GPS - Global Position System

G. P. S. BASICS http://gpsscales.com/intro.htm Global Positioning System (GPS) technology is a great boon to anyone who has the need to navigate either great or small distances. This wonderful navigation technology was actually first available for government use back in the late 1970s. In the past ten or so years, It has been made available to the general public in the form of handheld receivers that use this satellite technology provided by the U.S. government. Through the use of these handheld receivers, one can navigate back to a starting point or other predetermined locations without the use of maps or any other equipment. In conjunction with accurate maps like ones provided by the USGS, and other basic tools like a compass and Lat/Long or UTM scales, one can navigate to identified locations on maps or take readings from a location that they are at or have been at and plot those locations on a map. All of these features make it a very desirable and useful technology for a mirid of activities including Search and Rescue, Aviation and Nautical navigation, hiking, hunting, camping, fishing, and many more. All of these various GPS users have unique needs which require different levels of understanding and skill in using this technology. At the most basic level, the GPS user needs to be able to set-up and initialize the unit and SAVE and GOTO a waypoint. For many users, this is all that they really need to do. For others, it is important to understand the coordinate grid systems and to be able to plot and read position coordinates on a map. Being able to plot and read position coordinates, enables the user to make the optimum use of this technology for more sophisticated applications. In Chapter one, we will cover the fundamental concepts of what makes GPS work, the basics of setting up the receiver, taking a position fix, and activating the GOTO navigation function to a given waypoint. For some, this may be all they need to know for the applications that they are interested in. The second Chapter will cover the concepts of the Latitude and Longitude Grid system and how to plot and read the coordinates of positions on a map. Chapter three covers these same concepts using the Universal Transverse Mercator (UTM) metic grid system. AN INTRODUCTION TO DIFFERENTIAL GPS http://maps.unomaha.edu/Peterson/gis/Final_Projects/1996/Bohrer/DGPS.HTM GPS GPS stands for Global Positioning System and it allows users to determine their location on land, sea, and in the air around the Earth. It does this using satellites and receivers. There are currently 24 satellites in orbit operated by the US Department of Defense that provide worldwide coverage 24 hours a day, 7 days a week, in all weather. How the system works is by the satellites sending information to receivers. This information includes time, position, and satellite strength among other things. The receivers pick up this information and use it to determine the users location. Using the signals from at least 4 satellites, a receiver can determine latitude, longitude, and elevation. Some receivers can then convert the latitude and longitude into other coordinate system values. The accuracy of GPS depends on several factors such as which receiver is being used, the surroundings it''s being used in, and Selective Availability. Selective Availability is the Department of Defense deliberately interfering with the satellite signals to reduce positional accuracy to around 30m - 100m. With Selective Availability receivers are divided into two types: precise positioning systems (PPS) and standard positioning systems (SPS). PPS receivers are used by the military and are not affected by Selective Availability. Currently there are efforts under way to end the use of Selective Availability. DGPS Differential GPS uses position corrections to attain greater accuracy. It does this by the use of a reference station. The reference station (or base station) may be a ground based facility or a geosynchronous satellite, in either case it is a station whose position is a known point. When a station knows what it''s precise location is it can compare that position with the signals from the GPS satellites and thus find the SA error. These corrections can then be immediately transmitted to mobile GPS receivers (real time DGPS), or the receiver positions can be corrected at a later time (post processing). The use of DGPS can greatly increase positional accuracy (in general, the better it is the more expensive it is). Some surveying systems can give subcentimeter readings. There are a lot of different differential providers that supply real time and post processing corrections, many are private companies. The availability of these services varies greatly depending on what part of the country you are in, but the US Coast Guard covers the US coastline and the number of private and governmental providers is increasing, so I imagine that someday the entire US will be covered. Here are some links for brief explanations of (D)GPS. THE STARLINK DGPS PAGES --This site is currently under construction, but it does have a description of how real time DGPS works, along with a couple graphics. USCG DIFFERENTIAL GPS --This site gives a brief introduction to GPS including: how it works, how it is used, the Coast Guard DGPS system, and the latest US and Canadian Coast Guard status reports. Check out the NAVCEN homepage for links to lots of documents related to GPS. THE GLOBAL POSITIONING SYSTEM FROM ELIRIS --This site gives a brief introduction that covers the space segment, the control segment, the user segment, SPS/PPS, and has a few graphics. THE GLOBAL POSITIONING SYSTEM (GPS) --A brief overview with some information about receivers for boaters. GPS BY