2016 Vol. 41, No. 1
We review the development of the satellite gravity measuring techniques. The research status of present satellite gravity missions, including CHAMP, GRACE, and GOCE, are discussed and the main shortcomings of GRACE are analyzed. Some suggestions for determining gravity field at higher precision and higher resolution in space and time are proposed, which include reduction of the current level of aliasing, the elimination of systematic distortions (non-isotropic sensitivity of a single pair low-low SST), and the improvement in the separability of the observed geophysical signals are discussed. The progress of the next four international satellite gravity missions: E.motion, GRACE-FO, NGGM, GETRIS, are presented and the role and status of the SWARM satellite constellation mission on the earth's gravity field research are summarized. Finally, several suggestions about the implementation of the future satellite gravity missions in China are put forward. The next generation satellite gravity systems should be capable of global determination of changes in the Earth's gravity field from global down to regional spatial scales and at time scales of two weeks or shorter.
OTL(ocean tide loading) displacement inversion based on GNSS has a unique advantage given the rapid development of GNSS technology. It is more convenient, economic, accurate, and faster in comparison to gravity and VLBI (very long baseline interferometry) measurement on ocean tide loading. We conduct a systematic review on the progress of GNSS ocean loading research as this approach has drawn a lot of attention at home and abroad. We introduce the OTL displacement model for improving the accuracy of GNSS positioning, and obtain ocean tide loading information using GNSS. We point out the problems existing in the current OTL displacement models (the overall accuracy of the model is low, especially in the shallow water area) and the drawbacks of GNSS OTL displacement inversion methods (the emergence of pseudo anniversary, the existence of the system errors and the coupling effect between some tidal constituent and GNSS residual errors).
The strategic position of Arctic area and its rich natural resources are becoming increasingly important, while the northeast and northwest passages through the Arctic are receiving much attention as glaciers continue to melt. Hence, GNSS will play a significant role due to its unique advantages as Arctic navigation and positioning are particularly significant to guarantee the security of activities in that environment. The Beidou regional satellite navigation system was established successfully, and is developing into a global system enveloping the Polar Regions. In order to evaluate Beidou performance in the polar area, Beidou availability with present and future constellations, and the combination of Beidou with GPS are analyzed. The latent challenges to the navigation services provided by GNSS in polar area are discussed and possible solutions provided. We argue that the current BD2 can not satisfy the navigation requirements, but with a global BDS, it is possible to provide navigation services independently in Arctic area.
A semi-analytical (SA) approach is proposed to recover a gravity field model from the GOCE tensor invariant observations. The SA approach is more efficient than the least-squares method, and could provide the posterior variance of estimated coefficients. The formulas for gravity field model determination from tensor invariant observations based on SA approach were derived and a corresponding procedure for estimation of the satellite gravity model from satellite gravity gradient observations are described. Testing results from the simulated error free observations along the circular orbit with the inclination 89.5° show that applying the SA approach to process tensor invariant observations is theoretically rigorous. Using the simulated observations with or without errors in gradiometer reference frame (GRF) along the one repeat cycle GOCE real orbit, we recovered the gravity field models from the I2 and Vzz components respectively. Numerical analysis results show that the solution from I2 was slightly better than the one from Vzz component, which verified the feasibility of the method.
Recurrence relations for fully normalized associated Legendre functions and their derivatives and integrals are often used when studying the Earth gravity field. Fully normalized associated Legendre functions are all adopted in current popular Earth disturbance potential models make the calculations of various disturbance gravity field elements highly efficient and easy when such models and related recurrence relations are used. New generation satellite gravity exploration missions have been successfully implemented, and high or ultrahigh degree Earth gravity field models have been quickly developed. This study area has received much attention in academic community; our investigation has a special meaning as it addresses the recurrence relations of fully normalized associated Legendre functions. Building upon previous research, several new recurrence relation expressions are derived in detail based on elementary calculus as the mathematical tool for the derivation. Formulas for assessing the values of fully normalized associated Legendre functions, their derivatives, and integrals are also deduced. This study involves the mathematical properties of the spherical harmonic expansion series of the geo-potential, thus is related to fundamental theory research.
