【導(dǎo)讀】移動系統(tǒng)跨越世界性成功標(biāo)志是越來越朝著個人化、方便化方向發(fā)展。在快速的個人生活中，移動電話已成為一種必須，而不僅僅是為了方便。信系統(tǒng)的革命而發(fā)生變化。和重量以及網(wǎng)絡(luò)的花費和質(zhì)量。如果上述因素實現(xiàn)有困難，特別是前兩個，那么市場的發(fā)。固定電話的服務(wù)是全球的，相互聯(lián)系的范圍從同軸電纜到光纖，以及人造衛(wèi)星。通信標(biāo)準(zhǔn)是不同的，但隨著普通接口以及對接口轉(zhuǎn)化，相互之間的聯(lián)系能發(fā)生改變。第二代由第一個非常標(biāo)準(zhǔn)的計劃支配這個計劃由歐洲特殊移動通信系統(tǒng)委員會。制定，這個設(shè)計作為全球移動通信系統(tǒng)。于允許頻率復(fù)用。存器中，用戶寄存器使GSM系統(tǒng)檢查需要使用網(wǎng)絡(luò)的用戶，允許接入并建立收費功能等。GSM在頻帶為900MHz下用124個無線信道傳遞突發(fā)串。由于GSM支持通用標(biāo)準(zhǔn)所以蜂窩用戶在整。個GSM服務(wù)區(qū)也能夠使用他們的手機，能夠在GSM覆蓋區(qū)域內(nèi)自動漫游。GSM還提供一些新服務(wù)：如告訴數(shù)據(jù)通信，傳真和短消息服務(wù)。無繩電話對此作出響應(yīng)。蓋的范圍已達(dá)幾百米。

　　

【正文】 GPS is to use satellites in space as reference points for locations here on earth. That’s a big idea geometrically, as by very, very accurately measuring our distance from three satellites we can ―triangulate‖ our position anywhere on earth. Suppose we measure our distance from a satellite and find it to be 11,000 miles. Knowing that we’re 11,000 miles from a particular satellite narrows down all the possible locations we could be in the whole universe to the surface of a sphere that is centered on this satellite and has a radius of 11,000 miles. Next, say we measure our distance to a second satellite and find out that it’s 12,000 miles away. That tells us that we’re not only on the first sphere but we’re also on a （論文） 19 sphere that’s 12,000 miles from the second satellite. Or in other words, we’re somewhere on the circle where these two spheres intersect. If we then make a measurement from a third satellite and find that we’re 13,000 miles from that one, that narrows our position down even farther, to the two points where the 13,000 mile sphere cuts through the circle that’s the intersection of the first two spheres. So by ranging from three satellites we can narrow our position to just two points in space. To decide which one is our true location we could make a fourth measurement. But usually one of the two points is a ridiculous answer(either too far from Earth or an impossible velocity) and can be rejected without a measurement. Step 2: Measuring distance from a satellite We saw in the last section that a position is calculated from distance measurements to at least three satellites. But how can you measure the distance to something that’s floating around in space? We do it by timing how long it takes for a signal sent from the satellite to arrive at our receiver. In a sense, the whole thing boils down to those ―velocity times travel time‖ math problems we did in high school. In the case of GPS we’re measuring a radio signal so the velocity is going to be the speed of light or roughly 186,000 miles per second. The problem is measuring the travel time. But assuming we have precise clocks, how do we measure travel time? To explain it let’s use a goofy analogy: Suppose there was a way to get both the satellite and the receiver to start playing ―The StarSpangled Banner‖ at precisely 12 Noon. If sound could reach us from space (which, of course, is ridiculous), （論文） 20 then standing at the receiver we’d hear two versions of ―The StarSpangled Banner,‖ one from our receiver and one from the satellite. These two versions would be out of sync. The version ing from the satellite would be a little delayed because it had to travel over 11,000 miles. If we wanted to see just how delayed the satellite’s version was, we could start delaying the receiver’s version until they fell into perfect sync. The amount we have to shift back the receiver’s version is equal to the travel time of the satellite’s version. So we just multiply that time times the speed of light and BLNGO. We’ve got our distance to the satellite. That’s basically how GPS works. Only instead of ―The StarSpangled Banner‖ the satellites and receivers use something called a ―Pseudo Random Code‖which is probably easier to sing than ―The StarSpangled Banner‖. The Pseudo Random Code(PRC) is a fundamental part of GPS. Physically it’s just a very plicated digital code, or in other words, a plicated sequence of ―on‖ and ―off‖ pulses. The signal is so plicated that it almost looks like random electrical noise. Hence the name ―PseudoRandom.‖ Step 3: Getting perfect timing If measuring the travel time of a radio signal is the key to GPS then our stop watches had better be very good, because if their timing is off by just a thousandth of a second, at the speed of light, that translates into almost 200 miles to error! On the satellite side, timing is almost perfect because they have incredibly precise atomic clocks on board. But what about our receivers here on the ground? （論文） 21 Remember that both the satellite and the receiver need to be able to precisely synchronize their pseudorandom codes to make the system work. If our receivers needed atomic clocks(which cost upwards of $50K to $100K) GPS would be a lame duck technology. Nobody could afford it. Luckily the designers of GPS came up with a brilliant little trick that lets us get by with much less accurate clocks in our receivers. This trick is one of the key elements of GPS and as an added side benefit it means that every GPS receiver is essentially an atomicaccuracy clock. The secret to perfect timing is to make an extra satellite measurement. If three perfect measurements can locate a point in 3dimensional space, then four imperfect measurements can do the same thing. If everything were perfect than all of our satellite ranges would intersect at a single point. But imperfect clocks, a fourth measurement, done as a crosscheck, will NOT intersect with the first three. So the receiver’s puter says ―Uhoh!‖ There is a discrepancy in my measurements. I must not be perfectly synced with universal time. Since any offset from universal time will affect all of our measurements, the receiver looks for a single correction factor that it can subtract from all its timing measurements that would cause them all to intersect at a single point. That correction brings the receiver’s clock back into sync with universal time, and bingo! – you’ve got atomic accuracy time right in the palm of your hand. Once it has that correction it applies to all the rest of its measurements and now we’ve got precise positioning. One consequence of this principle is that any decent GPS receiver will need to have at least four channels so that it can make the four measurements （論文） 22 simultaneously. Step 4: Knowing where a satellite is i