全文获取类型
收费全文 | 132562篇 |
免费 | 1941篇 |
国内免费 | 946篇 |
专业分类
测绘学 | 3048篇 |
大气科学 | 9238篇 |
地球物理 | 26355篇 |
地质学 | 46566篇 |
海洋学 | 11945篇 |
天文学 | 29647篇 |
综合类 | 329篇 |
自然地理 | 8321篇 |
出版年
2021年 | 1285篇 |
2020年 | 1475篇 |
2019年 | 1607篇 |
2018年 | 3194篇 |
2017年 | 2999篇 |
2016年 | 3674篇 |
2015年 | 2086篇 |
2014年 | 3576篇 |
2013年 | 6830篇 |
2012年 | 3846篇 |
2011年 | 5222篇 |
2010年 | 4693篇 |
2009年 | 6160篇 |
2008年 | 5431篇 |
2007年 | 5339篇 |
2006年 | 5141篇 |
2005年 | 3937篇 |
2004年 | 3905篇 |
2003年 | 3679篇 |
2002年 | 3584篇 |
2001年 | 3141篇 |
2000年 | 3090篇 |
1999年 | 2569篇 |
1998年 | 2599篇 |
1997年 | 2446篇 |
1996年 | 2164篇 |
1995年 | 2108篇 |
1994年 | 1846篇 |
1993年 | 1743篇 |
1992年 | 1641篇 |
1991年 | 1564篇 |
1990年 | 1689篇 |
1989年 | 1490篇 |
1988年 | 1332篇 |
1987年 | 1608篇 |
1986年 | 1417篇 |
1985年 | 1736篇 |
1984年 | 2010篇 |
1983年 | 1913篇 |
1982年 | 1750篇 |
1981年 | 1655篇 |
1980年 | 1444篇 |
1979年 | 1408篇 |
1978年 | 1417篇 |
1977年 | 1266篇 |
1976年 | 1218篇 |
1975年 | 1191篇 |
1974年 | 1172篇 |
1973年 | 1226篇 |
1972年 | 751篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
841.
Pipia L. Fabregas X. Aguasca A. Lopez-Martinez C. 《Geoscience and Remote Sensing Letters, IEEE》2008,5(1):88-92
In this letter, a coherence-based technique for atmospheric artifact removal in ground-based (GB) zero-baseline synthetic aperture radar (SAR) acquisitions is proposed. For this purpose, polarimetric measurements acquired using the GB-SAR sensor developed at the Universitat Politecnica de Catalunya are employed. The heterogeneous environment of Collserola Park in the outskirts of Barcelona, Spain, was selected as the test area. Data sets were acquired at X-band during one week in June 2005. The effects of the atmosphere variations between successive zero-baseline SAR polarimetric acquisitions are treated here in detail. The need to compensate for the resulting phase-difference errors when retrieving interferometric information is put forward. A compensation technique is then proposed and evaluated using the control points placed inside the observed scene. 相似文献
842.
Martinez-Espla J.J. Martinez-Marin T. Lopez-Sanchez J.M. 《Geoscience and Remote Sensing Letters, IEEE》2008,5(2):147-151
This letter presents a phase-unwrapping (PU) algorithm for synthetic aperture radar interferometry based on a grid-based filter. The proposed PU algorithm, which is based on state-space techniques, simultaneously performs noise filtering and PU. The formulation of this technique provides independence from noise statistics and is not constrained by the nonlinearity of the problem. Results obtained with synthetic data show a significant improvement with respect to other conventional PU algorithms in some situations. 相似文献
843.
Mudanyal O. Yldz S. Semerci O. Yapar A. Akduman I. 《Geoscience and Remote Sensing Letters, IEEE》2008,5(2):180-184
A microwave imaging method for nondestructive testing of perfectly conducting surfaces beyond a layered media is presented. The method is an adaptation of the surface reconstruction approach by Yapar et al. to the present problem. It is based on the analytical continuation of the measured data to the surface under test through a special representation of the scattered field in terms of Fourier transform and Taylor expansion. Then the problem is reduced to the solution of a nonlinear equation which is solved iteratively via the Newton method and regularization in the least squares sense. Numerical simulations show that defects as small as lambda/500 can be recovered through the presented algorithm. 相似文献
844.
Accuracy assessment of lidar-derived digital elevation models 总被引:2,自引:0,他引:2
Despite the relatively high cost of airborne lidar-derived digital elevation models (DEMs), such products are usually presented without a satisfactory associated estimate of accuracy. For the most part, DEM accuracy estimates are typically provided by comparing lidar heights against a finite sample of check point coordinates from an independent source of higher accuracy, supposing a normal distribution of the derived height differences or errors. This paper proposes a new methodology to assess the vertical accuracy of lidar DEMs using confidence intervals constructed from a finite sample of errors computed at check points. A non-parametric approach has been tested where no particular error distribution is assumed, making the proposed methodology especially applicable to non-normal error distributions of the type usually found in DEMs derived from lidar. The performance of the proposed model was experimentally validated using Monte Carlo simulation on 18 vertical error data-sets. Fifteen of these data-sets were computed from original lidar data provided by the International Society for Photogrammetry and Remote Sensing Working Group III/3, using their respective filtered reference data as ground truth. The three remaining data-sets were provided by the Natural Environment Research Council's Airborne Research and Survey Facility lidar system, together with check points acquired using high precision kinematic GPS. The results proved promising, the proposed models reproducing the statistical behaviour of vertical errors of lidar using a favourable number of check points, even in the cases of data-sets with non-normally distributed residuals. This research can therefore be considered as a potentially important step towards improving the quality control of lidar-derived DEMs. 相似文献
845.
