Global Positioning System (GPS) An Overview (Module 1) Location: Determining a basic position. Navigation: Getting from one location to another. Tracking: Monitoring the movement of people and things. Mapping: Creating maps of the world. Timing: Bringing precise timing to the world. 1
Basic GPS Defined • The basic GPS is defined as the constellation of satellites, the navigation payloads which produce the GPS signals, ground stations, data links, and associated command control facilities which are operated and maintained by the Department of Defense (Do. D) in the United States. • The Standard Positioning Service (SPS) is the civil and commercial service provided by the basic GPS. • Augmentations are those systems based on the GPS that provide real-time accuracy greater than the SPS. • GPS permits land, sea, and airborne users to determine their three dimensional position, velocity, and time, 24 hours a day in all weather, anywhere in the world. 2
More about the GPS The GPS is an all weather, worldwide, continuous coverage, satellite-based radio navigation system. It is developed by the US Department of Defence under its NAVSTAR satellite program. GPS receivers measure time delays and decode messages from in -view satellites to determine the information necessary to complete position and time bias calculations. There alternative position-determination systems in addition to the GPS. However, GPS is the first uniform-accuracy worldwide system and GPS receivers are available at a very reasonable price. 3
Location! • The original theory behind location based services (LSB) is to help find out where objects are. There are several ways of determining location, including GPS, cell locations, triangulation and other methods. • The GPS is a network of 24 NAVSTAR satellites orbiting Earth at 20, 200 km. Originally established by the US Do. D. • Access to GPS is free to all users. The system’s positioning and timing data are used for a variety of applications, including air, land sea navigation, vehicle and vessel tracking, surveying and mapping, and asset and natural resource management. • With military accuracy restrictions partially lifted in March 1996 and fully lifted in May 2000, GPS can now pinpoint the location of objects as small as a penny anywhere on the earth’s surface. 4
• The first GPS satellite was launched in 1978. The first 10 satellites were development satellites, called Block I. From 1989 to 1993, 23 satellites, called Block II were launched. The launch of the 24 th satellite in 1994 completed the system. • The Do. D keeps 4 satellites in reserve to replace any destroyed or defective satellites. The satellites are positioned so that signals from six of them can be received nearly 100 percent of the time at any point on earth. • GPS provides coded satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position, velocity and time. • Basically, GPS works by using four GPS satellite signals to compute positions in three dimensions (and the time offset) in the receiver clock. So by very accurately measuring our distance from these satellites, users can triangulate their position anywhere on earth. 5
Four GPS satellite signals are used to compute positions in three dimensions and the time offset in the receiver clock. Satellite 3 Satellite 4 Satellite 2 Satellite 1 Receiver 6
• GPS receivers have been miniaturized to just a few integrated circuits and so are becoming very economical. And that makes the technology accessible to virtually everyone. • GPS is finding its way into cars, boats, planes, construction equipment, movie making gear, farm machinery, even laptop computers. • This course shows the various features of GPS and the reasons why it may soon become almost as basic as the telephone. • There are three positioning methods for the GPS: – Standards GPS – Differential GPS – Carrier-phase GPS 7
GPS Works in the Following Steps • • A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2 dimensional position (latitude and longitude) and track movement. With four or more satellites in view, the receiver may determine the user’s 3 -dimensional position (latitude, longitude and altitude). Once the user’s position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more. The basis of GPS is “triangulation” from satellites. To “triangulate, ” a GPS receiver measures distance using the travel time of radio signals. To measure travel time, GPS needs very accurate timing which it achieves with some techniques. Along with distance, you need to know exactly where the satellites are in space. Finally, it is necessary to correct for any delays the signal experiences as it travels through the atmosphere. 8
Basic Satellite System 9
• Number of satellites visible: The more satellites a GPS receiver can "see, " the better the accuracy. Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all. GPS units typically will not work indoors, underwater or underground. • Satellite geometry/shading: This refers to the relative position of the satellites at any given time. Ideal satellite geometry exists when the satellites are located at wide angles relative to each other. Poor geometry results when the satellites are located in a line or in a tight grouping. • Intentional degradation of the satellite signal: Selective Availability (SA) is an intentional degradation of the signal once imposed by the US Do. D. SA was intended to prevent military adversaries from using the highly accurate GPS signals. The government turned off SA in May 2000, which significantly improved the accuracy of civilian GPS receivers. 10
Applications of GPS • GPS has a variety of applications on land, at sea and in the air. Basically, GPS is usable everywhere except where it is impossible to receive the signal such as inside most buildings, in caves and other subterranean locations, and underwater. • The most common airborne applications are for navigation by general aviation and commercial aircraft. • At sea, GPS is also typically used for navigation by recreational boaters, commercial fishermen, and professional mariners. Landbased applications are more diverse. • The scientific community uses GPS for its precision timing capability and position information. • Surveyors use GPS for an increasing portion of their work. GPS offers cost savings by drastically reducing setup time at the survey site and providing incredible accuracy. Basic survey units, costing thousands of dollars, can offer accuracies down to one meter. More expensive systems are available that can provide accuracies to within a centimeter. 11
