Hasil untuk "Geodesy"

W. Peltier, D. Argus, R. Drummond

W. McGrew, W. McGrew, Xiaogang Zhang et al.

The passage of time is tracked by counting oscillations of a frequency reference, such as Earth’s revolutions or swings of a pendulum. By referencing atomic transitions, frequency (and thus time) can be measured more precisely than any other physical quantity, with the current generation of optical atomic clocks reporting fractional performance below the 10−17 level1–5. However, the theory of relativity prescribes that the passage of time is not absolute, but is affected by an observer’s reference frame. Consequently, clock measurements exhibit sensitivity to relative velocity, acceleration and gravity potential. Here we demonstrate local optical clock measurements that surpass the current ability to account for the gravitational distortion of space-time across the surface of Earth. In two independent ytterbium optical lattice clocks, we demonstrate unprecedented values of three fundamental benchmarks of clock performance. In units of the clock frequency, we report systematic uncertainty of 1.4 × 10−18, measurement instability of 3.2 × 10−19 and reproducibility characterized by ten blinded frequency comparisons, yielding a frequency difference of [−7 ± (5)stat ± (8)sys] × 10−19, where ‘stat’ and ‘sys’ indicate statistical and systematic uncertainty, respectively. Although sensitivity to differences in gravity potential could degrade the performance of the clocks as terrestrial standards of time, this same sensitivity can be used as a very sensitive probe of geopotential5–9. Near the surface of Earth, clock comparisons at the 1 × 10−18 level provide a resolution of one centimetre along the direction of gravity, so the performance of these clocks should enable geodesy beyond the state-of-the-art level. These optical clocks could further be used to explore geophysical phenomena10, detect gravitational waves11, test general relativity12 and search for dark matter13–17. Improved techniques allow the measurement of a frequency difference with an uncertainty of the order of 10–19 between two independent atomic optical lattice clocks, suggesting that they may be able to improve state-of-the-art geodetic techniques.

J. Grotti, S. Koller, S. Vogt et al.

Optical atomic clocks, due to their unprecedented stability1–3 and uncertainty3–6, are already being used to test physical theories7,8 and herald a revision of the International System of Units9,10. However, to unlock their potential for cross-disciplinary applications such as relativistic geodesy11, a major challenge remains: their transformation from highly specialized instruments restricted to national metrology laboratories into flexible devices deployable in different locations12–14. Here, we report the first field measurement campaign with a transportable 87Sr optical lattice clock12. We use it to determine the gravity potential difference between the middle of a mountain and a location 90 km away, exploiting both local and remote clock comparisons to eliminate potential clock errors. A local comparison with a 171Yb lattice clock15 also serves as an important check on the international consistency of independently developed optical clocks. This campaign demonstrates the exciting prospects for transportable optical clocks.An atomic clock has been deployed on a field measurement campaign to determine the height of a mountain location 1,000 m above sea level, returning a value that is in good agreement with state-of-the-art geodesy.

A. Socquet, J. Hollingsworth, E. Pathier et al.

Francesca Baldacchino, Whyjay Zheng, Kunpeng Wu et al.

Glacier velocity is a sensitive indicator of mass balance and is key to understanding how glaciers respond to climate change. Monitoring glacier velocity at high temporal resolutions enables a better understanding of the drivers of glacier dynamics. Previous studies have found that the glaciers in High Mountain Asia (HMA) tend to slow down concomitant to losing mass at an accelerating rate on decadal timescales. However, few studies have explored seasonal variations in glacier velocities and have typically focused on large, fast-flowing glaciers. We select one debris-covered glacier, and four clean-ice glaciers in HMA. Sentinel-1 and -2 images are used to calculate the glacial velocities using the feature tracking module provided by the Cryosphere And Remote Sensing Toolkit (CARST). We develop a novel, regularised linear inverse model to extract the seasonally resolved glacial velocity time series (6-day intervals) with rigorous uncertainty estimates. Our results show that three of the five glaciers have strong seasonal signals in velocities, with faster velocities in spring and/or summer compared to winter. We also find an up-glacier propagation of the late spring and/or summer accelera-tions and a down-glacier propagation of the autumn accelerations. We suggest that changes in the subglacial hydrology efficiency drive the observed seasonal variations in velocities. We also highlight that icefalls may alter the glacier flow response by blocking the development of subglacial drainage channels and thus the seasonal propagation of velocities. Our methodol-ogy enables us to successfully extract seasonal signals in glaciers that flow slowly and provide a further understanding of glacier dynamics.

