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OBSERVATIONS ON POLARIS
OR OTHER STARS For most land surveying, the determination of astronomic azimuth by observing the sun is sufficient; however, in some cases, the required degree of accuracy may be such that observation of Polaris or another star may be required. Several observation methods and calculation procedures can be applied to determine azimuth from Polaris; however, we will not discuss them here. Instead you should refer to commercial publications, such as Surveying Theory and Practice, by Davis, Foote, Anderson, and Mikhail, or Elementary Surveying, by Wolf and Brinker. You should also refer to these or other similar publications for a more thorough discussion of field astronomy in general.This ends our discussion of field astronomy. Now lets take a brief look at a new development in surveying that is related to field astronomy and to triangulation which is the final topic in this chapter. SATELLITE SURVEYING SYSTEMS In the preceding discussion, you learned how the location of a point on the earth can be determined from observations taken on the sun or stars. A far more recent development uses satellites. Satellite surveying systems are an offshoot of the space program and the U.S. Navys activities related to navigation. Since their development, satellite surveying systems have been successfully used in nearly all areas of surveying and are capable of producing extremely accurate results.The first generation of satellite surveying systems was the Doppler positioning systems. The success of the Doppler systems led to the U.S. Department of Defense development of a new navigation and positioning system using NAVSTAR (Navigation Satellite Timing And Ranging) satellites. This development ushered in the second generation of satellite surveying systems known as the Global Positioning System (GPS).DOPPLER POSITIONING SYSTEMS Imagine, if you will, the continuously changing pitch of a train whistle as it approaches and passes you. This is a classic example of the Doppler phenomenon in which the change in frequency is a function of range or distance. This phenomenon is the underlying principle of the Doppler positioning systems.In the Doppler system, a precisely controlled radio frequency is continuously transmitted from a satellite as it orbits past an observers station. As the satellite draws nearer the receiver, the received frequency increases. Then as the satellite passes the receiver, the frequency decreases below the transmitted level. With the transmitting frequency, satellite orbit, and precise timing of observations known, you can then compute the position of the receiving station. The observer uses a specially designed receiver system that is manufactured by one of several commercial firms. Typically, the system is composed of an antenna to receive the transmitted frequency; a receiver to detect, amplify and decode the transmitted signal; a recording medium, such as a paper or magnetic tape; and a rugged carrying case. To determine point locations using the Doppler system, you can use three basic methods. They are the point-positioning, translocation, and short-arc methods.In the point-positioning method a receiver located at a single location of unknown position collects data from multiple satellite passes. From the measured data, the location of the receiver is determined using a coordinate system that is relative to the position of the satellite. Then the location is converted to a conventional coordinate system used by surveyors. In translocation, receivers located at two or more stations track a satellite. The location of one of the stationsthe control stationmust be known. The control station, although its position is known, is first treated as an unknown and its coordinates are deter-mined using the point-positioning method described above. The determined coordinates are then compared to the known coordinates and differences indicate errors in the system. Based on the errors, corrections are determined and applied to the positions of the unknown stations whose locations have also been determined using the point-positioning method. The short-arc method is the same as the trans-location method, except that corrections are also made for the orbital parameters of the satellite. |
 
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