4.2.1 GPS
1. Brief introduction to GPS
To better understand the concept of GPS, I want to anatomize the term: “Global positioning system” Global: Anywhere on earth. Well, almost anywhere, but not inside buildings, underground, in sever precipitation, under heavy tree canopy, or anywhere else not having a direct view of a substantial portion of the sky. The radio waves that GPS satellites transmit have very short lengths-about 20cm. A wave of this length is good for measuring because it follows a very straight path, unlike its longer cousins, such as AM and FM band radio waves, that may bend considerably. Unfortunately, short waves also do not penetrate very well. So the transmitter and the receiver must have much solid matter between them, or the waves are blocked, as light waves are easily blocked.
Positioning: Answering brand new and age-old human questions. Where are you? How fast are you moving and in what direction? What direction you should go to get to some other specific location, and how long would it take at your speed to get them? And, most importantly for GIS, where have you been?
System: A collection of components with connections among them. Components and links have characteristics.
The Earth
The first major component of GPS is Earth itself: its mass and its surface, and the space immediately above. The mass of the Earth holds the satellites in orbit. From the point of view of physics, each satellite is trying to fly by the Earth at four kilometers per second. The Earth’s gravity pulls on the satellite vertically so it falls. The trajectory of its fall is a track that is parallel to the curve of the Earth’s surface.
The surface of the Earth is studded with little “monuments”-carefully positioned mental or stone markers-whose coordinates are known quite accurately. These lie in the “numerical gratitude” which we all agree forms the basis for geographic position. Measurements in the units of the gratitude, and based on the positions of the monuments, allow us to determine the poison of any object we choose on the surface of the Earth. Earth-circling satellites
The U.S.GPS design calls for a total of 24 solar-powered transmitters, forming a constellation such that several are “visible” from any point on Earth at any given time. The first one was launched on February 22, 1978. In mid-1994 all 24 were broadcasting. The standard “ constellation” of 24 includes three “spares”.
The satellites are at a “middle altitude” of 20200 km, roughly 12600 statute miles or 10900 nautical miles, above the Earth’s surface.
This puts them above the standard orbital height of the space shuttle, most other satellites, and the enormous amount of space junk that accumulated. They are also well above Earth’s air, where they are safe from the effect of atmospheric drag. GPS satellites are below the geosynchronous satellites, usually used for communication and sending TV and other signals back to Earth-based fixed antennas. These satellites are 35420 km above the Earth, where they hang over the equator relaying signals from and to ground-based stations.
The NAVSTAR satellites are neither polar nor equatorial, but slice the Earth’s latitude at about 55 degree, Executing a signal revolution every 12 hours. Further, although each satellite is in a 12hour orbit, an observer on Earth will see it rise and set about 4 minutes earlier each day. They are four satellites in each of six distinct orbital planes. The orbits are almost exactly circular. The combination of the Earth’s rotational speed and the satellites’ orbits produces a wide variety of tracks across the Earth’s surface. Below is a view of tracks, which occurred during the first two hours after noon on St. Patrick’s Day, 1996. You are looking down on the Earth, directly at the equator, the north-south meridian passes through Lexington, Kentucky. As you can see, the tracks near the equator trend to the almost north-south.
GPS satellites move at a speed of 3.87 km/sec. Each weighs about 860 kilograms and has a size of about 8.7 meters with the solar panels extended. Space buffs might want to know that they usually get into orbit on top of Delta Ⅱ rockets fired from the Kennedy Spaceflight Center in Florida.
Ground-based stations
While the GPS satellites are free from drag by the air, their tracks are influenced by the gravitational effects of the moon and by the solar wind. Further, they are crammed with electronics. Thus, both their tracks and their innards require monitoring. This accomplished by four ground-based stations, located on Ascension Island, at Diego Garcia, in Hawaii, and kwajalein. Each satellite passes over at least one monitoring station twice a day. Information developed by the monitoring station is transmitting back to the satellites, which in turn rebroadcasts it to GPS receivers. Subjects of a satellite’s broadcast are the health of the satellite’s electronics, how the track of the satellites, and other, more esoteric subjects which need not concern us at this point. Other ground-based stations exist, primarily for uploading information to the satellites; the master control station is in Colorado Springs, Colorado.
Receivers
This is the part o the system with which you will become most familiar. In most basic form, the satellites receiver consist of 1.an antenna
2.electronics to receive the satellite signals
3.a microcomputer to process the data that determines the antenna position, and to record position valued 4. controls to provide user input to the receiver, and a screen to display information.
More elaborate units have computer memory to store position data points and the velocity of the antenna. This information may be uploaded into a computer and then installed in a geographic information system. Another elaboration on the basic GPS unit is the ability to receive data from and transmit data to other GPS receivers-a technique called “real time differential GPS” that may be used to considerably increase the accuracy of positioning finding.
Receiver manufacturers
In additional to being an engineering marvel and of great benefit to many concerned with spatial issues as complex as national defense or as mundane as refinding a great fishing spot, GPS is also big business. Dozens of GPS receiver builders exist from those who manufacture just the GPS “engine” to those who provide a complete unit for the end user. The United States Department of Defense
The U.S.DoD is charged by law with developing and maintaining NAVSTAR. It was, at first, secret. Five years elapsed from the first launch in 1978 until news of GPS came out in 1983. In the decade since-despite the fact that parts of the system remain highly classified-mere citizens have been cashing in on what one manufacturer calls “The Next Utility”. There is little question that the design of GPS would have been different had it been a civilian system “from the ground up”. But then, GPS might not have been developed at all. Many issues must be resolved in the coming years. For instance, the military deliberately corrupts the GPS signals so that a signal GPS unit, operating by itself (i.e., autonomously), cannot assure accuracy of better than 100 meters. But the DOD is learning to play nicely with the civilian world. They and we all hope, of course, that the civil uses of GPS will vastly outpace the military need. 2. GPS in use
This time, we decided to choose GPS Trimble 4800 and 4700 manufactured by the America. Its specified precision is 5mm+1ppm×D (D means the distance between the tow stations). The field work is strictly according to the requirements of the 《Code for GPS Survey》.Specifically speaking:
①. Before the observation was conducted, we carried out forecasts and
choose the advantageous period. As known to us all, measuring also entails performing a physical operation such as preparations (either instrument setting-up or calibration or both), centering, pointing, matching, setting, comparing and reading etc. ②. During the observation was conducted; we observed consecutively
120 minutes or so. Meanwhile, we looked for some information about
the satellites through the menu on the receiver. If we found some unusual phenomena, we would record them down onto the recorder. ③. After the observation was conducted, we transmitted data to the
computer to conduct adjustment of observations by some software.
4.2.2 LEICA DI 2002
As known to all of us, elctro-opitcal distance measuring techniques are also subject to a number of systematic effects whose sources and chatacterictics must be determined and alleviated. Of these errors, the following are mentioned here: a change in the signal frequency (due to variations in wave propagation velocity; the instrument (and sometimes the reflector or remote unit) may not be properly centered on the ends of the line to be measured; and the path of propagation of the signal may not conform to the straight line assumpted and may be deviated because of environmental and other factors.
To avoid or alleviate the above-mentioned effect on the results, we decided to take the following steps strictly according to certain codes. ①.Set the measuring instrument and reflector at the two ends of the line to be measured respectively, center.
②.Switch on the measuring instrument, target the reflector precisely, and check the signal reflected by the reflector. If qualified, begin to measure; if not qualified, repeat the above-mentioned steps. ③.To avoid the coarse errors, it is quite necessary to read twice per targeting. Then, repeat it! The reason why we followed such work was to minimize the effect of the error into accidental ones.
④.Record the data down to the booklet. Then, read the vertical angles, record them down into the correspondent parts. When measuring, remember to read the temperature displayed on the thermometer and the pressure displayed on the QIYTAJI.
⑤.When all the jobs were done, remember to conduct oblique correction according to the vertical angles and the atomospherical correction according to the T&P. Finally, we got the horizontal distance. 4.2.3 trigonometric leveling traverse observation
The routine is (BM01) TN01-TN02-SBY05-TN03-TN04 (BM04). This step was conducted strictly according to scale Ⅳ requirements specified by the 《Code for Hydraulic and Hydroelectric Construction》. 4.2.4 the horizontal displacement
In this step, we mainly utilize three sides forward intersection to calculate the three- dimensional coordinates (X, Y, Z) of the monitored points.
5. Vertical control network and vertical displacement monitoring
In this step, we mainly carried out the structure subsidence monitoring
by precise leveling. No attempt was made to carry out oblique monitoring etc. Specifically speaking: ①.Leveling routine (twice)
SBYII05-BM01-LD05-LD06-BM02-LD02-LD01-BM04-BM03-SBYII01-BM03-BM04-LD01-LD02-BM02-LD06-LD05-BM01-SBYII05;
BM03-LD02-LD01-BM04; BM01-LD05-LD06-BM02; ②.Rigorous adjustment
After this step, we got such the following results:
The primary elevation results for both working points and monitored points;
The primary plane and altitude results for both working points and monitored points;
The horizontal distance projected to elevation 300.
6.conclusion
The whole design is quite reasonable and practicable. Under the circumstances of disadvantageous working surroundings, we fulfilled one breakthrough by taking advantage of two kinds of surveying tools, namely, GPS and the conventional surveying tools, such as Leica DI 2002, Ni002 etc.
Practice proves that the design is reasonable, the work is tough but qualified, and the results are dependable!
译文(translation)
1. 工程概述
导流洞出口高边坡开挖，正面边坡有
9个开挖马道,马道高程从365米到
229.5米，开挖高度为160多米，侧面边坡有8个开挖马道，马道高程从350米到229.5米，开挖高度为150多米.导流洞出口下游方向，侧面边坡上方是1#公路，3#公路，5#公路和7#公路,各自延长段分别通往导流洞出口.
2. 坐标系统
依据收集到的现有资料及技术设计的要求，平面控制网的起算数据为SBY02，SBY05，SBY09，垂直位移监测的起算数据为SBY01，SBY05.因此变形监测利用的基准和系统为： (1).1954年北京平面坐标系 (2).高斯－克吕格投影3°带 (3).中央子午线111° (4).1956年吴淞高程系