• Recreational uses of GPS are almost as varied as the number of recreational sports available. GPS is popular among hikers, hunters, snowmobilers, mountain bikers, and cross-country skiers, just to name a few. • Anyone who needs to keep track of where he or she is, to find his or her way to a specified location, or know what direction and how fast he or she is going can utilize the benefits of the global positioning system. • GPS is now commonplace in automobiles as well. Some basic systems are in place and provide emergency roadside assistance at the push of a button (by transmitting the vehicle position to a dispatch center). • More sophisticated systems that show vehicle position on a street map are also available. Currently these systems allow a driver to keep track of the vehicle position and suggest the best route to follow to reach a designated location. 12
Geographic Information Systems (GIS) • GIS is a new technology that combines the world of database management with digital maps and graphics. • A Geographic Information System can be defined as: "A system of computer hardware, software and procedures designed to support the capture, management, analysis, modeling and display of geographically referenced data for decision making. It is a way in which to begin to represent and model the real world. “ • General Definition: A GIS is any manual or computer-based set of procedures used to store and manipulate geographically referenced data. • Specific Definition: A GIS is a computer-based system that provides four sets of capabilities to handle georeferenced data: 1) Input, 2) Data Management, 3) Manipulation and Analysis, 4) Output. 13
GIS Functions • • Data Input – Manual Digitizing (vector) – GPS (vector: point, line, area) – Scanning (raster) – Remote Sensing (raster) – Existing Digital Data (vector and/or raster) – Existing Digital Data: Data Management: store and retrieve data from the database. Data Manipulation and Analysis: Spatial vs. non-spatial analysis. Data Output: Data in the form of maps, tables, and text either as softcopy (on-screen or electronic file) or as hardcopy (paper or film). 14
The GIS View of the World • GIS provide powerful tools for addressing geographical and environmental issues. Consider the schematic diagram below. Imagine that the GIS allows us to arrange information about a given region or city as a set of maps with each map displaying information about one characteristic of the region. • In the case shown in the next slide, a set of maps that will be helpful for urban transportation planning have been gathered. Each of these separate thematic maps is referred to as a layer, coverage, or level. And each layer has been carefully overlaid on the others so that every location is precisely matched to its corresponding locations on all the other maps. The bottom layer of this diagram is the most important, for it represents the grid of a locational reference system (such as latitude and longitude) to which all the maps have been precisely registered. 15
Once these maps have been registered carefully within a common locational reference system, information displayed on the different layers can be compared analyzed in combination. Transit routes can be compared to the location of shopping malls, population density to centers of employment. In addition. single locations or areas can be separated from surrounding locations, as in the diagram below, by simply cutting all the layers of the desired location from the larger map. Whether for one location or the entire region. 16
GPS Cell Phones • Of all the applications of GPS, the one offering the biggest and most broad-based business opportunities, and some of the greatest challenges may be the cell phone. • Mobile phones enabled with GPS are expected to increase in number at an astonishing rate over the next years. These numbers have entrepreneurs and telecommunications service providers salivating at the possibility of creating an entirely new class of revenue-generating location-based services. • But there is a problem! Users of mobile phones and other portable devices have been conditioned to expect reliable voice, messaging and Internet service in a range of physical locations, including inside structures, and even below ground in basements or transportation tunnels. But even though users will expect GPS cell-phone services to operate with this same level of reliability and ubiquity, today’s satellite-driven system, reliant largely on line-of-site communications, is not up to the task. 17
Segments of the GPS • The GPS is comprised of three segments: satellite constellation (space), ground control/monitoring network, and user receiving equipment: – The satellite constellation contains 24 GPS satellites in orbit that provide the ranging signals and data messages to the user equipment. – The operational control segment (OCS) tracks and maintains the satellites in space. The OCS monitors satellite condition and signal integrity and maintains the orbital configuration of the satellites. Furthermore, the OCS updates the satellite clock corrections as well as numerous other parameters essential to determining user position, velocity, and time (PVT). – The user GPS receiver equipment performs the navigation, timing or other related functions (e. g. surveying). 18
GPS Segments 19
GPS satellite constellation • The satellite constellation consists of the nominal 24 -satellite constellation. They transmit signals (at 1575. 42 MHz) that can be detected by receivers on the ground. Satellites are identified by space vehicle (SV) number or pseudo-random noise (PRN) number. • The satellites are positioned in six 12 -h (11 h and 58 min) Earthcentered orbital planes with four satellites in each plane. This means that signals from six of them can be received 100 percent of the time at any point on earth. The nominal orbital period of a GPS satellite is one half of a sidereal day or 11 hr 58 min. • The orbits are nearly circular and equally spaced about the equator at a 60° degree separation with an inclination relative to the equator of nominally 55° degrees. The orbital radius is approximately 26, 600 km (i. e. , distance from satellite to centre of mass of the earth). 20
GPS Satellites • Name: NAVSTAR • Manufacturer: Rockwell International • Altitude: 10, 900 nautical miles • Weight: 1900 lbs (in orbit) • Size: 17 ft with solar panels extended • Orbital Period: 12 hours • Orbital Plane: 55 degrees to equitorial plane • Planned Lifespan: 7. 5 years • Current constellation: 24 Block II production satellites • Future satellites: 21 Block IIrs developed by Martin Marietta 21
• The GPS spacecraft is 820 kg and three axis stabilized. It contains two Sun sensors, an Earth sensor, and three gyros for attitude determination along with 22 thrusters and four reaction wheels for attitude control. • It relies on a digital control electronics assembly for guidance and sends its clock information to the users via a telemetry, tracking, and commanding receiver. • GPS satellites transmit two low power radio signals, designated L 1 and L 2. Civilian GPS uses the L 1 frequency of 1575. 42 MHz in the UHF band. • A GPS signal contains three different bits of information, a pseudorandom code, ephemeris data and almanac data. • The pseudo-random code is simply an ID code that identifies which satellite is transmitting information. • Ephemeris data tells the GPS receiver where each GPS satellite should be at any time throughout the day. 22
• Each satellite transmits ephemeris data showing the orbital information for that satellite and for every other satellite in the system. • Almanac data, which is constantly transmitted by each satellite, contains important information about the status of the satellite (healthy or unhealthy), current date and time. This part of the signal is essential for determining a position. • Several different notations are used to refer to the satellites in their orbits. One particular notation assigns a letter to each orbital plane (i. e. , A, B, C, D, E, and F) with each satellite within a plane assigned a number from 1 to 4. Thus, a satellite referenced as B 3 refers to satellite number 3 in orbital plane B. • A second notation used is a NAVSTAR satellite number assigned by the U. S. Air Force. This notation is in the form of a space vehicle number (SVN) 11 to refer to NAVSTAR satellite 11. 23
Satellite Orbits • GPS satellites occupy six orbital planes inclined 55 o from the equatorial plane, with four or more satellites per plane. • As the GPS satellites are in nearly circular orbits, at an altitude of approximately 20200 km above the earth, this has a number of immediate effects which make the prediction of satellite location comparatively easy: – Their orbital period is approximately 11 hrs 58 min, so that each satellite makes two revolutions in one sidereal day (the period taken for the earth to complete one rotation about its axis with respect to the stars). – At the end of a sidereal day (approximately 23 hrs 56 min in length) the satellites are again over the same position on earth. – Reckoned in terms of a solar day (24 hrs in length), the satellites are in the same position in the sky about four (4) min earlier each day. – The orbit ground track approximately repeats each day, except that there is a small drift of the orbital plane to the west (-0. 03 per day). 24
Operational Control Segment (OCS) • • • The OCS has responsibility for maintaining the satellites and their proper functioning. This includes maintaining the satellites in their proper orbital positions (called station keeping) and monitoring satellite subsystem condition and status. The OCS also monitors the satellite solar arrays, battery power levels, and propellant levels used for manoeuvres and activates spare satellites. The overall structure of the operational ground/control segment is as follows: Remote monitor stations constantly track and gather C/A and P(Y) code from the satellites and transmit this data to the Master Control Station, which is located at Falcon Air Force Base, Colorado Springs with backup at Gaithersburg, MD. There is also the ground uplink antenna facility, which provides the means of commanding and controlling the satellites and uploading the navigation messages and other data. The unmanned ground monitor stations are located in Hawaii, Kwajalein in the Pacific Ocean, Diego Garcia in the Indian Ocean, Ascension Island in the Atlantic and Colorado Springs, Continental United States. Ground antennas are located in these areas also. These locations have been selected to maximize satellite coverage 25
Control Segment • The Control Segment consists of a system of tracking stations located around the world. The Master Control facility is located at Schriever Air Force Base (formerly Falcon AFB) in Colorado. • Master Control Station in Colorado Spring Backup at Gaithersburg Remote Monitor Stations and Ground Antennas in Cape Canaveral, Hawaii, Ascension Island, Diego Garcia, Kwajalien. 26
User Receiving Equipment • The user receiving equipment, typically referred to as a GPS receiver consists of an antenna, RF end, and receiver-processors that measure and decode the satellite transmissions to provide positioning, velocity, and precise timing information to the user. • There has been a significant evolution in the technology of GPS receiving sets since they were initially manufactured in the mid-70’s. Initially, they were large, bulky and heavy analog devices primarily used for military purposes. • With today’s technology, a GPS receiver of comparable or more capability typically weighs a few pounds or ounces, and occupies a small volume. The smallest of today’s are those of a wrist watch size, while the largest is a naval shipboard unit (weighing about 32 kgs). • The basic structure of a receiver is the antenna, the receiver and processor, the display and a regulated dc-power supply. These receivers can be mounted in ships, planes and cars, and provide exact position information, regardless of weather conditions. 27
User Segment • The GPS User Segment consists of the GPS receivers and the user community. GPS receivers convert SV signals into position, velocity, and time estimates. Four satellites are required to compute the four dimensions of X, Y, Z (position) and time. • GPS receivers are used for navigation, positioning, time dissemination, and other research. • Navigation in three dimensions is the primary function of GPS. Navigation receivers are made for aircraft, ships, ground vehicles, and for hand carrying by individuals. 28
