Purpose The intent of this module is to provide you with an overview of the Global Positioning System. Objectives Explain the basic concept of GPS. Identify various applications of GPS. Describe how GPS works. Describe the pseudo-random satellite signals as they apply to GPS satellites and ground stations. Content 24 pages 4 questions Learning Time 35 minutes Module Introduction The intent of this module is to provide you with an overview of the Global Positioning System (GPS). You will learn about the GPS concept and current applications, as well as how GPS is emerging throughout the world. You will also learn about the constellation of satellites for GPS that orbit the Earth and the signals they generate, send, and receive.
What is GPS? GPS stands for Global Positioning System. It is a system of satellites and groundbased transmitting stations. Satellites find their own position with respect to the fixed ground stations. Then, people on the ground using a GPS receiver can locate their own position from signals and data sent by the satellites. Let s start with a brief overview of GPS. The GPS is a system of satellites orbiting the Earth that provide a method for GPS receivers to locate themselves with respect to the satellites. The satellites know their own position with respect to fixed points on Earth. The GPS receivers can locate themselves using the position data from the satellites. The GPS system is maintained by the United States Department of Defense (DOD). However, anyone can use the GPS system. There are two modes in the system. There is a low resolution system that anyone can use and a much higher resolution system that the DOD can make available. This is referred to as Selective Availability (SA). Currently, anyone can use the high resolution system. However, the DOD can disable the high resolution system at any time it desires. There is also a very high accuracy military version. However, this version is only available to the military and other governmental users. It is not available for civilian use.
Global Positioning Systems Mouse over the question to learn more. What are some typical uses of GPS? Backpackers hiking in a dense forest Police dispatcher trying to locate emergency vehicles Airplane navigation Ship navigation Locating stolen vehicles Locating lost people Military applications GPS is used to locate nearly anything. Of course, its primary applications involve locating non-stationary objects. Users, such as airplanes, cars, ships, and people, can carry GPS receivers to identify their exact location. For example, GPS can guide an airplane to an airport, or locate a hiker in a forest so they can find their own location on a map. GPS can also be used to keep track of the location of a fleet of delivery vehicles. The same is true for police cars and other emergency vehicles. In fact, with GPS, you can keep track of your own car and locate it instantly if it is stolen. For a more complete list of typical GPS users, roll your mouse pointer over the question on the screen.
Question Is the following statement true or false? Click Done when you are finished. GPS is a system of satellites orbiting the Earth that provide a method for GPS receivers to locate themselves with respect to the satellites. Users such as airplanes, cars, ships, and people can carry GPS receivers to identify their exact location. True False Consider this question regarding GPS. Correct. The GPS satellites know their own position with respect to fixed points on Earth. This enables GPS receivers to locate themselves using the position data from the satellites. GPS is used to locate almost anything. However, its primary applications involve locating non-stationary objects.
Asset, Vehicle, People, and Pets Tracking Mouse over the bulleted points to learn more. Key Features of Instant GPS for Tracking Applications Low power consumption (sleep modes) High sensitivity (track indoors, parking facilities, obstructed areas, in vehicle) Small footprint Low price Opportunities Multi-purpose radio systems Wristwatch-sized child tracker Ankle locks Collars for pets Hidden tracking devices Container tracking Truck tracking + IO As mentioned earlier, GPS can accurately track assets, vehicles, people, and pets. Roll your mouse pointer over the bulleted points for more information. From a marketing standpoint, GPS opportunities are very price elastic. Marketers know this means that lowering the price will greatly increase sales. For instance, putting a GPS unit on an animal might not be feasible at $200, but it might be no problem if the price was $40. Because the price of a GPS receiver is decreasing, the opportunities are rapidly increasing.
Instant GPS Market Opportunities Cellular Handsets and Wireless Devices Vehicle and Asset Tracking Personal Safety Homeland Security OEM Products and Consumer Electronics E911 Safety Tracking Security Location Based Services Consumer Electronics Here, you can see some of the instant GPS market opportunities. For cellular handsets and wireless devices, the network assists GPS performance to 153 dbm for indoor GPS units. GPS units used for vehicle and asset tracking offer low power consumption and high jamming immunity. GPS units used for personal safety offer a fast satellite acquisition and a fast time to first fix. GPS units for OEM products and consumer electronics make it easy to add them to other existing products. GPS presents a solution to some hard-to-solve problems. For instance, when an emergency happens requiring police assistance, a dispatcher can use GPS to locate every police unit in the area. Likewise, knowing the location of all available fire fighting and emergency response vehicles is very important when trying to dispatch the nearest available unit. Safety is another issue. It is very easy to lose track of children at large amusement parks. Locating them is easy if they are wearing a GPS receiver. Hazardous material being shipped by trucks can be monitored. Low-level nuclear waste can be tracked. Armored cars carrying large sums of money are another high-risk vehicle. And then, there are applications where you simply want to know where you are. GPS units can supply map coordinates for users to look up on a map.