We propose a coseismic slip distribution inversion method based on variance component estimation (VCE), this method not only determines the weight of different data sets but also can get the smoothing factor value. To illustrate and validate this inversion method, we conducted inversions on synthetic data sets. Based on the Bayesian framework and according to the posterior probability density distribution of the unknown parameters, the proposed solution is comparted to one of the linear-non-linear inversion solutions to further demonstrate the use of the proposed method. We conduct inversions on the 2003 Bam earthquake with the proposed method and linear-non-linear inversion method, the spatial cross correlation of the resulting slip distribution was about 0.999, the depth of the max slip of the both were 3.04 m and the depth had little difference, the former was 4.50 km, the latter was 4.47 km. The mean slip in the former was 0.714 m, the latter was 0.718 m. But the inversion time of the former was 227 s, and only about 1/8 000 of the later (reaching 1.5×106 s, about 17 d).These results show that the VCE method is effective and faster than the linear-non-linear inversion method.
We calculated the key parameters of ground-based GPS water vapor inversion in each grid point and got the time series of the conversion factor Π-1by using the global zenith wet delay (ZWD) grid data with a spatial resolution of 2.5°×2°(longitude × latitude) provided by global geodetic observing system (GGOS) Atmosphere and precipitable water vapor (PWV) products with a same spatial resolution provided by European Centre for Medium-Range Weather Forecasts (ECMWF) from 2005 to 2011, followed by the analysis of temporal and spatial characteristic. We establish a global empirical model of Π which is related to the site's latitude, longitude, altitude and the day of year (doy) without meteorological data. Then we used the GGOS Atmosphere and ECMWF grid data that not involved in modeling in 2012, 661 radiosonde stations' data in 2012 to test accuracy of the model. The results show that the mean deviation (Bias) is -0.179 mm and root mean square error (Root Mean Square Error, RMS) is 1.806 mm from the grid data; the Bias is 0.465 mm, RMS is 0.789 mm from radiosonde data. Both of them indicate the accuracy and stability of the model is high with a small systematic bias on a global scale.
The vertical datums for hydrography and their relations were analysed and the technical approaches for expressing a chart datum on the height datum and Earth ellipsoidal surface were demonstrated. Among the three approaches for vertical datum transportation, developing a set of transportation models recent research has only focused on those relying on the single and discrete tidal station observations and calculations. The accuracy of the mean sea level(MSL) and the chart datum defined with the lowest normal low water was determined while different time spans were estimated with long period tidal station observations. This study indicates that the MSL under 5 cm can be obtained with two years of observations, and a CD under 7 cm root mean squares, along the coast of China. As an example, the CD model over the South China Sea and its separation from the height datum and Earth ellipsoidal surface are established through tide, mean sea surface, and sea surface topography models. Suggestions for unification tidal station CD calculation and establishment of a network for vertical datum control are put forward.
Terrestrial water storage anomaly from the Gravity Recovery and Climate Experiment (GRACE) and precipitation observations from the Tropical Rainfall Measuring Mission satellite (TRMM) are combined and applied to assess regional flood potential in Afghanistan. The results show that this is a useful remote-sensing method to improve estimates of terrestrial water storage capacity and flood potential. The improved monthly storage deficit variable obtained from the terrestrial water storage anomaly from GRACE was used to assess the terrestrial water storage capacity. A flood potential index model was established by combining it with TRMM. The results were compared with those from National Climate Center of China (NCCC), showing that the experimental flood potential index for Afghanistan was consistent with the NCCC data and accurate from the perspective of time and space. The proposed combination method using GRACE and TRMM, can effectively estimate the flood potential in Afghanistan, and provides a useful new tool for regional flood warnings.
The Earth rotation parameters (ERP) and the variation of length of day (ΔLOD) are estimated by processing the seventeen IGS stations' data collected in September 2005, January and February 2006 with GAMIT, the estimated results of which are compared with the IGS solutions. Secondly, the ERP and the ΔLOD are estimated by processing VLBI data during the same period with OCCAM 6.2, the results of which are compared with the IVS solutions. Thirdly, the results of GPS and VLBI are combined in a weighted way, based on their internal accord accuracy and IERS 08C04 respectively. Finally, the conclusion not only shows that the interpolation methods have a significant impact on the VLBI results, but also shows that the combining of VLBI and GPS in the second way can improve the stability and reliability of ERP and ΔLOD, which can make up for the shortcoming of the single technique as VLBI or GPS.
