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Chapter 1 - Introduction CHAPTER 1 INTRODUCTION The Global Positioning System (GPS) is a satellite-based radio-navigation system established by the U.S. Department of Defense for military positioning applications and as a by-product, has been made available to the civilian community. Navigation, surveying and integration with Geographic Information Systems (GIS) are just a few of the fields which have seen the successful application of GPS technology. GPS is a complex system which can be used to achieve position accuracies ranging from 100 m to a few millimetres depending on the equipment used and procedures followed. In general, higher accuracies correspond with higher costs and more complex observation and processing procedures. Therefore it is important for users to understand what techniques are required to achieve desired accuracies with the minimal cost and complexity. The objective of these guidelines is to provide the background and procedural information needed to effectively apply GPS technology. These guidelines contain four main parts geared towards achieving this objective. The fundamentals of GPS are explained in Chapter 2, basic positioning concepts are presented in Chapter 3, GPS positioning techniques are described in Chapter 4 and procedures for the application of GPS are discussed in Chapter 5. Although there are significant links between each of these chapters, one may prefer to reference any segment of these guidelines individually with the aid of the Table of Contents. The fundamental GPS concepts presented in Chapter 2 provide a starting point for those seeking to gain a better understanding of what GPS is all about. The discussion of GPS signals in this chapter is of particular importance since it is these signals which are at the root of the varied positioning techniques and their associated accuracies. The other concepts presented in Chapter 2 include a description of the system, general classifications of the types of GPS positioning, satellite visibility and errors. The significance of the basic positioning concepts presented in Chapter 3 should not be underestimated. An awareness of the various measures of accuracy used with respect to GPS is essential if one hopes to compare what is achievable with different techniques and equipment. A positioning concept of particular importance is the difference in the height system used by GPS satellites and the commonly used mean sea level heights. This is presented in Chapter 3 along with a description of coordinate systems and datums. Perhaps what might be the most interesting for those desiring to apply GPS are the positioning techniques summarized in Chapter 4. The beginning of the chapter commences by tabulating representative accuracies which can be achieved if the designated technique is successfully applied. Descriptions of each of these techniques follow. When reviewing these techniques, one should note that new methods are continually under development. An understanding of the general concepts of the methods presented herein, should make it easier to understand new techniques as they become available. The final chapter deals with procedures for carrying out a GPS project from initial conception to final returns. Since every project to be carried out and each set of equipment will require different procedures it would be impossible to address all contingencies in this chapter. Instead, general considerations and procedures which would be common to almost any GPS positioning project are presented. For specific detailed instructions one is wise to consult with manufacturers' documentation. The last section of Chapter 5 addresses special considerations which must be made when determining elevations with GPS. The appendices of these guidelines also provide a wealth of information. They include a glossary for all the terms included in the main portion of the text which are in italics, sources of information which may be beneficial when carrying out a GPS project, and suggested reading to learn more about GPS and its uses. A set of guidelines such as these cannot hope to address all queries regarding the huge and rapidly expanding industry of positioning with GPS. However it is hoped that they will help users appreciate the incredible benefits of the system and successfully employ it to satisfy their positioning needs. CHAPTER 2 GPS - BASIC CONCEPTS In this chapter, basic concepts of the Global Positioning System are presented. GPS can provide a wide range of accuracies, depending on the type of measurements used and procedures followed. In general, the higher the accuracy required, the higher the cost and the greater the complexity of using GPS. For users to understand which techniques are most suited for their requirements and why, it is important that the basic underlying concepts of GPS are understood. The main segments of GPS are described, followed by an explanation of GPS satellite signal components, general positioning techniques, satellite visibility and GPS error sources. 2.1 SYSTEM DESCRIPTION The Global Positioning System (GPS) consists of a constellation of radio-navigation satellites, a ground control segment which manages satellite operation and users with specialized receivers who use the satellite data to satisfy a broad range of positioning requirements (Figure 2.1). The system was established by the United States Department of Defense (DoD) to fulfill defence positioning needs and as a by-product, to serve the civilian community. The satellite constellation, which is expected to be fully operational by the end of 1993, will consist of 21 satellites and three active spares positioned 20,000 km (about three times the earth's radius) above the earth. The satellites will be distributed in a manner that ensures at least four satellites are visible almost anywhere in the world at any time (Figure 2.2). Each satellite receives and stores information from the control segment, maintains very accurate time through on-board precise atomic clocks and transmits signals to the earth. Figure 2.1 Three Segments of GPS The ground control segment (Figure 2.1) operates the satellite system on an on-going basis. It consists of five tracking stations distributed around the earth of which one, located in Colorado Springs, is a Master Control Station. The control segment tracks all satellites, ensures they are operating properly and computes their position in space. Figure 2.2 GPS Satellite Constellation If a satellite is not operating properly the ground control segment may set the satellite "unhealthy" and apply measures to correct the problem. In such cases, the satellite should not be used for positioning until its status is returned to "healthy". The computed positions of the satellites are used to derive parameters, which in turn are used to predict where the satellites will be later in time. These parameters are uploaded from the control segment to the satellites and