CAPTAIN ROD AND SUSIE STEBBINS --This site is boating oriented and gives an overview that includes: how it works, accuracy, DGPS, monitoring and control, information sources, a map datum explanation, and navigation responsibilities. Check out the Suncoast Boating link to find help in up/downloading Magellan receivers to PC''s. K&L GPS AND DIFFERENTIAL GPS --This site gives an introduction to GPS and covers the space segment, the control segment, and the user segment. It also has a link to a page of GPS related links and differential providers. NAVIGATIONAL SYSTEM FOR THE VISUALLY IMPAIRED --This site covers an introduction, satellite triangulation, timing, frequencies, DGPS, and has some graphics. It is part of a project using GPS to aid the visually impaired. DIFFERENTIAL GLOBAL POSITIONING SYSTEMS --A site from the Ontario Ministry of Agriculture, Food and Rural Affairs that gives an overview of DGPS with an emphasis on satellite and differential signals. It also has some information about differential providers, such as their addresses, phone numbers, hardware, signal formats, and receiver prices. Some sites with more detailed explanations of (D)GPS. GLOBAL POSITIONING SYSTEM INFORMATION --This site from the US Naval Observatory covers GPS policy, signal characteristics, selective availability, system segments, system time, system transfer, and the current constellation. THE GLOBAL POSITIONING SYSTEM (GPS) --A great site full of information from the Department of Geography at the University of Texas. It covers just about everything and has links from the table of contents to the rest of the document, graphics, and a bunch of links to other sites. RADIONAVIGATION --This site is part of a book for community emergency services planners by Rex Buddenberg. This page gives an introduction to GPS, LORAN, DGPS, and covers advantages, limitations, and integration of multiple systems. It also has a few graphics. INTRODUCTION TO GPS APPLICATIONS --This is an excellent site by John Beadles that is currently under construction (but is color coded to give construction status). It has many links and covers how GPS works, policy issues, applications, vendors, equipment, services, and related sites. DGPS APPLICATIONS Differential GPS is currently being used for many things, and it is a growing technology. One of it''s more popular applications is in air navigation. By using it a pilot can receive constant information about where the plane is in 3 dimensions. It is also becoming a hot topic in precision farming. (MORE) (MORE) Farmers can use DGPS to map out their crops, map crop yields, and control chemical applications and seeding. It is also proving to be useful in ground and hydrographic surveying. Another application is in weather forecasting, where atmospheric information can be gained from it''s effects on the satellite signals. There has also been at least one experiment where it was used for beach morphology and monitoring. DGPS can also be used for train control for such things as avoiding collisions and routing. There is even been research into using it to help the visually impaired in getting around in cities. There is also at least one project that is working on using DGPS for car navigation. In the sports world it is finding a place in balloon and boat racing. I think that the future of DGPS applications will only be limited by imagination and money, and I predict that it will eventually become an integral part of much of our technology. Global Positioning System Overview http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html GPS is a Satellite Navigation System GPS is funded by and controlled by the U. S. Department of Defense (DOD). While there are many thousands of civil users of GPS world-wide, the system was designed for and is operated by the U. S. military. GPS provides specially coded satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position, velocity and time. Four GPS satellite signals are used to compute positions in three dimensions and the time offset in the receiver clock. An Article by William C. Dias, IBM GIS http://giswww.pok.ibm.com/gps/gpsweb.html#Header_3 GPS SYSTEM SEGMENTS http://tycho.usno.navy.mil/gpsinfo.html#seg The GPS consists of three major segments: SPACE, CONTROL and USER. The SPACE segment consists of 24 operational satellites in six orbital planes (four satellites in each plane). The satellites operate in circular 20,200 km (10,900 nm) orbits at an inclination angle of 55 degrees and with a 12-hour period. The position is therefore the same at the same sidereal time each day, i.e. the satellites appear 4 minutes earlier each day. The CONTROL segment consists of five Monitor Stations (Hawaii, Kwajalein, Ascension Island, Diego Garcia, Colorado Springs), three Ground Antennas, (Ascension Island, Diego Garcia, Kwajalein), and a Master Control Station (MCS) located at Schriever AFB in Colorado. The monitor stations passively track all satellites in view, accumulating ranging data. This information is processed at the MCS to determine satellite orbits and to update each satellite''s navigation message. Updated information is transmitted to each satellite via the Ground Antennas. The USER segment consists of antennas and receiver-processors that provide positioning, velocity, and precise timing to the user. GPS SYSTEM TIME GPS system time is given by its Composite Clock (CC). The CC or "paper" clock consists of all operational Monitor Station and satellite frequency standards. GPS system time, in turn, is referenced to the Master Clock (MC) at the USNO and steered to UTC(USNO) from which system time will not deviate by more than one microsecond. The exact difference is contained in the navigation message in the form of two constants, A0 and A1, giving