OBSERVATIONS ON POLARISOR OTHER STARSFor most land surveying, the determination ofastronomic azimuth by observing the sun is sufficient;however, in some cases, the required degree of accuracymay be such that observation of Polaris or another starmay be required. Several observation methods andcalculation procedures can be applied to determineazimuth from Polaris; however, we will not discussthem here. Instead you should refer to commercialpublications, such as Surveying Theory and Practice,by Davis, Foote, Anderson, and Mikhail, or ElementarySurveying, by Wolf and Brinker. You should also referto these or other similar publications for a morethorough discussion of field astronomy in general.This ends our discussion of field astronomy. Nowlet’s take a brief look at a new development in surveyingthat is related to field astronomy and to triangulationwhich is the final topic in this chapter.SATELLITE SURVEYING SYSTEMSIn the preceding discussion, you learned how thelocation of a point on the earth can be determined fromobservations taken on the sun or stars. A far more recentdevelopment uses satellites.Satellite surveying systems are an offshoot of thespace program and the U.S. Navy’s activities related tonavigation. Since their development, satellite surveyingsystems have been successfully used in nearly all areasof surveying and are capable of producing extremelyaccurate results.The first generation of satellite surveying systemswas the Doppler positioning systems. The success ofthe Doppler systems led to the U.S. Department ofDefense development of a new navigation andpositioning system using NAVSTAR (NavigationSatellite Timing And Ranging) satellites. Thisdevelopment ushered in the second generation ofsatellite surveying systems known as the GlobalPositioning System (GPS).In the Doppler system, a precisely controlled radiofrequency is continuously transmitted from a satellite asit orbits past an observer’s station. As the satellite drawsnearer the receiver, the received frequency increases.Then as the satellite passes the receiver, the frequencydecreases below the transmitted level. With thetransmitting frequency, satellite orbit, and precisetiming of observations known, you can then computethe position of the receiving station.The observer uses a specially designed receiversystem that is manufactured by one of severalcommercial firms. Typically, the system is composed ofan antenna to receive the transmitted frequency; areceiver to detect, amplify and decode the transmittedsignal; a recording medium, such as a paper or magnetictape; and a rugged carrying case.To determine point locations using the Dopplersystem, you can use three basic methods. They arethe point-positioning, translocation, and short-arcmethods.In the point-positioning method a receiver locatedat a single location of unknown position collects datafrom multiple satellite passes. From the measured data,the location of the receiver is determined using acoordinate system that is relative to the position of thesatellite. Then the location is converted to aconventional coordinate system used by surveyors.In translocation, receivers located at two or morestations track a satellite. The location of one of thestations—the control station—must be known. Thecontrol station, although its position is known, is firsttreated as an unknown and its coordinates are deter-mined using the point-positioning method describedabove. The determined coordinates are then comparedto the known coordinates and differences indicate errorsin the system. Based on the errors, corrections aredetermined and applied to the positions of the unknownstations whose locations have also been determinedusing the point-positioning method.The short-arc method is the same as the trans-location method, except that corrections are also madefor the orbital parameters of the satellite.DOPPLER POSITIONING SYSTEMSGLOBAL POSITIONING SYSTEMSImagine, if you will, the continuously changingpitch of a train whistle as it approaches and passes you.This is a classic example of the Doppler phenomenonin which the change in frequency is a function of rangeor distance. This phenomenon is the underlyingprinciple of the Doppler positioning systems.Because of its superiority, the global positioningsystem is phasing out the use of the Doppler positioningsystem; however, like the Doppler system, the globalpositioning system is based on observations of satellites.GPS satellites are in near-circular orbits around the15-23
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business