中图分类号：TU74 文献标识码：A 文章编号：
　　1.Introduction
　　ITRF2008 is the new realization of the International Terrestrial Reference System. The ITRF2008 uses as input data time series of station positions and Earth Orientation Parameters (EOPs) provided by the Technique Centers of the four space geodetic techniques (GPS, VLBI, SLR, DORIS). Based on completely reprocessed solutions of the four techniques, the ITRF2008 is expected to be an improved solution compared to ITF2005[1].
　　2. Input data and Computation strategy of ITRF2008
　　 Space geodesy solutions from 4 space geodesy techniques. The used time series of solutions of space geodesy are summarized in the following table, indicating for each one, the time span and the type of constraints
　　
　　
　　The strategy adopted for the ITRF2008 generation consists in the following steps, illustrated in Figure 1.:
　　①Apply minimum constraints equally to all loosely constrained solutions: this is the case of SLR solutions
　　②Apply No-Net-Translation and No-Net-Rotation condition to IVS solutions provided in the form of Normal Equation
　　③Use as they are minimally constrained solutions: this is the case of IGS and IDS weekly solutions
　　④Form per-technique combinations (TRF + EOP), by rigorously staking the time series, solving for station positions, velocities, EOPs and 7 transformation parameters for each weekly (daily in case of VLBI) solution w.r.t the per-technique cumulative solution.
　　⑤Identify and reject/de-weight outliers and properly handle discontinuities using piece-wise approach.
　　⑥Combine the per-technique cumulative solutions adding local ties in co-location sites.
　　This last step yields the final ITRF2008 solution comprising station positions, velocities and EOPs. Note that EOPs start in the early eighties with VLBI and SLR, while DORIS contribution starts from 1993 and GPS from 1997.0.
　　3. Datum Definition
　　Origin:
　　The ITRF2008 origin is defined in such a way that there are null translation parameters at epoch 2005.0 and null translation rates between the ITRF2008 and the ILRS SLR time series.
　　Scale:
　　The ITRF2008 scale is defined in such a way that there null scale and scale rate between ITRF2008 and the average of VLBI and SLR scales/rates.
　　Orientation:
　　The ITRF2008 orientation is defined in such a way that there are null rotation parameters at epoch 2005.0 and null rotation rates between the ITRF2008 and ITRF2005. These two conditions are applied over a core network .14 transformation parameters between ITRF2005 and ITRF2008 have been estimated using 171 stations listed in Table 2 and located at 131 sites shown on Figure 2.
　　
　　
　　Table 2: Transformation parameters at epoch 2005.0 and their rates from ITRF2008 to ITRF2005 (ITRF2005 minus ITRF2008)
　　4.ITRF2008 solutions and transformation between previous ITRFS
　　ITRF2008 solutions include Scale, Geocenter time series and solution files. The solution files are provided by in the format of tables and SINEX files of ITRF2008 coordinates and velocities. For example, GNSS station positions/velocities table (ssc format).
　　
　　
　　Now the GNSS station SHAO coordinate and velocity transformate between ITRF2005 and ITRF2008.
　　①Unify epoch
　　
　　②Transformation between ITRFS
　　
　　An example Form ITRF2008 to ITRF2005
　　
　　 =
　　The coordinates of ITRF2005 framework:a2,b2,c2
　　Coordinate difference：
　　 =
　　From the foregoing：The conversion of the two frameworks can achieve centimeter order of magnitude，shows a higher accuracy.
　　5. Conclusions
　　ITRF2008 is the new realization of International Terrestrial Reference System, is the international reference framework to achieve the optimal,transformation with the previous frame shows a more accurate.
　　References：
　　[1] 張西光，吕志平.ITRF2005的实现与改进[J]，测绘通报，2007年07期
　　[2] 朱文耀，熊福文，宋淑丽.ITRF2005简介和评析[J]，天文学进展，2008年01期
　　作者简介：赵玉江，男，1985年生人，汉族，山东潍坊高密市人，助理工程师，本科，2008年7月毕业于山东科技大学，2008年7月—2011年12月在山东省城乡建设勘察院从事测绘工作；2012年2月至今在济南润土工程设计有限公司工作。