ISSN:0028-1522    

DOI:10.1002/j.2161-4296.1984.tb00855.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

ABSTRACTSince NAVSTAR/GPS has extensive application, users may have diverse requirements. Here we study how to select the satellites at every instant.In GPS navigation, users usually obtain their positions and clock biases from four visible satellites. The selection of these four satellites has a great influence on the accuracy of a navigation fix. The degree of this influence is well represented by GDOP. In this paper GDOP is expressed by the simplest form of a ratio of polynomials using a well‐chosen coordinate. Using this expression, it is shown that the minimum GDOP is realized when the four satellites are located in symmetric directions with respect to the user position, and that the minimum achievable value of GDOP is 1.5811. The reason GDOP tends to be inversely proportional to the volume of the tetrahedron formed by the points of unit vectors from user toward the satellites is also revealed. An optimization under a visible condition is discussed. On the basis of these analyses, a user's practical method of selecting the four satellites at every instant is proposed and it's performance is evaluated by simulation. As the result of this evaluation, it is clarified that the proposed method provides sufficiently small GDOP in spite of its small requirement of computatio

ISSN:0028-1522    

DOI:10.1002/j.2161-4296.1984.tb00856.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

ABSTRACTThe next generation weather radar system (NEXRAD) is expected to be an important element in the improved air traffic control plan being currently shaped by the Federal Aviation Administration. This paper assesses the probable impact of the proposed NEXRAD system on flight safety as well as on air traffic control. Weather hazards to aviation are reviewed. The problems specific to flight in terminal and enroute areas are outlined. Aircraft are susceptible to weather hazards at all altitudes, but are particularly vulnerable during low‐level flight. The NEXRAD, by virtue of its coherent or Doppler processing capability, is expected to detect low‐altitude wind shear, which poses a serious hazard during landing and takeoff. It can also provide quantitative estimates of precipitation activity and turbulence levels. This information, together with hailstorm warning, will be helpful at all flight altitudes. NEXRAD, being a multiservice system, involves considerable design compromises. The characteristics that are desirable from an aviation point of view are discussed in the pa

ISSN:0028-1522    

DOI:10.1002/j.2161-4296.1984.tb00857.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

ABSTRACTThe mechanical 3‐arm protractor is used to answer (graphically) the question: “Given three positions (landmarks) and two angles, what is present position?”. A marine sextant is typically used to obtain the angles while a 3‐arm protractor is used to determine present position from a chart. However, while the sextant is fairly accurate, the 3‐arm protractor is awkward to use and sacrifices much of the accuracy gained by careful sextant work. Fortunately, the function that the 3‐arm protractor performs graphically can be implemented on a pocket calculator once this function is described mathematically. Thus, savings of time, accuracy, and money may be realized along with the elimination of plotting hardware. The math is presented here along with an example for flat earth calculations. An iterative approach is also presented for spherical earth c

ISSN:0028-1522    

DOI:10.1002/j.2161-4296.1984.tb00858.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

Book reviewed in this article:SHIPHANDLING FOR THE MARINER, by Captain David H. Mac Elrevey, with illustrations by Earl Mc Millin, Cornell Maritime Press, Centreville, MD, 1983.RADIONAVIGATION, by P. Fonnbonne, Massoro S.A. Paris, 1983 (In French)BEHAVIOR AND HANDLING OF SHIPS, by Captain Henry H. Hooyer, Cornell Maritime Press, Centreville, MD 1983.SEAMANSHIP, by Peter Kemp, Van Nostrand Reinhold, New York, 1983

ISSN:0028-1522    

DOI:10.1002/j.2161-4296.1984.tb00859.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

ISSN:0028-1522    

DOI:10.1002/j.2161-4296.1984.tb00860.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

ISSN:0028-1522    

DOI:10.1002/j.2161-4296.1984.tb00861.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY