Estimating the Baseline Error of Wide-Swath Altimeters Using Nadir Altimeters via Numerical Simulation

The baseline roll and length errors for wide-swath altimeters are major error sources in sea surface measurements that exhibit strong spatial characteristics in the cross-track direction.These errors can be identified and estimated in accordance with height differences at crossover points generated with nadir altimeters after excluding the interference from other error sources.Most of the wide-swath altimeter baseline estimation methods considered only the roll error in previous studies.A numerical simulation was conducted in this study using nadir altimeters to estimate the roll and length errors simultaneously to provide a selectable scheme for baseline error estimation and correction for future wide-swath altimeters.Results based on the parameters of the surface water and ocean topography mission and Sentinel-3A show that the correlation coefficient of the roll error between the estimated and simulated values is 0.89,while the correlation coefficient of the length error is 0.85.The sea surface height root mean square error(RMSE)can be reduced from 12.18 cm to 6.45 cm based on the two estimated results.The estimation effect can be increased by using multiple nadir altimeters to form an observation constellation.The numerical simulation of the five nadir altimeter constellation shows that the correlation coefficients of the roll and length errors would increase to 0.97,which reduces the sea surface height RMSE to 2.88 cm.In addition,the stability of this method is indicated in simulation experiments,which introduce different degrees of sea state errors.

Research on changes and circulations of sea-ice in Eurasian in recent 50 years

Using the monthly 1° × 1° sea-ice concentration data of Hadley center and the monthly NCEP geopotential height data from January 1953 to February 2003, temporal and spatial changing characters of sea-ice arc examined. The results show almost all of the sea-ice of eight regions was decreasing, especially all seasons in Europe. But in Asia part, those display some increasing trends in spring and winter, Abrupt times of sea-ice in Europe were at end of 1970＇ s and in Asia the times in summer/fall（spring/winter） were at end of 1980＇ s.

Performance analysis of multi-GNSS static and RTK techniques in estimating height differences

Establishing reliable elevation differences is imperative for most geoscience and engineering applications.This work has traditionally been accomplished through spirit leveling techniques;however,surveyors have been utilizing satellite positioning systems in measuring height differences for more than a decade.Yet the quality of these heights needs to be evaluated in order to adopt them in different applications.In this article,we present the outcome of an accuracy assessment of height differences obtained with static and RTK surveys.Twenty control points with an average baseline length of 1 km were occupied with dual-frequency GNSS receivers for different time periods.Collected signals were processed using open-source software and verified with an online processing tool.Heights were estimated by processing the GPS and the GLONASS data individually,and combined(i.e.GNSS).Height differences were determined and compared with those measured by spirit levels and corrected through geoid models.Best results were achieved by combining GPS and GLONASS solutions for both static and RTK surveys.Solutions with either GPS or GLONASS satellites were comparable,but in most cases,the GPS solutions performed better.For the static surveys,longer occupation provided much accurate height differences.Inconsistencies among 10 different RTK surveys were minimum for the GPS+GLONASS solutions and worst for the GLONASS solutions.The ANOVA,LSD,F,and χ^(2) statistical tests confirmed our findings at the 95%confidence level.

Characterization of pavement texture by means of height difference correlation and relation to wet skid resistance

Driving safety is of utmost importance in the automobile industry and is acknowledged by the introduction of the tire wet grip index as part of the EU tire label. The rubber pavement interaction is determined by the viscoelastic properties of the rubber as well as by the pavement texture. Nowadays available optical surface profiling instruments allow for a detailed measurement of surface roughness covering several length scales. This enables the validation of a mathematical statistical description of pavement texture within the framework of self-affine surfaces and hence provides a holistic characterization of surface roughness covering several length scales within a few characteristic parameters. We deduce within this article the correlation between classical surface roughness pa- rameters and the parameter set of self-affine surfaces. These parameters allow for a detailed understanding of the relationship between pavement texture and its wet skid resistance. We present wet skid resistance measurements with the British pendulum and a linear friction tester device on different pavement textures. We demonstrate that the so- called estimated texture depth does not correlate to the surface skid resistance measured with the British pendulum. Finally, we deduce a dependency of wet skid resistance on pavement texture which is supported by current models for hysteresis friction.