GPS Global Positioning System

Transcription

1 GPS Global Positioning System

2 GPS Basics 1. What is GPS? 2. How does it work? 3. Brief history of GPS These topics will be covered in this slide set.

3 NAVSTAR GPS (Navigation Satellite Timing and Ranging system) 24 Satellites orbiting the earth Provides accurate positioning, navigation and timing Operates 24 hrs/day, in all weather Can be used for any application that requires location information Wikipedia GPS constellation animation, NOAA orbit animation (requires Macromedia Flash Player) What is GPS? GPS is a satellite-based system, operated and maintained by the U.S. Department of Defense (DoD), that provides accurate location and timing information to people worldwide. The system transmits radio signals that can be used by GPS receivers to calculate position, velocity and time anywhere on earth, any time of day or night, in any kind of weather. The NAVSTAR GPS concept was developed in the early 1970's to meet the U.S. military s need for improved navigation and positioning. The first Block I GPS satellite was launched in 1978 and Full Operational Capability (FOC) was achieved in The Global Positioning System is a National resource and an international utility for positioning, navigation and timing.

4 Space GPS Segments The GPS consists of 3 segments: space, control and user. User Control The space segment is the satellite constellation, consisting of 24 or more satellites. The first Block I satellite was launched in early The 1986 Challenger disaster slowed the GPS constellation development. In February 1989 the first Delta 2 launch took place. The constellation is now fully operational and consists of 24 or more satellites (currently, there are 31). The control segment is operated by the U.S. Department of Defense (DoD) which tracks and maintains the satellites. The Department of Transportation (DoT) now has management responsibility, along with DoD. The user segment consists of both military and civilian users. Military uses of GPS include navigation, reconnaissance, and missile guidance systems. Civilian use of GPS developed at the same time as military use, and has expanded far beyond anyone's original expectations.

5 Power Sun-seeking solar panels Nicad batteries Timing 4 atomic clocks Space Segment GPS Satellites The GPS satellites weigh about 900 kg and are about 5 meters wide with the solar panels fully extended. They are built to last about 7.5 years, but many have outlasted their original estimated life-span. The solar panels provide primary power; secondary power is provided by Nicad batteries. On board each satellite are four highly accurate atomic clocks. Quartz clock accuracy: quartz clocks are accurate to approximately 1 second per month Atomic clock accuracy: atomic clocks are accurate to roughly 1 second per 20 million years National standards agencies maintain an accuracy of 10-9 seconds per day (approximately 1 part in )

6 24 satellites in 6 orbital planes Orbit the earth at ~ 20,200 km (11,000 nautical miles) Satellites complete an orbit in approx. 12 hours Satellites rise (and set) about 4 minutes earlier each day Satellite Orbits There are four satellites in each orbit plane, and each plane is inclined 55 degrees relative to the equatorial plane (the satellite path crosses the equator at a 55 degree angle). The high altitude insures that satellite orbits are stable, precise and predictable, and that the satellites' motion through space is not affected by atmospheric drag. It also insures satellite coverage over large areas. GPS satellites orbit around the earth, in contrast to TV satellites which are in geostationary orbits (they rotate with the earth). The GPS satellites cross over any point on the earth approximately twice per day.

7 GPS satellites originally broadcast messages via radio signals on 2 frequencies L1: Mhz (C/A and P/Y code) L2: Mhz (P/Y code) Two levels of service Standard Positioning Service (SPS) Precise Positioning Service (PPS) (Anti-spoofing (AS) guards against fake transmissions of satellite data by encrypting the P-code to form the Y-code) Satellite Signals The radio signals travel at the speed of light: 300,000 km per second (186,000 miles per second). It takes 6/100ths of a second for a GPS satellite signal to reach earth. These signals are transmitted at a very low wattage (about watts in the microwave spectrum). C/A code (Coarse Acquisition code) is available to civilians as the Standard Positioning Service (SPS). Before selective availability (SA) was turned off, SPS provided a predictable positioning accuracy of 100 meters horizontally, 156 meters vertically, and time transfer accuracy to UTC within 340 nanoseconds (95 percent). SPS now provides average horizontal accuracy of 7.8 meters 95% of the time and average vertical accuracy of <= 15 meters 95% of the time. The Precise Positioning Service (PPS), available only to the military (and other authorized users), provides higher accuracy via the P code.

