1、英文文献 The Global Positioning System The global Positioning System (GPS) is revolutionizing surveying technology, Like its predecessor , the TEANSIT Doppler system, GPS shifts the scene of surveying operations from ground-to-ground measurements to ground-to-sky , with obvious implications : intervisib
2、ility of marks is no longer a criteion for their location ； operations are possible in nearly all kinds of weather and be performed during day or night ； and the skills required to utilise the technology are different both in field operations and data processing . But GPS is not merely a replacement
3、 for TRANSIT . The simultaneous visibility of multiple satellites allows effective cancellation of the major sources of error in satellite observations , with the result that with GPS, relative positioning accuracies of one part per million(ppm) or better over distances from one kilometer to thousan
4、ds of kilometers are possible . This means that GPS can compete with terrestrial techniques over short distances, and can achieve more accurate results in less time than TRANSIT observations over longer distances . GPS was designed primarily as a navigation system, to satisfy both military and civil
5、ian needs for real-time positioning. This positioning is accomplished through the use of coded information, essentially clever timing signals, transmitted by the satellites. Each GPS satellite transmits a unique signal on two L-band frequencies: A at 1575.42 MHz and B at 1227.60 MHz(equivalent to wa
6、velengths of approximately 19 and 24 cm, respectively).The satellite signals consist of the L-band carrier waves modulated with a Standard or S code (formerly called the C/A code),a Precise or P code and a Navigation Message containing, amongst other things ,the coordinates of the satellites as func
7、tions of time-the Broadcast Ephemerides. The S code which is intended mainly for civilian use , yields a range measurement precision of about 10 meter, The navigation service provided by this code is referred to as the Standard Positioning Service, The p code is intended for military and selected ci
8、vilian use only and yields a measurement precision of about 1 meter, The navigation service provided by the P code is therefore referred to as the Precise Positioning Service (PPS) Although both codes can be used for surveying , a more accurate method is to measure the phase of the carrier signal ,
9、For this reason , we will not discuss the detailed characteristics of the codes in this monograph . There are currently eight usable satellites in orbit. These are the experimental, ”Block 1” satellites, which will be progressively replaced as the “block 2”, operational satellites are placed into or
10、bit beginning in 1986.By 1989 the system should be complete, with 18 satellites in six orbital planes-at about 20200 km altitude, allowing for simultaneous visibility of at least four satellites at any time of day almost anywhere in the world. The present constellation of satellites is configured to
11、 provide the most favorable geometry for testing the system over North America. As it happens, the observation geometry is equally favorable in Australia, and it is possible now to obtain surveying accuracies equal to those obtainable when the system is fully configured, but only for about six hours
12、 per day, At the time of writing (November 1985),the period of maximum mutual visibility of the satellites in eastern Australia is between 6 pm and mid-night local time The period regresses by 4minutes per day (or 2 hours per month), returning to the same times a year from now. This period of useful
13、 visibility will increase as additional satellites are launched from late 1985. As with TRANSIT , much higher accuracies are obtained in relative positioning from observations made simultaneously at two observing stations. Consequently , unless otherwise indicated , all discussion concerning data ac
14、quisition and processing will assume a two-receiver configuration. This is often referred to as the differential mode. The position differences so determined constitute the baseline vector or simply the baseline between the points occupied by two receivers . All satellite positioning systems provide
15、 ground coordinates of a receiver (or the baseline vector between a pair of receivers) in an earthcentered coordinate system, The orientation of the system is determined by the tabulated coordinates or ephemeredes of the GPS satellites. In order to relate coordinates determined by GPS surveying to t
16、he local geodetic datum a transformation relationship needs to be established. The following factors influence the final positioning accuracy obtainable with GPS: (1) The precision of the measurement and the receiver-satellite geometry. (2) The measurement processing technique adopted. (3) The accur
17、acy with which atmospheric and ionospheric effects can be modeled. (4) The accuracy of the satellites ephemeredes. Each of these factor is discussed briefly in the next three sections. GPS Measurement Types. GPS measurement can be made using either the carrier signal or the codes. Code measurements
18、are called pseudo-ranges and can be based on either the P code or the S code. Knowledge of the properties of each of these types of measurements is necessary for understanding and evaluating GPS instruments. Pseudo-ranges are the simplest to visualize geometrically , as they are essentially a measur
19、ement of distance contaminated by clock errors. Throughout this monograph, we use the terms clock , frequency standard and oscillator to denote the same thing , namely , a device for precisely measuring a time interval. When four satellites are observed simultaneously , it is possible to determine t
20、he three-dimensional position of the ground receiver, and the receiver clock offset, at a single epoch . This is simply resection by distance, in surveying terminology , with the satellites serving as the control station, As with the resection technique, the precision is a function of the geometry o
21、f the receiver in relation to the four visible satellites. The best geometry would be when the satellites are in each of the four quadrants and each at an elevation angle of 40-70above the horizon. However , pseudo-range measurements are not nearly as precise as phase measurements of the carrier wav
22、e itself . In order to achieve position accuracies of 10 meter from P code measurements or 100 meter from S code measurements ( adequate for navigation ) , it was only necessary to design a code structure which allowed metre level measurement precision . Morever , the more precise P code will likely be encrypted , and may therefore not be available for non-military use , when the system becomes fully operational in 1989 . An additional
