33 Using GPS for GIS data capture A F LANGE AND C GILBERT Depending on the particular equipment utilised and the techniques used, Global Positioning Systems (GPS) are capable of recording position to a high level of accuracy. Differential GPS is required to obtain the accuracy required by many GIS data capture applications. Differential GPS requires the use of a base station at a known location to remove systematic errors from the GPS signal. Modern GPS/GIS data collection devices have the capability to collect a wide range of GIS attribute data in addition to position of the feature of interest; attributes include point, line, and area features. Since GPS can be used in many diverse GIS data capture applications (including applications as diverse as road centreline mapping, utility pole mapping, and wetland boundary mapping), it is important to understand the limitations of GPS in order to derive full benefits from the technology. 1 FUNDAMENTALS OF GPS satellite-based radio-navigation system that is capable of providing extremely accurate worldwide, The explosion in interest in GIS as a management tool 24-hour, 3-dimensional (latitude, longitude, and has been accompanied by the development of a elevation) location data (Wells 1987). It is one of two number of enabling technologies, one of the more such satellite systems, although its Russian important of which is the Global Positioning System counterpart, GLONASS, will not be discussed here. (GPS). While GIS technology offers tremendous The system was designed and is maintained by the capabilities for more informed management decision- US Department of Defense (DoD) as an accurate, making, rendering competent decisions still depends all-weather navigation system. Though designed as a on having reliable data. In order to realise the benefits military system, it is available with certain of GPS, and not to mis-apply the technology, it is restrictions to civilians. The system has reached its important to understand its limitations. This chapter full operational capability, with a complete set of at describes how GPS works and how to obtain reliable least 24 satellites orbiting the Earth in a carefully data using it. Accuracy (in the surveying sense) of designed pattern. GPS data is an important consideration. The first part of this chapter covers how the GPS accuracy is 1.1.1 The fundamental components of GPS obtained and the second part discusses how to use The NAVSTAR GPS has three basic segments: GPS for GIS data capture. The term ‘GPS’ is used space, control, and user. The space segment consists interchangeably here to refer to satellite-based of the orbiting satellites making up the constellation. navigation systems in general and the ground-based This constellation is comprised of 24 satellites, each geographical data collection instruments in particular. orbiting at an altitude of approximately 20 000 kilometres, in one of six orbital planes inclined 55 degrees relative to the Earth’s equator. Each 1.1 Overview of GPS satellite broadcasts a unique coded signal, known as The Navigation Satellite Timing And Ranging pseudo random noise (PRN) code, that enables GPS Global Positioning System, or NAVSTAR GPS, is a receivers to identify the satellites from which the 467 A F Lange and C Gilbert signals are coming (Hurn 1989). The satellites broadcast the PRN codes as modulation on two carrier frequencies, L1 and L2. The L1 frequency is 1575.5 MHz and the L2 frequency is 1227.6 MHz. With 24 satellites in the constellation, and the design of the orbits and the spacing of the satellites in the orbital planes, most users will have six or more satellites available at all times. There are a number of different programs used to plot the satellite availability for any geographical location. The universal reference locator (URL) for a World Wide Web (WWW) site with a GPS satellite visibility program on-line is http://www.trimble.com/satviz. The control segment, under the United States Department of Defense’s direction, oversees the building, launching, orbital positioning, and monitoring of the system. It provides two classes of GPS service: Standard Positioning Service (SPS) and Precise Positioning Service (PPS). Monitoring and ground control stations, located around the globe near the equator, constantly monitor the performance of each satellite and the constellation as a whole. A master control station updates the information component of the GPS signal with Fig 1. A hand-held 12-channel GPS receiver. satellite ephemeris data and other messages to the users. This information is then decoded by the dramatically in recent years, because of the receiver and used in the positioning process. Of the emergence of low-cost portable GPS receivers two classes of service, the PPS is only available to (Figure 1 and Plate 23) and the ever-expanding areas authorised government users while the SPS is of applications in which GPS has been found to be available for civilian