Technical description of solution usn2018a
2018 March
1. Purpose of solution: TRF/CRF for session EOP
2. Analysis center: USN ( United States Naval Observatory )
3. Short narrative description of solution:
Solution usn2018a estimates station position and velocity
parameters to define the TRF/CRF for computing EOP time
series. Source positions are also estimated. The TRF is attached
to ITRF2014 by imposing no-net-rotation and no-net-translation
conditions for the positions of a subset of stations and,
similarly, no-net-rotation and no-net-translation conditions for
the velocities of a subset of stations. The CRF is attached to
the ICRF by a no-net-rotation condition using the 295 ICRF2
defining sources [1].
Parameters are split into three groups:
a) global - parameters estimated over all sessions;
b) local - parameters estimated for each 24-hour session;
c) segmented - parameters estimated over 60 minute time spans.
Mean site gradients were computed from a GSFC Data Assimilation
Office (DAO) model for met data for ICRF2 [1] (gsfc_dao_gmao.mgr).
Atmospheric gradient delay is modeled as
tau = m_grad(el,az) * [GN*cos(az)+GE*sin(az)],
where el and az are the elevation and azimuth of the the
observation and the gradient mapping function is m_grad. The
gradient vector has east and north components GE and GN. Refer to
[2], [3].
3a. Differences with respect to previous solution:
3b. Handling of earthquakes:
TICOCONC was affected by a large nearby earthquake in Chile on Feb. 27,
2010, and experienced an episodic offset of more than 3 meters, mostly in
the westward direction. An episodic break cannot model the motion properly.
Therefore, we have continued to use a user partial program to estimate
offsets at TIGOCONC for each session after the earthquake.
A large earthquake also ocurred of the coast of Japan on March 11, 2011.
The motions of several Japanese stations have been non-linear since then,
and so the same user partial program was also used to estimate the
positions of TIGOCONC, TSUKUB32, KASHIM34, KASHIM11, VERAMZSW, SINTOTU3,
USUAD64, and KOGANEI for each session after the March 11, 2011 earthquake.
Several episodic breaks are solved for.
4. Estimated parameters:
a. celestial frame: right ascension, declination (global and
local)
b. terrestrial frame: X, Y, Z, Xdot, Ydot, Zdot (global)
c. Earth Orientation: X-pole, Y-pole, UT1-TAI, Xdot, Ydot, UT1dot,
X-nutation, Y-nutation (local parameters).
d. zenith troposphere: continuous piece-wise linear; 20 min
interval; rate constraint generally 50 ps/hr; NMF wet partial
derivative (segmented)
e. troposphere gradient: 6 hour east and north piece-wise
continuous at all stations except a set of 110 stations;
offset constraint 0.5 mm, rate constraint 2.0 mm/day
(segmented)
f. station clocks: quadratic + continuous piece-wise linear with 60
min interval; rate constraint generally 5.0E-14 (segmented)
g. baseline clocks: set in initial analysis - usually used (local)
h. other: global antenna axis offsets
5. Celestial reference frame:
a. a priori source positions: ICRF2
b. source positions adjusted in solution: yes
If yes,
c. definition of orientation: no-net-rotation tie to the ICRF2
using only ICRF2 defining sources
6. Terrestrial reference frame:
a. a priori station positions: ITRF2014
b. a priori station velocities: ITRF2014
c. reference epoch for site positions: 2014.0
d. station positions/velocities adjusted in solution: yes
If yes,
e. definition of origin, orientation, and their time evolution:
no-net-translation and no-net-rotation of position with respect to
ITRF2014.
no-net-translation and no-net-rotation of velocity with respect to
ITRF2014.
f. station parameter estimation: X, Y, Z, Xdot, Ydot, Zdot
globally for all stations, some with constraints
g. stations with constraints:
A priori velocity of U, E, and N components for
selected stations were constrained to the ITRF2014
velocities with reciprocal weights 0.1, 3.0, and 3.0 mm/yr
respectively because the stations have too short history of
observations, in many cases only one occupation.