the time difference and rate of system time against UTC(USNO,MC). UTC(USNO) itself is kept very close to the international benchmark UTC as maintained by the BIPM, and the exact difference, USNO vs. BIPM is available in near real time. The latest individual satellite measurements are updated daily. (Data format explanation.) The best current measure of the difference, UTC(USNO MC) - GPS is based on filtered and smoothed data over the past two days. -------------------------------------------------------------------------------- GPS TIME TRANSFER GPS is at the present time the most competent system for time transfer , the distribution of Precise Time and Time Interval (PTTI). The system uses time of arrival (TOA) measurements for the determination of user position. A precisely timed clock is not essential for the user because time is obtained in addition to position by the measurement of TOA of FOUR satellites simultaneously in view. If altitude is known (i.e. for a surface user), then THREE satellites are sufficient. If time is being kept by a stable clock (say, since the last complete coverage), then TWO satellites in view are sufficient for a fix at known altitude. If the user is, in addition, stationary or has a known speed then, in principle, the position can be obtained by the observation of a complete pass of a SINGLE satellite. This could be called the "transit" mode, because the old TRANSIT system uses this method. In the case of GPS, however, the apparent motion of the satellite is much slower, requiring much more stability of the user clock. What is GPS http://infohost.nmt.edu/~mreece/gps/whatisgps.html GPS is a satellite-based navigation system originally developed for military purposes and is maintained and controlled by the United States Department of Defense. GPS permits land, sea, and airborne users to determine their three-dimensional position, velocity, and time. It can be used by anyone with a receiver anywhere on the planet, at any time of day or night, in any type of weather. This is an amazing capability! There are two GPS systems: NAVSTAR - United State''s system, and GLONASS - the Russian version. The NAVSTAR system is often referred to as the GPS (at least in the U.S.) since it was generally available first. Many GPS receivers can use data from both NAVSTAR and GLONASS; this report focuses on the NAVSTAR system. Segments GPS uses radio transmissions. The satellites transmit timing information and satellite location information. The system can be separated into three parts: Space Segment Control Segment User Segment This page includes several figures to help describe the system. The following figure illustrates how the three segments fit together (Figure 1) Introduction to the Global Positioning System for GIS and TRAVERSE http://www.cmtinc.com/gpsbook/chap7.html Measuring GPS Accuracy As discussed above, there are several external sources which introduce errors into a GPS position. While the errors discussed above always affect accuracy, another major factor in determining positional accuracy is the alignment, or geometry, of the group of satellites (constellation) from which signals are being received. The geometry of the constellation is evaluated for several factors, all of which fall into the category of Dilution Of Precision, or DOP. DOP DOP is an indicator of the quality of the geometry of the satellite constellation. Your computed position can vary depending on which satellites you use for the measurement. Different satellite geometries can magnify or lessen the errors in the error budget described above. A greater angle between the satellites lowers the DOP, and provides a better measurement. A higher DOP indicates poor satellite geometry, and an inferior measurement cofiguration. ch7 Introduction to the Global Positioning System for GIS and TRAVERSE http://www.cmtinc.com/gpsbook/chap6.html The GPS system has been designed to be as nearly accurate as possible. However, there are still errors. Added together, these errors can cause a deviation of +/- 50 -100 meters from the actual GPS receiver position. There are several sources for these errors, the most significant of which are discussed below: Atmospheric Conditions The ionosphere and troposphere both refract the GPS signals. This causes the speed of the GPS signal in the ionosphere and troposphere to be different from the speed of the GPS signal in space. Therefore, the distance calculated from "Signal Speed x Time" will be different for the portion of the GPS signal path that passes through the ionosphere and troposphere and for the portion that passes through space. Ephemeris Errors/Clock Drift/Measurement Noise As mentioned earlier, GPS signals contain information about ephemeris (orbital position) errors, and about the rate of clock drift for the broadcasting satellite. The data concerning ephemeris errors may not exactly model the true satellite motion or the exact rate of clock drift. Distortion of the signal by measurement noise can further increase positional error. The disparity in ephemeris data can introduce 1-5 meters of positional error, clock drift disparity can introduce 0-1.5 meters of positional error and measurement noise can introduce 0-10 meters of positional error. Selective Availability Ephemeris errors should not be confused with Selective Availability (SA), which is the intentional alteration of the time and epherimis signal by the Department of Defense. SA can introduce 0-70 meters of positional error. Fortunately, positional errors caused by SA can be removed by differential correction. Multipath A GPS signal bouncing off a reflectilve surface prior to reaching the GPS receiver antenna is referred to as multipath. Because it is difficult to completely correct multipath