毕业外文资料原文与翻译测绘类Word格式文档下载.docx

1、院 系：专业班级：学 号：学生姓名：指导老师：Automatic Position Determination1 Position DeterminationThe traditional method for the direct determination of position has been by astronomical observation. The relative positions and movements of the stars as well catalogued and so with a combination of altitude, direction a

2、nd time observations to the stars, the position in terms of latitude and longitude of a ground station can be calculated. There if less call for direct position determination nowadays since most countries are covered by a primary horizontal control scheme, and the absolute position of any new local

3、survey word can be established by including a national reference point in the local survey.In very remote areas, or where the terrain is totally unsuitable for the classic survey methods of triangulation and traverse, there is, however, a need for direct position determination. For the majority of t

4、his work, visual observation to the stars has been replaced by electromagnetic measurements to or from artificial earth satellites. In addition, the relative positions of survey stations can be determined directly by inertial techniques originally developed for aircraft navigation.1.1 Inertial Posit

5、ioningA pair of gyros are incorporated in a position fixing device known as the Auto-Surveyor manufactured by Litton Systems. It has originated from an aircraft navigation system, which was developed for military applications into its present three-dimensional survey system. Hardware consists of （i）

6、the inertial measuring unit, （ii）a computer ,（iii）a cassette recorder,（iv）a display and control unit and （v） the power supply .These items can be placed anywhere within a road vehicle or helicopter providing that the operator is close to （iv）.The measuring unit contains a gyro-stabilized platform wi

7、th two air-bearing gyroscopes in four gimbal mountings which keep its three orthogonal axis oriented in space in a north-east=downwards relationship . Each axis also has a sensor-torquer type of acclerometer which defines acceleration in the particular direction via a quantitizer system feeding the

8、computer .This is pre-programmed to compute the survey as it is carried out and to control the system （see Fig.1）Before staring the survey an alignment drill carried out at the origin station. This is said to have a duration of about one hour （that for the survey is restricted to some four hours or

9、so） and the platform levels itself with respect to the local vertical and aligns its nothing axis with the local meridian so that the accelerometers are in the directions of north, east and down. Simultaneously the computer monitors the biases of the system and evaluates the initial conditions for a

10、 Kalman filter enables an evaluation of the performance of the system by comparison of the errors arising during the survey to a priori data relating to the statistical nature of errors which navigation instruments are likely to contain.When the alignment is complete the known coordinates and elevat

11、ion of the station are fed in by the operator and the traverse can begin. The orthogonal movements are sensed by the accelerometers and, at intervals of sixteen milliseconds, the signals to the computer are doubly integrated into distances of displacement with respect to the origin. In the Auto-Surv

12、eyor z-increments accumulate as elevation differences, whilst the other two increments are used to compute values of geodetic latitude and longitude on the pre-programmed reference spheroid. The computer also directs the platform gimbal torques to place the platform tangential to the spheroid with t

13、he north axis correctly oriented.At approximately four minute intervals the vehicle is halted a zero velocity updating is carried out. This takes about thirty seconds and the platform is re-leveled, with respect to local vertical, and effectively resets vertical zero. As mentioned above, previous ca

14、lculations and platform torquing are with respect to the reference spheroid so that the amount of torque used for the re-leveling is a measure of the directional change of deviation from the vertical. Since the vehicle is stationary, the accelerometers should read zero as should the velocities by mt

15、egration so that when the Kalman filter scrutinizes these values it can establish accelerometer errors due to their drift, which is non-linear, and it uses the velocity errors to establish the rate of change of drift; precise integration is not possible after four minutes due to that drift. Platform

16、 re-leveling takes place after the adjustment.The survey now continues with successive updating stops and also with halts at the survey stations where the same process occurs. At the terminal station known coordinates and elevations are entered as updating information and a zero velocity update is c

17、arried out; the Kalman filter carries out a smoothing adjustment data and results of the smoothing are registered on the recorder.Traverse lines are normally measured in two directions, occupying the same stations , discrepancies then indicating effects of gyrodrift on platform alignment .Standard e

