全文获取类型
收费全文 | 986篇 |
免费 | 13篇 |
专业分类
公路运输 | 157篇 |
综合类 | 312篇 |
水路运输 | 314篇 |
铁路运输 | 150篇 |
综合运输 | 66篇 |
出版年
2023年 | 1篇 |
2022年 | 5篇 |
2021年 | 6篇 |
2020年 | 12篇 |
2019年 | 11篇 |
2018年 | 14篇 |
2017年 | 16篇 |
2016年 | 17篇 |
2015年 | 19篇 |
2014年 | 55篇 |
2013年 | 42篇 |
2012年 | 49篇 |
2011年 | 67篇 |
2010年 | 64篇 |
2009年 | 106篇 |
2008年 | 69篇 |
2007年 | 101篇 |
2006年 | 103篇 |
2005年 | 57篇 |
2004年 | 48篇 |
2003年 | 45篇 |
2002年 | 35篇 |
2001年 | 21篇 |
2000年 | 12篇 |
1999年 | 8篇 |
1998年 | 1篇 |
1997年 | 4篇 |
1996年 | 3篇 |
1995年 | 3篇 |
1994年 | 1篇 |
1993年 | 1篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1990年 | 1篇 |
排序方式: 共有999条查询结果,搜索用时 343 毫秒
11.
随着智能网联汽车的迅速发展,汽车对外通信性能的重要性显著提高,但汽车体积大、重量沉,无法直接借鉴传统通信行业的天线系统测试方法。本文以智能网联汽车的天线系统为研究对象,研究出一种改进的近远场变换算法,并基于该方法设计出一种合理的验证方案,从而降低因汽车测试位置偏移对试验结果的影响。研究表明,当以整车作为被测件时,车载天线在测试系统中心和整车几何中心在测试系统中心的情况下,近场测试数据存在明显的较大差异;然而通过正确的NF-FF变换处理,两者远场辐射方向图都是一致的。说明了在球面近场系统测量加近远场变换的处理下,可以得到整车远场辐射信息。该结论对推动整车无线性能测试提供了实验和验证基础。 相似文献
12.
在城市轨道交通中,当列车运行跨越由多个AP构成的大型无线网络时,存在切换的问题,切换过程中不可避免引起时延和丢包率问题。安全性是轨道交通永恒的主题,过长的时延和重要数据丢失都会引起行车安全。提出了基于位置的切换算法,减少切换时延的同时实现在相邻AP点间接力数据传输,消除丢包率,实现准无缝切换。 相似文献
13.
14.
介绍了一个用于现代物流业的位置服务系统LTLBS,并着重就系统设计中的关键技术和与现代物流业有关的问题进行了分析。针对现代物流的特殊应用,在LTLBS中设计了一些特殊的功能,如货物跟踪,到达时间估计等。系统的性能分析表明定位精度和数据传输时延完全满足要求。 相似文献
15.
17.
18.
Recently connected vehicle (CV) technology has received significant attention thanks to active pilot deployments supported by the US Department of Transportation (USDOT). At signalized intersections, CVs may serve as mobile sensors, providing opportunities of reducing dependencies on conventional vehicle detectors for signal operation. However, most of the existing studies mainly focus on scenarios that penetration rates of CVs reach certain level, e.g., 25%, which may not be feasible in the near future. How to utilize data from a small number of CVs to improve traffic signal operation remains an open question. In this work, we develop an approach to estimate traffic volume, a key input to many signal optimization algorithms, using GPS trajectory data from CV or navigation devices under low market penetration rates. To estimate traffic volumes, we model vehicle arrivals at signalized intersections as a time-dependent Poisson process, which can account for signal coordination. The estimation problem is formulated as a maximum likelihood problem given multiple observed trajectories from CVs approaching to the intersection. An expectation maximization (EM) procedure is derived to solve the estimation problem. Two case studies were conducted to validate our estimation algorithm. One uses the CV data from the Safety Pilot Model Deployment (SPMD) project, in which around 2800 CVs were deployed in the City of Ann Arbor, MI. The other uses vehicle trajectory data from users of a commercial navigation service in China. Mean absolute percentage error (MAPE) of the estimation is found to be 9–12%, based on benchmark data manually collected and data from loop detectors. Considering the existing scale of CV deployments, the proposed approach could be of significant help to traffic management agencies for evaluating and operating traffic signals, paving the way of using CVs for detector-free signal operation in the future. 相似文献
19.
In this research, a Bayesian network (BN) approach is proposed to model the car use behavior of drivers by time of day and to analyze its relationship with driver and car characteristics. The proposed BN model can be categorized as a tree-augmented naive (TAN) Bayesian network. A latent class variable is included in this model to describe the unobserved heterogeneity of drivers. Both the structure and the parameters are learned from the dataset, which is extracted from GPS data collected in Toyota City, Japan. Based on inferences and evidence sensitivity analysis using the estimated TAN model, the effects of each single observed characteristic on car use measures are tested and found to be significant. The features of each category of the latent class are also analyzed. By testing the effect of each car use measure on every other measure, it is found that the correlations between car use measures are significant and should be considered in modeling car use behavior. 相似文献
20.
This paper investigates the market potential and environmental benefits of replacing internal combustion engine (ICE) vehicles with battery electric vehicles (BEVs) in the taxi fleet in Nanjing, China. Vehicle trajectory data collected by onboard global positioning system (GPS) units are used to study the travel patterns of taxis. The impacts of charger power, charging infrastructure coverage, and taxi apps on the feasibility of electric taxis are quantified, considering taxi drivers’ recharging behavior and operating activities. It is found that (1) depending on the charger power and coverage, 19% (with AC Level 2 chargers and 20% charger network coverage) to 56% (with DC chargers and 100% charger network coverage) of the ICE vehicles can be replaced by electric taxis without driving pattern changes; (2) by using taxi apps to find nearby passengers and charging stations, drivers could utilize the empty cruising time to charge the battery, which may increase the acceptance of BEVs by up to 82.6% compared to the scenario without taxi apps; and (3) tailpipe emissions in urban areas could be significantly reduced with taxi electrification: a mixed taxi fleet with 46% compressed-natural-gas-powered (CNG) and 54% electricity-powered vehicles can reduce the tailpipe emissions by 48% in comparison with the fleet of 100% CNG taxis. 相似文献