The effect of look angle on the accuracy performance of fixed-baseline interferometric synthetic aperture radar is studied. It is shown that there exists an optimal look angle that minimizes the variance of the surface height estimate for a resolution cell, and it depends upon the system as well as surface parameters. Numerical analysis confirming the existence of the optimal look angle is presented 相似文献
846.
An Efficient Reordering Prediction-Based Lossless Compression Algorithm for Hyperspectral Images 总被引:8,自引:0,他引:8
In this letter, we propose an efficient lossless compression algorithm for hyperspectral images; it is based on an adaptive spectral band reordering algorithm and an adaptive backward previous closest neighbor (PCN) prediction with error feedback. The adaptive spectral band reordering algorithm has some strong points. It can adaptively determine the range of spectral bands needed to be reordered, and it can efficiently find the optimum branches. Hyperspectral images have a large number of spectral bands, which express the same land cover structure and have high correlation. The adaptive backward PCN prediction with error feedback can sufficiently make use of this correlation. Experiments show that implementing both the reordering of the spectral bands before prediction and the prediction with error feedback improve compression performance 相似文献
847.
Amitai E. Nystuen J.A. Anagnostou E.N. Anagnostou M.N. 《Geoscience and Remote Sensing Letters, IEEE》2007,4(3):406-410
Deep-water acoustical measurements of rainfall are compared to high-resolution ground radar observations for the first time. The measurements of underwater ambient sound were made from a subsurface mooring near Methoni, Greece, in 2004. The acoustical measurements were at 60-, 200-, 1000-, and 2000-m depths. Simultaneous ground-based polarimetric -band radar observations were made over the acoustic mooring. Comparisons show acoustic detection of rain events and storm structure that are in agreement with the radar observations. Results from a comparison between the underwater sound pressure level at different depths and the observed radar reflectivities are presented. 相似文献
848.
Min Zhu Wadge G. Holley R.J. James I.N. Clark P.A. Changgui Wang Woodage M.J. 《Geoscience and Remote Sensing Letters, IEEE》2007,4(3):401-405
Propagation delay due to variable tropospheric water vapor (WV) is one of the most intractable problems for radar interferometry, particularly over mountains. The WV field can be simulated by an atmospheric model, and the difference between the two fields is used to correct the radar interferogram. Here, we report our use of the U.K. Met Office Unified Model in a nested mode to produce high-resolution forecast fields for the 3-km-high Mount Etna volcano. The simulated precipitable-water field is validated against that retrieved from the Medium-Resolution Imaging Spectrometer (MERIS) radiometer on the Envisat satellite, which has a resolution of 300 m. Two case studies, one from winter (November 24, 2004) and one from summer (June 25, 2005), show that the mismatch between the model and the MERIS fields ( rms = 1.1 and 1.6 mm, respectively) is small. One of the main potential sources of error in the models is the timing of the WV field simulation. We show that long-wavelength upper tropospheric troughs of low WV could be identified in both the model output and Meteosat WV imagery for the November 24, 2004 case and used to choose the best time of model output. 相似文献
849.
Testing satellite and ground thermal imaging of low-temperature fumarolic fields: The dormant Nisyros Volcano (Greece) 总被引:1,自引:0,他引:1
E. Lagios S. Vassilopoulou V. Sakkas V. Dietrich B.N. Damiata A. Ganas 《ISPRS Journal of Photogrammetry and Remote Sensing》2007,62(6):447-460
The Nisyros Volcano (Greece) was monitored by satellite and ground thermal imaging during the period 2000–2002. Three night-scheduled Landsat-7 ETM+ thermal (band 6) images of Nisyros Island were processed to obtain land surface temperature. Ground temperature data were also collected during one of the satellite overpasses. Processed results involving orthorectification and 3-D atmospheric correction clearly show the existence of a thermal anomaly inside the Nisyros Caldera. This anomaly is associated mainly with the largest hydrothermal craters and has land surface temperatures 5–10 °C warmer than its surroundings. The ground temperature generally increased by about 4 °C inside the main crater over the period 2000–2002. Ground thermal images of the hydrothermal Stephanos Crater were also collected in 2002 using a portable thermal infrared camera. These images were calibrated to ground temperature data and orthorectified. A difference of about 0–2 °C was observed between the ground thermal images and the ground temperature data. The overall study demonstrates that satellite remote sensing of low-temperature fumarolic fields within calderas can provide a reliable long-term monitoring tool of dormant volcanoes that have the potential to reactivate. Similarly, a portable thermo-imager can easily be deployed for real-time monitoring using telemetric data transfer. The operational costs for both systems are relatively low for an early warning system. 相似文献
850.
P. J. G. Teunissen 《Journal of Geodesy》2007,81(12):759-780
In this contribution, we extend the existing theory of minimum mean squared error prediction (best prediction). This extention
is motivated by the desire to be able to deal with models in which the parameter vectors have real-valued and/or integer-valued
entries. New classes of predictors are introduced, based on the principle of equivariance. Equivariant prediction is developed
for the real-parameter case, the integer-parameter case, and for the mixed integer/real case. The best predictors within these
classes are identified, and they are shown to have a better performance than best linear (unbiased) prediction. This holds
true for the mean squared error performance, as well as for the error variance performance. We show that, in the context of
linear model prediction, best predictors and best estimators come in pairs. We take advantage of this property by also identifying
the corresponding best estimators. All of the best equivariant estimators are shown to have a better precision than the best
linear unbiased estimator. Although no restrictions are placed on the probability distributions of the random vectors, the
Gaussian case is derived separately. The best predictors are also compared with least-squares predictors, in particular with
the integer-based least-squares predictor introduced in Teunissen (J Geodesy, in press, 2006). 相似文献