David Idiata, Ngozi Kayode - Ojo, Ehizonomhen Okonofua

This study investigates the geochemical and geotechnical properties of soils from Uwelu, Benin City, Nigeria (6.3861°N, 5.5827°E, 107 m altitude), to assess their engineering relevance. Samples from two sites underwent tests including specific gravity, sieve analysis, Atterberg limits, compaction, and California Bearing Ratio (CBR), following ASTM and AASHTO standards. X-ray fluorescence (XRF) was used to determine the presence of major oxides and trace elements. The soils, classified as A-2-4 and A-2-6 by AASHTO, had specific gravities of 2.55 and 2.54. The optimum moisture content was 10%, with Maximum Dry Densities (MDD) of 2.01 and 2.06 g/cm³. CBR results showed higher strength in unsoaked samples (20.11%, 6.38%) than soaked ones (9.69%, 3.24%). SiO₂ dominated the geochemistry (57.33%, 48.36%), with notable Al₂O₃ and Fe₂O₃. The findings emphasize the value of integrating geochemical and geotechnical analyses in construction.

Deren Li, Mi Wang, Haonan Guo et al.

China’s Earth Observation(EO) System has undergone significant development since the 1970s, as China has dedicated substantial efforts to advancing remote sensing technology. With fifty years of development, China has successfully narrowed the remote sensing technology gap with foreign countries through collaborative endeavors of the government and enterprises. At present, China has constructed a comprehensive EO system that has been proven indispensable for driving economic growth and facilitating sustainable development. This paper provides an overview of the development, missions, andapplications of China’s EO system, while also exploring future directions and technical trends of China’s EO system.

Zhao Li, Weiping Jiang, Tonie van Dam et al.

Nonlinear variations in the coordinate time series of global navigation satellite system (GNSS) reference stations are strongly correlated with surface displacements caused by environmental loading effects, including atmospheric, hydrological, and nontidal ocean loading. Continuous improvements in the accuracy of surface mass loading products, performance of Earth models, and precise data-processing technologies have significantly advanced research on the effects of environmental loading on nonlinear variations in GNSS coordinate time series. However, owing to theoretical limitations, the lack of high spatiotemporal resolution surface mass observations, and the coupling of GNSS technology-related systematic errors, environmental loading and nonlinear GNSS reference station displacements remain inconsistent. The applicability and capability of these loading products across different regions also require further evaluation. This paper outlines methods for modeling environmental loading, surface mass loading products, and service organizations. In addition, it summarizes recent advances in applying environmental loading to address nonlinear variations in global and regional GNSS coordinate time series. Moreover, the scientific questions of existing studies are summarized, and insights into future research directions are provided. The complex nonlinear motion of reference stations is a major factor limiting the accuracy of the current terrestrial reference frame. Further refining the environmental load modeling method, establishing a surface mass distribution model with high spatiotemporal resolution and reliability, exploring other environmental load factors such as ice sheet and artificial mass-change effects, and developing an optimal data-processing model and strategy for reprocessing global reference station data consistently could contribute to the development of a millimeter-level nonlinear motion model for GNSS reference stations with actual physical significance and provide theoretical support for establishing a terrestrial reference frame with 1 mm accuracy by 2050.

Xingxing Li, Jiande Huang, Xin Li et al.

T. E. Mehlstäubler, G. Grosche, C. Lisdat et al.

We review experimental progress on optical atomic clocks and frequency transfer, and consider the prospects of using these technologies for geodetic measurements. Today, optical atomic frequency standards have reached relative frequency inaccuracies below 10−17, opening new fields of fundamental and applied research. The dependence of atomic frequencies on the gravitational potential makes atomic clocks ideal candidates for the search for deviations in the predictions of Einstein’s general relativity, tests of modern unifying theories and the development of new gravity field sensors. In this review, we introduce the concepts of optical atomic clocks and present the status of international clock development and comparison. Besides further improvement in stability and accuracy of today’s best clocks, a large effort is put into increasing the reliability and technological readiness for applications outside of specialized laboratories with compact, portable devices. With relative frequency uncertainties of 10−18, comparisons of optical frequency standards are foreseen to contribute together with satellite and terrestrial data to the precise determination of fundamental height reference systems in geodesy with a resolution at the cm-level. The long-term stability of atomic standards will deliver excellent long-term height references for geodetic measurements and for the modelling and understanding of our Earth.

A. Nothnagel, T. Artz, D. Behrend et al.

Wei Liu, Zihui Lin, Yuan Hu et al.

