GPS理论与应用05.ppt

GPS理论与应用 （5.伪随机码测距原理） GPS理论与应用5.伪随机码测距原理 授课内容 1.GPS卫星的测距码信号 2.GPS测距原理 3.伪随机码基础 4.GPS中的伪码扩频 5.伪随机码测距原理 GPS理论与应用5.伪随机码测距原理 1.GPS卫星的测距码信号 GPS卫星发送的信号采用L波段的两种载频作载波，分别被称作L1的主频率和L2的次频率。 这些载波频率由扩频码（每一颗卫星均有专门的伪随机序列）和导航电文所调制。所有卫星均在这两个相同的载波频率上发射，但由于伪随机码调制不同，因此无明显的相互干扰。 GPS理论与应用5.伪随机码测距原理 GPS理论与应用5.伪随机码测距原理 Each satellite transmits its ranging signal on two different radio frequencies: 1575.42 Megahertz (part of the so-called “L-Band”) which is referred to as the L1 Carrier, and 1227.60 Megahertz (also of the L-Band) designated as the L2 Carrier. Superimposed on these radio carrier wave signals are pseudo-random, binary, bi-phase modulation codes called PRN (Pseudo Random Noise) codes that are unique to each individual satellite. This simply means that the carrier signal is modulated (varied) by changing its phase (up-down position of the waves) back and forth (bi-phase) at a regular and programmed rate and interval. GPS理论与应用5.伪随机码测距原理 This modulation of the signal, which is really just a series of “dots and dashes,” is very long and complicated. So complicated, in fact, that if you were just to look at it without knowing what it was, it would simply look like a bunch of random noise that made no sense at all. But it really does make sense to those in the know. Thus the term pseudo-random noise. There are two different pseudo-random code strings used by the GPS: the Coarse Acquisition Code (C/A-code), sometimes called the “Civilian Code” , and the Precise, or Protected Code (P-Code). GPS理论与应用5.伪随机码测距原理 选择L波段的优点是： (1) 减少拥挤，避免“撞车”。目前L波段的频率占用率低于其他波段，与其他工作频率不易发生“撞车”现象，有利于全球性的导航定位测量。 (2) 适应扩频，传送宽带信号。GPS卫星采用扩频技术发送卫星导航电文，其频带高达20 MHz左右，在占用率较低的L波段上， 易于传送扩频后的宽带信号。 (3)大气衰减小，有利于研制用户设备。GPS卫星采用L波段，避开了大气的谐振吸收，衰减较小，且电离层延迟的影响小，有利于用较经济的接收设备测量GPS信号。而采用两个载频，目的在于测量出或消除掉由于电离层效应而引起的延迟误差。 GPS理论与应用5.伪随机码测距原理 When a radio transmits a signal, it is in the form of a simple sine wave that has a particular frequency (the number of “humps” on the sine wave that pass a fixed point per