全文获取类型
收费全文 | 71229篇 |
免费 | 1127篇 |
国内免费 | 659篇 |
专业分类
测绘学 | 1714篇 |
大气科学 | 5029篇 |
地球物理 | 14214篇 |
地质学 | 25227篇 |
海洋学 | 6164篇 |
天文学 | 16332篇 |
综合类 | 208篇 |
自然地理 | 4127篇 |
出版年
2022年 | 396篇 |
2021年 | 704篇 |
2020年 | 777篇 |
2019年 | 825篇 |
2018年 | 1893篇 |
2017年 | 1759篇 |
2016年 | 2235篇 |
2015年 | 1318篇 |
2014年 | 2176篇 |
2013年 | 3766篇 |
2012年 | 2276篇 |
2011年 | 3118篇 |
2010年 | 2605篇 |
2009年 | 3504篇 |
2008年 | 3254篇 |
2007年 | 3045篇 |
2006年 | 2876篇 |
2005年 | 2353篇 |
2004年 | 2253篇 |
2003年 | 2110篇 |
2002年 | 1927篇 |
2001年 | 1800篇 |
2000年 | 1716篇 |
1999年 | 1377篇 |
1998年 | 1470篇 |
1997年 | 1387篇 |
1996年 | 1100篇 |
1995年 | 1142篇 |
1994年 | 962篇 |
1993年 | 872篇 |
1992年 | 855篇 |
1991年 | 759篇 |
1990年 | 856篇 |
1989年 | 717篇 |
1988年 | 650篇 |
1987年 | 814篇 |
1986年 | 663篇 |
1985年 | 855篇 |
1984年 | 922篇 |
1983年 | 862篇 |
1982年 | 831篇 |
1981年 | 707篇 |
1980年 | 663篇 |
1979年 | 606篇 |
1978年 | 603篇 |
1977年 | 552篇 |
1976年 | 539篇 |
1975年 | 499篇 |
1974年 | 506篇 |
1973年 | 468篇 |
排序方式: 共有10000条查询结果,搜索用时 16 毫秒
411.
SVLBI (space very long baseline interferometry) has some important potential applications in geodesy and geodynamics, for which one of the most difficult tasks is to precisely determine the orbit of an SVLBI satellite. This work studies several technologies that will possibly be able to determine the orbit of a space VLBI satellite. Then, according to the types and charac- teristics of the satellite and the requirements for geodetic study and the geometry of the GNSS (GPS, GALILEO) satellite to track the space VLBI satellite, the six Keplerian elements of the SVLBI satellite (TEST-SVLBI) are determined. A program is designed to analyze the coverage area of space of different altitudes by the stations of the network, with which the tracking network of TEST-SVLBI is designed. The efficiency of tracking TEST-SVLBI by the network is studied, and the results are presented. 相似文献
412.
Ocean tide loading (OTL) displacements from global and local grids: comparisons to GPS estimates over the shelf of Brittany, France 总被引:1,自引:0,他引:1
Stavros A. Melachroinos R. Biancale M. Llubes F. Perosanz F. Lyard M. Vergnolle M. -N. Bouin F. Masson J. Nicolas L. Morel S. Durand 《Journal of Geodesy》2008,82(6):357-371
In this paper we examine OTL displacements detected by GPS stations of a dedicated campaign and validate ocean tide models.
Our area of study is the continental shelf of Brittany and Cotentin in France. Brittany is one of the few places in the world
where tides provoke loading displacements of ∼10–12 cm vertically and a few cm horizontally. Ocean tide models suffer from
important discrepancies in this region. Seven global and regional ocean tide models were tested: FES2004 corrected for K2,
TPXO.7.0, TPXO.6.2, GOT00.2, CSR4.0, NAO.99b and the most recent regional grids of the North East Atlantic (NEA2004). These
gridded amplitudes and phases of ocean tides were convolved in order to get the predicted OTL displacements using two different
algorithms. Data over a period of 3.5 months of 8 GPS campaign stations located on the north coast of Brittany are used, in
order to evaluate the geographical distribution of the OTL effect. We have modified and implemented new algorithms in our
GPS software, GINS 7.1. GPS OTL constituents are estimated based on 1-day batch solutions. We compare the observed GPS OTL
constituents of M2, S2, N2 and K1 waves with the selected ocean tide models on global and regional grids. Large phase-lag and amplitude discrepancies over
20° and 1.5 cm in the vertical direction in the semi-diurnal band of M2 between predictions and GPS/models are detected in the Bay of Mont St-Michel. From a least squares spectral analysis of the
GPS time-series, significant harmonic peaks in the integer multiples of the orbital periods of the GPS satellites are observed,
indicating the existence of multipath effects in the GPS OTL constituents. The GPS OTL observations agree best with FES2004,
NEA2004, GOT00.2 and CSR4.0 tide models. 相似文献
413.
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
414.
Spin rate estimation of sounding rockets using GPS wind-up 总被引:1,自引:1,他引:1
Carrier phase wind-up is a well-known effect that arises from the relative rotation between a transmitting and receiving antenna.
In GPS measurements at L1 frequency, this effect translates into an error of 19.029 cm per full relative rotation of antennas.
