The empirical model GPT (Global Pressure and Temperature), which is based on spherical harmonics up to degree and order nine, provides pressure and temperature at any site in the vicinity of the Earth’s surface. It can be used for geodetic applications such as the determination of a priori hydrostatic zenith delays, reference pressure values for atmospheric loading, or thermal deformation of Very Long Baseline Interferometry (VLBI) radio telescopes. Input parameters of GPT are the station coordinates and the day of the year, thus also allowing one to model the annual variations of the parameters. As an improvement compared with previous models, it reproduces the large pressure anomaly over Antarctica, which can cause station height errors in the analysis of space-geodetic data of up to 1 cm if not considered properly in troposphere modelling. First tests at selected geodetic observing stations show that the pressure biases considerably decrease when using GPT instead of the very simple approaches applied to various Global Navigation Satellite Systems (GNSS) software packages so far. GPT also provides an appropriate model for the annual variability of global temperature.

Short Note: A global model of pressure and temperature for geodetic applications

The development and numerical values of the new absolute phase-center correction model for GPS receiver and satellite antennas, as adopted by the International GNSS (global navigation satellite systems) Service, are presented. Fixing absolute receiver antenna phase-center corrections to robot-based calibrations, the GeoForschungsZentrum Potsdam (GFZ) and the Technische Universitat Munchen reprocessed more than 10 years of GPS data in order to generate a consistent set of nadir-dependent phase-center variations (PCVs) and offsets in the z-direction pointing toward the Earth for all GPS satellites in orbit during that period. The agreement between the two solutions estimated by independent software packages is better than 1 mm for the PCVs and about 4 cm for the z-offsets. In addition, the long time-series facilitates the study of correlations of the satellite antenna corrections with several other parameters such as the global terrestrial scale or the orientation of the orbital planes with respect to the Sun. Finally, completely reprocessed GPS solutions using different phase-center correction models demonstrate the benefits from switching from relative to absolute antenna phase-center corrections. For example, tropospheric zenith delay biases between GPS and very long baseline interferometry (VLBI), as well as the drift of the terrestrial scale, are reduced and the GPS orbit consistency is improved.

/pdf/generation-of-a-consistent-absolute-phase-center-correction-5afza4lgcw.pdf

Generation of a consistent absolute phase-center correction model for GPS receiver and satellite antennas

We analyse geodetically estimated deformation
across the Nepal Himalaya in order to determine the geodetic
rate of shortening between Southern Tibet and India,
previously proposed to range from 12 to 21 mm yr^(−1). The
dataset includes spirit-levelling data along a road going from
the Indian to the Tibetan border across Central Nepal, data
from the DORIS station on Everest, which has been analysed
since 1993, GPS campaign measurements from surveys
carried on between 1995 and 2001, as well as data from continuous
GPS stations along a transect at the logitude of Kathmandu
operated continuously since 1997. The GPS data were processed in International Terrestrial Reference Frame 2000
(ITRF2000), together with the data from 20 International
GNSS Service (IGS) stations and then combined using quasi observation
combination analysis (QOCA). Finally, spatially
complementary velocities at stations in Southern Tibet, initially
determined in ITRF97, were expressed in ITRF2000.
After analysing previous studies by different authors, we
determined the pole of rotation of the Indian tectonic plate to
be located in ITRF2000 at 51.409±1.560°N and−10.915±
5.556°E, with an angular velocity of 0.483±0.015°. Myr^(−1).
Internal deformation of India is found to be small, corresponding
to less than about 2 mm yr^(−1) of baseline change
between Southern India and the Himalayan piedmont. Based
on an elastic dislocation model of interseismic strain and
taking into account the uncertainty on India plate motion,
the mean convergence rate across Central and Eastern Nepal
is estimated to 19 ± 2.5 mm yr^(−1), (at the 67% confidence
level). The main himalayan thrust (MHT) fault was found to
be locked from the surface to a depth of about 20km over a
width of about 115 km. In these regions, the model parameters
are well constrained, thanks to the long and continuous
time-series from the permanent GPS as well as DORIS data.
Further west, a convergence rate of 13.4 ± 5 mm yr^(−1), as
well as a fault zone, locked over 150 km, are proposed. The
slight discrepancy between the geologically estimated deformation
rate of 21 ± 1.5 mm yr^(−1) and the 19 ± 2.5 mm yr^(−1)
geodetic rate in Central and Eastern Nepal, as well as the
lower geodetic rate in Western Nepal compared to Eastern
Nepal, places bounds on possible temporal variations of the
pattern and rate of strain in the period between large earthquakes
in this region.

