3 research outputs found
Observability and estimation of geocentre motion using multi-satellite laser ranging
PhD ThesisArtificial satellites orbit about the Earthâs system centre of mass, a point known as the
geocentre that conventionally defines the long-term origin of the terrestrial reference frame
(TRF). In a frame attached to the Earthâs crust, the geocentre exhibits motions on subdaily
to secular time scales due to various geophysical processes. Annual variations induced
by the redistribution of fluid mass in the Earthâs surface layer are most prominent
and can bias ice mass balance and sea level change estimates if neglected. Theoretically,
these annual variations are directly observable by any satellite geodetic technique, but
orbit modelling complications affect the retrieval of geocentre motion from Global Navigation
Satellite Systems (GNSS) and Doppler Orbitography and Radiopositioning Integrated
by Satellite (DORIS) data. This study focuses on Satellite Laser Ranging (SLR),
the only technique proven to yield reliable geocentre motion estimates via translational
approaches.
By means of collinearity diagnosis applied to the determination of geocentre motion
using the network shift approach, it is shown that, subject to certain parameterisation constraints,
the low Earth orbiters (LEOs) Starlette, Ajisai and the Laser Relativity Satellite
(LARES) can beneficially supplement the traditionally employed pair Laser Geodynamics
Satellite (LAGEOS) 1 and 2. In particular, the combination of LAGEOS-1 and 2 with
LARES data can improve the observability of the geocentre coordinates by 25â30% on
average compared to LAGEOSâonly solutions due to both the larger number of observations
and the proven higher sensitivity of LARES to geocentre motion. Tests involving
different satellite combinations show that the contribution of Stella is minor owing to its
quasi-polar orbit, whereas observations to the medium Earth orbiters (MEOs) Etalon-1
and 2 are too infrequently acquired to benefit the retrieval of geocentre motion and possibly
other parameters of geophysical interest. An analysis of SLR data spanning two
decades partitioned in weekly batches reveals that geocentre motion estimates derived
from LAGEOSâStarletteâStellaâAjisai combinations are contaminated by modelling errors
to a larger extent than in LAGEOSâonly solutions and, without considerable advances
in orbit modelling, the exploitation of the high sensitivity of Starlette and Ajisai to geocentre
motion appears remote. Compounded by the short tracking history of LARES, a
conclusive assessment of the long-term quality of LAGEOSâLARES solutions is infeasible
at present.
iv
Similar to other geodetic parameters, the geocentre coordinates exhibit temporal correlations
that have been typically neglected in previous studies. The power spectral
densities (PSDs) of weekly derived geocentre coordinates display a power-law behaviour
at long periods and white noise flattening for frequencies above 4 cycles per year (cpy).
When temporal dependencies are appropriately modelled using one of the readily available
maximum likelihood estimation (MLE) software implementations, the uncertainties
of the annual amplitude and phase estimates inflate by an average factor of 1.6 for weekly
time series over 12 years in length. The formal errors of the linear and quadratic trend
estimates amplify by a larger factor of 2.2â2.3. First-order autoregressive noise plus white
noise and power-law noise are the preferred stochastic models in most cases based on
model-selection criteria. As demonstrated through the analysis of independent time series,
for sampling periods longer than one week the first-order autoregressive model becomes
more competitive on its own due to the suppression of white noise at high frequencies,
but the power-law noise model is also occasionally preferred.