Examples of GPS Receivers The GPS receiver is a line of sight system. If the path between the receiver and a satellite is blocked, then the satellite signal will not be received. See this video clip: http: //www. gps. tv/mpage/roadmate/01 m. html 29
Receiver Classes • Code based – Positions are determined from range estimates (pseudo-ranges) from at least 4 satellites. – Pseudo-ranges are derived from the time interval from when the signal was sent by the satellite to when the signal arrived at the receiver. • Carrier phase based – Positions are determined by setting one receiver on a point with an established coordinate and then solving for a 3 D vector between it and a receiver at an unknown point. – Pseudo-ranges are derived by the determination of the number of signal wavelengths plus any remaining fraction of a wavelength that separates the satellite and the receiver. 30
Characteristics of Code Based Receivers • Pseudo-ranges are determined from time difference calculations. • Smaller, more portable, typically 1 to 12 channels – Guaranteed horizontal accuracy of locations is < 15 meters (95% confidence). – Code-correlating techniques using Differential GPS allow us to achieve accuracies of 2 -3 m in static mode and 5 -10 m in dynamic mode. Better accuracies require a data correction resource that supplies data for differential GPS. • The best code based receivers can achieve sub-meter results under favorable conditions. • Costs range from $100 to $10, 000. • Applications: Navigation, mapping/GIS, vehicle tracking 31
Positioning with Code Based Receivers • Each satellite transmits data including the satellite’s location, the exact time the signal was transmitted, and the satellite's unique pseudo-random noise (PRN) code. • The same PRN code sequences are generated in the GPS receiver simultaneously. • See: http: //www. pbs. org/wgbh/nova/longitude/gpsgame. html. 32
Methods of Positioning with Code Based Receivers Three methods, application dependent Autonomous (uncorrected): Single receiver used. Capable of providing <15 meter positioning. Generally not recommended for GIS mapping. Differential (post-processed): Two receivers used, one set to collect at a known location (base receiver). A sample of points (fixes) are collected by the rover receiver for each feature. The points are then differentially corrected using data from the base receiver and GPS correction software. Accuracies typically range from 5 meters down to sub-meter. Differential (real time): Rover receiver must be used with integrated radio receiver, pager, or modem that receives broadcast pseudo-range corrections for each satellite. The rover stores and/or displays corrected positions. Slightly less accurate than post-processed. 33
Carrier-Phase GPS • This is a new version of GPS that can eliminate errors even better than other forms. Recall that a GPS receiver determines the travel time of a signal from a satellite by comparing the pseudo random code it is generating, with an identical code in the signal from the satellite. The receiver slides its code later and later in time until it syncs up with the satellite’s code. The amount it has to slide the code is equal to the signal’s travel time. The problem is that the bits (or cycles) of the pseudo random code are so wide that when the signals sync up there is room for error. 34
• Survey receivers are better as they start with the pseudo random code and then move on to measurements based on the carrier frequency for that code. This carrier frequency is much higher so its pulses are much closer together and therefore more accurate. At the speed of light the 1. 57 GHz GPS signal has a wavelength of roughly twenty cm, so the carrier signal can act as a much more accurate reference than the pseudo random code by itself. And if it can get to within one percent of perfect phase like you expect with code-phase receivers you can (theoretically) obtain 3 or 4 mm accuracy. 35
• In essence this method is counting the exact number of carrier cycles between the satellite and the receiver. The problem is that the carrier frequency is hard to count because it's so uniform. • Every cycle looks like every other. The pseudo random code on the other hand is intentionally complex to make it easier to know which cycle you're looking at. • But Carrier-phase GPS tackles this problem by using code-phase techniques to get close. If the code measurement can be made accurate to say, a meter, then we only have a few wavelengths of carrier to consider as we try to determine which cycle really marks the edge of our timing pulse. 36
• Resolving this carrier phase ambiguity for just a few cycles is a much more tractable problem and as the computers inside the receivers increase in processing power and functionality it is becoming possible to make this kind of measurement without all the steps that survey receivers go through. 37
Characteristics of Carrier Phase Based Receivers • Pseudo-ranges are determined from the count of complete wavelengths plus any fractional portion of the wavelength that separates the satellite and the receiver. • Typically 9 to 12 channels. • Receiver must work simultaneously with at least one other carrier phase receiver that is collecting data on a known point. • Processing software enables the determination of delta x, delta y, and delta z values between the known point(s) and the unknown point. • Depending on the type of receiver and accuracy requirements, site occupation can be as short as a few seconds or as long as several hours. • Costs range from $10, 000 to $40, 000. 38
How Carrier Phase Based Receivers Work? • Carrier Phase Differential works on the principle of the Doppler phenomenon. The phenomena can be explained in terms of the movement of a sound source in relation to a stationary observer. If the sound source remains at a constant distance from the observer, the pitch will remain constant. • If the sound source travels in a straight line towards and away from an observer, (i. e. a speeding car or train), then the pitch will increase and then decrease, respectively. This is due to the sound waves traveling a shorter distance towards the observer and a farther distance away from the observer. • The same analogy applies for an observer on the ground a GPS satellite in orbit. The difference is that the sound source is replaced by the carrier frequency produced by the GPS satellite. 39