Snow depth monitoring is crucial for hydrology, climate research, and avalanche prediction. While traditional global navigation satellite system (GNSS) reflectometer methods offer cost-effective snow thickness retrieval, they suffer from poor accuracy and robustness, especially in complex terrains and extreme weather. This study proposes an innovative snow depth retrieval technique employing a time-series recurrent neural network with bidirectional gated recurrent units (Bi-GRUs). Unlike traditional methods using signal-to-noise ratio (SNR) features, our algorithm utilizes the detrended SNR as Bi-GRU input, aiming to enhance accuracy, particularly in low snow depths and complex terrains. SNR observations from GPS L1 carriers at stations P351 and AB33 were analyzed. The Bi-GRU algorithm demonstrated high consistency with true snow depths at station P351 (coefficient of determination: 0.9766), with the root-mean-square error (RMSE) and the mean absolute error (MAE) of 9.1559 and 6.4185 cm, respectively. Compared to traditional methods, the Bi-GRU model improved the RMSE by 30.9% and the MAE by 44.5%. At station AB33, where snow depth variations were significant, accuracy improvements of 65.6% (RMSE: 7.4905 cm) and 63.2% (MAE: 5.6074 cm) were observed. In addition, the Bi-GRU model exhibited greater robustness compared to long short-term memory. These findings highlight the efficacy of the Bi-GRU-based approach, suggesting its superiority and broader applicability.

Masayuki Kano, Yusuke Tanaka, Daisuke Sato et al.

Abstract Monitoring and predicting fault slip behaviors in subduction zones is essential for understanding earthquake cycles and assessing future earthquake potential. We developed a data assimilation method for fault slip monitoring and the short-term prediction of slow slip events, and applied to the 2010 Bungo Channel slow slip event in southwest Japan. The observed geodetic data were quantitatively explained using a physics-based model with data assimilation. We investigated short-term predictability by assimilating observation data within limited periods. Without prior constraints on fault slip style, observations solely during slip acceleration predicted the occurrence of a fast slip; however, the inclusion of slip deceleration data successfully predicted a slow transient slip. With prior constraints to exclude unstable slip, the assimilation of data after slow slip event occurrence also predicted a slow transient slip. This study provides a tool using data assimilation for fault slip monitoring and prediction based on real observation data. Graphical Abstract

M. Poland, H. Zebker

D. Izzo, P. G'omez

Asteroids’ and comets’ geodesy is a challenging yet important task for planetary science and spacecraft operations, such as ESA’s Hera mission tasked to look at the aftermath of the recent NASA DART spacecraft’s impact on Dimorphos. Here we present a machine learning approach based on so-called geodesyNets which learns accurate density models of irregular bodies using minimal prior information. geodesyNets are a three-dimensional, differentiable function representing the density of a target irregular body. We investigate six different bodies, including the asteroids Bennu, Eros, and Itokawa and the comet Churyumov-Gerasimenko, and validate on heterogeneous and homogeneous ground-truth density distributions. Induced gravitational accelerations and inferred body shape are accurate, resulting in a relative acceleration error of less than 1%, also close to the surface. With a shape model, geodesyNets can even learn heterogeneous density fields and thus provide insight into the body’s internal structure. This adds a powerful tool to consolidated approaches like spherical harmonics, mascon models, and polyhedral gravity. Izzo and Gómez present a machine learning-based method for obtaining accurate density models of even irregular celestial bodies using minimal prior information. The work is validated on uniform and non-uniform density models of several visited asteroids.

L. Iess, S. Asmar, P. Cappuccio et al.

The Mercury Orbiter Radio Science Experiment (MORE) of the ESA mission BepiColombo will provide an accurate estimation of Mercury’s gravity field and rotational state, improved tests of general relativity, and a novel deep space navigation system. The key experimental setup entails a highly stable, multi-frequency radio link in X and Ka band, enabling two-way range rate measurements of 3 micron/s at nearly all solar elongation angles. In addition, a high chip rate, pseudo-noise ranging system has already been tested at 1-2 cm accuracy. The tracking data will be used together with the measurements of the Italian Spring Accelerometer to provide a pseudo drag free environment for the data analysis. We summarize the existing literature published over the past years and report on the overall configuration of the experiment, its operations in cruise and at Mercury, and the expected scientific results.

A. Hines, Andrea Nelson, Yanqi Zhang et al.

We present a novel optomechanical inertial sensor for low-frequency applications and corresponding acceleration measurements. This sensor has a resonant frequency of 4.715 (1) Hz, a mechanical quality factor of 4.76(3) × 105, a test mass of 2.6 g, and a projected noise floor of approximately 5 × 10−11 ms−2/Hz at 1 Hz. Such performance, together with its small size, low weight, reduced power consumption, and low susceptibility to environmental variables such as magnetic field or drag conditions makes it an attractive technology for future space geodesy missions. In this paper, we present an experimental demonstration of low-frequency ground seismic noise detection by direct comparison with a commercial seismometer, and data analysis algorithms for the identification, characterization, and correction of several noise sources.

