|                  |    1.E-6, and 2.0 meters for the x-, y-, and z-        | 
|                  |    component respectively.                             | 
|                  | 2) Center of mass coordinates are estimated for each   |
|                  |    3-days solution but with heavy constraints of       |
|                  |    1.E-6 meters for each coordinate.                   |
|                  | 3) Drifts in the corrections "Deps" and "Dpsi" to the  |
|                  |    IAU 1980 nutation model. Heavily constrained for    |
|                  |    the computation of the IGS products.                |
|                  | The saved normal equations are later used for multi-   |
|                  | day solutions of these parameters.                     |
----------------------------------------------------------------------------- 

----------------------------------------------------------------------------- 
|                            REFERENCE FRAMES                               |
|---------------------------------------------------------------------------| 
| Inertial         | Geocentric; mean equator and equinox of 2000 Jan 1     |
|                  | at 12:00 (J2000.0)                                     |
|---------------------------------------------------------------------------| 
| Terrestrial      | ITRF92 reference frame realized through the set of 13  |
|                  | station coordinates and velocities as given in IGSMAIL#|
|                  | 430 as well as the antenna offsets for the above       |
|                  | stations given in /igscb/station/tie/localtie.tab      |
|                  | available from IGS CB (sideshow.jpl.nasa.gov)          |
|---------------------------------------------------------------------------| 
| Interconnection  | Precession: IAU 1976 Precession Theory                 | 
|                  |--------------------------------------------------------|
|                  | Nutation: IAU 1980 Nutation Theory                     |
|                  |--------------------------------------------------------| 
|                  | Relationship between UT1 and GMST: USNO Circular       |
|                  | No. 163 (IAU Resolution)                               |
|                  |--------------------------------------------------------| 
|                  | ERP interpolated from IERS Bulletin A (updated every   |
|                  | week). No celestial pole corrections.                  |
|                  |--------------------------------------------------------| 
|                  | Tidal variations in UT1: periods <= 35 d applied       |
----------------------------------------------------------------------------- 

REFERENCES:

  Beutler, G. (1990). Numerische Integration gewoehnlicher Differential-
    gleichungssysteme: Prinzipien und Algoithmen. Mitteilungen der 
    Satelliten-Beobachtungsstation Zimmerwald, No. 23, Druckerei der 
    Universitaet Bern, 1990.

  Fliegel, H., T. Gallini and E. Swift (1992). Global Positioning System 
    radiation force model for geodetic applications. J.Geophys.Res. 97(B1), 
    pp. 559-568, January 1992.

  McCarthy, D.D. (ed.) (1992). IERS Standards (1992). IERS Technical Note 
    13, Observatoire de Paris, July 1992.

  Rothacher, M., G. Beutler, W. Gurtner, E. Brockmann, L. Mervart (1993).
    Bernese GPS Software Version 3.4, Documentation May 1993, Druckerei
    der Universitaet Bern, May 1993.

  Wu, J.T., S.C. Wu, G.A. Hajj, W.I. Bertiger, S.M. Lichten (1993). 
    Effects of antenna orientation on GPS carrier phase. Manuscripta 
    Geodaetica 18, 1993, pp. 91-98.

[Mailed From: MARKUS ROTHACHER <[email-masked]>]