error, even in high precision GPS units, multipath error is a serious concern to the GPS user. The chart below lists the most common sources of error in GPS positions. This chart is commonly known as the GPS Error Budget: GPS Error Budget Source Uncorrected Error Level Ionosphere 0-30 meters Troposphere 0-30 meters Measurement Noise 0-10 meters Ephemeris Data 1-5 meters Clock Drift 0-1.5 meters Multipath 0-1 meter Selective Availability 0-70 meters ch6 Introduction to the Global Positioning System for GIS and TRAVERSE http://www.cmtinc.com/gpsbook/chap5.html Four (4) Satellites to give a 3D position In the previous example, you saw that it took only 3 measurements to "triangulate" a 3D position. However, GPS needs a 4th satellite to provide a 3D position. Why?? Three measurements can be used to locate a point, assuming the GPS receiver and satellite clocks are precisely and continually synchronized, thereby allowing the distance calculations to be accurately determined. Unfortunately, it is impossible to synchronize these two clocks, since the clocks in GPS receivers are not as accurate as the very precise and expensive atomic clocks in the satellites. The GPS signals travel from the satellite to the receiver very fast, so if the two clocks are off by only a small fraction, the determined position data may be considerably distorted. The atomic clocks aboard the satellites maintain their time to a very high degree of accuracy. However, there will always be a slight variation in clock rates from satellite to satellite. Close monitoring of the clock of each satellite from the ground permits the control station to insert a message in the signal of each satellite which precisely describes the drift rate of that satellite''s clock. The insertion of the drift rate effectively synchronizes all of the GPS satellite clocks. The same procedure cannot be applied to the clock in a GPS receiver. Therefore, a fourth variable (in addition to x, y and z), time, must be determined in order to calculate a precise location. Mathematically, to solve for four unknowns (x,y,z, and t), there must be four equations. In determining GPS positions, the four equations are represented by signals from four different satellites. ch5 Introduction to the Global Positioning System for GIS and TRAVERSE http://www.cmtinc.com/gpsbook/chap4.html Computing the Distance Between Your Position and the GPS Satellites GPS determines distance between a GPS satellite and a GPS receiver by measuring the amount of time it takes a radio signal (the GPS signal) to travel from the satellite to the receiver. Radio waves travel at the speed of light, which is about 186,000 miles per second. So, if the amount of time it takes for the signal to travel from the satellite to the receiver is known, the distance from the satellite to the receiver (distance = speed x time) can be determined. If the exact time when the signal was transmitted and the exact time when it was received are known, the signal''s travel time can be determined. In order to do this, the satellites and the receivers use very accurate clocks which are synchronized so that they generate the same code at exactly the same time. The code received from the satellite can be compared with the code generated by the receiver. By comparing the codes, the time difference between when the satellite generated the code and when the receiver generated the code can be determined. This interval is the travel time of the code. Multiplying this travel time, in seconds, by 186,000 miles per second gives the distance from the receiver position to the satellite in miles. Introduction to the Global Positioning System for GIS and TRAVERSE http://www.cmtinc.com/gpsbook/chap3.html How the Current Locations of GPS Satellites are Determined GPS satellites are orbiting the Earth at an altitude of 11,000 miles. The DOD can predict the paths of the satellites vs. time with great accuracy. Furthermore, the satellites can be periodically adjusted by huge land-based radar systems. Therefore, the orbits, and thus the locations of the satellites, are known in advance. Today''s GPS receivers store this orbit information for all of the GPS satellites in what is known as an almanac. Think of the almanac as a "bus schedule" advising you of where each satellite will be at a particular time. Each GPS satellite continually broadcasts the almanac. Your GPS receiver will automatically collect this information and store it for future reference. The Department of Defense constantly monitors the orbit of the satellites looking for deviations from predicted values. Any deviations (caused by natural atmospheric phenomenon such as gravity), are known as ephemeris errors. When ephemeris errors are determined to exist for a satellite, the errors are sent back up to that satellite, which in turn broadcasts the errors as part of the standard message, supplying this information to the GPS receivers. By using the information from the almanac in conjuction with the ephemeris error data, the position of a GPS satellite can be very precisely determined for a given time. Introduction to the Global Positioning System for GIS and TRAVERSE http://www.cmtinc.com/gpsbook/chap2.html In a nutshell, GPS is based on satellite ranging - calculating the distances between the receiver and the position of 3 or more satellites (4 or more if elevation is desired) and then applying some good old mathematics. Assuming the positions of the satellites are known, the location of the receiver can be calculated by determining the distance from each of the satellites to the receiver. GPS takes these 3 or more known references and measured distances and “triangulates” an additional position. As an example, assume that I have asked you to find me at a stationary