GPS'nin gelişimi ve geleceği

Development of global positioning system (GPS) its future

Tez No: 46275
Yazar: CENGİZ ŞAHİN
Danışmanlar: Y.DOÇ.DR. MUHAMMED ŞAHİN
Tez Türü: Yüksek Lisans
Konular: Jeodezi ve Fotogrametri, Geodesy and Photogrammetry
Anahtar Kelimeler: Belirtilmemiş.
Yıl: 1995
Dil: Türkçe
Üniversite: İstanbul Teknik Üniversitesi
Enstitü: Fen Bilimleri Enstitüsü
Ana Bilim Dalı: Belirtilmemiş.
Bilim Dalı: Belirtilmemiş.
Sayfa Sayısı: 155

Özet

ÖZET Günümüzde çok çeşitli amaçlar için kullanılan uydu sistemleri vardır ve bu sistemlerden bir tanesi de GPS'tir. GPS Amerikan Hava Kuvvetleri tarafından askeri uçakların, gemilerin ve kara araçlarının konumunu belirlemek üzere Dünyanın neresinde ve ne zaman olursa olsun anlık navigasyonu sağlayacak şekilde geliştirilmiştir. GPS sivil kullanıma açılmış olmasına rağmen askeri güvenlik gerekçeleri ile sivil kullanıcıların sistemi tam olarak kullanmalar sınırlanmıştır. GPS sisteminde kullanılan iki tip ölçü vardır. Uydu sinyalinin alıcıya geliş zamanı ölçülür ve ışık hızı ile çarpılıp uzaklık bulunur. Bu yöntem daha çok navigasyon amaçlı olarak kullanılır. Navigasyonda 100 metre presizyon ile konum doğruluğu elde edilir. GPS'te ikinci ölçü ve aynı zamanda yüksek doğruluk gerektiren çalışmalarda kullanılan faz ölçmeleridir. Uydu sinyali ile alıcı sinyali arasındaki fark ölçülür ve uzunluk elde edilir. Çözünürlük dalga boyunun %1'idir ve uygulamanın amacına göre santimetre ile milimetre seviyesinde doğruluk elde edilir. GPS'in sivil kullanıma açılması ile birlikte birçok ölçme ile elde edilen doğruluğun üstündedir. GPS ölçüler daha kısa zamanda ve daha az maliyet ile tamamlanmaktadır. Statik ölçme türünde alıcılar sabit olarak durur ve 30 dakikalık gözlem süresi ile milimetre seviyesinde konum doğruluğu elde edilir. Bu yöntem haritacılığı ilgilendirir ve klasik ölçülerin yerini almaktadır. Kinematik ölçmede bir alıcı sabit, diğeri hareketlidir. Faz denklemleri kullanılır ve gezici alıcının üç boyutlu koordinatları santimetre doğruluğunda elde edilir. Bu yöntem topografik haritaların eş yükselti eğrilerinin belirlenmesinde kullanılır. GPS ölçmeleri ile gerçek zamandaki üç boyutlu deformasyon ölçüleri yapılır; örneğin petrol platformlarının ve kabuk hareketlerinin belirlenmesi gibi. GPS haritacılığı ilgilendiren tüm alanlarda kullanım olanağı bulmaktadır. Bu olanaklar; kadarstro, mühendislik ölçmeleri ve aplikasyonları, hidrografik ölçmeler, deformasyonların izlenmesi, coğrafi bilgi sistemleri ve sayısal arazi modellerini içine alan hemen hemen konum belirleme ile ilgili tüm alanlardır. GPS diğer birçok sistemle birlikte kullanılmaktadır. GPS planlanırken bu kadar kullanım alanı bulacağı tahmin edilmiyordu. GPS ile elde edilen sonuçların doğruluğu arttıkça yeni uygulama alanları gelişecektir. Fakat gelişen bu yeni uygulama alanlarını takip edebilmek için GPS'in teorisi iyi anlaşılmalıdır. Bu tezde GPS ile ilgili olarak hem teorik hemde pratik uygulamalar açıklanmıştır. x

Özet (Çeviri)