8 Unique pseudorandom code Ephemeris data Clock behavior Clock corrections System time Status messages Almanac The almanac data can be transferred to the office computer and used to display a graphic showing the locations of all satellites. This information can also be used to predict satellite availability for a specific mapping time and date. Information Contained in Satellite Signals Each satellite transmits a radio signal containing its unique pseudorandom (appears to be random but is not) code. This code identifies the satellite and distinguishes it from other satellites. The signal also contains the precise location of the satellite (ephemeris data), its clock behavior and clock corrections, system time (highly accurate because of the atomic clocks on-board the satellites) and status messages (usually referring to satellite health). In addition, an almanac is also provided which gives the approximate location data for each active satellite. The almanac is automatically downloaded from the satellites to the GPS receiver when the receiver is operating outside. It takes about 12 minutes to receive an almanac.

9 Satellite signals require a direct line to GPS receivers Signals cannot penetrate water, soil, walls or other obstacles Signal Behavior Trees, buildings, bridges, mountain ranges, your hand (over the receiver antenna) or your body can all block the satellite signals. Heavy forest canopy causes interference, making it difficult to compute positions. In canyons (and "urban canyons" in cities) GPS signals are blocked by mountains or buildings.

10 Sent along with position and timing messages Prediction of all satellite orbits Needed to run satellite availability software Valid for about 30 days Satellite Almanac The almanac has information about the orbits of all 24+ satellites. A GPS receiver uses the almanac (for quick acquisition of satellite positions), along with satellite data messages, to precisely establish the position of each satellite it is tracking. Satellite availability software uses the almanac to make graphs of satellite locations overhead and to calculate the best times to survey in a particular area. This graph shows the number of satellites available during a specific mapping time period. GPS receivers automatically collect a new almanac each time they are turned on for more than about 15 minutes. It is important to use an up-to-date almanac when viewing satellite availability in planning software. Almanac data are valid for about 30 days, but a new almanac should be transferred to satellite availability software as frequently as possible.

11 Control Segment US DoD Monitoring The control segment consists of five Monitor Stations (Hawaii, Kwajalein [West Pacific], Ascension Island [South Atlantic], Diego Garcia [Indian Ocean], Colorado Springs), three Ground Antennas (Ascension Island, Diego Garcia, Kwajalein) and a Master Control Station (MCS) located at Schriever Air Force Base in Colorado. Earthmap:NASA During August and September 2005, six additional monitor stations of the NGA (National Geospatial-Intelligence Agency) were added. Now, every satellite can be seen from at least two monitor stations. This allows more precise calculation of orbits and ephemeris data. For the end user, this improvement translates to better precision. In the near future, five more NGA stations will be added so that every satellite can be seen by at least three monitor stations. This improves integrity monitoring of the satellites and thus the whole system.

12 Orbits are precisely measured Discrepancies between predicted orbits (almanac) and actual orbits (ephemeris) are transmitted back to the satellites So satellites can transmit their correct locations US DoD Monitoring The monitor stations 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 GPS satellites send satellite data messages (position and timing), an almanac, and orbital corrections they receive from the Master Control Station. The GPS receiver uses all this information to compute positions.

13 User Segment The user segment consists of receivers that provide positioning, velocity and precise timing to users worldwide. Civilian applications of GPS exist in almost every field, from surveying to transportation to natural resource management to agriculture. The civilian community is a powerful political force in influencing GPS policy decisions. Examples of civilian GPS applications include 1) GPS on a helicopter to identify the location of victims in search and rescue operations, 2) GPS on a tractor/combine linked to a yield monitor to generate yield maps (precision farming), 3) GPS used for aircraft navigation or to mark where rangeland weeds have been sprayed, 4) GPS used for recreational sailing navigation, 5) Emergency services response system: a combined GPS/GIS system used to dispatch emergency vehicles and find the quickest route to a destination (GPS is also being used for pizza delivery systems!), 6) GPS to help a backpacker navigate in the woods. 90% of the data created in the world today has some type of geographic component, and civilian users outnumber military users worldwide.

14 GPS receiver calculates its position by measuring the distance to satellites (satellite ranging) How Does GPS Work? GPS satellites are constantly transmitting signals that contain orbit data and timing information. Receivers pick up those signals and use the information to compute positions. Note: Receivers don t send signals back to satellites, contrary to what many people think. They are receivers not transceivers. The distance measurement calculated by a GPS receiver is referred to as a range or pseudorange. Because of errors in the system, it is not a true range measurement. In order to compute a position, we START by measuring the distance between the receiver and the satellites. The satellites are the known points; the GPS receiver on the ground is the unknown point. The range (actually pseudorange: estimate of range) is measured as elapsed transit time.

