Technical description of sinex solution gsf2016a 2016.09.26 1. Purpose of solution: TRF/CRF and EOP series. 2. Analysis center: GSF (NASA Goddard Space Flight Center). 3. Short narrative description of solution: Solution gsf2016a_snx is the daily Sinex file version of quarterly update solution gsf2016a. All regular X/S dual-band Mark-3/Mark-4/Mark-5/K3/K4/VLBA VLBI observations from 1979.08.03 through 2016.08.22, with durations of 18 hours or longer (5938 sessions, 10,807,922 group delays) were used in the 2016a solution. The series will be updated with new sessions using the same control file. The positions of all stations were estimated as local parameters. Station velocities are not estimated. A priori site positions and velocities are from the ITRF2014 [1] catalog. Source positions are estimated for each session and included in the sinex files. The a priori source positions are from the gsf2016a_astro solution, with the positions of the 295 ICRF2 [2] defining sources replaced with their official ICRF2 positions. In the gsf2016a_astro solution, most of the source positions were estimated globally using a no-net-rotation constraint on the 295 ICRF2 defining sources. However, the positions of the 39 most unstable sources (the 'special handling' sources of ICRF2) were estimated in each session individually. Also, all sources which did not have at least 3 good observations in at least one session were excluded from the solution. Three gravitational lenses (0218+357/0218+35A/0218+35B, 1830-21A/1830-21B/1830-211, and 1422+231) were also excluded from the dataset. Mean site gradients were applied. These were computed from the GSFC Data Assimilation Office (DAO) model from met data from 1990-95. Residual East and North gradients were then estimated at 6 hr intervals. Refer to references [3] and [4] for more details. 4. Estimated parameters: a. Source positions: Most a priori source positions are from the gsf2016a_astro solution, with the positions of the 295 ICRF2 defining sources replaced with their ICRF2 positions. Source positions are estimated for each session and included in the Sinex files. Sources with fewer than 3 good observations in the global solution version and 3 gravitational lenses were excluded from the single session Sinex solutions. b. Station positions: X,Y,Z station positions for each session. c. Earth Orientation: X-pole, Y-pole, UT1-TAI, Xdot, Ydot, UT1dot, X-nutation and Y-nutation for each session. The IAU2000/2006 Precession/Nutation model was applied. d. Zenith Troposphere: Linear spline 20-min interval; rate constraint with reciprocal weights generally 50 ps/hr; VMF wet partial derivative (segmented). e. Troposphere Gradient: 6-hour East and North linear splines at all stations using offset and rate constraints with reciprocal weights of 0.5 mm and 2.0 mm/day (segmented). f. Station Clocks: Quadratic + linear spline with 1-hr interval (segmented); rate constraint with reciprocal weights, generally 5.0E-14. g. Baseline Clocks: As set in initial analysis usually used. 5. A priori Earth orientation: a. A priori precession/nutation model: IAU2006/2000A Precession/Nutation, IERS 2010 [5] implementation in Calc 11. b. A priori short-period in X-pole, Y-pole and UT1 due to short period tidal and libration effects were applied. These were computed by Calc 11, as recommended in the IERS 2010 Conventions [5], chapter 5. 6. A priori geophysical models: a. Troposphere: VMF1 total (dry+wet) mapping function; Saastamoinen zenith delay calculated using logged pressure and temperature; a priori mean gradients from DAO weather model. b. Solid Earth tide: IERS Conventions 2010 [5], chapter 7, steps 1 and 2, including tides of the 2-nd and 3-rd order. c. Ocean loading: 3D ocean loading displacements computed using the Hardisp model. Ocean loading coefficients computed using the TPXO7.2 model. These were obtained from the ocean loading web site at http://holt.oso.chalmers.se/loading/, which is provided by H.-G. Scherneck, Onsala Space Observatory. d. Pole tide: Mean pole coordinates used for computation of pole tide deformation were set to the IERS 2010 Conventions [5] recommended values (Chapter 7, p. 114-116). e. Ocean pole tide loading: An ocean pole tide loading correction was applied, using the model of Desai 2002 [6]. These were computed by Calc 11, and are described in the IERS Conventions (2010), chapter 7, pages 116-118. This small correction should have an approximately 14 month period. f. Antenna thermal deformation: Antenna heights were adjusted, based on the average daily temperatures, using the IVS antenna thermal deformation model of Nothnagel 2008 [7]. g. Axis offsets: Taken from 2016a.axof, from the 2016a quarterly. 7. Data type: Group delays only. 8. Data editing: 5 deg elevation cutoff. 9. Data weighting: Weights are defined as follows: 1/sqrt ( f**2 + a**2 ). Quantity "f" is the formal uncertainty of the ionosphere free linear combination of group delays at X- and S-band obtained by fringe fitting. The station-dependent parameter "a" was computed for each session by an iterative procedure such that the ratio of the sum of the squares of the weighted residuals to the estimate of their mathematical expectation is about 1.0. 10. Software: Calc 11, SOLVE revision date 2016.09.07. References: 1. Altamimi, Z., P. Rebischung, L. Mtivier, and X. Collilieux, "ITRF2014: A new release of the International Terrestrial Reference Frame modeling nonlinear station motions, J. Geophys. Res. Solid Earth, vol. 121, Issue #8, pp. 6109-6131, 2016. doi:10.1002/2016JB013098. 2. IERS Technical Note 35, 'The Second Realization of the International Celestial Reference Frame by Very Long Baseline Interferometry'; A.L. Fey, D. Gordon, C.S. Jacobs, editors; 2009. http://www.iers.org/IERS/EN/Publications/TechnicalNotes/tn35.html 3. MacMillan, D.S. and C. Ma, "Atmospheric gradients from very long baseline interferometry observations", Geophys. Res. Lett., 22, 1041-1044, 1995. 4. MacMillan, D.S. and C. Ma, "Atmospheric gradients and the VLBI terrestrial and celestial reference frames", Geophys. Res. Lett., 24, 453-456, 1997. 5. Petit, Gerard and Luzum, Brian, 'IERS Conventions (2010), IERS Technical Note 36, 2010. 6. Desai, S.D., "Observing the pole tide with satellite altimetry," J. Geophysical Research, vol. 107 (C11), p.3186. doi:10.1029/2002JC001224. 7. Nothnagel, A., "Short Note: Conventions on Thermal Expansion Modelling of Radio Telescopes for Geodetic and Astrometric VLBI," Journal of Geodesy, DOI: 10.1007/s00190-008-0284-z, 2008.