Since this effect is independent of the satellite elevation for pure rotation about the antenna boresight axis, it is usually
absorbed by the clock estimation in navigation algorithms. Therefore, the impact of wind-up is usually neglected for applications
that do not require accuracies to the cm level like RTK. However, in receiving platforms with high rotation rate, the accumulated
wind-up value can be important and actually be larger than receiver noise or even ionospheric variations. Therefore, in such
scenarios, the wind-up contribution can be isolated and used as a source of information to compute the spin rate of such platforms
using an appropriate combination of GPS observables. This work shows some results of a coarse, yet simple, approach to monitor
the rotation angle and spin-rate of spin stabilized sounding rockets flown by DLR. 相似文献
415.
This paper describes the spatial and functional evolution of a central place system as market conditions change with population growth. Utilizing a partial equilibrium optimization model, we examine the spatial response of two economic sectors to increases in market populations resulting from natural increase and migration. Response in both sectors is conditioned by threshold demand, with factor prices also affecting one of the sectors. As the central place system evolves it exhibits spatial and functional characteristics that are initially consistent with a Löschian landscape, then a Christallerian landscape at higher populations, while at even larger populations Krugman’s landscape emerges. 相似文献
416.
Kristine M. Larson Eric E. Small Ethan Gutmann Andria Bilich Penina Axelrad John Braun 《GPS Solutions》2008,12(3):173-177
Measurements of soil moisture are important for studies of climate and weather forecasting, flood prediction, and aquifer
recharge studies. Although soil moisture measurement networks exist, most are sparsely distributed and lack standardized instrumentation.
Measurements of soil moisture from satellites have extremely large spatial footprints (40–60 km). A methodology is described
here that uses existing networks of continuously-operating GPS receivers to measure soil moisture fluctuations. In this technique,
incoming signals are reflected off and attenuated by the ground before reception by the GPS receiver. These multipath reflections
directly affect signal-to-noise ratio (SNR) data routinely collected by GPS receivers, creating amplitude variations that
are a function of ground reflectivity and therefore soil moisture content. After describing this technique, multipath reflection
amplitudes at a GPS site in Tashkent, Uzbekistan are compared to estimates of soil moisture from the Noah land surface model.
Although the GPS multipath amplitudes and the land surface model are uncalibrated, over the 70-day period studied, they both
rise sharply following each rainfall event and slowly decrease over a period of ∼10 days. 相似文献
417.
This paper presents a continuous version of the model of distribution dynamics to analyse the transition dynamics and implied
long-run behaviour of the EU-27 NUTS-2 regions over the period 1995–2003. It departs from previous research in two respects:
first, by introducing kernel estimation and three-dimensional stacked conditional density plots as well as highest density regions
plots for the visualisation of the transition function, based on Hyndman et al. (J Comput Graph Stat 5(4):315–336, 1996), and second, by combining Getis’ spatial filtering view with kernel estimation to explicitly account for the spatial dimension of the
growth process. The results of the analysis indicate a very slow catching-up of the poorest regions with the richer ones,
a process of shifting away of a small group of very rich regions, and highlight the importance of geography in understanding
regional income distribution dynamics.
相似文献
Manfred M. FischerEmail: |
418.
H.B. Chauhan R.M. Dwivedi 《International Journal of Applied Earth Observation and Geoinformation》2008,10(2):181-1
Coastal zone assumes importance due to high productivity of ecosystems, man-made developmental activities, natural hazards and dynamic nature of the coast. As costal ecosystems are unique and fragile, understanding the impact of developmental activities on the sustainability of the coastal zone is very important. Remote sensing, because of repetitive and synoptic nature is an ideal tool for studying this. Time series data analyses for monitoring coastal zone require different type of sensors. Present study deals with atmospheric correction of satellite data, reflectance, selection of coastal features like, mudflat, mangroves, vegetated dune, coastal water, etc. and their inter-comparison using different sensor data of RESOURCESAT sensors. Reflectance values give better separateability for various coastal features in comparison to DN values. LISS IV can be used in place of LISS III or merged (LISS III + PAN) for long-term coastal zone studies. 相似文献
419.
High-dimensional image data open new possibilities in remote sensing digital image classification, particularly when dealing with classes that are spectrally very similar. The main problem refers to the estimation of a large number of classifier's parameters. One possible solution to this problem consists in reducing the dimensionality of the original data without a significant loss of information. In this letter, a new approach to reduce data dimensionality is proposed. In the proposed methodology, each pixel's curve of spectral response is initially segmented, and the digital numbers (DNs) at each segment are replaced by a smaller number of statistics. In this letter, the proposed statistics are the mean and variance of the segment's DNs, which are supposed to carry information about the segment's position and shape, respectively. Tests were performed by using Airborne Visible/Infrared Imaging Spectrometer hyperspectral image data. The experiments have shown that this methodology is capable of providing very acceptable results, in addition of being computationally efficient 相似文献
420.
In the proposed approach, the well-known enhanced Lee filter is modified to allow the integration of feature outlines-previously extracted from segmented optical images. The filter is applied to several ENVISAT ASAR images that cover urban, agricultural, and forest areas during different plant phenological stages. The performance of this segment-based speckle filter is compared to those of other filters using ratio images, visual interpretation, and statistical indexes. The approach reduces the loss of radiometry and spatial information. It performs comparable to more complex methods and outperforms common techniques 相似文献