W. Weber, D. Bortoluzzi, P. Bosetti et al.

Like gravitational wave detection, inter-spacecraft geodesy is a measurement of gravitational tidal accelerations deforming a constellation of two or more orbiting reference test masses (TM). The LISA TM system requires TM in free fall with residual stray accelerations approaching the fm/s2/Hz1/2 level in the mHz band, as demonstrated in the LISA Pathfinder “Einstein’s geodesic explorer” mission. Current geodesy missions are limited by accelerometers with 100 pm/s2/Hz1/2 level, due to intrinsic design limitations, as well as the challenging low Earth orbit environment and operating conditions. A reduction in the TM acceleration noise could lead to an important improvement in the scientific return of future geodesy missions focusing on mass change, especially in a scenario with multiple pairs of geodesy satellites. We present here a preliminary assessment of how the LISA TM system, known as the “gravitational reference sensor” (GRS), could be adapted for use in future geodesy missions aiming at residual TM accelerations noise at the pm/s2/Hz1/2 level, addressing the major design issues and performance limitations. We find that such a performance is possible in a geodesy GRS that is simpler and smaller than that used for LISA, with a lighter, sub-kg TM and gaps reduced from 4 mm to less than 1 mm. Acceleration noise performance limitations will likely be closely tied to the required levels of applied actuation forces on the TM.

Jinyun Guo, C. Hwang, X. Deng

The satellite altimetry concept was first proposed in 1969. Since then, many satellite altimetry missions have been implemented. With the development of the satellite altimetry technique, altimetry modes have been created for ocean and land observations, such as the traditional pulse-limited radar, the synthetic aperture radar (SAR), the laser mode, the three-dimensional imagingmode, and the global navigation satellite system refection (GNSS-R)mode. Figure 1 gives an overview of all satellite altimetry missions. China also developed the ocean dynamic environment satellite missions, HY-2A/2B/2C/2D. These three missions (HY-2B/2C/2D) are currently simultaneously collecting global marine information and monitoring changes in ocean states. Altimeter data quality may be affected by an error in instruments, atmospheric delay, sea state bias, geophysical environment correction (e.g., solid earth tide correction, ocean tide correction, inverse barometric effect, etc.), and precise orbit determination, The coastal waveforms may be seriously contaminated by the land and seabed. Therefore, these systematic errors should be corrected and presented in geophysical data records (GDRs). Researchers can then apply alternative correction models and new waveform retracking algorithms to further improve the quality of satellite altimetry data. Radar altimetry missions include the exact repeat mission (ERM) and the geodetic mission (GM). The oceanic environment can be continuously monitored with ERM data. The GM data are mainly used to study marine geodesy and geophysics. Laser altimeter, SAR altimeter, and three-dimensional imaging altimeter collect massive ocean data. Once fused, multi-source altimeter data can provide high-resolution and precise ocean information. In marine geodesy, geophysics, and oceanography, altimeter data have been used in several studies including marine gravity, geoid, mean sea surface, mean dynamic topography, sea levels rising, ocean currents, geostrophic, sea wind, and wave, bathymetry, and seabed tectonics. This Research Topic includes 17 papers in the field of satellite altimeter data processing, exploring applications to marine geodesy and geophysics. A summary of these papers is given below.

A. Genova, H. Hussmann, T. Hoolst et al.

In preparation for the ESA/JAXA BepiColombo mission to Mercury, thematic working groups had been established for coordinating the activities within the BepiColombo Science Working Team in specific fields. Here we describe the scientific goals of the Geodesy and Geophysics Working Group (GGWG) that aims at addressing fundamental questions regarding Mercury’s internal structure and evolution. This multidisciplinary investigation will also test the gravity laws by using the planet Mercury as a proof mass. The instruments on the Mercury Planetary Orbiter (MPO), which are devoted to accomplishing the GGWG science objectives, include the BepiColombo Laser Altimeter (BELA), the Mercury orbiter radio science experiment (MORE), and the MPO magnetometer (MPO-MAG). The onboard Italian spring accelerometer (ISA) will greatly aid the orbit reconstruction needed by the gravity investigation and laser altimetry. We report the current knowledge on the geophysics, geodesy, and evolution of Mercury after the successful NASA mission MESSENGER and set the prospects for the BepiColombo science investigations based on the latest findings on Mercury’s interior. The MPO spacecraft of the BepiColombo mission will provide extremely accurate measurements of Mercury’s topography, gravity, and magnetic field, extending and improving MESSENGER data coverage, in particular in the southern hemisphere. Furthermore, the dual-spacecraft configuration of the BepiColombo mission with the Mio spacecraft at higher altitudes than the MPO spacecraft will be fundamental for decoupling the internal and external contributions of Mercury’s magnetic field. Thanks to the synergy between the geophysical instrument suite and to the complementary instruments dedicated to the investigations on Mercury’s surface, composition, and environment, the BepiColombo mission is poised to advance our understanding of the interior and evolution of the innermost planet of the solar system.