position based upon a few clues which I am willing to give you. First, I tell you that I am exactly 10 miles away from your house. You would know I am somewhere on the perimeter of a sphere that has an origin as your house and a radius of 10 miles. With this information alone, you would have a difficult time to find me since there are an infinite number of locations on the perimeter of that sphere. Second, I tell you that I am also exactly 12 miles away from the ABC Grocery Store. Now you can define a second sphere with its origin at the store and a radius of 12 miles. You know that I am located somewhere in the space where the perimeters of these two spheres intersect - but there are still many possibilities to define my location. Adding additional spheres will further reduce the number of possible locations. In fact, a third origin and distance (I tell you am 8 miles away from the City Clock) narrows my position down to just 2 points. By adding one more sphere, you can pinpoint my exact location. Actually, the 4th sphere may not be necessary. One of the possibilities may not make sense, and therefore can be eliminated. For example, if you know I am above sea level, you can reject a point that has negative elevation. Mathematics and computers allow us to determine the correct point with only 3 satellites. Introduction to the Global Positioning System for GIS and TRAVERSE http://www.cmtinc.com/gpsbook/chap1.html What is GPS? The Global Positioning System (GPS) is a location system based on a constellation of 24 satellites orbiting the earth at altitudes of approximately 11,000 miles. GPS was developed by the United States Department of Defense (DOD), for its tremendous application as a military locating utility. The DOD''s investment in GPS is immense. Billions and billions of dollars have been invested in creating this technology for military uses. However, over the past several years, GPS has proven to be a useful tool in non-military mapping applications as well. GPS satellites are orbited high enough to avoid the problems associated with land based systems, yet can provide accurate positioning 24 hours a day, anywhere in the world. Uncorrected positions determined from GPS satellite signals produce accuracies in the range of 50 to 100 meters. When using a technique called differential correction, users can get positions accurate to within 5 meters or less. Today, many industries are leveraging off the DOD''s massive undertaking. As GPS units are becoming smaller and less expensive, there are an expanding number of applications for GPS. In transportation applications, GPS assists pilots and drivers in pinpointing their locations and avoiding collisions. Farmers can use GPS to guide equipment and control accurate distribution of fertilizers and other chemicals. Recreationally, GPS is used for providing accurate locations and as a navigation tool for hikers, hunters and boaters. Many would argue that GPS has found its greatest utility in the field of Geographic Information Systems (GIS). With some consideration for error, GPS can provide any point on earth with a unique address (its precise location). A GIS is basically a descriptive database of the earth (or a specific part of the earth). GPS tells you that you are at point X,Y,Z while GIS tells you that X,Y,Z is an oak tree, or a spot in a stream with a pH level of 5.4. GPS tells us the "where". GIS tells us the "what". GPS/GIS is reshaping the way we locate, organize, analyze and map our resources. Introduction to the Global Positioning System for GIS and TRAVERSE http://www.cmtinc.com/gpsbook/chap8.html Using Differential GPS to Increase Accuracy As powerful as GPS is, +/-50 - 100 meters of uncertainty is not acceptable in many applications. How can we obtain higher accuracies? A technique called differential correction is necessary to get accuracies within 1 -5 meters, or even better, with advanced equipment. Differential correction requires a second GPS receiver, a base station, collecting data at a stationary position on a precisely known point (typically it is a surveyed benchmark). Because the physical location of the base station is known, a correction factor can be computed by comparing the known location with the GPS location determined by using the satellites. The differential correction process takes this correction factor and applies it to the GPS data collected by a GPS receiver in the field. Differential correction eliminates most of the errors listed in the GPS Error Budget discussed earlier. After differential correction, the GPS Error Budget changes as follows: GPS Error Budget Source Uncorrected With Differential Ionosphere 0-30 meters Mostly Removed Troposphere 0-30 meters All Removed Signal Noise 0-10 meters All Removed Ephemeris Data 1-5 meters All Removed Clock Drift 0-1.5 meters All Removed Multipath 0-1 meters Not Removed SA 0-70 meters All Removed By eliminating many of the above errors, differential correction allows GPS positions to be computed at a much higher level of accuracy Introduction to the Global Positioning System for GIS and TRAVERSE http://www.cmtinc.com/gpsbook/chap9.html Levels of GPS Accuracy There are three types of GPS receivers which are available in today''s marketplace. Each of the three types offers different levels of accuracy, and has different requirements to obtain those accuracies. To this point, the discussion in this book has focused on Coarse Acquisition (C/A code) GPS receivers. The two remaining types of GPS receiver are Carrier Phase receivers and Dual Frequency receivers. C/A Code receivers C/A Code receivers typically provide 1-5 meter GPS position accuracy with differential correction. C/A Code GPS receivers provide a sufficient degree of accuracy to make them useful