全文获取类型
收费全文 | 5012篇 |
免费 | 37篇 |
国内免费 | 18篇 |
专业分类
航空 | 2528篇 |
航天技术 | 1782篇 |
综合类 | 17篇 |
航天 | 740篇 |
出版年
2018年 | 73篇 |
2017年 | 64篇 |
2016年 | 53篇 |
2014年 | 88篇 |
2013年 | 123篇 |
2012年 | 106篇 |
2011年 | 156篇 |
2010年 | 112篇 |
2009年 | 183篇 |
2008年 | 238篇 |
2007年 | 122篇 |
2006年 | 93篇 |
2005年 | 129篇 |
2004年 | 130篇 |
2003年 | 156篇 |
2002年 | 99篇 |
2001年 | 162篇 |
2000年 | 100篇 |
1999年 | 93篇 |
1998年 | 139篇 |
1997年 | 95篇 |
1996年 | 133篇 |
1995年 | 166篇 |
1994年 | 179篇 |
1993年 | 95篇 |
1992年 | 121篇 |
1991年 | 58篇 |
1990年 | 60篇 |
1989年 | 131篇 |
1988年 | 51篇 |
1987年 | 52篇 |
1986年 | 76篇 |
1985年 | 158篇 |
1984年 | 136篇 |
1983年 | 117篇 |
1982年 | 115篇 |
1981年 | 155篇 |
1980年 | 51篇 |
1979年 | 45篇 |
1978年 | 51篇 |
1977年 | 45篇 |
1975年 | 51篇 |
1974年 | 45篇 |
1973年 | 41篇 |
1972年 | 37篇 |
1971年 | 46篇 |
1970年 | 34篇 |
1969年 | 40篇 |
1968年 | 34篇 |
1967年 | 36篇 |
排序方式: 共有5067条查询结果,搜索用时 15 毫秒
271.
本文研究了人力资源管理与服务质量的关系。透过问卷调查及数据分析,确立人力资源的重点、结构与相互关系,促进服务质量。研究选择了零售服务为试点,给管理人员提供了一个更直接、更清晰的指引,发挥人力资源本身的素质。研究结果把希士杰等质量大师在美国哈佛商报中提出的“服务利润连接(Serviceprofitchain)”具体化,使管理人员可以直接在企业内运用。 相似文献
272.
A New Mathematical Formulation for Strapdown Inertial Navigation 总被引:12,自引:0,他引:12
A differential equation is developed for the orientation vector relating the body frame to a chosen reference frame. The time derivative of this vector is the sum of the inertially measurable angular velocity vector and of the inertially nonmeasurable noncommutativity rate vector. It is precisely this noncommutativity rate vector that causes the computational problems when numerically integrating the direction cosine matrix. The orientation vector formulation allows the noncommutativity contribution to be isolated and, therefore, treated separately and advantageously. An orientation vector mechanization is presented for a strap down inertial system. Further, an example is given of the applica tion of this formulation to a typical rigid body rotation problem. 相似文献
273.
Miles R.B. Kimball R.B. Frey W.H. 《IEEE transactions on aerospace and electronic systems》1971,(4):578-584
Outdated and rigorous curricula are not only producing engineers unsuited to the challenges of the 1970's, but they are scaring off many of the most creative and intelligent students. Engineering education has become an endurance test; the excitement has been lost. Most students strive only to get out. Engineering schools have not yet realized the importance of the involvement of engineers in current environmental problems. They continue to present only the dry technical courses and seem unconcerned about the student's sense of social perspective or his motivation. This paper proposes a freshman awareness seminar plus project oriented learning in the following years as a remedy for this sad situation. 相似文献
274.
Comments on "Optimum Bandwidth of a Low-Pass Filter for Detection of a Pulse in Nonstationary Noise"
In a recent correspondence1 a calculation of the optimum bandwidth of a low-pass RC filter for the detection of a pulse signal in nonstationary noise was presented. The purpose of this correspondence is: 1) to point out additional references to the work which has been conducted in the stationary noise case, and 2) to present an interesting alternate derivation of the expected output noise power for the nonstationary noise case. 相似文献
275.