Plate Motion of India and Interseismic Strain in the Nepal Himalaya from GPS and DORIS Measurements

PPP-RTK is integer ambiguity resolution-enabled precise point positioning. In this contribution, we present the principles of PPP-RTK, together with a review of different mechanizations that have been proposed in the literature. By application of $$\mathcal {S}$$
 -system theory, the estimable parameters of the different methods are identified and compared. Their interpretation is essential for gaining a proper insight into PPP-RTK in general, and into the role of the PPP-RTK corrections in particular. We show that PPP-RTK is a relative technique for which the ‘single-receiver user’ integer ambiguities are in fact double-differenced ambiguities. We determine the transformational links between the different methods and their PPP-RTK corrections, thereby showing how different PPP-RTK methods can be mixed between network and users. We also present and discuss four different estimators of the PPP-RTK corrections. It is shown how they apply to the different PPP-RTK models, as well as why some of the proposed estimation methods cannot be accepted as PPP-RTK proper. We determine analytical expressions for the variance matrices of the ambiguity-fixed and ambiguity-float PPP-RTK corrections. This gives important insight into their precision, as well as allows us to discuss which parts of the PPP-RTK correction variance matrix are essential for the user and which are not.

/pdf/review-and-principles-of-ppp-rtk-methods-2qrskwdxim.pdf

Review and principles of PPP-RTK methods

On April 17, 2011, the International GNSS Service (IGS) stopped using the IGS05 reference frame and adopted a new one, called IGS08, as the basis of its products. The latter was derived from the latest release of the International Terrestrial Reference Frame (ITRF2008). However, the simultaneous adoption of a new set of antenna phase center calibrations by the IGS required slight adaptations of ITRF2008 positions for 65 of the 232 IGS08 stations. The impact of the switch from IGS05 to IGS08 on GNSS station coordinates was twofold: in addition to a global transformation due to the frame change from ITRF2005 to ITRF2008, many station coordinates underwent small shifts due to antenna calibration updates, which need to be accounted for in any comparison or alignment of an IGS05-consistent solution to IGS08. Because the heterogeneous distribution of the IGS08 network makes it sub-optimal for the alignment of global frames, a smaller well-distributed sub-network was additionally designed and designated as the IGS08 core network. Only 2 months after their implementation, both the full IGS08 network and the IGS08 core network already strongly suffer from the loss of many reference stations. To avoid a future crisis situation, updates of IGS08 will certainly have to be considered before the next ITRF release.

IGS08: the IGS realization of ITRF2008

Bernese GPS Software Version 5.0

SLR tracking data allow for a completely independent validation of GNSS orbits that are derived from microwave data. SLR validation results show mean range residuals of several centimeters for both, GPS and GLONASS satellites, as well as significant seasonal variations for the two GPS satellites that are equipped with retroreflector arrays. It was, however, not clear whether these systematic effects could be assigned to orbit modeling deficiencies or to SLR tracking biases. We present new SLR validation results, which point to serious GPS orbit modeling problems. Moreover, we address the question, whether it would make sense to perform a combined analysis of microwave and SLR data for GNSS orbit determination. With the available low number of SLR observations no significant improvement of the orbit accuracy is found. An a priori variance-covariance analysis shows an improvement of the situation, if continuous SLR tracking data of already a very small number of globally distributed SLR sites were available.

/pdf/calibrating-gnss-orbits-with-slr-tracking-data-55ikcevjw7.pdf

Stefan Schär

Papers

Bernese GPS Software Version 5.0

Calibrating GNSS orbits with SLR tracking data