New Coordinates for CORS Sites The New Multi-year CORS Solution Dr. Giovanni Sella Edited & Presented by David Newcomer, PE, PLS National Geodetic Survey Tallahassee, FL Coordinates and Velocities they move!
Preliminary CORS Multiyear soln. Plate fixed Current CORS Coord. and Vel. Mixed coordinates and velocities from: last reprocessing (1994-2002), and adjustments using IGS ref. sites Mixed HTDP vs. computed vel.; Vertical vel. = 0 mm/yr NGSs global frame was ITRF00 epoch 1997.0 projecting 13 yrs was unrealistic;
NAD 83(CORS96) epoch 2002.0 projecting 8 yrs. became a problem Metadata issues, discontinuities/offsets Significant software changes Absolute vs relative antenna phase center values Needed to revise CORS coordinates and velocities Solution: multiyear Multiyear effort began 6 years ago Began with a revision of PAGES and processing strategy
driven by weak NGS orbit contribution to IGS. IGS proposed re-processing all data to re-compute station coordinates and orbits from 1994-present Richard Snay provided resources and managed the effort of the multiyear effort till May 2010 Current Team: K. Choi, M. Cline, B. Dillinger, B. Dulaney, J. Griffiths, S. Hilla, B. Kass, J. Ray, J. Rohde, G. Sella, R. Snay, T. Soler, M. Schenewerk, and G. Ray Frame Definition and Nomenclature
ITRF frame (global) multi-techniquevlbi, slr, doris, gnss IGS frame (global) GNSS only NAD frame (plate fixed) related to ITRF Critical to pay attention to frametags and epoch dates and antenna calibration values
Tying CORS to Global Network (~1600 sites in recent weekly CORS+glbl SNX files) Global tracking network used for estimating - GPS satellite orbits (15 minute intervals) - Terrestrial framework Earth Orientation (EOPs)
Global station positions U.S. CORS tied to global framework via single baselines radiating from global stations - Minimizes frame distortions from local effects in dense regional
networks CORS IGS08 Velocity Field What is the new NAD 83 frame? Same datum, so no need for transformation between NAD 83 (CORS96) epoch 2002.0 and NAD 83(2011) epoch 2010.0 NAD 83 (2011) velocities should be used to move positions through time
So, first to assess differences, need to compare at same epoch date (2002.0) using NAD 83(2011) velocities to move new positions to 2002 Changes in Horizontal NAD 83 Positions NAD 83(2011) epoch 2002.0 NAD 83(CORS96) epoch 2002.0 approx. 2 cm error expected avg. horizontal shifts: E =-0.18 (1.86) cm N = 0.14 (1.07) cm
Changes in Vertical NAD 83 Positions NAD 83(2011) epoch 2002.0 NAD 83(CORS96) epoch 2002.0 avg. vertical shift: U = 0.71 cm (2.11) cm The NAD 83 datum is the same So what will be the shifts caused by changing reference epoch to 2010? Previous 2 slides show consequences of new realization:
Approx. 1-2 mm avg. horiz. shift Less than 1 cm avg. vert. shift BUT reference epoch will change from 2002.0 to 2010.0 Velocities are therefore critical Lets compare NAD 83(CORS96) positions @ 2002.0 to NAD 83(2011) positions @ 2010.0 Differences dominated by effects of crustal motion, i.e., NAD 83 velocities are non-zero
e.g. 2 mm/yr velocity after 8 years = 1.6 cm change in position Changes in Horizontal NAD 83 Positions Different Epochs NAD 83(2011)epoch2010.0NAD 83(CORS96) epoch 2002.0 Approx. 2 cm error expected @ 2005.0 (based on old solution) Overall avg. horizontal shifts: E =0.09 (5.84) cm N = 2.03 (5.98) cmE =0.09 (5.84) cm E =0.09 (5.84) cm N = 2.03 (5.98) cmN = 2.03 (5.98) cm - Combination of position and velocity differences - Due mostly to updated velocities (including up to 8 more years of data) Changes in Vertical NAD 83 Positions Different Epochs
NAD 83(2011) epoch2010.0 NAD 83(CORS96) epoch 2002.0 Overall avg. vertical shifts: U =-0.79 (2.09) cm Combination of position and velocity differences from additional data, tectonics Assuming vertical velocity 0.00 in NAD 83(CORS96) Key Changes Change from relative to absolute antenna phase center values (PCV) NGS global frame pos/vels are aligned to full global frame
(IGS08) Distinction between computed and modeled velocities must be maintained and emphasized to users Ability to robustly/consistently re-compute pos/vels More robust discontinuity identification and resolve metadata issues quickly (site operators are key) Change in pos/vel NAD 83(2011) change epoch from 2002.0 to 2010.0 QUESTIONS?
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