Positioning with Carrier Phase Based Receivers • Pseudo-ranges determined by counting the number of whole signal wavelengths and measuring the partial (phase) signal wavelength. • Once the number of wavelengths is known, a pseudo-range may be calculated by multiplying this number by the wavelength of the signal (L 1 and/or L 2, 19 cm and 24. 4 cm respectively) plus the partial wavelength. • A baseline distance and azimuth between any pair of receivers operating simultaneously may be computed. • With one receiver collecting at a known point and with the calculated baseline, the coordinate for the unknown point can be determined. 40
Methods of Positioning with Carrier Phase Receivers • Static - used for control surveys over large areas; long observation times; most accurate - 0. 5 cm. • Fast static - used for control surveys over small areas; shorter occupation time; accurate to 0. 5 -1 cm. • Kinematic - used for collecting measurements while in motion; accurate to within a few cm. • Real time kinematic - used for topographic, construction, stakeout, and location surveys in open areas; accurate to within a few cm. 41
GPS Signal • The basic idea behind GPS is to use satellites in space as reference points for locations on earth. With GPS, signals from the satellites arrive at the exact position of the user and are triangulated. This triangulation is the key behind accurate location determining and is achieved through several steps. • The GPS satellites transmits ranging codes and navigation data by using code-division multiple access (CDMA) on the same two carrier frequencies, L 1 (1575. 42 MHz) and L 2 (1227. 60 MHz). • The carrier frequencies are modulated by spread spectrum signals to carry information to the user. • Three pseudorandom noise (PRN) ranging codes are associated with each satellite. 42
The C/A Code • Binary data that is modulated or "superimposed" on the carrier signal is referred to as Code. Two main forms of code are used with NAVSTAR GPS: C/A or Coarse/Acquisition Code (also known as the civilian code) and the P-code. • The coarse acquisition (C/A) code modulates the L 1 carrier phase. This code has a length of 1023 bits and a 1. 023 -bit rate resulting in a period of 1 ms. There is a different C/A PRN code for each satellite and each PRN code is nearly orthogonal to all other C/A PRN codes. • Although all satellites are broadcasting on the same two frequencies, a GPS receiver is able to lock on to a particular satellite and discriminate between satellites by correlating an internally generated version of the C/A code of a satellite with the received signal. • The GPS space vehicles are often identified by their unique PRN code number. 43
The Precise (P) and L 1 CA Codes • The precise (P) code modulates both L 1 and L 2 carrier phases. The P code is very long (7 day) 10. 23 MHz PRN code. This code is intended for military users and can be encrypted. When it is encrypted it is called "Y" code. • Since P code is more complicated than C/A it is more difficult for receivers to acquire. That is why many military receivers start by acquiring the C/A code first and then move on to P code. • The navigation message also modulates the L 1 C/A code signal. The navigation message is a 50 -bit/s signal consisting of data bits that a GPS receiver decodes into satellite orbit, clock correction, and other system parameters. 44
Factors that Degrade the GPS Signal • Ionosphere and troposphere delays: The satellite signal slows as it passes through the atmosphere. The GPS system uses a built-in model that calculates an average amount of delay to partially correct for this type of error. • Signal multipath: This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby causing errors. • Receiver clock errors: A receiver’s built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors. • Orbital errors: Also known as ephemeris errors, these are inaccuracies of the satellite's reported location. 45
GPS Services The GPS provides two levels of services • Standard-Positioning Service (SPS) – This serves is based on the C/A code which is available to all users in a continuous, worldwide basis with no direct charge. – This service is available on the L 1 frequency which contains the C/A code and navigation-data message. – Predictable accuracy of the SPS is 100 m horizontal and 156 m vertical and 340 ns time. Timing accuracy is with respect to Universal Coordinated Time (Greenwich Mean Time). 46
• Precise-Positioning Service (PPS) – More accurate positioning, velocity, and timing service that is available to users authorized by the Us government. – Access to this service is controlled by two techniques known as AS and selective availability (SA). – AS is implemented by replacing the P code with the Y code. – SA is implemented by degrading the satellite clock and ephemeris data available to non-authorized users. – Predictable accuracy of PPS is 22 m horizontal, 27. 7 m vertical and 200 ns time. 47
Positioning or Location • The first and most obvious application of GPS is the simple determination of a "position" or location. GPS is the first positioning system to offer highly precise location data for any point on the planet, in any weather. • Knowing the precise location of something, or someone, is especially critical when the consequences of inaccurate data are measured in human terms. • Other than common applications of the GPS, it is also being applied in Italy to create exact location points: the Italian Military Geographic Institute is creating what is reputed to be the first nationwide geodetic network. This project is paving the way for similar networks in Europe and possibly around the world. 48
Satellite Position Location 49
Mathematics of Positioning • Solving the following equation determine user position. 50
• If user velocity is of interest, the measurement can be processed by state-estimation techniques. • Additional processing in some positioning systems can yield Doppler velocity estimates. • A six-dimensional system state could include the two-dimensional position, velocity, and acceleration vectors. • Difficulties include: – Sensor noise limits the ability to estimate states that are related through derivatives to the measurement. – The rate at which new information enters the estimation problem is limited by the positioning system sampling rate. – State-estimation accuracy is related to the motion of the platform. – Navigation reliability is dependent on the reception of the external positioning system signals. 51