DEVELOPMENT OF GLOBAL POSITIONING SYSTEM (GPS) & ITS FUTURE SUMMARY Since the dawn of civilazition, men has looked to the heavens with awe searhcing for portentous signs. Some of these men became experts and developed rules to govern life. These men were the first surveyors. The chain of technical developments from these early astronomical surveyors to the present satellite geodetists reflects men's desire to able to master time and space to use science to further his society. The surveyor's role in society has remained unchanged from the early days; this role is to determine land boundaries, provide maps, and control the construction of public works. Triangulation technique was subsequently used by surveyors as the main means of determining accurate coordinates over continental distances. Triangulation technique was limited by line-of-sight. Because of this fact, each continent positionally isolated and their interrelationship was imprecisely known. Several attemps had been tried to determine the exact positions of continents. Optical global triangulation comprises taking photos of selected stars at two different points simultaneously. These oriented directions were used to construct a globeUnetwork that the scale was derived from terrestrial traverses. Later electromagnetic techniques used to connect continents. This technique is known as HIRAN, an electromagnetic High RANging system. This siytem was develop during World War II to position aircraft. And also this system was used to determine the relative difference between North American and European datum. The immediate predecessor of today's modern positioning system is the Navy Navigational Satellite System (NNSS), also called TRANSIT system. TRANSIT system was developed by U.S Military to determine the positions of vessels and aircrafts. Civilian use of this system was authorized and today TRANSIT satellites are being used by to determine the coordinates of selected datum points. And also this system is used by many small vessels and aircrafts for navigation. Global Positioning System was used to replace the TRANSIT- system. The main problem with TRANSIT system was large time gaps in coverage. The second problem is the low navigation accuracy. XIIn contrast, GPS gives exact answers to these qusetions“What time, What position, and What velocity is it? ”quickly, accurately and inexpensively anywhere on the globe at any time. The aim of navigation is instantaneous positioning and velocity determination. TRANSIT system was not able to provide continous positioning. To provide a continous global positioning capability, a scheme to orbit a sufficient number of satellites to ensure that four were always electronically visible was developed for GPS. Twentyone evenly spaced in circular twelve hours orbits inclined fiftyfive degrees to the equatorial plane would provide the desired coverage for the least expense. Depending on the selected elevation angle there will be more than the minimum number of satellites available for use and during these periods surveyors will able to perform kinematic or other kinds of surveys. Assuming ten degrees of elevation angle there will be up to ten satellites visible on earth. Consider the satellites frozen in space at a given instant, the coordinates i 6 s:). of the satellite relative to center of earth can be determined from broadcast ephemeris. If the ground receiver defined by the geocentric position vector ( Qs ) employed a clock that was set precisely to GPS system time, the true distance to each satellite could be accurately measured by recording the time required for the satellite signal to reach the receiver. Using this technique, ranges to only three satellites are sufficient because the intersection of three sphere yields three unknowns and could be determined from three range equations. q - !es-ej Modern GPS receivers apply a different technique. They use an inexpensive crystal clock which is set approximately to GPS system time. The clock of receiver is offset from true GPS time, and as a result, the distance to satellite is slighty longer or shorter than true range. The receiver can overcome this clock problem by observing four distances to four satellites simultaneously. These distances which have plus or minus a small extra distance aq resulting from clock error or bias Ö are called pseudoranges (R). #=Q+A6 = Q + CÖ c is the velocity of light. The point position can be solved by resection. We now need four pseudoranges to solve for the four unknowns: these are components of position and a clock bias. It is worth nothing that the range error aq could be eleminated by differecing the pseudoranges derived from one site to two satellites or two different positions of one satellite. The observation technique where both receivers remained fixed in position is called static surveying. The static method requires hours of observation and was the first technique that was developed for precise surveys. Kinematic XIItechnique involves one fixed and a roving receiver. The three dimensional position of the roving receiver can be determined as little as a few seconds of data and this technique can be used to produce a topographic map. Differential positioning technique involves placing a continous tracking receiver at a fixed site of known coordinates. Comparing computed pseudoranges with measured pseudoranges, the reference site can transmit corrections to a roving receiver to improve its measured pseudoranges. As explained before GPS was develop by U.S. Air Force for navigation. But U.S. Congress later promoted the civilian use of GPS. There are basically two methods for denying civilian users full use of the system. Selective availability has been accomplished by“dithering”the satellite clock frequency in a way that prevents civilian users from accurately measuring instantenous pseudoranges. The method of accuracy denial is to truncate the navigation message so that the coordinates of satellites can not be accurately computed. The error in satellite position translates to a like position error of the receiver. U.S. Department of Defence has also implemented Anti-Spoofing which denies full access to the P code. Under AS, the P code is mixed with the W code to produce Y code. GPS has three segments. These segments are space, control and user. Space segment comprises satellites. The satellites are 20200 km. above the earth and have 12 hours orbital periods. These satellites inclined 63° and 55° to the equatorial plane. Early satellites use two ribidium and cesium time standarts, but the new satellites have hydrogen masers. Hydrogen masers have precise frequency stability. These highly accurate frequency standarts produce L band frequency of 10.23 MHz. L1 and L2 carrier waves generated by multiplying the fundemental frequency by 154 and 120 respectively. On these carriers there are two modulated codes. These are P (Precision) and C/A (Coarse/Acquisition) codes. These codes contain messages such as Keplerian elements, GPS system time, ionospheric modeling coefficients, drift information and satellite ephemerides. The main operational tasks of control segment are: tracking of satellites for orbit and clock determination, prediction