in most GIS applications. C/A Code receivers can provide 1-5 meter GPS position accuracy with an occupation time of 1 second. Longer occupation times (up to 3 minutes) will provide GPS position accuracies consistently within 1-3 meters. Recent advances in GPS receiver design will now allow a C/A Code receiver to provide sub-meter accuracy, down to 30 cm. Carrier Phase receivers Carrier Phase receivers typically provide 10-30 cm GPS position accuracy with differential correction. Carrier Phase receivers provide the higher level of accuracy demanded by certain GIS applications. Carrier Phase receivers measure the distance from the receiver to the satellites by counting the number of waves that carry the C/A Code signal. This method of determining position is much more accurate; however, it does require a substatially higher occupation time to attain 10-30 cm accuracy. Initializing a Carrier Phase GPS job on a known point requires an occupation time of about 5 minutes. Initializing a Carrier Phase GPS job on an unknown point requires an occupation time of about 30-40 minutes. Additional requirements, such as maintaining the same satellite constellation throughout the job, performance under canopy and the need to be very close to a base station, limit the applicability of Carrier Phase GPS receivers to many GIS applications. Dual-Frequency receivers Dual-Frequency receivers are capable of providing sub-centimeter GPS position accuracy with differential correction. Dual-Frequency receivers provide "survey grade" accuracies not often required for GIS applications. Dual-Frequency receivers receive signals from the satellites on two frequencies simultaneously. Receiving GPS signals on two frequencies simultaneously allows the receiver to determine very precise positions. Introduction to the Global Positioning System for GIS and TRAVERSE http://www.cmtinc.com/gpsbook/chap10.html GPS and Canopy GPS receivers require a line of sight to the satellites in order to obtain a signal representative of the true distance from the satellite to the receiver. Therefore, any object in the path of the signal has the potential to interfere with the reception of that signal. Objects which can block a GPS signal include tree canopy, buildings and terrain features. Further, reflective surfaces can cause the GPS signals to bounce before arriving at a receiver, thus causing an error in the distance calculation. This problem, known as multipath, can be caused by a variety of materials including water, glass and metal. The water contained in the leaves of vegatation can produce multipath error. In some instances, operating under heavy, wet forest canopy can degrade the ability of a GPS receiver to track satellites. There are several data collection techniques which can mitigate the effects of signal blockage by tree canopy or other objects. For example, many GPS receivers can be instructed to track only the highest satellites in the sky, as opposed to those satellites which provide the best DOP. Increasing the elevation of the GPS antenna can also dramatically increase the ability of the receiver to track satellites. Unfortunately, there will be locations where GPS signals simply are not available due to obstruction. In these cases, there are additional techniques which can help to solve the problem. Some GPS receivers have the ability to collect an offset point, which involves recording a GPS position at a location where GPS signals are available while also recording the distance, bearing and slope from the GPS antenna to the position of interest where the GPS signals are not available. This technique is useful for avoiding a dense timber stand or building. Further, a traditional traverse program can be used to manually enter a series of bearings and ranges to generate positions until satellite signals can again be received. This position data can then be used to augment position data collected with the GPS receiver. Introduction to the Global Positioning System for GIS and TRAVERSE GPS for GIS Up to this point, the discussion has focused on describing how GPS determines a location on the surface of the Earth. Now the discussion can shift to the process of describing what is at the location. The "what" is the object or objects which will be mapped. These objects are referred to as "Features", and are used to build a GIS. It is the power of GPS to precisely locate these Features which adds so much to the utility of the GIS system. On the other hand, without Feature data, a coordinate location is of little value. Feature Types There are three types of Feature which can be mapped: Points, Lines and Areas. A Point Feature is a single GPS coordinate position which is identified with a specific Object. A Line Feature is a collection of GPS positions which are identified with the same Object and linked together to form a line. An Area Feature is very similar to a Line Feature, except that the ends of the line are tied to each other to form a closed area. Describing Features As stated above, a Feature is the object which will be mapped by the GPS system. The ability to describe a Feature in terms of a multi-layered database is essential for successful integration with any GIS system. For example, it is possible to map the location of each house on a city block and simply label each coordinate position as a house. However, the addition of information such as color, size, cost, occupants, etc. will provide the ability to sort and classify the houses by these catagories. These catagories of descriptions for a Feature are know as Attributes. Attributes can be thought of as questions which are asked about the Feature. Using the example above, the Attributes of the Feature "house" would be "color", "size", "cost" and "occupants". Logically, each question asked by the Attributes must have an answer. The answers to the questions posed by the Attributes are called Values. In the example above, an appropriate Value (answer) for the Attribute (question) "color" may be "blue". The following table illustrates the relationship between Features, Attributes and Values: Feature Attribute Value House Color Blue Size 3 BDR Cost $118K Occupants 5 By collecting the same type of data for each house which is mapped, a database is created. Tying this database to position information is the core philosophy underlying any GIS system. Feature Lists The field data entry process can be streamlined by the use of a Feature List. The Feature List is a database which contains a listing of the Features which will be mapped, as well as the associated Attributes for each Feature. In addition, the Feature List contains a selection of appropriate Values for each Attribute. The Feature List can be created on the CMT hand-held GPS data collector, or on a PC. Below is an example of a Feature List as it appears in PC-GPS: When a Feature List is used in the field, the first step is to select the Feature to be mapped. Once a Feature is selected, the Attributes for that Feature are automatically listed. A Value for each Attribute can then be selected from the displayed list of predetermined Values. The use of a Feature List streamlines the data entry process and also ensures consistent data entry among different users in the same organization. Exporting to a GIS System The final step in incorporating GPS data with a GIS system is to export the GPS and Feature data into the GIS system. During this process, a GIS "layer" is created for each Feature in the GPS job. For example, the process of exporting a GPS job which contains data for House, Road and Lot Features would create a House layer, a Road layer and a Lot layer in the GIS system. These layers can then be incorporated with existing GIS data. Once the GPS job has been exported, the full power of the GIS system can be used to classify and evaluate the data. http://www.cmtinc.com/gpsbook/chap11.html GPS accuracy http://www.romdas.com/technical/gps/gps-acc.htm Introduction If you were to simply read the advertisements of several GPS manufacturers, you could become very confused. Worse yet you could be mislead to think that you understand more than you really do. Whether intentional or otherwise, advertisements often do not convey an intelligible picture of GPS accuracy. In defense of the manufacturers, GPS accuracy is a complex topic involving a variety of technical factors. In an advertisement, a detailed outline of these factors would be, at best, out of place, and at worst, completely meaningless to many readers. The inherent complexity of the topic coupled with the desire to show the product in its best light results in most advertisements simply glossing over several important points. This month''s column will discuss a few of these points, in an attempt to increase your awareness of what the GPS manufacturers are trying to convey. General differences in Style If the same GPS system were to be described by different manufacturers, you would probably end up with varying descriptions; such differences may be attributed to what I will refer to as "style." Some manufacturers may use an aggressive style and state the best accuracy that they were able to achieve under optimal conditions (even though such an accuracy may have been achieved only 50 percent of the time). At the other extreme, some manufacturers may be overly conservative. A conservative manufacturer may characterize the receiver under difficult or extreme circumstances, then state an accuracy that reflect the observed results at 95 percent probability. The same receiver, described in two different ways could have two very different accuracy values. For example, imagine that a certain GPS receiver collected 1000 data points under ideal conditions. It is conceivable that the best data point could be accurate to better than 0.01 meters, and the worst, accurate to only 15 meters. Now imagine that the same receiver collected 1000 data points under difficult GPS conditions (such as the multipath rich environment typical of an urban city center) and under these difficult conditions, it is conceivable that the resulting accuracy varied from 1 meter to 5 meters. How would you describe the accuracy of this receiver given, say, 20 words in a small ad? How long is long enough? One of the more common "gotchas" in describing GPS accuracy is the occupation time required to achieve the claimed accuracy. Be wary if the ad does not explicitly state how long you must occupy a location in order to achieve a particular accuracy. In the best case scenario, the required occupation time might be as little as one second. However, several systems that tout sub-meter accuracy are only able to achieve this after a stationary occupation of at least several minutes. Expression of accuracy How are the accuracy values represented statistically? The accuracy can be expressed in a manner that describes the 50th percentile (e.g. half the data is better than the stated value, half the data is worse than the stated value). Alternatively, the accuracy may be described at the 95th percentile (95 percent of the data is better than the specification). The list below states the more common terms used to describe GPS accuracy: CEP (Circular Error Probable) - Values stated as CEP apply to horizontal accuracy only. Half of the data points fall within a circle of this radius centered on truth, half lie outside this circle. (As a nifty approximation, you may multiply CEP by 2.5 to obtain 2dRMS.) SEP (Spherical Error Probable) - Applies to combined horizontal and vertical accuracy. Half of the data points fall within a sphere of this radius centered on truth, half lie ou side this sphere. 