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Defense Information, Communication, and Space Technology
Published in Anna M. Doro-on, Handbook of Systems Engineering and Risk Management in Control Systems, Communication, Space Technology, Missile, Security and Defense Operations, 2023
A satellite in eastward equatorial orbit at this altitude therefore remains over the same point on the equator as both it and the earth go around. The satellite has line-of-sight contact with more than 80% of the hemisphere beneath it. To all observers who can see the satellite, it remains in the same position in the sky at all times. Several satellites spaced evenly around the equator give coverage of the entire world except the polar regions as depicted in Figure 5.2-B. The majority of U.S. and Russian communications satellites are in geosynchronous orbit.
Strength of Materials
Published in Daniel H. Nichols, Physics for Technology, 2019
A space elevator is a theoretical method of transportation for traveling from earth to space. It consists of a cable anchored to the earth at the equator, and stretching to space with a large mass attached to its end. The large mass would be placed at a distance from the earth to obtain a geosynchronous orbit so as to remain above the anchoring point on the earth. Centrifugal force will produce a tension in the cable to keep it taught, so an elevator car can ride up along it to space. For a geosynchronous orbit, the satellite will be at a distance r = 42.1 × 106 m with respect to the center of the earth or 35,786 km above mean sea level. The problem with realizing this design has been finding a cable with a high enough tensile strength to endure the extreme tension in the cable needed to function. The cable must be able to support its own weight plus that of the elevator. Each length of cable must support the section below it. Typical designs have a tapper where the cable has a larger diameter in space than the anchor point on the earth. The breaking length, also known as the self-supporting length, is defined to be the maximum length of a material of a fixed diameter that can support its weight hanging vertically. Carbon nanotubes, having an ultimate strength of 62,000 MPa, seem to be the leading candidate for the cable, but can be manufactured in lengths of only several centimeters. Clearly, we will not be riding a space elevator to space anytime soon (Figure 7.16).
G
Published in Philip A. Laplante, Comprehensive Dictionary of Electrical Engineering, 2018
geometric theory of diffraction a correction to geometrical optics that includes diffracted fields due to corners or edges and accounts for energy diffracted into the shadow region. geometric transformation transforms the pixel co-ordinates of an image to effect a change in the spatial relationships of elements in the image. The change often takes the form of a stretching or warping of the image. geometrical optics a high-frequency technique using ray tracing for tubes of rays to determine incident, reflected, and refracted fields. Most useful in real media when the wave amplitude varies slowly compared to the wavelength. geosynchronous orbit an orbit 22,753 miles above the earth in which an object will orbit the earth once every 24 hours above the equator and will appear to be stationary from the earth's surface. geothermal energy thermal energy in the form of hot water and steam in the earth's crust. germanium (Ge) an optical material for construction of components and systems at infrared
Combined BeiDou-2 and BeiDou-3 instantaneous RTK positioning: stochastic modeling and positioning performance assessment
Published in Journal of Spatial Science, 2020
Weikai Miao, Bofeng Li, Zhiteng Zhang, Xuewen Zhang
Since the regional BeiDou navigation satellite system (BDS-2) has been officially released at the end of 2012, its new phase of global constellation (BDS-3) has been constructing and proceeding well (Yang et al. 2019). The regional BDS-2 constellation consists of 14 satellites, of which 5 GEO satellites, 5 inclined geosynchronous orbit (IGSO) satellites, and 4 medium Earth orbit (MEO) satellites (CSNO 2013). By the end of 2018, the global constellation of BDS-3 was primarily built and officially released for global service, including total of 19 satellites (18 operational MEO satellites and 1 testing GEO satellite (CSNO 2018)). Compared to other global navigation satellite systems (GNSSs), BDS-3 system has two characteristics. First, it broadcasts the quad-frequency signals by all satellites and therefore many studied have investigated in BeiDou’s capabilities on ambiguity resolution and positioning (Odolinski et al. 2015, Li et al. 2015a, 2017a, Zhang et al. 2017, Tian et al. 2019). Second, BDS system has unique superiority of the short message communication, with which for example one can transmit the differential corrections to realise the high-precision long-baseline RTK positioning on ocean environment (Li et al. 2019).
Theoretical analysis on power stability and switch time of the non-mechanical beam steering using liquid crystal optical phased array
Published in Liquid Crystals, 2018
Xiangru Wang, Liang Wu, Xiaoxian He, Xinning Huang, Qinggui Tan
To successfully realise non-mechanical laser beam steering, optical phased arrays (OPA) have been developed through a number of techniques to reduce such detrimental situation of the current mechanical beam deflector on free-space optical (FSO) communication system, such as long acquisition time, high Size Weight and Power assumption (SWaP) and so on [1]. One of these techniques, liquid crystal optically phased array (LC-OPA), was firstly demonstrated to show great potential as a non-mechanical OPA through theoretical analysis and experimental verification [2–4]. In subsequent experimental reports using different liquid crystal material and device architecture, hundreds of literatures have been published to verify the validity of LC-OPA on the process of building a dynamic phase controlled optical link, including high precision, high resolution, wide range steering and high efficiency [5–7]. Meanwhile, a set of radiation tests on the condition of simulating the radiation environment on geosynchronous orbit (GEO) show that it is resilient to severe as a module in the lasercom terminal for 10 to 15 years [8,9].
GNSS/MIMU tightly coupled integrated with improved multi-state ZUPT/DZUPT constraints for a Land vehicle in GNSS-denied enviroments
Published in International Journal of Image and Data Fusion, 2021
Yipeng Ning, Wengang Sang, Guobiao Yao, Jingxue Bi, Shida Wang
where is a zenith-referenced undifferenced phase and code standard deviations (STD), is the satellite elevation angle and the values are selected to 3 mm for GPS/BDS carrier phase observations. For pseudorange observations, the values of are taken as 0.3 m for GPS and BDS medium earth orbit (MEO)/inclined geosynchronous orbit (IGSO) observations; a larger value of 0.5 m is used for geostationary earth orbit (GEO) observations in view of the multipath effects.