are referred to as broadcast ephemerides. The user segment includes all those who use GPS tracking equipment to receive GPS signals to satisfy specific positioning requirements. A wide range of equipment designed to receive GPS signals is available commercially, to fulfill an even wider range of user applications. Almost all GPS tracking equipment have the same basic components: an antenna, an RF (radio frequency) section, a microprocessor, a control and display unit (CDU), a recording device, and a power supply. These components may be individual units, integrated as one unit, or partially integrated (Figure 2.3). Usually all components, with the exception of the antenna, are grouped together and referred to as a receiver. Some GPS receivers being marketed now in fact only consist of computer cards which may be mounted in portable computers or integrated with other navigation systems. Figure 2.3 GPS Equipment 2.2 GPS SIGNALS Each GPS satellite continuously transmits signals which contain a wealth of information. Depending on the type and accuracy of positioning being carried out, a user may only be interested in a portion of the information included in the GPS signal. Similarly, a given GPS receiver may only enable use of a portion of the available information. It is therefore important for users to understand the content and use of GPS signals. The information contained in GPS signals includes the carrier frequencies, Coarse Acquisition (C/A) and Precise (P) codes and the satellite message. Descriptions of each of these signal components follow. Carrier Measurements Signals from GPS satellites are continuously transmitted on two carrier frequencies, 1575.42 MHz and 1227.60 MHz, and are referred to as L1 and L2 respectively. Since radio waves propagate through space at the speed of light, the wavelengths of the GPS carrier signals are computed as λ=c/f (2.1) where λ is the wavelength (i.e. the length of one cycle) in metres, c is the speed of light (approximately 3 × 108 m/s) and f is the carrier frequency in Hz (i.e. cycles per second). A snapshot of one section of carrier transmission which illustrates the definition of wavelength and cycles is shown in Figure 2.4. Figure 2.4 Carrier The frequency and wavelength of the L1 and L2 carriers (computed using equation (2.1)) are given in Table 2.1. GPS receivers which record carrier phase, measure the fraction of one wavelength (i.e. fraction of 19 cm for the L1 carrier) when the receiver first locks onto a satellite and continuously measure the carrier phase from that time. The number of cycles between the satellite and receiver at initial start up (referred to as the ambiguity) and the measured carrier phase together represent the satellite-receiver range (i.e. the distance between a satellite and a receiver). In other words, measured carrier phase = range + (ambiguity × wavelength) + errors оr ^ = p + Nλ+ errors, (2.2) where ^ is the measured carrier phase in metres, p is the satellite-receiver range in metres, N is the ambiguity (i.e. number of cycles) and Y is the carrier wavelength in metres. Note that a sign convention similar to that adopted by the Canadian GPS Associates (Wells et.al.) was used. The errors are as described in Section 2.5. Table 2.1 Carrier Frequencies and Wavelengths Carrier Frequency (f) Wavelength (Y) L1 1575.42 MHz 19 cm L2 1227.60 MHz 24 cm Code and satellite messages are piggy-backed on the carrier signal through modulation. The L1 carrier is modulated by a coarse acquisition code referred to as the CA code, a precise code referred to as the P code and the satellite message. The L2 carrier is modulated by the P code and the satellite message (Figure 2.5). Figure 2.5 Information Modulated on Each Carrier Code Measurements It is the code measurements (also referred to as pseudorange measurements) that enable instantaneous position determinations using GPS satellites. The code is composed of a series of chips which have values of 1 or 0. The C/A code has a frequency of 1.023 MHz (i.e. 1.023 million chips per second) and the P code has a frequency of 10.23 MHz. Example portions of C/A code and P code are shown in Figure 2.6. Figure 2.6 C/A and P Codes The chip lengths of 293 m and 29.3 m for the C/A code and P code respectively were computed using equation (2.1), letting λ be the chip length. Although the P code is generally ten times more accurate than the C/A code, it is expected to be unavailable for civilian use in 1993 when the full GPS constellation is complete (McNeff, 1991), meaning only C/A code is worthy of consideration for civilian GPS applications. Code measurements are the difference in time between when the code is transmitted from a satellite and received at a GPS receiver, multiplied by the speed of light. That is, measured code = speed of light x (reception time - transmission time) or P = c(tr - tt). (in metres) (2.3) where P is the measured code, c is the speed of light, tr is the signal reception time and tt is the signal transmission time. The code measurement is actually a direct measurement of satellite-receiver range (p), i.e.: measured code = range + errors Or P = p + errors. (in metres) (2.4) or The errors are as described in Section 2.5. Comparison of Code and Carrier Measurements At this point it is possible to make some brief comparisons of code and carrier measurements. Carrier wavelengths (19 cm for L1) are much shorter than the C/A code chip length (293 m) and consequently can be measured more accurately and used to achieve much higher positional accuracies than code measurements. Indeed the best relative accuracies achieved using code measurements are usually a few metres, and using carrier measurement are usually a few centimetres. The problem with using carrier observations instead of code observations is evident upon comparison of equations (2.2) and (2.4). With code observations a direct measure of the satellite-receiver range is attained. With carrier observations, the ambiguity term (number of whole cycles) must be estimated before one may take advantage of the carrier accuracy. Ambiguity estimation leads to complexities in the use of carrier phase observations which do not exist with code observations. The advantages and disadvantages of code and carrier observations are summarized in Table 2.2. Table 2.2 Key Advantages and Disadvantages of Code and Carrier Observations Code Carrier Advantages Non-ambiguous/simple High accuracy potential Disadvantages Low Accuracy Lot more complex Satellite Message The satellite message, which is modulated on both L1 and L2 frequencies, contains among other information, satellite broadcast ephemerides and health status. The ephemerides include the parameters necessary to compute a satellite's position in space for a given time and the health status indicates if a satellite is healthy. Almost all receivers use the broadcast ephemerides in conjunction with code observations, carrier observations or both to solve for a GPS receiver's position in space