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Cheng et al. Satell Navig (2021) 2:2 Page 3 of 12

Selection of control stations from the NNR-NUVEL-1A model and the other from an
It is the common practice that a new ITRF is aligned with ITRF combined solution, which can be downloaded from
its previous one by applying the no-net-translation and the ITRF website, are used to estimate the seven trans-
no-net-rotation conditions from transformation param- formation parameters for each plate between the two
eters in the global well distributed core stations that are associated frames with the two sets of data using the least
commonly used for the maintenance of the two ITRFs, squares method; then the supervised clustering method
and then the coordinates of these stations in the new is used to identify the stations to be excluded, whose
ITRF are estimated by a combined adjustment with the velocity or azimuth residuals are larger than their two
minimal constraints. Te seven transformation param- sigma values. A test on the selection of Chinese national
eters and their rates between the two ITRFs are estimated control stations was conducted. 92 stations were selected
by the least squares method based on the coordinates of from the initial 126 global IGS candidate stations and
these core stations. However, if the frame of a regional used in the estimation of the refned coordinates of the
GNSS network is aligned with an ITRF, it is unlikely to global GNSS network, from which the regional control
use the same core stations as the ones used in the ITRF, stations were further selected for the estimation of Chi-
e.g. like the case that some of the core stations have no nese national velocity feld. Figure 1 shows the distribu-
observations or have bad observations during the period tion (red stars) of the 92 stations, along with the other
of investigation. Tus the global control stations, which global IGS stations (blue dots) in the ITRF2005 frame.
determine a reference coordinate datum, are selected to For the detailed procedure refers to (Cheng et al. 2020).
better represent the relationship between the two frames.
Tese global control stations are used as control stations Coordinate refnement of regional control stations
for a global combined adjustment to estimate the coor- Similar to the selection of global control stations for the
dinates of all the global stations used in the adjustment determination of the coordinate datum of a global GNSS
for the ITRF construction. To align the regional GNSS network, regional control stations are needed in multi-
network tightly with the ITRF, regional control stations day combined adjustments for the regional network and
can be selected from the global control stations selected aligning the regional network with the associated ITRF.
above for a regional combined adjustment. Te strategy In this study, for the adjustment of the Chinese GNSS
for the selection of such a group of global control stations network, among the 92 global control stations selected
is proposed in this study. Since the International Ter- above, only the stations within the Chinese region and
restrial Reference System (ITRS) is defned as an earth- its surrounding areas were chosen as the regional con-
fx coordinate system, all GNSS stations are assumed trol stations. For the evaluation of the alignment of
to fx on the earth crust and have the same movements the regional network with the ITRF, the corrections of
theoretically. However, the earth crust comprises sev- the coordinates of these regional control stations with
eral plates, which have diferent movement trends and respect to their prior coordinates, i.e. the known coordi-
magnitudes, thus regional GNSS stations are most likely nates were used as an indicator. In this study the known
to move with their corresponding plates. Based on this coordinates were referred to the refned coordinates of
reasoning, a plate-fxed coordinate system is proposed the stations estimated in the global combined adjust-
in this study for a better alignment of a regional GNSS ment. Te smaller the corrections, the better the align-
network with the ITRF. In contrast to the general crite- ment accuracy is. Te coordinates of the regional control
ria for the selection of the ITRF stations, i.e. only based stations, which determine the regional coordinate datum
on the precisions of the coordinates and/or velocities of for the regional combined adjustment, can be estimated
the stations, we take into account the characteristics of through the combination of constraint-free normal equa-
the plate movements in addition to applying the general tion systems obtained from several IGS analysis centers,
criteria for selecting initial candidates. Among the initial or obtained based on the station velocities at the refer-
candidate stations only those whose motions are consist- ence epoch, provided in ITRF documentation. Here-
ent with the corresponding plate movements are selected after, the two datum or frames constructed are named
as the global control stations. A statistic method, called Epoch Reference Frame (ERF) and Derived Reference
supervised clustering, is employed to identify the con- Frame (DRF), respectively. Te former represents the
sistency between the movements of the stations and the real position of each station at a given epoch, while the
plate. Tis can ensure that all selected control stations are latter is the mean position of the station in a period of
in a stable area of the plate so that they can well represent time, and their diferences can reach several centimeters.
the real movement of the plate. Te diferences are mainly caused by the site’s non-linear
Te procedure for the selection of global control sta- movement driven by geophysical mechanism, such as
tions is as follows. Two sets of velocity data, one obtained seasonal changes, position jumps, high-frequency loads