坐标系相互转换.ppt

第二章 GPS定位相关基本知识 围绕卫星、地面测站的位置信息需要设定参考的空间和时间系统以及对卫星轨道精确计算。坐标系统与时间系统是描述卫星运动，处理观测数据和表达定位结果的数学与物理基础。因此，了解和掌握一些常用坐标系统和时间系统，熟悉它们各自间的转换关系，对GPS用户来说，是极为重要的。 地球的形状和大小 基本概念 1. 重力方向线 即铅垂线, 是测量工作的基准线 2. 水准面 自由静止的水面; 是等位面, 有无数个 §2-1 空间参考坐标系统介绍 空间系统即空间参考坐标系，需要确定原点、坐标轴指向、长度单位。 GPS测量与应用中，常采用直角坐标系统及其相应的大地坐标系，取地球的质心作为原点。根据坐标轴取向不同： 一类是地球坐标系，该系坐标系是固结在地球上的，随地球一起转动，故又称为地固坐标系。它是一种非惯性坐标系，对于表述点的位置和处理GPS观测结果是十分方便的。地固坐标系有多种表达形式，对GPS测量来说，最基本的是以地球质心为原点的地心坐标系。 第二类是天球坐标系，该类坐标系与地球自转无关，称空固坐标系，对于描述卫星的运行状态、确定卫星轨道是极其方便的。 坐标系 相互转换 Coordinate Systems Navigation: knowing where you are, where you want to go, and how to get there – Also useful: knowing how long it will take – To achieve these goals in a general way; a coordinate system is needed that allow quantitative calculations Reference Frames (describes coordinate system basis) –Definition –Realization (implementation of definition) Coordinate system definition Definition of a 3D set of axes requires: – An origin (3 quantities) – An orientation (3 quantities) – A scale (1 quantity) (A “Helmert ” transformation estimates these 7 quantities to relate two reference frames). For the Earth; terrestrial frames come in two forms: – Geometric (mathematical description) – Potential field based (gravity and magnetic) Simplest Global Reference Frame Geometric: Origin at the center of mass of the Earth; Orientation defined by a Z axis near the rotation axis; one “Meridian” (plane containing the Z-axis) defined by a convenient location such as Greenwich, England. Coordinate system would be Cartesian XYZ. Simple System The use of this type of simple system is actually a recent development and is the most common system used in GPS. Until the advent of modern “space-based geodetic systems” (mid-1950s), coordinate systems were much more complicated and based on the gravity field of the Earth. Potential based coordinate systems The basic reason is “realization”: Until distance measurements to earth-orbiting satellites and galactic-bas