Satellite gravimetry is considered as one of the most powerful techniques for determining the global gravity field at high resolution and accuracy. At present, our country is implementing a satellite gravity project which is necessary to develop a preprocessing method and software for satellite gravity gradient observations. The effects of tide and non-tide signals on GOCE satellite gravity gradient observations were systematically explored. The results show that: (1) The magnitude of time-varying correction was at the 0.1mE level, and slightly lower than the designed accuracy (3.2 mE) of the GOCE satellite. This colored noise must be removed in the pretreatment. (2) The magnitude of tide (0.1 mE) was bigger than non-tide effects(0.01 mE). Thus, the correction accuracy for the time-varying gravity field must be determined by the tide. (3) These results correspond to the official GOCE, which validates the effectiveness of our calculating method and results.
Ice temperature is one of the most important physical parameters, during the period from 2009 to 2011, observations of ice temperature were carried out on the Austre Lovénbreen glacier in the NY-Ålesund of the arctic region at the points B2, E2 and F, in the layer 20 m below the surface. Ice temperature curves at these three points showed a cold season in May and warm season in September. Below the 9 m depth,the ice temperature at the point E2 was lower than that at points B2 and F. The lower boundary depth of active ice temperature layer at the three points was 14 m deep, with annual mean temperature of -2.76、-3.23 and -2.84℃. With increasing altitude, the lapse rate in the ice temperature at the 20 m depth (0.3℃·100 m-1) was higher than that at 10m depth (0.15℃·100 m-1). Ice temperatures at the 10m depth were higher than the calculated mean annual air temperatures by 1~4℃, demonstrating the impact of refreezing melted glacier surface water on the release of latent heat, causing increasing ice temperature.
After having analyzed the disadvantage of existing automatic filtering algorithm, which means deleting the micro morphology or obstacle caused by devilishly pursuing the smoothing effect of artificial filtering method, aims at Ping as the processing cell of multi-beam bathymetric survey data, and by means of rolling circle transform's characteristic of identifying and analyzing the concave (convex) part quantitatively under specific precision, the paper details an algorithm for filtering noises in multi-beam data based on a rolling circle transform, and details the keystone and solution steps of the model. At last, in a VC++ environment, some experiments were done to validate the algorithm's validity. The experiments show that the algorithm can filter noises in multi-beam data; maintaining marine topography completeness and enhancing the efficiency in gross error detection.
The new GRAIL gravity model GRGM660PRIM has advanced in the aspect of resolution and degree when compared with historical lunar gravity field models. In this paper, we present a quantitative analysis of the orbit determination ability of the GRGM660PRIM in orbit prediction and precise orbit determination (POD). In a comparison with the LP50Q and SGM150j models, results show that the GRGM660PRIM is close to the level of the LP150Q and SGM150 in satellite lifetime prediction; in the Apollo 16 subsatellite POD, GRGM660PRIM shows its advantages at high degrees and precision, the residuals RMS of two Doppler decreases from 1.36~1.48 mm/s to 0.722 mm/s, and the three way Doppler residuals RMS decrease from 1.67~1.69 mm/s to 1.2 mm/s;the precision of orbit determination improves significantly. These conclusions can provide a reference for the upcoming Chinese Chang'E-5 mission and further lunar missions.
The paper introduces the bistatic SAR system of the TanDEM-X/TerraSAR-X, especially focusing on its flexible data acquisition model and the special advantages of a zero-time baseline. We present a bistatic InSAR processing method for DEM generation with TanDEM-X/TerraSAR-X data, highlighted by a phase difference strategy as a means of differencing bistatic interferograms and simulated phase from an external DEM, e.g. SRTM DEM. This strategy reduces the residual phase and improves the efficiency and the precision of phase unwrapping. We take account of the bistatic geometry and the corresponding zero Doppler time in SAR interferometric processing. The absolute phase offset was calibrated with some external ICESat points. A case study was carried out to generate the DEM product over the Puruogangri Ice Field in the Tibetan Plateau. Compared with the SRTM-X DEM and ICESat elevation, our preliminary results demonstrate the great potential of the TanDEM-X bistatic InSAR technology for mapping surface topography.