15 1. Measure time for signal to travel from satellite to receiver 2. Speed of light x travel time = distance Distance measurements to 4 satellites are required to compute a 3-D position (latitude, longitude and altitude) Measuring Distance to Satellites Since radio waves travel at the speed of light, we can multiply the travel time of the GPS signal by 300,000 kilometers per second (186,000 miles per second) to get the distance between the GPS satellite and the receiver. Once we have the distance measurements it's basically a problem of geometry: if we know where the 4 satellites are and how far we are from each satellite, we can compute our location through trilateration. Note: It takes about.06 seconds for a GPS radio signal to reach Earth.

16 3 Distance Measurements Trilateration Here is an example of trilateration in two dimensions (note that GPS works in 3 dimensions, but it s the same idea). Three ranges (distance measurements) will locate a point in two-dimensional space. If we know that our location is 127 miles from Great Falls, then we are somewhere on the red circle. If we also know that we are 122 miles from Billings, then our position is somewhere on the purple circle, and, if we are 80 miles from Helena, we are somewhere on the green circle. Considering the three range measurements together, our position must be where the three circles intersect, or, Bozeman! Not Triangulation! With GPS, trilateration refers to measuring the distance (lengths) from 3 satellites to establish a position on the Earth. So, receivers compute positions through trilateration (measuring distances to satellites, not angles, as in triangulation). And with GPS, we are calculating 3- dimensional positions and correcting for timing offset, so we need four measurements to determine latitude, longitude, altitude and timing offset. This will be explained further in the next several slides.

17 GPS receiver receives a chunk of code from the satellite and looks back to see how long ago it was generated Measuring Travel Time of Satellite Signals In order to measure the travel time of the satellite signal, we have to know when the signal left the satellite AND when the signal reached the receiver. Presumably our receiver "knows" when it receives a signal, because it can note the time of reception, but how does it know when the signal left the satellite? Time difference GPS satellites generate a complicated set of digital codes. These codes are complicated enough that they can be compared easily and unambiguously (look at the diagram of the code shown on the slide in red). They are "pseudo-random" sequences that actually repeat every millisecond. The trick is that the GPS satellites and our receivers are synchronized so they're generating the same code at exactly the same time. Satellite and GPS receiver generate the same codes at the same time (so the receiver knows when the signal left the satellite) synchronized codes So, when a GPS receiver receives codes from a satellite, it looks back to see how long ago it (the receiver) generated the same code. The time difference is how long the signal took to get from the satellite to the receiver. In other words, the receiver compares how "late" the received satellite code is, compared to the code generated by the receiver itself.

18 GPS receiver receives a chunk of code from the satellite and looks back to see how long ago it was generated Measuring Travel Time of Satellite Signals This slide shows the receiver sliding the code it received from the satellite to the left, to match up with the code it generated. The amount it has to slide is the time difference, or the time it took to travel from the satellite to the receiver antenna. Hint: go to the previous slide and then back to this one to see the slide. Satellite and GPS receiver generate the same codes at the same time (so the receiver knows when the signal left the satellite) synchronized codes

19 One measurement narrows down our position to the surface of a sphere How GPS Positions are Computed Now let s look graphically at how GPS positions are computed. The first measurement narrows down our position to the surface of a sphere. In this example we are 12,000 miles away from the satellite, and 12,000 miles is the radius of a sphere centered on the satellite. Our position could be anywhere on the surface of that sphere.

20 When we add a second measurement, we can narrow down our position a little more. The intersection of two spheres is a circle. Now we know that our position is somewhere on that circle.

21 Adding a third measurement narrows down our position even further. The three spheres intersect at only two points. We can discard one of the two points because it is nowhere near the earth. The computers in GPS receivers have various techniques for distinguishing the correct point from the incorrect one.

22 In theory, we should be able to nail down a 3 dimensional position (x, y, z or latitude, longitude, altitude) with 3 perfect measurements. However, there is a problem (timing offset) that causes error in the measurements. To solve this problem, we need a fourth measurement to establish an accurate 3-D position...

23 Receivers use accurate quartz clocks Satellites use highly accurate atomic clocks The next few slides illustrate the problem graphically. The illustration is shown in 2-D to make it easier to visualize remember that GPS works in 3-D (satellite range measurements are determined from spheres (3-D), not circles (2-D) the sphere represents the satellite signal transmitting in all directions from the satellite). But we will illustrate the problem (and it s solution) using circles because it is easier to see. What is Timing Offset? Timing offset refers to the difference in synchronization between the satellite clock and the receiver clock. Remember, the satellites have highly accurate atomic clocks on board. In order to be perfectly in sync, we would have to have an atomic clock in our receiver. But, atomic clocks are far too expensive to put in GPS receivers, so we have to live with the receiver clocks and satellite clocks being slightly out of sync. This difference causes the timing measurements to be slightly off. Remember that time is needed in order to calculate a range: the time it takes for the satellite signal to reach the receiver is multiplied by the speed of light to get the distance, or range. Without a correct time measurement, we can t calculate a correct range.