1dRMS (or RMS) - Approximately 68 percent of the data points occur within this distance of truth. It should be expressed clearly whether the accuracy value refers only to horizontal or to both horizontal and vertical. (Note that ldRMS can be double or tripled to obtain 2dRMS or 3dRMS.) 2dRMS - Approximately 95 percent of the data points occur with this distance of truth. It should be expressed clearly whether the accuracy value refers only to horizontal or to both horizontal and vertical. 3dRMS - Approximately 99.7 percent of the data points occur with this distance of truth. It should be expressed clearly whether the accuracy value refers only to horizontal or to both horizontal and vertical. With or without Selective Availability The vast majority of GPS-based data collection systems for GIS utilize the civilian C/A code (as opposed to the military P code). The U.S. military runs a program that almost always degrades this GPS C/A code. This governmental degradation of the GPS signal (known as Selective Availability, or S/A) has an equal impact on all C/A code GPS receivers. The specified accuracy of positions under the influence of S/A is that the horizontal coordinates will be within 100 meters of truth 95 percent of the time. This specification will hold true regardless of the manufacturer or model of C/A code receiver. It is true that the effects of S/A can be removed by using a process known as differential correction. However, without the benefit of differential correction all C/A code receivers are essentially the same accuracy, less than 100 meters 95 percent of the time. A less common, but very misleading, tactic is to advertise or display the hypothetical accuracy of the GPS receiver as if there were no S /A in effect. Some systems will display such a hypothetical accuracy even when S /A is in full force. When researching accuracy claims, compare the accuracy after differential correction - this is the only meaningful accuracy value. Don''t forget that statements regarding the uncorrected accuracy when there is no S/A are essentially meaningless since the user cannot "turn S/A off." Upgrade Costs Watch for accuracy claims that require the purchase of upgrades at additional cost since it is quite common that the standard system may have a lirnited accuracy. Generally this is not a big problem, however, the necessity of the upgrade should be made clear in the advertisements and literature. Maximum baseline length Differential correction requires at least two receivers. The distance between these two receivers will have an impact on the accuracy of your differential correction. Consider your application and whether your source of base data (usually a GPS base station) will typically be nearby or whether your source of base data will be located at a significant distance from where the rover data is collected . If your base station is typically more than 10-20 kilometers from the site of your rover data collection, you should consider this factor in your purchasing decision. The degradation of accuracy with distance is known as spatial decorrelation. Spatial decorrelation is often expressed in terms of parts-per-million (ppm) of the distance between the base and rover receivers. For example, if the distance between your base and rover is 200 kilometers, and the decorrelation of your GPS system is specified at 10 ppm; you may experience as much as 10 millionths of 200 kilometers of accuracy degradation, or 2 meters. On the other hand, using the same 200 kilometer example, with a decorrelation of only 2 ppm; your error will be limited to 2 millionths of 200 kilometers, or 0.4 meters. Spatial decorrelation values of GPS systems on the market today range from 1-2 ppm to as much as 20 p pm. Spatial decorrelation was discussed in more detail in the November 1995 issue. Memory requirements When GPS data is going to be used in a postprocessed differential correction, it is necessary to store much more information than merely position records. As a result, there is a relationship between desired accuracy and the amount of memory you will require to store that data. This is not the most common problem, however, it has stung a few GPS purchasers in past years. I mention this issue only because there has been historical precedent. There have been manufacturers who proclaim that their system can store many hundreds of thousand of positions. What the advertisements did not say was that in the operating mode that can store hundreds of thousands of positions, your data is limited to (uncorrected) 100 meter accuracy. The same system could store differentially correctable data (for 2-5 meter accuracy), however, in the correctable mode, the same amount of memory could only accommodate a few dozen positions. Summary The issue of GPS accuracy can be complex. There is rarely enough room in an advertisement to show the necessary detail for a complete picture of GPS accuracy. An ideal description of GPS accuracy will have reference to several factors. The most common factors that should be included in a complete description of accuracy include the following: Required occupation time Type of data recorded (phase or pseudorange) Type of processing (phase or pseudorange) Environmental conditions Maximum allowable PDOP Minimum allowable signal strength Maximum allowable distance between base and rover receivers Honzontal accuracy versus vertcal accuracy In summary, do not use advertisements as your guide to GPS accuracy. Whenever possible refer instead to independent technical reports, or manufacturers technica l data sheets that feature the system of interest. Maps fact http://www.mapfacts.com/?f