1、GPS理论与应用（ 5.伪随机码测距原理 ）刘瑞华 教授中国民航大学 电子信息工程学院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n 1.GPS卫星的测距码信号n 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。n 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
2、理GPS理论与应用5.伪随机码测距原理n 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.n Superimposed on these radio carrie
3、r wave signals are pseudo-random, binary, bi-phase modulation codes called PRN (Pseudo Random Noise) codes that are unique to each individual satellite. n 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) a
4、t a regular and programmed rate and interval. GPS理论与应用5.伪随机码测距原理n 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 bun
5、ch 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.n 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
6、 Protected Code (P-Code).GPS理论与应用5.伪随机码测距原理n 选择 L波段的 优点 是： n (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。n (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。n (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
7、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n 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 unit of time-usually given as Hertz, or times per second), wavelength (the di
8、stance between “humps” or any matching successive point on the sine wave), and amplitude (the “height” of the “humps”). n Radio wavelengths can range from tens of kilometers down to tiny fractions of micrometer. Frequencies, intrinsically linked to wave-lengths, also have wide ranges, from only a fe
9、w per hour (low frequency) to billions per second (high frequency).n A basic carrier sine wave is illustrated at the top of the diagram.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n By itself, the carrier wave carries no information other than its frequency, wavelength, and amplitude. n If we want to transm
10、it any useful information on that carrier wave, we have to modulate or vary it at a regular rate. n The second line in the diagram represents a string of zeros (offs) and ones (ons) that we want to send on the carrier wave, much like Morse-code. n There are several methods of transmitting that infor
11、mation on a carrier wave.GPS理论与应用5.伪随机码测距原理n The first is by varying (modulating) the amplitude, or how “high” and “low” the sine “humps” go. If youve ever listened to AM radio, youve heard Amplitude Modulation.n You could also vary, just slightly, the frequency of the carrier wave around a central
12、“flat” frequency. That concept is illustrated by the line second from the bottom in the diagram. This is how FM, or Frequency Modulation, radio works.n Finally, you could modulate the phase of the carrier. The phase is the relative up/down position of the sine “humps.” By regularly reversing the ups
13、 and downs you can transmit your “Morse-code” information. This is how GPS transmits data on its two carriers. This is illustrated in the bottom line of the diagram.GPS理论与应用5.伪随机码测距原理n Two Morse-code-like signal strings are transmitted by each satellite. They are the Coarse Acquisition, or C/A-code,
14、 and the Precise, or Protected Code - more commonly referred to as simply the P-Code.n GPS使用 L频段的 两种载频 为（其中 f0是卫星信号发生器的基准频率）： L1载波： fL1=154f0=1575.42 MHz，波长1=19.032 cm； L2载波： fL2=120f0 =1227.6MHz，波长2=24.42 cm。 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n The C/A-code is a sequence of 1,023 bi-phase modulat
15、ions of the carrier wave. n Each opportunity for a phase-reversal modulation, or switch from a zero to a one, is called a “Chip“ (whether or not the phase is actually reversed at that moment). n This entire sequence of 1,023 chips is repeated 1,000 times each second, resulting in a “Chip-Rate” of 1.
16、023 MHz or one (opportunity for a) phase switch (chip) every one-millionth of a second. n Each satellite carries its own unique code string. The C/A-code is the code used for the Standard Positioning Service(PPS).GPS理论与应用5.伪随机码测距原理n The Precise (P) code is similar to the C/A-code, but instead of a s
17、equence of 1023 chips, the chip-count runs to the millions. n As a result, the complete sequence for the P-code takes 267 days to complete, rather than the one one-thousandth of a second for the C/A-code. One-week segments of the 267-day string are assigned to each satellite and are changed weekly.
18、n The P-code is the code used for the Precise Positioning Service(PPS). GPS理论与应用5.伪随机码测距原理n The chip rate of the P-code is an order of magnitude higher than for the C/A-code, running at phase-reversal chip rate of 10.23 MHz, or one phase switch (chip) opportunity every one ten-millionth of a second.
