全文获取类型
收费全文 | 1598篇 |
免费 | 479篇 |
国内免费 | 210篇 |
专业分类
航空 | 914篇 |
航天技术 | 646篇 |
综合类 | 95篇 |
航天 | 632篇 |
出版年
2024年 | 17篇 |
2023年 | 53篇 |
2022年 | 72篇 |
2021年 | 87篇 |
2020年 | 124篇 |
2019年 | 86篇 |
2018年 | 113篇 |
2017年 | 79篇 |
2016年 | 98篇 |
2015年 | 83篇 |
2014年 | 103篇 |
2013年 | 110篇 |
2012年 | 98篇 |
2011年 | 136篇 |
2010年 | 116篇 |
2009年 | 105篇 |
2008年 | 91篇 |
2007年 | 87篇 |
2006年 | 97篇 |
2005年 | 86篇 |
2004年 | 70篇 |
2003年 | 57篇 |
2002年 | 49篇 |
2001年 | 45篇 |
2000年 | 39篇 |
1999年 | 40篇 |
1998年 | 27篇 |
1997年 | 17篇 |
1996年 | 13篇 |
1995年 | 13篇 |
1994年 | 18篇 |
1993年 | 7篇 |
1992年 | 10篇 |
1991年 | 17篇 |
1990年 | 9篇 |
1989年 | 5篇 |
1988年 | 6篇 |
1987年 | 3篇 |
1986年 | 1篇 |
排序方式: 共有2287条查询结果,搜索用时 453 毫秒
491.
《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2018,61(6):1573-1587
This paper explores two types of mathematical functions to fit single- and full-frequency waveform of spaceborne Global Navigation Satellite System-Reflectometry (GNSS-R), respectively. The metrics of the waveforms, such as the noise floor, peak magnitude, mid-point position of the leading edge, leading edge slope and trailing edge slope, can be derived from the parameters of the proposed models. Because the quality of the UK TDS-1 data is not at the level required by remote sensing mission, the waveforms buried in noise or from ice/land are removed by defining peak-to-mean ratio, cosine similarity of the waveform before wind speed are retrieved. The single-parameter retrieval models are developed by comparing the peak magnitude, leading edge slope and trailing edge slope derived from the parameters of the proposed models with in situ wind speed from the ASCAT scatterometer. To improve the retrieval accuracy, three types of multi-parameter observations based on the principle component analysis (PCA), minimum variance (MV) estimator and Back Propagation (BP) network are implemented. The results indicate that compared to the best results of the single-parameter observation, the approaches based on the principle component analysis and minimum variance could not significantly improve retrieval accuracy, however, the BP networks obtain improvement with the RMSE of 2.55 m/s and 2.53 m/s for single- and full-frequency waveform, respectively. 相似文献
492.
Shyh-Biau Jiang Tse-Liang Yeh Li-Wu Chen Jann-Yenq Liu Ming-Hsuan Yu Yu-Qin Huang Chen-Kiang Chiang Chung-Jen Chou 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2018,61(10):2673-2679
In this study, we construct a photomultiplier calibration system. This calibration system can help scientists measuring and establishing the characteristic curve of the photon count versus light intensity. The system uses an innovative 10-fold optical attenuator to enable an optical power meter to calibrate photomultiplier tubes which have the resolution being much greater than that of the optical power meter. A simulation is firstly conducted to validate the feasibility of the system, and then the system construction, including optical design, circuit design, and software algorithm, is realized. The simulation generally agrees with measurement data of the constructed system, which are further used to establish the characteristic curve of the photon count versus light intensity. 相似文献
493.
低成本IMU/GPS组合导航系统研究 总被引:2,自引:0,他引:2
GPS/INS组合导航系统已在一定领域得到应用,而INS昂贵的价格限制了它的广泛应用,本文采用低成本惯性测量组件(IMU)设计了低成本IMU/GPS组合系统的硬件和相应的软件,进行了静态实验和数据处理,系统的俯仰角和横滚角误差均小于0.2°,实验结果表明,低成本组合系统的性能较好,而其性能价格比更高。 相似文献
494.
佟鑫 《中国民航学院学报》2000,18(1):30-32,37
随着民航机场建设的加强,机场导航台遥控方式的选择渐渐提上议程。针对无线控遥的特点和功能,提出了利用以PC机为主的无线遥控原理的硬件模型。 相似文献
495.
Real-time and accurate fault detection is essential to enhance the aircraft navigation system’s reliability and safety. The existent detection methods based on analytical model draws back at simultaneously detecting gradual and sudden faults. On account of this reason, we propose an online detection solution based on non-analytical model. In this article, the navigation system fault detection model is established based on belief rule base (BRB), where the system measuring residual and its changing rate are used as the inputs of BRB model and the fault detection function as the output. To overcome the drawbacks of current parameter optimization algorithms for BRB and achieve online update, a parameter recursive estimation algorithm is presented for online BRB detection model based on expectation maximization (EM) algorithm. Furthermore, the proposed method is verified by navigation experiment. Experimental results show that the proposed method is able to effectively realize online parameter evaluation in navigation system fault detection model. The output of the detection model can track the fault state very well, and the faults can be diagnosed in real time and accurately. In addition, the detection ability, especially in the probability of false detection, is superior to offline optimization method, and thus the system reliability has great improvement. 相似文献
496.
497.
在捷联惯导制导工具误差分离过程中引入交叉验证检验制导工具误差分离精度,并基于均方误差准则建立量化检验指标.首先建立简化的制导工具误差分离模型,并将数据划分成训练集和检验集;然后利用训练集进行制导工具误差分离;最后利用检验集和量化检验指标对分离效果进行检验.数学仿真表明,提出的误差分离模型正确可靠,精度较高,交叉验证能够对分离效果进行有效检验. 相似文献
498.
J.G. Cerqueira Jr. J.H. Fernandez J.J. Hoelzemann N.M.P. Leme C.T. Sousa 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2014
Due to the high costs of commercial monitoring instruments, a portable sun photometer was developed at INPE/CRN laboratories, operating in four bands, with two bands in the visible spectrum and two in near infrared. The instrument calibration process is performed by applying the classical Langley method. Application of the Langley’s methodology requires a site with high optical stability during the measurements, which is usually found in high altitudes. However, far from being an ideal site, Harrison et al. (1994) report success with applying the Langley method to some data for a site in Boulder, Colorado. Recently, Liu et al. (2011) show that low elevation sites, far away from urban and industrial centers can provide a stable optical depth, similar to high altitudes. In this study we investigated the feasibility of applying the methodology in the semiarid region of northeastern Brazil, far away from pollution areas with low altitudes, for sun photometer calibration. We investigated optical depth stability using two periods of measurements in the year during dry season in austral summer. The first one was in December when the native vegetation naturally dries, losing all its leaves and the second one was in September in the middle of the dry season when the vegetation is still with leaves. The data were distributed during four days in December 2012 and four days in September 2013 totaling eleven half days of collections between mornings and afternoons and by means of fitted line to the data V0 values were found. Despite the high correlation between the collected data and the fitted line, the study showed a variation between the values of V0 greater than allowed for sun photometer calibration. The lowest V0 variation reached in this experiment with values lower than 3% for the bands 500, 670 and 870 nm are displayed in tables. The results indicate that the site needs to be better characterized with studies in more favorable periods, soon after the rainy season. 相似文献
499.
500.