Determining the Position • Suppose we measure our distance from a satellite and find it to be 11, 000 miles (how it is measured is covered later). Knowing that we are 11, 000 km from a particular satellite narrows down all the possible locations we could be in the whole universe to the surface of a sphere that is centered on this satellite and has a radius of 11, 000 km. • Next, let us say we measure our distance to a second satellite and find out that it is 12, 000 km away. That tells us that we are not only on the first sphere but we are also on a sphere that is 12, 000 km from the second satellite, i. e. somewhere on the circle where these two spheres intersect. 52
• If we then make a measurement from a third satellite and find that we are 13, 000 km from that one, that narrows our position down even further, to the two points where the 13, 000 km sphere cuts through the circle that's the intersection of the first two spheres. • So by ranging from three satellites we can narrow our position to just two points in space. To decide which one is our true location we could make a fourth measurement. But usually one of the two points is a ridiculous answer (either too far from Earth or moving at an impossible velocity) and therefore can be rejected without a measurement. 53
Triangulation Position is calculated from distance measurements (ranges) to satellites. Mathematically we need four satellite ranges to determine exact position. Distance from first satellite Two possible positions Distance from a second satellite Distance from a third satellite 54
Measuring Your Distance • How the satellites actually measure the distance is quite different from determining the position and essentially involves using the travel time of a radio message from the satellite to a ground receiver. • To make the measurement we assume that both the satellite and our receiver are generating the same pseudo-random code at exactly the same time. This pseudo-random code is a digital code unique to each satellite, designed to be complex enough to ensure that the receiver does not accidentally sync up to some other signal. • Since each satellite has its own unique pseudo-random code this complexity also guarantees that the receiver won’t accidentally pick up another satellite's signal. So all the satellites can use the same frequency without jamming each other. And it makes it more difficult for a hostile force to jam the system, as well as giving the Do. D a way to control access to the system. 55
• By comparing how late the satellite’s pseudo-random code appears compared to our receiver's code, we determine how long it took to reach us. Multiply that travel time by the speed of light and you obtain the distance between the receiver and the satellite. • However this calls for precise timing to determine the interval between the code being generated at the receiver and received from space. • On the satellite side, timing is almost perfect due to their cesium and/or rubidium atomic clocks installed within each satellite. However as it would be extremely uneconomical for receiver to use atomic clocks a different technique must be found. • GPS solves this problem by using an extra satellite measurement for the following reason: If our receiver's clocks were perfect, then all our satellite ranges would intersect at a single point - our position. 56
• But with imperfect clocks, a fourth measurement, will not intersect with the first three satellite ranges. So the receiver’s computer will then calculate a single correction factor that it can subtract from all its timing measurements that would cause them all to intersect at a single point. • That correction brings the receiver’s clock back into sync with universal time , ensuring (once the correction is applied to all the rest of the receivers’ measurements) precise positioning. 57
Navigation • GPS helps the user determine exactly where he/she is, but sometimes important to know how to get somewhere else. GPS was originally designed to provide navigation information for ships and planes. So while this service is appropriate for navigating on water, it is also very useful in the air and on the land. 58
Tracking • If navigation is the process of getting something from one location to another, then tracking is the process of monitoring it as it moves along. • Commerce relies on fleets of vehicles to deliver goods and services either across a crowded city or through nationwide corridors. So, effective fleet management has direct bottom-line implications, such as telling a customer when a package will arrive, spacing buses for the best scheduled service, directing the nearest ambulance to an accident, or helping tankers avoid hazards. • GPS used in conjunction with communication links and computers can provide the backbone for systems tailored to applications in agriculture, mass transit, urban delivery, public safety, and vessel and vehicle tracking. So it is no surprise that police, ambulance, and fire departments are adopting systems like GPS-based AVL (Automatic Vehicle Location) Manager to pinpoint both the location of the emergency and the location of the nearest response vehicle on a computer map. With this kind of clear visual picture of the situation, dispatchers can react immediately and confidently. 59
• Flying a single-engine airplane or a commercial jumbo jet requires the same precise navigation information, and GPS puts it all at the pilot’s fingertips as safely as possible. • By providing more precise navigation tools and accurate landing systems, GPS not only makes flying safer, but also more efficient. With precise point-to-point navigation, GPS saves fuel and extends an aircraft’s range by ensuring pilots do not stray from the most direct routes to their destinations. • GPS accuracy will also allow closer aircraft separations on more direct routes, which in turn means more planes can occupy our limited airspace. 60