The velocities of colocated stations
were constrained to be the same.
j. relativity scale:
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.
k. permanent tide correction: yes
"Yes" means that both the permanent and the periodic tides have
been included in the model, 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
7. Earth orientation:
a. A priori precession/nutation model: IAU2006/2000A Precession/Nutation,
IERS Conventions 2010 [6] as implementated in Calc 11.
b. A priori short-period tidal variations in X, Y, UT1 due to short
period tidal and nutation effects were applied. These were computed by
Calc 11, as recommended in the IERS 2010 Conventions [6], chapter 5,
p. 50-51.
c. EOP estimation: Two tables are given:
usn2018a.eoxy: X, Y, UT1, Xdot, Ydot, UT1dot, X-nutation, Y-nutation,
each session. Using a priori error of 45 mas for pole and 3 ms for UT1,
45 mas/day and 3 ms/day for pole rate and UT1 rate to allow estimation
for one-baseline sessions; X-nutation and Y-nutation are relative to
IAU2000A/2006 Nutation/Precession models. Time tag of the EOP series
is the middle epoch of the observing session.
usn2018a.eops: X, Y, UT1, Xdot, Ydot, UT1dot, Deps, Dpsi, each session.
Deps and Dpsi are relative to the IAU 1976 precession and IAU 1980
nutation models.
{Internally, Calc/Solve estimated offsets to the X and Y
precession/nutation quantities, relative to the IAU2000A/2006
nutation/precession models, using the IERS 2010 Conventions [6]
implementation. These were converted to classical Dpsi and Deps nutation
offsets relative to the IAU 2000A nutation model. These were then
converted to the IAU 1976/1980 precession/nutation (Wahr) model by
adding the following terms:
Deps: -25.24*Cent - 6.8192 (m-arc-sec), and
Dpsi: -299.65*Cent - 41.775 (m-arc-sec), where Cent is the epoch
in fractional centuries since 2000.0 (Julian date 2451545.0).
This conversion is not quite correct though. There are some long
term drifts that are not accounted for. See reference [7].}
High frequency variations in polar motion and UT1, as computed by
Calc 11, were added to the a priori EOP during the Solve/Globl
solution. The reported values of polar motion and UT1 are the sum of
the adjustments and the apriori EOP without contribution due to the
high frequency variations. Thus, the final series of polar motion and
UT1 do not contain contributions due to high frequency variations.
8. A priori geophysical models:
a. troposphere: NMF dry mapping function; Saastamoinen zenith delay
calculated using logged pressure, temperature;
a priori mean gradients from DAO weather model.
b. Solid Earth tide: IERS Conventions 2010 [6], chapter 7, 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 by Leonid Petrov. Harmonic model 2007b_oclo.hps
d. atmosphere loading: 3D displacements computed by convolving global
surface pressure field on 2.5x2.5 degrees grid with 6 hour temporal
resolution using the NCEP Reanalysis model by Leonid Petrov.
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].
9. Data type: group delays
10. Data editing: 5 degree elevation cutoff
11. 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. The station-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.
12. Standard errors reported: all errors derived from least-squares
estimation propagated from the data weights and the constraints applied
to the estimated parameters.
13. Software: Calc 11, SOLVE revision date 2017.01.03.
14. Other information:
Mean pole coordinates used for computation of pole tide
deformation were set to 0.0, 0.0
References:
1. 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
2. MacMillan, D.S. and C. Ma, Atmospheric gradients from very
long baseline interferometry observations, Geophys. Res.
Lett., 22, 1041-1044, 1995.
3. MacMillan, D.S. and C. Ma, Atmospheric gradients and the
VLBI terrestrial and celestial reference frames, Geophys.
Res. Lett., 24, 453-456, 1997.
4. Takashima, K., et al., "Status and Results of GSI Domestic
VLBI Network", Bulletin of the Geographical survey Institute,
Vol. 46, March 2000, p. 1-9.
5. Petrov, L. and J.-P. Boy, "Study of the atmospheric pressure loading
signal in VLBI observations", J. Geophys. Res., 10.1029/2003JB002500,
vol. 109, No. B03405, 2004.
6. Petit, Gerard and Luzum, Brian, 'IERS Conventions (2010), IERS Technical
Note 36, 2010.
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.