Kinematic estimates of geocentre coordinates are highly coherent with network shift
results across the entire frequency range only when station positions are simultaneously
solved for. Additionally, network shift estimates are more coherent with kinematic results
when the scale parameter is omitted from the functional model of the similarity transformation
linking the quasi-instantaneous frames and the secular frame. In addition to
draconitic errors related to solar radiation pressure modelling, long-period tidal aliases
due to mismodelled tidal constituents also contaminate geocentre motion estimates. Independent
geodetic estimates and geophysical model predictions validating the results from
this study agree that the annual geocentre motion signals have amplitudes of 2â3 mm in
the equatorial components and 4â6 mm in the Z component. The maximum geocentre
vector magnitude of about 7 mm is attained in July.partly funded by Newcastle University, the Dinu Patriciu Foundation
and the RomanianâAmerican Foundation
Altimetry for the future: Building on 25 years of progress
In 2018 we celebrated 25 years of development of radar altimetry, and the progress achieved by this methodology in the fields of global and coastal oceanography, hydrology, geodesy and cryospheric sciences. Many symbolic major events have celebrated these developments, e.g., in Venice, Italy, the 15th (2006) and 20th (2012) years of progress and more recently, in 2018, in Ponta Delgada, Portugal, 25 Years of Progress in Radar Altimetry. On this latter occasion it was decided to collect contributions of scientists, engineers and managers involved in the worldwide altimetry community to depict the state of altimetry and propose recommendations for the altimetry of the future. This paper summarizes contributions and recommendations that were collected and provides guidance for future mission design, research activities, and sustainable operational radar altimetry data exploitation. Recommendations provided are fundamental for optimizing further scientific and operational advances of oceanographic observations by altimetry, including requirements for spatial and temporal resolution of altimetric measurements, their accuracy and continuity. There are also new challenges and new openings mentioned in the paper that are particularly crucial for observations at higher latitudes, for coastal oceanography, for cryospheric studies and for hydrology. The paper starts with a general introduction followed by a section on Earth System Science including Ocean Dynamics, Sea Level, the Coastal Ocean, Hydrology, the Cryosphere and Polar Oceans and the ââGreenâ Ocean, extending the frontier from biogeochemistry to marine ecology. Applications are described in a subsequent section, which covers Operational Oceanography, Weather, Hurricane Wave and Wind Forecasting, Climate projection. Instrumentsâ development and satellite missionsâ evolutions are described in a fourth section. A fifth section covers the key observations that altimeters provide and their potential complements, from other Earth observation measurements to in situ data. Section 6 identifies the data and methods and provides some accuracy and resolution requirements for the wet tropospheric correction, the orbit and other geodetic requirements, the Mean Sea Surface, Geoid and Mean Dynamic Topography, Calibration and Validation, data accuracy, data access and handling (including the DUACS system). Section 7 brings a transversal view on scales, integration, artificial intelligence, and capacity building (education and training). Section 8 reviews the programmatic issues followed by a conclusion
Altimetry for the future: building on 25 years of progress
In 2018 we celebrated 25âŻyears of development of radar altimetry, and the progress achieved by this methodology in the fields of global and coastal oceanography, hydrology, geodesy and cryospheric sciences. Many symbolic major events have celebrated these developments, e.g., in Venice, Italy, the 15th (2006) and 20th (2012) years of progress and more recently, in 2018, in Ponta Delgada, Portugal, 25 Years of Progress in Radar Altimetry. On this latter occasion it was decided to collect contributions of scientists, engineers and managers involved in the worldwide altimetry community to depict the state of altimetry and propose recommendations for the altimetry of the future. This paper summarizes contributions and recommendations that were collected and provides guidance for future mission design, research activities, and sustainable operational radar altimetry data exploitation. Recommendations provided are fundamental for optimizing further scientific and operational advances of oceanographic observations by altimetry, including requirements for spatial and temporal resolution of altimetric measurements, their accuracy and continuity. There are also new challenges and new openings mentioned in the paper that are particularly crucial for observations at higher latitudes, for coastal oceanography, for cryospheric studies and for hydrology.
The paper starts with a general introduction followed by a section on Earth System Science including Ocean Dynamics, Sea Level, the Coastal Ocean, Hydrology, the Cryosphere and Polar Oceans and the âGreenâ Ocean, extending the frontier from biogeochemistry to marine ecology. Applications are described in a subsequent section, which covers Operational Oceanography, Weather, Hurricane Wave and Wind Forecasting, Climate projection. Instrumentsâ development and satellite missionsâ evolutions are described in a fourth section. A fifth section covers the key observations that altimeters provide and their potential complements, from other Earth observation measurements to in situ data. Section 6 identifies the data and methods and provides some accuracy and resolution requirements for the wet tropospheric correction, the orbit and other geodetic requirements, the Mean Sea Surface, Geoid and Mean Dynamic Topography, Calibration and Validation, data accuracy, data access and handling (including the DUACS system). Section 7 brings a transversal view on scales, integration, artificial intelligence, and capacity building (education and training). Section 8 reviews the programmatic issues followed by a conclusion