Mapping • Using GPS to survey and map precisely saves time and money in this most stringent of all applications. Today, GPS makes it possible for a single surveyor to accomplish in a day what used to take weeks with an entire team. And they can do their work with a higher level of accuracy than ever before. • The effect on surveying in general has been considerable. Mapping is the art and science of using GPS to locate items, then create maps and models of everything in the world. And we do mean everything. Mountains, rivers, forests and other landforms. Roads, routes, and city streets. Endangered animals, precious minerals and all sorts of resources. • GPS is mapping the world! GPS can help generate maps and models of everything in the world mountains, sea, rivers, cities and help manage endangered animals, archaeological treasures, precious minerals and all sorts of resources , as well as accurately managing the effect of damage and disasters. 61
Timing • Although GPS is well-known for navigation, tracking, and mapping, it is also used to disseminate precise time, time intervals, and frequency. Time is a powerful commodity, and exact time is more powerful still. Knowing that a group of timed events is perfectly synchronized is often very important. GPS makes the job of "synchronizing our watches" easy and reliable. • There are three fundamental ways we use time. As a universal marker, time tells us when things happened or when they will. As a way to synchronize people, events, even other types of signals, time helps keep the world on schedule. And as a way to tell how long things last, time provides and accurate, unambiguous sense of duration. • GPS satellites carry highly accurate atomic clocks. And in order for the system to work, GPS receivers here on the ground synchronize themselves to these clocks. That means that every GPS receiver is, in essence, an atomic accuracy clock. 62
GPS Errors • GPS errors are a combination of noise, bias, blunders: • Noise errors are the combined effect of PRN code noise (around 1 meter) and noise within the receiver noise (around 1 meter). • Bias errors result from Selective Availability and other factors • Blunders can result in errors of hundred of kilometers: – Control segment mistakes due to computer or human error can cause errors from one meter to hundreds of kilometers. – User mistakes, including incorrect geodetic datum selection, can cause errors from 1 to hundreds of meters. – Receiver errors from software or hardware failures can cause blunder errors of any size. • Noise and bias errors combine, resulting in typical ranging errors of around fifteen meters for each satellite used in the position solution. 63
Sources of Error • Atmosphere: Ionospheric and tropospheric refraction can delay the signal and cause ranging errors. • Multipath: Reflecting or bouncing signals not traveling directly to the antenna can cause ranging errors. • Satellite Geometry: Bad satellite geometry can result in weak positional. • Selective Availability: The US government’s ability to degrade positional accuracy by “dithering” or slightly altering the satellite clocks and by changing the broadcast ephemeris to report a slightly different satellite position. • Anti Spoofing: To prevent hostile outside sources from degrading the P-code, the (Y) code replaces the P code, creating an encryption that can only be demodulated by special hardware. 64
Some of the Causes of Position Errors in the GPS System 65
Error Correction • A variety of different errors can occur within the system, some of which are natural, whilst others are artificial. First of all, a basic assumption, the speed of light, is not constant as this value changes as the satellite signals travel through the atmosphere. • As a GPS signal passes through the charged particles of the ionosphere and then through the water vapour of the troposphere it gets slowed down, and this creates the same kind of error as bad clocks. This problem is tackled by attempting to use modeling of the atmospheric conditions of the day, and using dual-frequency measurement, i. e. comparing the relative speeds of two different signals. • Another problem is multipath error , this is when the signal may bounce off various local obstructions before it gets to our receiver. Sophisticated signal rejection techniques are used to minimize this problem. 66
• There also potential problems at the satellites. Minute time differences can occur within the on-board atomic clocks, and sometimes position (ephemeris) errors can occur. These other errors can be magnified by a high GDOP "Geometric Dilution of Precision". • This is where a receiver picks satellites that are close together in the sky, meaning the intersecting circles that define a position will cross at very shallow angles. That increases the grey area or error margin around a position. • If the receiver picks satellites that are widely separated the circles intersect at almost right angles and that minimizes the error region. Obviously good receivers determine which satellites will give the lowest GDOP. 67
• Finally up to recently there was another, man-made source of errors. The US was very mindful of the fact that terrorists and unfriendly governments could use the accurate positioning provided by GPS and so intentionally degraded GPS’s accuracy. • This policy is called Selective Availability or SA. This involves the DOD introducing some "noise" into the satellite's clock data which, in turn, adds noise (or inaccuracy) into position calculations. • The DOD may also has been sending slightly erroneous orbital data to the satellites which they transmit back to receivers on the ground as part of a status message. Together these factors made SA the biggest single source of inaccuracy in the system. Military receivers used a decryption key to remove the SA errors and so they were considerably more accurate. 68
Forms of Real Time Differential Codes There are currently several popular forms of Real time code differential available to the consumer. – Radio Beacon Correction: A land-based radio correction usually controlled by the Coast Guard. Satellite Corrections: A subscription-based service that provides the user with corrections from a geo-stationary satellite. – WAAS EGNOS and MSAS: The Wide Area Augmentation System and its sister corrections in Europe and Japan are a new satellite-based differential that is free of charge, but still of questionable reliability. These systems are designed to provide a higher confidence level in autonomous GPS positioning for use in aviation. Unlike radio and satellite differential, WAAS corrects the atmospheric and orbital data so that autonomous calculations can better determine true position. – User Defined: Higher end units can be used to create their own differential by employing two receivers (a reference and a rover) and communicating via radio, Internet, or cellular phone. 69