24 2-D Illustration of Timing Offset 4 seconds A Note: The explanation of timing offset will be shown in two dimensions for illustration. Remember that in reality we are working with spheres, not circles, and we need 3 perfect measurements to calculate a 3-D position, not 2. B 6 seconds Remember that establishing the travel time of the GPS signal is the first step in calculating the distance between the satellite and receiver (travel time x speed of light = distance). In an ideal situation there would be no timing error. Let's say we're 4 seconds from satellite A and 6 seconds from satellite B: our position is where the 2 circles intersect (we can throw out the other position because it is nowhere near the earth remember, in 3-D, it takes 3 measurements to get to this point). Note: the times we are using here are not accurate in reality, it takes about 6/100ths of a second for a satellite signal to reach the earth.

25 2-D Illustration of Timing Offset If the receiver clock is one second fast (it's ahead one second from the satellite clock) the receiver will "think" the distance from satellite A is 5 seconds and the distance from satellite B is 7 seconds. And it "thinks" our position is where the two dotted circles intersect. We obviously have an incorrect position here because of timing offset. Note: The explanation of timing offset will be shown in two dimensions for illustration. Remember that in reality we are working with spheres, not circles, and we need 3 perfect measurements to calculate a 3-D position, not 2.

26 2-D Illustration of Timing Offset A B But if we add an extra imperfect measurement, we now have enough information to figure out the amount that the clocks are out of sync. 5 seconds (wrong time) 9 seconds (wrong time) 7 seconds (wrong time) When the receiver gets a series of measurements that cannot intersect at a single point, it finds the adjustment to all measurements that lets the ranges go through one point. In this example, subtracting 1 second from all three measurements makes the circles intersect at a point. C So, by adding one extra measurement (the 3 rd measurement in this 2-D example) we can cancel out any consistent clock error the receiver might have. Remember that in 3 dimensions this means we really need 4 measurements to cancel out the error.

27 The first three measurements narrow down our position A fourth measurement is needed to correct for the difference in synchronization between satellite and receiver clocks Correcting for Timing Offset The four measurements are used to solve for four variables: latitude, longitude, altitude and timing offset. By the way, GPS can also provide a measurement of velocity and heading, which is important for navigation. 4 measurements: 4 variables Latitude Longitude Altitude Time (timing offset)

28 GPS Timeline GPS conceived by U.S. Department of Defense (DoD) funding approved by Congress Dept. of Transportation (DoT) became involved in management of GPS to respond to civil needs Selective Availability (SA) activated Selective Availability (SA) deactivated, GPS Modernization Begins GPS Modernization Continues 1970s 1980s 1990s 2000s 2010s 2020s 1974 First satellite launched 1978 First Block, I satellite launched Aug 1990 Jun 1991 SA deactivated during Persian Gulf War 1994 GPS declared fully operational 1996 Presidential Decision Directive (PDD) strengthened federal policy for GPS and provided strategic vision for its management & use 1997 GPS Modernization Planning Begins 2005 First modernized Block IIR-M satellite launched (with new L2C signal)

29 GPS is the global standard 40-year old system Satellite expected life 7.5 years opportunity to upgrade with replenishment Improved ground (monitoring) facilities Modernization to improve capabilities for civil and military users Removal of selective availability (SA) was first step GPS Modernization GPS is the first Global Navigation Satellite System (GNSS), and the U.S. Government wants it to be the best in the world. The system is almost 40 years old and upgrades are needed. As old satellites are taken out of operation, there is an opportunity to replace them with upgraded satellites. GPS is being modernized in order to further improve positioning, navigation and timing capabilities for both civil and military users. The modernization initiative includes improved ground and space facilities, and will result in substantial improvements in GPS positioning accuracy. Removal of selective availability (SA) in May 2000 was the first step in the GPS Modernization initiative. This action immediately improved GPS accuracy from 100 meters to less than 5 meters.