19、 This means that there are ten million individual opportunities for a phase reversal each and every second. n Since distance is a direct function of time, the radio wave clearly cant travel very far in only one ten-millionth of a second. Consequently, the P-code is considerably more precise than C/A
20、-code. As well see, this fact is critical in understanding how GPS determines distance and why one service is so much more accurate than the other. GPS理论与应用5.伪随机码测距原理n 2.GPS测距原理n The code is the key to understanding how GPS determines distance between the satellite and receiver, both for the Standar
21、d Positioning Service as well as the Precise Positioning Service. n Both use their respective codes essentially the same way: they simply derive different levels of precision by using different chip strings. Conceptually, both work identically.n The basic concept is illustrated in the diagram. Each
22、receiver has in its own memory each of the satellites unique codes. The receiver uses this information to internally generate an exact replica of the satellites code at the same instant that the satellite generates its “real” code.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n Because it took some finite amo
23、unt of time for the signal from the satellite to reach the receiver, the two signals dont quite match up: theres a tiny delay, or lag time. Its that time delay that is used to determine the distance between the satellite and receiver. n This method of range measurement by comparing the delay between
24、 two copies of the code is called “Code Correlation.”n Distances derived in this manner, before any kind of error correction is applied to the signal (which well talk about shortly) are called “Pseudo-Ranges.”GPS理论与应用5.伪随机码测距原理n You might ask “Why the ultra-complex chip string? Why not a simple, reg
25、ular beep for example? Wouldnt that do the same thing?”n Imagine for a moment that youre standing on the goal line of a football field and a colleague of yours is standing 100 yards away at the other goal line. n At the 50-yard line, theres a referee. Its agreed upon that at the exact moment the ref
26、eree drops a flag, you and your colleague will begin yelling “HEY!” to each other at a pace of once per second. n What would you hear at your end? GPS理论与应用5.伪随机码测距原理n Obviously, you would hear yourself yell “HEY!” n A moment later you would hear your colleagues “HEY!” n You could then measure how lo
27、ng after you yelled your “HEY!” that your colleagues “HEY!” got to you. n Assuming that you knew the speed of sound under your current conditions, calculation of your distance from your colleague would be straightforward.GPS理论与应用5.伪随机码测距原理n But there could be a problem here. n How do you know that t
28、he “HEY!” you hear from your colleague is the right one to match your “HEY!”? In other words, what if, for example, it took 2 seconds for his “HEY!” to reach you? n You wouldnt hear his “HEY!” until between your second and third one. You could quickly loose track and might even think that he was onl
29、y second away because, after all, one of his “HEY!” did come only second after one of your “HEY!”s, just the wrong one!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n Now imagine instead that at the same moment you both started yelling a count: “ONE!, TWO!, THREE!.” and so on. n Now when you heard any “number
30、” that he yelled, youd instantly know which equivalent number of your own you would need to measure the time delay against.n This would allow you to jump in anywhere and know right away where you were in the count-string. n Conceptually, thats how GPS measures distance with the C/A- and P-codes. Of
31、course, GPS doesnt use numbers; instead, it uses those unique strings of ons and off s: zeros and ones.GPS理论与应用5.伪随机码测距原理n In the “real” world of GPS, its easy to find out where you are in the C/A-code string since the whole string “passes by” in only l/l,000 of a second. n Theres a problem, however
32、, when trying to figure out where youre at in the 267-day long P-code string. n Thus the term: Coarse Acquisition: because P-code receivers use the C/A-code to “get close” to where they need to look in the P-code string, or to “ramp up” to P-code “lock-on.” n If the C/A-code can tell the receiver wh
33、ere its at within a few hundred meters, then it only has to look at a very small part of the P-code.GPS理论与应用5.伪随机码测距原理n It turns out that just knowing how far away you are from the requisite four satellites isnt enough. n The ranges to the satellites only tell you where you are relative to the satel
34、lites. But where are the satellites? It is also necessary to know where each satellite is in space.n Fortunately, thats not too tough.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n In the first place, the military is very careful about where it sticks it very expensive space hardware. n Once in place in spac
35、e, the satellites orbits tend to be very stable through time because they are far above virtually all of the atmosphere and the drag that it can induce. n Variations in orbits that are due to gravitational forces are fairly easy to predict and compensate for.GPS理论与应用5.伪随机码测距原理n To compensate for the
36、 inevitable unpredictable perturbations in the satellites orbits, they are constantly monitored from the ground. n Corrections for any orbital variations that are identified are quickly uploaded from ground antennas to the satellites which then send the information back down to each receiver thats t
37、uned in to them. n This satellite position and orbital information is called the “Ephemeris,” or, as plural, “Ephemerides.”(Orbital position is constantly changing, thus the term, based on the word “ephemeral,” meaning lasting only a short time.)GPS理论与应用5.伪随机码测距原理n The ephemeris is part of the Navig
38、ation/System data message (the “NAV-msg”) that is also superimposed on the L1 and L2 carriers, in a sense acting as a modulation of the modulation that weve already talked about.n In addition to the corrected satellite orbital and position data (the ephemeris data), the NAV-msg also carries a correction for any clock bias, or error in the atomic clocks, on board the satellites so that the receivers on the ground can compensate for these errors.