Satellite clock error plays an important role in precision positioning, so it is essential to monitor its anomalies in real time. On the premise of modeling satellite clock error, this paper proposes a real-time satellite clock anomaly monitoring algorithm, based on the forgetting factor least square algorithm (RFFLS algorithm). The principles of this algorithm are explained. Using the IGS final precision product, comparisons are made between the RFFLS algorithm, LS algorithm, and FFLS algorithm for satellite clock error prediction precision, time consumption, and anomaly monitoring performance. The results show that the time consumption of RFFLS algorithm is only one tenth of that of LS algorithm or FFLS algorithm, and the anomaly monitoring ability of RFFLS algorithm is the best. In conclusion, the RFFLS algorithm is simple, flexible, and shows a clear advantage in real-time application.
This paper approaches the problem of detecting and recognizing ship targets in color images based on sea-sky background. We propose a method based on multi-scale feature cluster robust to the changes in scale, orientation and illumination of the image and can fast detect and recognize ship targets. For ship detection, a "group feature" detecting method that can adapt to the multi-scale of the target and improve the adaptive of the canny edge detection is proposed that combines the canny edge detection algorithm, image pyramid algorithm and minimum spanning tree clustering algorithm. For ship recognition, a "probability tree" recognition method that can recognize a ship target in multi-angle, multi-target approach is proposed, combining with the probability tree classifier and principal component analysis algorithm. The recognition results are calculated by the total probability formula.
The pyramid model is the basis for visualization and processing of massive remote sensing data in a virtual globe system. It produces large amounts of data tiles in the process of building pyramids however, and the traditional serial CPU algorithm has difficulties satisfying requests for large images. This paper proposes a parallel global pyramid approach that exploits GPU capabilities and multi-threading to enable memory-intensive and computation-intensive tasks. A two-level decomposition strategy was developed to migrate the performance bottlenecks in data transfers among the GPU, CPU, and disk. A multi-threading strategy was used to reduce the delay between CPU and disk, and pin-memory for GPU global delay. Finally, the GPU is used to complete large-scale parallel resample computing and constructing an octree structure used to maximize reuse of data in the graphic memory. Experimental results show that the proposed method can significantly improve pyramid construction speed for large remote sensing images.
Spatial relation research has important applications in conflict detection, cartographic generalization, queries and analysis. Refinement and integrated description of spatial relationships to solve practical problems have become an active direction spatial relationship research. This paper analyzes the shortcomings of current integrated methods to express spatial relationships between lines and regions. We analyze and classify simple topological relations between lines and regions and propose the meta-relation concept. Then, the position, orientation and connecting mode of meta-relations are discussed and an integrated representation model of line-region is detailed. Evaluation criteria for the integrated representation model of spatial relations are brought forward and the proposed model is analyzed using these evaluation criteria.
Hilltops are one of the important terrain features. In view of defects of existing hilltop extraction algorithms based on DEMs, we establish a hilltop extraction model based on the maximum fluctuation threshold according to the terrain morphological feature of hilltops, followed by a relational analysis between different grid sizes of DEM and the accuracy in hilltop extraction with source data from a 1:50 000 DEM of the loess hills taken as an example. A hill top extraction was carried out for a Chinese inland area based on a DEM of 1:1 million; the amount and distribution of hilltops is discussed. The conclusion is: ① The hilltop extraction model based on the maximum fluctuation threshold has a very high application value because the extracted results of the hilltops accord to the actual relief quite well. ② For a terrain with a known generalization scale, there exists an optical DEM grid resolution; when the grid resolution of a DEM is not lower than this threshold, the extracted hilltops have a very good accuracy, otherwise, there would be uncertainty errors, not only in the amount of the extracted hilltops, but also in their spatial distribution. ③ There are a total of 40 038 hilltops in the Chinse inland study area with the altitude greater than 500m and the relative fluctuation greater than 200 m, according to the extraction results based on the DEM of 1:1 million.
Intersections are the critical points of urban transportation, acting as bottlenecks and clog points in urban traffic. The travel time through intersections is highly uncertain and comprises a large proportion of the overall travel time. Detecting the intersection travel time in different turning directions could contribute to improved efficiency in urban transportation. Based on low-frequency spatial-temporal GPS trajectory data, this paper presents a method to detect the intersection travel time. We analyzed four different travel patterns of vehicles according to the trajectory points through intersections. An improved point density method was used to determine the range of an intersection with different travel patterns, reasonably and dynamically. A fuzzy regression model was established to detect intersection travel time accurately. Traffic free flow speed and delays can also be obtained from the proposed method. Wuhan road network and GPS trajectory data were tested in experiments, and the results illustrate the effectiveness of the proposed method in detecting intersection travel time.