Technical description of gsf2012a Intensive series UT1 solution
2012.July.09
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 gsf2012a 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 2012.07.02. 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 5614
sessions, of which 273 were 3- or 4-station sessions for which the
gsf2012a.eopi file contains 4 or more UT1 values - from the full network
and from each separate baseline.
Note: 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 a user program, which uses individual GPS and VLBI measurements.
4. Differences with respect to previous (gsf2011a) solution:
a. The site and velocity mod files are from the gsf2012a solution
and are based on ITRF2008 [1].
b. The source position mod file is also from the gsf2012a solution
and is based on ICRF2 [2].
c. Seismic offsets on 2008 June 14 at TSUKUB32 and KASHIM34 are
included. The dispalcements seen are ~14 mm at TSUKUB32 and ~27
mm at KASHIM34, as solved for in the gsf2012a 24-hr session
solution. They are accounted for in the Intensive solution via
the eccentricity file.
d. TSUKUB32's position for each session after the 2011 March 11
earthquake is obtained by a user program.
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/solutions/2012a/gsf2012a.src.
This catalog was created in the gsf2012a solution and is based on
ICRF2 [2]. The positions of the 295 ICRF2 defining sources were
replaced by their ICRF2 positions in gsf2012a.src.
b. Source positions adjusted in solution: No
7. Terrestrial reference frame:
a. A priori station positions: /500/solutions/2012a_int/2012a.sit.
This catalog was created in the gsf2012a solution and is based on
ITRF2008 [1].
b. A priori station velocities: /500/solutions/2012a_int/2012a.vel.
This catalog was created in the gsf2012a solution.
c. Reference epoch: 2008.0.
d. Station positions/velocities adjusted in solution: No.
j. Relativity scale: The terrestrial reference frame is defined using
the following metric tensor:
G_oo = -(1 - (2W/c^2 + W^2/c^4) + 2L_g )
G_oa = -4W^a/c^3
G_ab = \delta_ab (1 + 2W/c^2 - 2L_g) )
Specifically the old formula 29 in IERS Conventions 1992, page 127-136
was used, although it is known to have a deficiency.
THIS METRIC TENSOR DOES NOT CONFORM TO IAU 2000 RESOLUTIONS!
k. 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: IAU2000A Precession-Nutation, IERS
Conventions 2003 [3] implementation, modified to use the IAU2006
Precession model.
b. A priori short-period tidal variations in X, Y, UT1 were computed
in Calc 10.0, IERS Conventions 2003 [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 gsf2012a.eops series
over the period 2000.01.05 - 2012.06.29.
9. A priori geophysical models:
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 did not use the VMF model because the
necessary data is usually not available at the time
of processing of new Intensive sessions.]
b. Solid Earth tide: IERS Conventions 2003 [3], chapter 7, p. 9, steps
1 and 2, including tides of the 2-nd and 3-rd order.
c. Ocean loading: 3D ocean loading displacements computed by SPOTL
software. The model of displacements caused by ocean loading contains
18 constituents. The following ocean tide models were used:
Harmonic Frequency rad/sec Model
k2-a 1.324501D+00 1.458530140651D-04 GOT00 admittance
k2 3.506941D+00 1.458423171028D-04 GOT00
s2 6.283185D+00 1.454441043329D-04 GOT00
s2-a 4.312500D-02 1.452450074576D-04 GOT00 admittance
m2 2.169437D+00 1.405189027044D-04 GOT00
m2-a 1.210284D+00 1.405082057420D-04 GOT00 admittance
n2 6.097067D+00 1.378796996516D-04 GOT00
k1-a 1.141827D+00 7.293185551375D-05 GOT00 admittance
k1 3.324267D+00 7.292115855138D-05 GOT00
k1-b 2.365113D+00 7.291046158901D-05 GOT00 admittance
p1 2.958919D+00 7.252294578148D-05 GOT00
p1-a 3.002044D+00 7.232384890619D-05 GOT00 admittance
o1 5.128356D+00 6.759774415297D-05 GOT00
o1-a 1.027610D+00 6.758704719061D-05 GOT00 admittance
q1 2.772800D+00 6.495854110023D-05 GOT00
q1-a 4.955240D+00 6.494784413786D-05 GOT00 admittance
mtm-a 4.652212D+00 7.973314413516D-06 NAO99.l admittance
mtm 5.514660D-01 7.962617451151D-06 NAO99.l
mf-a 2.296657D+00 5.334111360775D-06 NAO99.l admittance
mf 4.479096D+00 5.323414398410D-06 NAO99.l
msf 9.721550D-01 4.925201628510D-06 NAO99.l
mm 5.497148D+00 2.639203052741D-06 NAO99.l
msm 4.899785D+00 2.285998575769D-06 NAO99.l
ssa 3.653480D-01 3.982127698995D-07 NAO99.l
paw 5.012885D+00 1.991063797295D-07 equilibrium
sa 3.098467D+00 1.990968752920D-07 NAO99.l
pcw 2.003605D+00 1.671771314171D-07 equilibrium
18.6 4.100746D+00 1.069696236521D-08 equilibrium
d. Pole tide: Mean pole coordinates used for computation of pole tide
deformation were set to the IERS 2003 Conventions [3] recommended
values (Chapter 7, p. 15).
e. 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].
f. 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: Manual, 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 10.0/10.01/10.02, SOLVE revision date 2011.11.15.
References:
1. Altimimi, Z., X. Collilieux, and L. Metivier, " ITRF2008: an improved
solution of the international terrestrial reference frame", Journal of
Geodesy, 2011, DOI 10.1007/s00190-011-0444-4.
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. McCarthy, D.D., Petit, G., IERS Technical Note 32, IERS Conventions
(2003), 2003.
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.