Technical description of gsf2016a Intensive series UT1 solution
2016-Oct-04
1. Purpose of solution: Determination of UT1 from short, ~1.0 hour,
NEOS Intensive VLBI sessions.
2. Analysis center: GSF (NASA Goddard Space Flight Center).
3. Short narrative description of solution:
Intensive solution gsf2016a is for the estimation of UT1 from the 1-hour
NEOS and K4 Intensive sessions. Most sessions consist of ~12-30
observations on one baseline. The solution includes all usable Intensive
sessions from 1991.10.01 through 2016.10.03. The primary single-baseline
session networks have been: WESTFORD/WETTZELL, NRAO85-3/WETTZELL,
NRAO20/WETTZELL, KOKEE/WETTZELL, and TSUKUB32/WETTZELL. Beginning in 2005,
some sessions contain 3 stations, primarily NYALES20/TSUKUB32/WETTZELL
and KOKEE/SVETLOE/WETTZELL and a handful contain 4 stations. Each session
was processed independently. The initial solution used a total of 7181
sessions, of which several hundred were 3- or 4-station sessions for which
the gsf2016a.eopi file contains 4 or more UT1 values - from the full
network and from each separate baseline. With multiple baselines from
3 and 4 station sessions, there are 8802 estimates of UT1.
Japan experienced a large earthquake off its coast on 11-March-2011.
Many Japanese VLBI stations underwent large episodic motions from the
earthquake, and have not yet returned to their pre-earthquake tectonic
motions. The position used for TSUKUB32 after the earthquake is provided
by the post-seismic models of ITRF2014.
4. Differences with respect to previous (gsf2014a) solution:
a. The site mod file comes from the gsf2016 solution but has the episodic
offsets from ITRF2014 inserted. The velocity mod file is from the
gsf2016a solution. The gsf2016a solution was based on ITRF2014 [1].
b. The source position mod file is from the gsf2016a_astro solution
and is based on ICRF2 [2].
5. Estimated parameters:
a. UT1 angle.
b. Atmosphere path delay offset at each station.
c. Coefficients of the second order polynomial of clock functions.
6. Celestial reference frame:
a. A priori source positions: /500/oper/solve_save_files/2016a.src
This catalog was created for the gsf2016a_astro solution and is based
on ICRF2 [2]. The positions of the 295 ICRF2 defining sources were
replaced by their ICRF2 positions.
b. Source positions adjusted in solution: No
7. Terrestrial reference frame:
a. A priori station positions: /500/oper/solve_save_files/2016a_int.sit.
This catalog was created from the gsf2016a solution and has offsets
at TSUKUB32 due to earthquakes in 2008 and 2011 from ITRF2014. The
post-seismic model from ITRF2014 [1] is also applied to site positions.
b. A priori station velocities: /500/oper/solve_save_files/2016a.vel.
This catalog was created in the gsf2016a solution and is based on
ITRF2014 [1].
c. Reference epoch: Not applicable.
d. Station positions/velocities adjusted in solution: No.
e. Permanent tide correction: Yes.
"Yes" means that both the permanent and the periodic tides have
been modeled, so that the output station position is for after the
removal of both the permanent and the periodic tidal effect.
The model used includes tide displacements for zero frequency with
Love numbers h2(freq=0) = 0.6078, l2(freq=0) = 0.0847
8. Earth orientation:
a. A priori Precession-Nutation: IAU2006/2000A Precession/Nutation, IERS
Conventions 2010 [3] implementation in Calc 11.
b. A priori short-period tidal variations in X-pole, Y-pole, and UT1
due to tidal and libration effects were computed in Calc 11,
IERS Conventions 2010 [3] implementation.
c. EOP estimation: UT1 (without any constraints).
d. A priori UT1 and polar motion: usno_finals.data
http://gemini.gsfc.nasa.gov/solve_save/usno_finals.erp obtained
by adding small linear shift and drift to the left columns of
original finals.data file generated by USNO,
ftp://maia.usno.navy.mil/ser7/finals.data in such a manner that there
is no shift or secular drift with respect to the gsf2016a.eops series
over the period 2000.01.05 - 2016.09.07.
9. A priori geophysical models and other corrections:
a. Troposphere: NMF total mapping function; Saastamoinen zenith delay
calculated using logged pressure and temperature; a priori mean
gradients from VLBI data or DAO weather model. [Note: We do not use
the VMF1 model because the necessary data is usually not available
at the time of processing of new Intensive sessions.]
b. Solid Earth tide: IERS Conventions 2010 [3], chapter 7, steps 1 and 2,
including tides of the 2-nd and 3-rd order.
c. Ocean loading: The Hardisp model in Calc11 is applied. 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 [3] recommended
values (Chapter 7).
e. Ocean pole tide loading: An ocean pole tide loading correction was
applied, using the model of Desai 2002 [8]. 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 ~14 month period.
f. Mean site gradients were computed from the GSFC Data Assimilation
Office (DAO) model for met data from 1990-95. The atmospheric gradient
delay is modeled as:
tau = m_grad(el) * [GN*cos(az)+GE*sin(az)],
where el and az are the elevation and azimuth of the observation and
the gradient mapping function is m_grad. The gradient vector has East
and North components GE and GN. Refer to references [4] and [5].
g. Antenna thermal deformation: Antenna heights were adjusted, based
on the average daily temperatures, using the IVS antenna thermal
deformation model of Nothnagel 2008 [6].
10. Data type: Group delays only.
11. Data editing: Elevation cutoff 5 degrees.
12. Data weighting. Weights are defined as follows: 1/sqrt ( f**2 + a**2 )
where "f" is formal uncertainty of the ionosphere free linear combination
of group delays at X- and S-band obtained by fringe fitting on the base
of achieved signal to noise ratio. Session-dependent parameter "a" was
computed for each session by an iterative procedure such that the ratio
of the sum of squares of weighted residuals to the estimate of their
mathematical expectation is about unity.
13. Standard errors reported: All errors derived from least-squares
estimation propagated from the data weights and the constraints applied
to the troposphere, clock and EOP parameters.
14. Software: CALC 11, SOLVE revision date 2016.10.03.
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. Petit, Gerard and Luzum, Brian, 'IERS Conventions (2010), IERS Technical
Note 36, 2010.
4. MacMillan, D.S. and C. Ma, "Atmospheric Gradients from Very Long Baseline
Interferometry Observations", Geophys. Res. Lett., 22, 1041-1044, 1995.
5. MacMillan, D.S. and C. Ma, "Atmospheric Gradients and the VLBI Terrestrial
and Celestial Reference Frames", Geophys. Res. Lett., 24, 453-456, 1997.
6. 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.