美国国家天基PNT概况

简要介绍美国国家天基PNT推出背景、管理机构、相关政策和PNT体系结构及对GPS系统的影响和GPS的改进活动,并从PNT政策和GPS系统的安全两个角度分析了PNT的安全。     

PAPER CONTROL AND THE E.S.R.

Abstract

Readers will notice that the weight of the paper used in this issue of the Review has been substantially reduced when compared with the weight of that used in previous issues. The reason for this is the necessity for further economy in the use of paper. A reduction in the number of pages from sixty-four to forty-eight was made in the January and April numbers, but even these savings are not sufficient in themselves and commencing with the October issue the number of pages will have to be reduced still further from forty-eight to forty. However, by reducing the sizes of the margins, so as to give an increase in the actual printing area, it will be possible to include more printed matter on each page than appears at present. In addition, it is proposed no longer to print the whole of the Colonial Office List of Recent Appointments and Promotions in the Colonial Service, but only to include those parts of it that are of immediate interest to surveyors. As a result of these measures, it is estimated that the total amount of matter to appear in the forty pages will be almost the same as that now included in forty-eight.     

Signal and noise in S.L.R. data

The problem of judging on model deficiency in S.L.R. data treatment is considered. In particular the correlation analysis of residuals of adjusted orbits is used to show the presence of unmodelled signals. This analysis seems suitable at least for a proper preprocessing of S.L.R. data, in the sense of being an efficient procedure of outliers rejection.     

CAPTAIN WILLIAM BLIGH,R.N., HYDROGRAPHIC SURVEYOR

Abstract

The name of Bligh will for all time be associated with the incident of the Bounty Mutiny during his first mission of transporting the Bread Fruit to the West Indies. This was, however, but one episode in his eventful career in the Royal Navy. At various times he took part in scientific expeditions and voyages of discovery, commanded fighting ships, etc. Between 1797 and 1803 he fulfilled the duty of hydrographic surveyor and carried out a number of surveys of limited areas in the British Isles and adjacent coasts of Europe. These examinations, of which the River Humber, Dublin Bay, Dungeness and the mouth of the River Schelde are the most detailed, were valuable additions to hydrographic knowledge, coming as they did before organized surveying as we know it today had fully established itself.     

Comparison of the Graf sea gravimeter with the Vening Meinesz apparatus on board the submarineU.S.S. Becuna

Measurements of gravity were made on boardU.S.S. Becuna (SS 319) with a Vening Meinesz pendulum apparatus and a Graf sea gravimeter at approximately the same time. Comparison of data uncorrected for depth of submergence, E?tvos correction and second order effects of horizontal acceleration showed that there was a change related to time of observation. These corrections were not made as they would be the same for both sets of observations, and no provision had yet been made to take care of the horizontal accelerations for the Graf sea gravimeter. The variation with otime could be caused by instrumental drift or scale calibration. After removal of this effect by visually fitting the data trend with a straight line, there were three observations with large discrepancies, seventeen with discrepancies of 3–9 mgal and thirty-nine with discrepancies of 0–3 mgal, showing close approach to a statistical distribution. The three large discrepancies may be dismissed because of very bad depth control during the observations. The discrepancies 3–9 mgals are larger than expected and perhaps are attributable to depth control and inadequate observation time for the Graf sea gravimeter. It is concluded that the Graf sea gravimeter shows great promise for use on a submarine. An apparatus to take into account the horizontal acceleration effects must be added, and suitable drift characteristic obtained. Despite its advantages of ruggedness, ease of operation, ease of data reduction, reduced size and weight, many more comparisons of the Graf sea gravimeter with the Vening Meinesz pendulum equipment should be made before the latter are displaced Probably the Graf sea gravimeter will be usable on a stable platform on a surface vessel at least in calm sea states.