Wide Area Augmentation System (WAAS) • WAAS is a system of satellites and ground stations that provide GPS signal corrections, giving even better position accuracy (of up to five times better). • A WAAS-capable receiver can provide a position accuracy of better than three meters 95 percent of the time. No fee is required to utilize WAAS. • The Federal Aviation Administration (FAA) and the Department of Transportation (Do. T) are developing the WAAS program for use in precision flight approaches. Currently, GPS alone does not meet the FAA’s navigation requirements for accuracy, integrity, and availability. • WAAS corrects for GPS signal errors caused by ionospheric disturbances, timing, and satellite orbit errors, and it provides vital integrity information regarding the health of each GPS satellite. 70
• WAAS consists of approximately 25 ground reference stations positioned across the US that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message. • This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere. The corrected differential message is then broadcast through one of two geostationary satellites, or satellites with a fixed position over the equator. • The information is compatible with the basic GPS signal structure, which means any WAAS-enabled GPS receiver can read the signal. • Currently, WAAS satellite coverage is only available in North America. 71
• For some users in the US, the position of the satellites over the equator makes it difficult to receive the signals when trees or mountains obstruct the view of the horizon. WAAS signal reception is ideal for open land marine applications. • WAAS provides extended coverage both inland offshore compared to the land-based DGPS (differential GPS) system. Another benefit of WAAS is that it does not require additional receiving equipment, while DGPS does. • Other governments are developing similar satellite-based differential systems. In Asia, Japan is developing the Multi-Functional Satellite Augmentation System (MSAS), while Europe has the Euro Geostationary Navigation Overlay Service (EGNOS). • GPS users around the world will have access to precise position data using these and other compatible systems. 72
Post-Process Differential • There also different kinds of DGPS, for use when users do not need precise positioning immediately. This is termed Post Processing DGPS, and is used when the roving receiver just needs to record all of its measured positions and the exact time it made each measurement. • Then later, this data can be merged with corrections recorded at a reference receiver for a final clean-up of the data, meaning you do not need the radio link required in real-time systems. 73
Inverted DGPS • Another form of DGPS, called Inverted DGPS, which is used to save money when operating a large fleet of users. • With an inverted DGPS system the users would be equipped with standard GPS receivers and a transmitter and would transmit their standard GPS positions back to the tracking station (the main office). Then at the tracking station the corrections would be applied to the received positions. It requires a computer to do the calculations, a transmitter to transmit the data but it gives you a fleet of very accurate positions for the cost of one reference station, a computer and a lot of standard GPS receivers. • Using the inverted DGPS would help the tourist helpers become more available to the general public with lower cost. 74
GPS Competitors • GLONASS: 1602. 56 -1615. 5 MHz / 1246. 44 -1256. 5 MHz; Space based; 100 m accuracy; Global. – GLONASS is the Russian Federation’s satellite navigation system. In many ways it is very similar to GPS: there are 24 satellites in the full constellation; each satellite transmits various data on two L-band carriers; there is one navigation signal that has been authorized for civilian use, and further navigation signals that have been reserved solely for Russian military use; there is a ground segment that monitors and control the satellites; and users passively receive its signals and are able to navigate with accuracy’s of few tens of meters or better. – There are some key differences between the GPS and GLONASS. In many ways, GLONASS is a more elegant and economical system design than GPS. Unfortunately there a number of quality control issues and much more serious funding problems. The GLONASS system was declared fully operational on 1996. Not all satellites are operational now. 75
• Galileo – The Galileo satellite radio navigation system is an initiative launched by the European Union and the European Space Agency (ESA). The project architects plan deployment in 2006 -7, becoming operational in 2008 at a yearly cost of € 220 m. It is projected by 2008 a constellation of 30 satellites should be available. – The technology behind Galileo is designed to be more accurate and more reliable than GPS or GLONASS. This will allow safetycritical systems - such as air traffic control, and ship and car navigation - to be run on the technology. The system should also guarantee coverage to previously inaccessible areas such as those that are either blocked by buildings or isolated areas at high latitudes. – With some of the member countries facing recession, now might not be the time to fund the idea. Britain and the Netherlands have already questioned the timing, and Germany has raised issues over costs. In contrast, France and Italy whose aerospace industries both stand to benefit from the system are looking forward to a go-ahead. 76
Other Systems System Frequency Implementation Accuracy Range LORAN C/D Omega VOR / DME Decca MLS Transit Glonass 100 k. Hz 10 -13 k. Hz 108 -118 MHz 70 -90 k. Hz 1. 5 GHz 150, 400 MHz 1602. 56 -1615. 5 MHz / 1246. 441256. 5 MHz 1575. 42 -1227. 6 MHz Ground Based Ground based Ground Based Space Based 460 m 3. 7 -7. 4 km 185 m 20 -50 m 5 -10 m 500 m 100 m 2900 km Global 370 km 224 km 56 km Global 100 m Global NAVSTAR GPS 77
References • Global Positioning Systems by MS Grewal, LR Weill, AP Andrews, Wiley, 2001. • Peter Rinder and Nicolaj Bertelsen, Design of a Single GPS Software Receiver, Aalborg University 2004. • http: //www. trimble. com/gps/index. html • http: //www. trimble. com/gps/ • http: //www. garmin. com/ • http: //www. nj. gov/dep/gis/gpsweb/train 00_files/frame. htm 78
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