18、rrors of 0.20 m in position of points at 10 km separation are achievable with a similar value for heighting when a road vehicle is used.Ferranti manufacture a corresponding system in the UK, and reference can be made to a report in the New Civil Engineer, 17 January 1980, on its value to the highway

19、 engineer. This report refers specifically to their Inertial Road Surveyor although it indicates that systems for land surveys, borehole surveys and underwater surveys are available too.2 GPS IN NAVIGATIONAL APPLICATIONS 2.1 INTRODUCTION Global Positioning System （GPS） has for several years been a b

20、uzzword for professionals in many fields including surveying, geodesy, GIS, meteorology, and geodynamics. The reason for this GPS wonder perhaps lies in the superior capability of GPS: it offers solutions to many problems that we could not or felt difficult to solve, and also enables us to do many t

21、hings better than before. Navigation is one of these things, which has been greatly changed from the development of GPS.This paper will provide an overview of GPS as applied to navigation. It will first describe briefly the principles of GPS .The different GPS based positioning methods in navigation

22、 will then be discussed, followed by an review of GPS based systems for air, land and marine navigation.2.2 PRINCIPLES OF GPS POSITIONINGGPS is a satellite based passive positioning system that was initially designed primarily for military use .It was developed and has been maintained by the United

23、States Department of Defense （US DoD）. The system is now used by both the military and civilian users to obtain high accuracy position, velocity and time information, 24 hours a day, under all weather conditions, and anywhere in the world. The system was 1993 and full operational capability （FOC） in

24、 April 1995.2.2.1 The Components of GPSOne common way to look at GPS is to resolve it into three segments:The space segment refers to GPS satellites that are orbiting at an altitude of about 20,200 km above the earth surface. The full operational capacity of GPS is achieved with 24 active satellites

25、. There are currently 27 operational satellites, three of that are the active spares that can be used as replacements when the active satellites are out of services. The key components in satellite are the antennas sending and receiving signals, two large wings covered with solar cells to generate p

26、ower for the satellite to consume, and atomic clocks that are accurate to about 1 second in 3,000,000 years.The control segment consists of 5 monitor stations, 3 ground antennas, and 1 master control station. The monitor stations passively track all satellites in view, accumulating ranging data. The

27、 tracked data are processed at the master control station to determine satellite orbits and to update each satellites Navigation Message. The updated information is transmitted to each satellite via the ground antennas.The user segment is anybody who has a GPS receiver. The surveyors, the navigators

28、 and the GIS data collectors are examples of the users.The signals that GPS satellites send out consist of two codes, the coarse acquisition （C/A） code and the precise （P） code, and a Navigation Message. The GPS codes are just like a series of 1s and 0s that are arranged into certain sequences, Figu

29、re 1. The C/A code is used for the standard positioning service （SPS） available to all users. The service offers a positional accuracy of about 100 m horizontally and 156 m vertically at the 95% probability level. The P code is used for the Precise Positioning Service （PPS） and can bi accessed only

30、by authorized users such as the US military and its allies. The service provides a positional accuracy of about 15 m horizontally and 25 m vertically at the 95% probability level.The GPS Navigation Message contains such information as the orbital elements of the satellites, clock behavior, and an al

31、manac that gives the approximate data for each active satellite. Two carrier frequencies on L-band, L1 and L2 are used to carry the signals described above. L1 has a wave-length of about 19 cm （1575.42MHz） and L2 a wave-length of about 24 cm （1227.60MHz）. Both L1 and L2 are microwave frequencies and

32、 can penetrate the atmosphere. L1 carries both the C/A and the p codes and L2 the p code only. The Navigation Message is carried on both of the two frequencies.To the more sophisticated users such as the surveyors, positioning using the code information cannot fulfill their accuracy requirements, say at the centimeter or millimeter level. In this case, the L1 or L2, or both L1 and L2 carrier phases are also observed and used for positioning. 2.2.2 The Working Principles of GPS GPS measures positions by measuring distances. GPS satellites have known orbits