Hopper D.J. Andryczyk R.W. 《IEEE transactions on aerospace and electronic systems》1971,(6):1147-1150
The design concept for the traveling wave tube amplifier converter for possible use in the Thermoelectric Outer Planet Spacecraft (TOPS) is presented. An unusual combination of semiconductors and magnetics were utilized to achieve very stable voltage regulation on a number of separate outputs to satisfy the requirements of a high-power traveling wave tube (TWT), and at the same time operate at an efficiency of better than 90 percent from a 30-volt source. The circuitry consists of an output filter, an auxiliary Jensen oscillator driving a high-reactance transformer to provide current limiting to the heater, a variable time delay, a main Jensen oscillator driving the power transformer with a maximum step-up ratio of 120 to 1, and series transistorized post regulators to provide precise voltage adjustment and low output impedance. This paper discusses the design of the high-reactance transformer and the high step-up ratio transformer, as well as the high-voltage series regulators that are limited in range and operate at the top of the unregulated output voltage. Test data is presented, and details of current transients caused by charging the filter circuits, input current ripple, and output voltage ripples are considered. The circuit provides better than 0.5 percent regulation against load change, input voltage change, and over-operating temperature range of from -20 to + 80°C, with output ripple voltage of less than 2 volts peak-to-peak on top of the 3600-Vdc output. The measured efficiency was typically 87 percent. and recommendations are included to improve this to in excess of 90 percent. 相似文献
276.
Power hybrid microelectronic design techniques have been developed to satisfy the requirements of 10-ampere 100-watt series voltage regulators. Application of these new techniques has resulted in a package occupying roughly the same volume as a TO-63 power transistor, yet containing two 30-ampere power transistors, selectable external components, and associated control circuitry. 相似文献
277.
W. I. Axford 《Space Science Reviews》1968,8(3):331-365
Observations bearing on the nature and properties of the interplanetary plasma are reviewed, and consideration is given to possible fruitful directions for further work. The observations are classified according as they involve traditional (comet tail, optical, geomagnetic, cosmic ray), radio (solar noise, radar, radio-source scattering and scintillation, space-probe transmission) or direct (space-probe) measurements. A fairly complete set of references up to September 1967 is given for the cases of comet tail, radar, radio-source scattering and scintillation, and space-probe measurements.An important development concerns observations of the composition of the solar wind. High-resolution measurements of the positive ion energy per charge spectra have been made using the Vela-3 satellites (Bame
et al., 1968). Ionic components other than H+ and He++ have been detected, notably the various ions of oxygen, O+5, O+6, O+7, (Hundhausen
et al., 1968). A promising technique for unambiguously distinguishing H+ and He++ ions, based on velocity as well as energy per unit charge, has been flown successfully on the satellite IMP-F by Ogilvie and Williamson (1968).This research was supported by the Advanced Research Projects Agency (Project DEFENDER) and was monitored by the U.S. Army Research Office — Durham under Contract DA-31-124-ARO-D-257. 相似文献
278.
Bates Martin R. Fletcher Harold K. Michnik Lewis Prast Johannes W. 《IEEE transactions on aerospace and electronic systems》1968,(2):238-256
Present-day collision avoidance systems (CAS) of the time-frequency variety employ modes of operation similar to those of airborne equipment which has been operating in military aircraft since 1959. A fleet operational evaluation of these systems began in 1961 in aircraft of U. S. Navy Helicopter Squadron HS-4, based on the aircraft carrier U.S.S. Yorktown. This equipment utilized a local clock in each aircraft, a separate time slot for each aircraft's interrogation signal, air-to-air coarse synchronization of all stations, and sufficient free-drift stability for time slot keeping. Operational use of one-way ranging with elimination of propagation delay offsets, higher clock stability for open-loop time keeping in the SNS-64 ... AN/APN-169 family of systems began in 1964 in U. S. Air Force C-130E turboprop aircraft. In 1965, use of the EROS I collision avoidance system began during flight testing of F-4 Phantom supersonic aircraft. 相似文献
279.
The collision avoidance system (CAS) defined by the ATA provides for the exchange of range, range-rate, and altitude data, as well as the less significant items of altitude rate and north-south and east-west velocity vector data. This paper discusses the CAS message format and presents data (based on McDonnell-Douglas in-flight experience with its EROS system) on the accuracy of gathering the CAS data. 相似文献
280.
A coherent CW superheterodyne radar system operating at frequencies of 9, 17, 35, and 70 GHz is described. The radars are installed on a free-flight range to study backscattering from wakes of hypersonic-velocity projectiles. Each radar is equipped with a focused-lens antenna oriented at an angle of approximately 45° to the flight axis. Amplitude and phase of the received signal are recorded separately. Some typical results are given to demonstrate the capabilities of the equipment. 相似文献