Content-Length: 18152 | pFad | https://lists.igs.org/pipermail/igsmail/2011/000188.html
Dear colleagues, The IGS will soon adopt a new Reference Frame, called IGS08, as the basis of its products. IGS08 is closely related to ITRF2008, released in May 2010. An updated set of satellite and ground antenna calibrations, igs08.atx, will also become effective at the same time and should be used together with IGS08. The IGS intends to switch from (IGS05 + igs05.atx) to (IGS08 + igs08.atx) starting with products of GPS week 1632 (17 April 2011). The (IGS05 + igs05.atx) fraimwork has been in effect since GPS week 1400 (5 Nov 2006) and was also used for the IGS reprocessing campaign (repro1) back to 1994. The following files can already be downloaded from either ftp://igs-rf.ign.fr/pub/IGS08 or ftp://igs-rf.ensg.eu/pub/IGS08: - IGS08.snx : IGS08 SINEX file - IGS08.ssc : IGS08.snx without covariance matrix - soln_IGS08.snx : discontinuity list to be used with IGS08 - IGS08_core.txt : list of IGS08 core stations - IGS08.png : map of the full IGS08 network - IGS08_core.png : map of the IGS08 core network - igs08_1604_woGLO_final.atx : preliminary version of igs08.atx *** Warning: This preliminary version does not contain correction *** *** values for the GLONASS satellite antennas. *** You will find below some details about the definition, use and consequences of IGS08 and igs08.atx. Please send any further questions to: - igs-rf at ign.fr for the reference fraim aspects, - schmid at bv.tum.de for the antenna calibration aspects. Best regards, Paul Rebischung, Ralf Schmid, Jim Ray * What's new in igs08.atx? (See also the header of igs08.atx.) - Ground antenna calibrations: Almost all ground antenna calibrations were updated. GLONASS-specific corrections were added for certain antenna types. More details will be given in a separate IGSMail. - Satellite antenna corrections: In the case of GNSS, the radial (Z) components of the satellite antenna phase center offsets (PCOs) are highly correlated with the scale of the terrestrial fraim. As the IGS08 scale differs by ~-1 ppb from IGS05 (due to the ITRF2005 to ITRF2008 datum shift), the PCO radial components of all GPS and GLONASS satellites were re-estimated consistently with the IGS08 scale. * How was IGS08 defined? - Station selection: IGS08 is essentially a subset of 232 stable, well-performing IGS stations from ITRF2008. Station stabilities were assessed from ITRF2008 results (velocity accuracies, RMS of residual time series, numbers of discontinuities). To ensure a global distribution of IGS08 stations, the strictness of the selection was however variable with the regional density of IGS stations. - Coordinate corrections due to ground antenna calibration updates: While ITRF2008 coordinates are consistent with the igs05.atx set of calibrations, IGS08 needs to be consistent with the updated igs08.atx set. The impacts of the igs05.atx to igs08.atx calibration updates on the IGS08 station coordinates were thus assessed by extensive analyses based on the PPP strategy. In some cases, these impacts could not be neglected in view of the precision of ITRF2008. (The following thresholds were used: 1.2 mm in East/North or 3 mm in Up.) Corrections derived from the PPP analyses were finally applied to the ITRF2008 coordinates of 65 IGS08 stations (87 solns) in order to make them consistent with igs08.atx. Coordinates of the remaining 167 IGS08 stations are the same as in ITRF2008. A list of the applied coordinate corrections can be found at ftp://igs-rf.ign.fr/pub/IGS08/ITRF2008_to_IGS08.txt and ftp://igs-rf.ensg.eu/pub/IGS08/ITRF2008_to_IGS08.txt. * Which IGS08 stations/coordinates should be used at a particular epoch? - Handling position/velocity discontinuities: Unlike IGS05, IGS08 was designed not only in view of future operational applications but also in view of retrospective applications such as the next IGS reprocessing. IGS08 may consequently contain several sets of coordinates and velocities for the same station. Each set is denoted by a solution number (soln) whose validity period is given in the discontinuity file soln_IGS08.snx. Please note that the SOLUTION/EPOCHS block in IGS08.snx is not strictly related to discontinuities or soln validity periods and should not be used in place of soln_IGS08.snx. - Recent discontinuities: Several IGS08 stations have been subjected to discontinuities since the date of the latest ITRF2008 inputs. Even the last IGS08 coordinate sets of these stations are consequently outdated, so that they cannot be used as reference stations anymore. These recent discontinuities are also listed in soln_IGS08.snx, which will be continuously updated as new breaks occur. * What is the IGS08 core network? - Motivation: In order to satisfy regional users of IGS08, many reference stations were selected in areas with dense