use. useful. Such applications include surveying, The user segment is comprised of all of the users mapping, agriculture, navigation, and vehicle making observations with GPS receivers. The tracking (Figure 2; Plate 24). The civilian users of civilian GPS user community has increased GPS greatly outnumber the military users. Fig 2. Off-road navigation using GPS. 468 Using GPS for GIS data capture 1.1.2 The limitations of GPS for a 2-dimensional reading (latitude and longitude), Although in theory GPS can provide worldwide, 3- and at least four satellites for a 3-dimensional dimensional positions, 24 hours a day, in any type of position (latitude, longitude, and altitude). The GPS weather, the system does have some limitations. First, receiver ‘knows’ where each of the satellites is at the there must be a (relatively) clear ‘line of sight’ between instant in which the distance was measured. These the receiver’s antenna and several orbiting satellites. distances will intersect at only one point, the position Anything shielding the antenna from a satellite can of the GPS antenna. How does the receiver ‘know’ potentially weaken the satellite’s signal to such a the position of the satellites? Well, this information degree that it becomes too difficult to make reliable comes from the broadcast orbit data that are received positioning. As a rule of thumb, an obstruction that when the GPS receiver is turned on. The GPS receiver can block sunlight can effectively block GPS signals. performs the necessary mathematical calculations, Buildings, trees, overpasses, and other obstructions then displays and/or stores the position, along with that block the line of sight between the satellite and any other descriptive information entered by the the observer (GPS antenna) all make it impossible to work with GPS. Urban areas are especially affected by operator from the keyboard. these types of difficulties. Bouncing of the signal off The way in which a GPS receiver determines nearby objects or the ground may create another distances to the satellites depends on the type of problem called multi-path interference. Multi-path GPS receiver. There are two broad classes: code interference is caused by the inability of the receiver to based and carrier phase based. distinguish between the signal coming directly from the satellite and the ‘echo’ signal that reaches the 1.1.4 Code-based receivers receiver indirectly. In areas that possess these type of Although less accurate than their carrier phase characteristics, as in the urban core of a large city, counterparts, code-based receivers have gained inertial navigation techniques must be used to widespread appeal for applications such as GIS data complement GPS positioning. capture. Their popularity stems mainly from their The receiver must receive signals from at least four relatively low cost, portability, and ease of use. satellites in order to be able to make reliable position Code-based receivers use the speed of light and measurements. In addition, this number of satellites the time interval that it takes for a signal to travel must be in a favourable geometrical arrangement, from the satellite to the receiver to compute the relatively spread out. In areas with an open view of distance to the satellites. The time interval is the sky, this will almost always be the case because of determined by comparing the time in which a the way the satellite orbits were designed. The specific part of the coded signal left the satellite with dilution of precision, or DOP, is a measure of the the time it arrived at the antenna. The time interval geometry of the GPS satellites. When the satellites is translated to a distance by multiplying the interval are spaced well apart in the sky, the GPS position by the speed of light. Position fixes are made by the data will be more accurate than when the satellites receiver roughly every second, and the GPS receivers are all in a straight line or grouped closely together. designed for use in GIS data capture applications An additional limitation of GPS is the DoD enable the user to store the positions in a file that policy of selective availability (SA). This policy limits the full autonomous accuracy only to official can be downloaded to a computer for post- government users. This policy, and methods used to processing or analysis. ameliorate its effects, are described in section 1.1.6. Under normal circumstances, autonomous A Presidential Directive of May 1996 has declared position fixes made by code-based receivers would that the policy of SA will likely be discontinued be accurate to within 25 metres. This is the specified within a period of more than four years and less accuracy for the SPS. The DoD, however, began than ten years. However, even if SA is eliminated, imposing its selective availability policy in July 1992, GPS will not be accurate enough for many mapping and this limits position fix accuracy to within and GIS data capture applications.