30 This graph shows the immediate effect of SA removal. Notice the difference in horizontal and vertical error before and after SA was eliminated. Selective Availability (SA) SA made it difficult to determine which highway a car was on, in areas where several highways run in parallel this caused problems for in-car navigation systems. Now, it is possible to determine in which lane a car is traveling. Removal of SA significantly benefits emergency vehicle response to E-911 calls. SA removal has also benefited fleet management making tracking the locations of taxis, buses, tractor trailers and boxcars more efficient, especially in crowded parking lots and railway yards. Today, the Standard Positioning Service (SPS) provides horizontal positioning accuracy of 7.8 meters (95 percent). Shaw, Michael, Kanwaljit Sandhoo and David Turner, Modernization of the Global Positioning System, GPS World Magazine, September 2000 Removal of SA has increased the safety of GPS for non-precision runway approaches and generally improved pilot situational awareness. Recreational benefits include the ability to more precisely locate favorite fishing holes, boating obstacles, and game left for future retrieval. Fisherman can more accurately locate lobster pots and other fishing gear.

31 x 1990 April 2000 SA on: 100 meter spread Selective Availability (SA) The first step in GPS modernization was the removal of SA on May 1, 2000, by President Clinton. This immediately increased the accuracy of stand-alone GPS receivers from meters to about 10 meters. May 1, 2000 SA off: 3 meter spread x These graphs show 1-hour base files (5 second logging interval) from the old MSU GPS Base Station on the roof of Leon Johnson Hall before and after SA was eliminated. Each point you see in the graph is a GPS measurement (latitude, longitude and altitude) of the stationary GPS base station. The spread across the widest part of the points while SA was implemented is 100 meters. After SA was turned off, the spread across the points was only 3 meters. The black x in each map shows the true location of the base station.

32 GPS satellites broadcast messages via radio signals on 3 frequencies (L1, L2 and L5) L1: Mhz (C/A and P/Y code) L2: Mhz (P/Y code) L2C: Mhz ( ) Non-safety critical applications More sophisticated code Higher power L5: Mhz ( ) For safety of life applications (civil aviation) Higher power Wider bandwidth L1C: Mhz (2016) Interoperability between GPS and other GNSSs Improved reception in cities and other challenging environments New Satellite Signals Three new civil signals (L2C, L5 and L1C) will be added to future satellites. The new signals will significantly improve the robustness and reliability of GPS for civil users. Higher power and wider bandwidth signals will make it much easier to acquire and track GPS signals under tree canopy and indoors. Estimated accuracy is one meter or better in real-time, without augmentations. This new capability will spur new applications for GPS, further expanding the rapidly growing market for GPS equipment and services worldwide. For more information on GPS Modernization, see the GPS Modernization slide show.

33 First satellite launched Other Global Navigation Satellite Systems Other Global Navigation Satellite Systems (GNSS) exist. They will be discussed in the next few slides. This slide shows the launch year for the first satellite in each system

34 Galileo Galileo is Europe's contribution to the next generation Global Navigation Satellite System (GNSS). Unlike GPS, which is funded by the public sector and operated by the U.S. Air Force, Galileo will be a civil-controlled system that draws on both public and private sectors for funding. The service will be free at the point of use, but a range of chargeable services with additional features will also be offered. These additional features would include improved reception, accuracy and availability. The Galileo system will consist of 30 satellites positioned in three orbit planes at 23,333 km altitude. Galileo is expected to be fully operational by Galileo s first two operational satellites were launched in Galileo currently has 2 test satellites and 2 in-orbit validation (IOV) satellites (Source: The Almanac, By: Richard B. Langley GPS World, December 1, 2011, tem/almanac/almanac-4265 )

35 GLONASS GLONASS is the Russian satellite navigation system which, like GPS, consists of 24 satellites. However, there are configuration and signal structure differences. On December 8, 2011, the GLONASS constellation was completed. Russia is now discussing and planning GLONASS modernization.

36 BeiDou In an effort to shake off dependence on foreign systems, China is now building its own global satellite navigation system. The initial constellation of three geostationary Earth orbit (GEO) satellites was completed in A fourth GEO satellite was launched in The initial regional Beidou system (Beidou-1) is being expanded, in stages, into a global system known as Beidou-2 or Compass. It will include five GEO satellites, 27 medium Earth orbit (MEO) satellites, and five inclined geosynchronous orbit (IGSO) satellites. The system will cover the Asia-Pacific region by 2012 with the global system expected to be fully completed by 2020.

37 6 Things to Take Away Today 1. 3 GPS segments 2. Satellites transmit radio signals containing Unique pseudorandom code Ephemeris data Clock behavior and clock corrections System time Status messages Almanac 3. Formula for satellite ranging (D = t v) 4. 4 satellites to compute an accurate 3-D position (the 4 th measurement is needed to correct for timing offset) 5. GPS is an evolving system with major improvements coming in the next 10 years 6. We are not the only country with a GNSS And, there s a lot more to it than this we ll progress through the rest of the important info in the next few weeks.