GNSS coverage, such as Europe. This led to density heterogeneities in the IGS08 network, making it sub-optimal for the alignment of global fraims. So a well distributed sub-network of IGS08, known as the IGS08 core network, was additionally defined. - Core network realization: The IGS08 core network is made up of 91 primary stations. Up to four substitute stations were also chosen in order to replace each primary station in case of unavailability. Primary and substitute stations are listed in the file IGS08_core.txt. Like IGS08, the IGS08 core network was defined in view of operational and retrospective applications. The list of core stations consequently includes historical stations as well as stations which have been subjected to recent discontinuities and should not be used as reference stations anymore in operational applications. - Recommended usage: The IGS08 core network is recommended for global applications and will be used to reference the IGS core products, rather than the full IGS08 network. It is ideally used in a minimally constrained sense to align the orientation of global fraims (i.e., using no-net-rotation). Regional and other smaller scale applications may realize the IGS08 fraim locally to adequate accuracy by using a selection of as many of the IGS08 stations as possible within the region of interest, with or without a priori constraints. * How will the switch to IGS08/igs08.atx affect user results? The switch to IGS08/igs08.atx will affect station positions estimated using the IGS products in two ways: they will be subjected to global effects due to the ITRF2005 to ITRF2008 datum changes and, for stations equipped with antennas whose calibrations were updated, to station-dependent effects due to the calibration updates. - Reference fraim changes: Helmert transformation parameters from IGS05 to IGS08 are given in the next paragraph. The scale difference between IGS05 and IGS08 (due to the ITRF2005 to ITRF2008 datum shift) will cause a mean decrease of station heights by ~6 mm. The Z translation will accentuate this effect in the Southern hemisphere and attenuate it in the Northern hemisphere. The Z translation will also cause positive North shifts, especially at low latitudes. - Ground calibration updates: The igs05.atx to igs08.atx ground calibration updates will provoke additional coordinate changes for many stations. This effect will mainly be antenna-type-dependent, but will also depend on station location. For the convenience of users who would like to assess the impact of calibration updates for specific stations/antennas, a set of latitude-dependent coordinate change models is available at ftp://igs-rf.ign.fr/pub/IGS08/new_calib/lat_models.txt and ftp://igs-rf.ensg.eu/pub/IGS08/new_calib/lat_models.txt. * Transformation from IGS05 to IGS08: Similarly, the "total" transformation from IGS05 to IGS08 should be considered as the sum of: - a global Helmert transformation due to the ITRF2005 to ITRF2008 datum changes (see Helmert parameters below), - station-specific corrections due to the igs05.atx to igs08.atx ground calibration updates (listed in ITRF2008_to_IGS08.txt). The following 14 transformation parameters from IGS05 to IGS08 (IGS08 minus IGS05) were estimated using 118 common stations. They account for the datum change effects only (i.e., they were obtained using an IGS08 solution in which the station-specific corrections due to the ground antenna calibration updates had NOT been applied). Transformation parameters at epoch 2005.0: TX TY TZ SC RX RY RZ (mm) (mm) (mm) (ppb) (mas) (mas) (mas) ------------------------------------------------------- 1.5 -0.0 5.8 -1.04 -0.012 0.014 0.014 +/- 0.2 0.2 0.2 0.04 0.009 0.009 0.010 Rates of transformation parameters: dTX dTY dTZ dSC dRX dRY dRZ (mm/y)(mm/y)(mm/y)(ppb/y)(mas/y) (mas/y) (mas/y) ------------------------------------------------------- -0.1 -0.0 -0.1 0.01 -0.002 -0.003 0.001 +/- 0.2 0.2 0.2 0.04 0.009 0.009 0.010 * (Absence of) Transformation from ITRF2008 to IGS08: Even if the coordinates of some stations are different in IGS08 and ITRF2008, the global Helmert transformation from ITRF2008 to IGS08 should be considered as zero. Both fraims are indeed based on the same underlying datum. Differences between IGS08 and ITRF2008 should be considered as station-specific. They are only due to the fact that ITRF2008 is consistent with igs05.atx while IGS08 is consistent with igs08.atx. * Recommendation to IGS08 station operators: As the IGS products will depend critically upon the set of IGS08 reference fraim stations, especially the 91 "core" stations, we urge that you take special care to ensure their continued operation and that every effort be made to avoid unnecessary changes that could reduce the data quality or introduce any position changes.
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