modeling, time synchronization of satellites and upload of data message to the satellites. There are two kinds of users. These are military user and civilian user. Both user perform every kind of surveying modes. But military users especially use the system for navigation, on the other hand civilian users use the system to produce all kinds of maps. GPS uses pseudoranges derived from the broadcast satellite signal. The pseudorange is either derived from measuring the travel time of signal and multiplying it by its velocity or by measuring the phase of signal. In both cases, clocks of the receiver and satellite are employed. Since these clocks are never perfectly synchronized, instead of true ranges“pseudoranges”obtained where the synchronization error is taken into account. XIIIConsequently, these pseudorange equations comprises four unknowns: the desired point's three coordinates and a clock error. Four satellites are necessary to solve four unknowns. GPS concept provides four or more satellites on earth 24 hours a day. The solution becomes more complicated when using measured phase. This observable is ambigious by an integer number of signal wave lengths so that the model for phase pseudoranges is augmented by an initial bias called integer ambiguity. The code pseudoranges and phase pseudoranges are affected by both, systematic errors or biases and random noise. The systematic errors can be modeled and give rise to additional terms in the observation equations. Systematic effects can also be eleminated by appropriate combinations of the observables. Differencing between receivers eleminates satellite specific biases. Double differenced pseudoranges are, to high degree, free of systematic errors originating from satellites and receivers. Ionospheric refraction can be virtually eleminated by an adequate combination of the dual frequency data. Phase differencing means subracting one equation from another. There are three kinds of phase differencing: between receiver, between satellite and between epoch. The most popular one is between receiver differencing. In single difference, two receivers and one satellite are involved. To get a single difference, one phase equation is subtracted from another. In this type, satellite clock bias cancels. In GPS software, double differenced equations are used to get the exact solutions. Here, two points and two satellites are involved. To get a double difference, individually formed singel differences to each satellite are subtracted. In this type receiver and satellite clock biases cancel. Multipath is "mainly caused by reflecting surfaces near by the receiver. _Multipath is described like that: a satellite emitted signal arrives the receiver more than one path. While choosing occupation points for GPS, this must be considered. Desinging a GPS network gives rise to several questions which are just as important as theorotical aspects of GPS. GPS equipments can be used for several surveying modes. But to get the desired level of accuracy, first you must choose your equipment carefully. For particular projects, observation technique does not change. The organization scheme must be well. Observation window must be analyzed carefully and sessions must last long as to obtain the desired solution. On the other hand GPS observer must be instructed how the project would proceed. General uses of GPS are numerous, in order to classify: global, reginol and local uses of GPS can be considered. Navigation was the planned primay use of GPS. Both military and civilian uses of the system in this mode are similar in that users wish to know their XIVspatial locations as precisely as possible. For example, all types of aircraft and vessels will use GPS for an route navigation and the denial of accuracy does not materially affect this use. Global applications of GPS is a powerful tool for geodesy. This science is involved in monitoring global geodynamical phenomena such as earth rotation and plate tectonics. Reginol applications of GPS navigation are enormous and include: exploration, transportation management, structural monitoring and various types of automation. Geodesists have long desired to measure crustal movements for various scientific purposes. One use would be to predict earthquakes by measuring certain precursor ground movement. GPS is an ideal tool for such studies in that the equipment is relatively inexpensive, portable and highly accurate. A topographic site survey and boundary determination requires the establishment of many temporary and permanent points all connected by conventional survey measurements. Two GPS receivers placed on two widely spaced points in this local scheme could collect data during the time the survey crew made conventional measurements, the coordinates of all points in the scheme would be determined and also all bearings in the scheme would be precisely related to true north. An additional local use of GPS is to perform topographic surveys by using the kinematic mode. In areas relatively few obstructions, a roving GPS receiver can either be carried or placed on a vehicle and the terrain traversed by a series of cross sections. The horizantal position and the height of points can typically be determined every second, so a high density of point determinations will result even when the receiver is vehicle mounted travels at high speed. The processing and plotting of the kinematic cross sections is automated so that the field to finish time can be minimized. Several organizations have used GPS kinematic surveying to determine the coodinates of the photocenter during aerial mapping flights. When the GPS receiver is carefully synchronized with the mapping camera, accuracies of a few centimeters have been obtained. The knowledge of an accurate air base significantly reduces the ground control that will be needed to perform even large-scale mapping. One such application is the use of GPS to automate various types of machinery. For example it should be possible to automate the grading and paving equipment used for road building. Equipment could be run around the clock without operators with GPS performing all motion operations based upon a digital terrain model stored in the equipments computer. Similarly, ships and planes could be operated in an automated mode with take offs and landings being performed by integrated GPS/computer units. Considering the present status of GPS and taking into account all the improvements previously discussed leads to an exciting outlook of the future of GPS. The enlarged number of next generation satellites will meet all the xvrequirements of navigation, surveying and timing. Substantially improved hardware and software components will provide the desired results more quickly or even in real-time. The accuracy is expected to be increased by at least one order of magnitude with lower costs. The future of GPS is really fascinating. However, what will happen within, let us say, the next decade can never be predicted exactly. Instead of relying on general predictions, we must observe present developments in order to keep abreast of this rapidly changing technology. XVI

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