Location Based Services Cellular, affordable GPS, inside-the-vehicle sensitivity Location Based Services (LBS) emerging in Europe & Asia Mouse over the bulleted points to learn more. Worldwide Market Japan and Korea offering applications that demand higher performance than E911 Hutchinson in UK offering LBS on 3g phones Bell Mobility (Canada) announced LBS AT&T Friend Finder Applications = emergency, safety, security, and convenience Key Features & LBS Opportunities: Concierge High sensitivity for use in vehicles while driving Turn by turn directions minimal host servicing, MIPS for speech or mapping Buddy list notification low cost, small footprint; can fit in a memory stick Targeted advertising based upon geofencing software supports all network assist modes and autonomous GPS Now, let s look at GPS as it is used for Location Based Services (LBS). In response to a demand for cellular, affordable GPS, and sensitivity inside vehicles, LBS are now emerging in Europe and Asia. To learn about LBS in the worldwide market and to see LBS opportunities and key features, roll your mouse pointer over the bulleted points. Regardless of where you are located in the world, it s important to know your location. Standards are being developed for E911 responses just like the regular 911 response network is used now. With an E911 equipped phone, the responding authority can locate the caller using GPS. For instance, a person who has been injured in an accident can press an emergency response button on the phone and the location can be transmitted to the responding authority. This concept can be further enhanced by having a telephone inside an automobile automatically call E911 when the air bags deploy and transmit the cars location to an E911 operator.
Consumer Electronics Instant GPS Solution: A fast and easy way of offering additional capabilities to wireless and stand-alone electronics products. Mouse over the photo to learn more. Key Features of Instant GPS for Consumer Electronics Easily integrated (pop, de-pop) in new or existing systems Indoor or in-vehicle use (after market telematics, hands-free kits) Network assist & autonomous capabilities Small footprint, low part count bill of material Fractional synthesizer allows the receiver to use the Host Temperature Compensated Crystal Very low host service requirements Here is a brief overview of how GPS can be used in consumer electronics. The small size and low power consumption of modern GPS units, such as the Instant GPS solution from Motorola, along with high performance and low cost, provides consumer electronics developers with a fast and easy way of providing additional capabilities to their wireless and stand-alone electronics products. To learn about the key features of Instant GPS for consumer electronics, roll your mouse pointer over the photo. The GPS system using the MG4100 can be manufactured very economically and in a very small size. The attributes of small size and economical cost open up tremendous marketing opportunities. Modules can be manufactured that automatically determine GPS coordinates of the unit. Then, software running on a host computer can use this data for various location-based applications. GPS units can be designed that implement connections to a host computer or network where needed GPS data can be obtained quickly. This includes error correction data, data about where the GPS satellites are located, and other factors. In fact, it is possible for the GPS unit to transmit the satellite data it receives back to a network host and have the host do the location calculations. The ability of a GPS unit to access remote assistance greatly speeds up the amount of time required for the GPS receiver to initially find its location after being powered up. This is referred to as Time to First Fix (TTFF).
Heading or pointing functions provided Real-Time tracking capability Easy integration Populate or de-populate Two-Way Radios Mouse over the bulleted point to learn more. Key Features of Instant GPS for Two-Way Radios Easily integrated (pop, de-pop) in new or existing systems Indoor or in-vehicle use (after market telematics, hands-free kits) Autonomous GPS small footprint, low part count bill of material Fractional synthesizer allows reuse of host Temperature Compensated Crystal (TCXO) Very low host service requirements Let s briefly examine two-way radios because they are a natural application for GPS. They already have the communications function to communicate with remote GPS resources, such as a computer located on a network that has error correction data. In addition, the MG4100 GPS system is economical and small enough to easily fit into a handheld radio. GPS capability can add heading and pointing functions to two-way radios. Because of the small size of modern GPS receivers, these receivers can be an add-on feature that can be populated or depopulated, as required, to the radios. For information on the key features of instant GPS for two-way radios, roll your mouse pointer over the bulleted points.
Navigation and Positioning Standard GPS systems are very computation-intensive. The 1kHz service requirement is handled within the MG4100. The host processor is now available for additional features. Key Features of Instant GPS for GPS Products Low power consumption Low price Small bill of material Small footprint Ease of integration Generally speaking, GPS units must run on local power a battery, 12-volt automotive power, or something similar. Low power consumption is important so that reasonable size batteries can be used. Remember, batteries are heavy and big batteries are even heavier. Lightweight is also a very important attribute for portable devices. Here, you can see the key features of Instant GPS for GPS products. This includes low power consumption, low price, small bill of material, small footprint, and ease of integration.
E911 FCC Requirements for US Cellular Carriers & Cellular Handset Manufacturers: E911 Fast TTFF Mouse over the bulleted points below to learn more. FCC mandate driving location technology in U.S. 100% of installed base by December, 2005 Application = emergency response only Key Features E911 Low IF front end GPS Acquisition Module (GAM) -153 dbm sensitivity Robust single chip solution E911 is coming. Cellular phones will be required to have E911 capability. The FCC will require US cellular carriers and cellular handset manufacturers to implement cellular phones and systems that provide a caller s position and time to Emergency Service Providers. Obviously, in an emergency, it is important for the GPS module to determine its location quickly. Many factors affect the amount of time required for the GPS unit to locate itself. The FCC is proposing a time of 6 seconds for Time To First Fix from a cold start. The MG4100 chip has hardware and firmware to perform this operation. To learn more about E911, roll your mouse pointer over the bulleted points. With a few external filter components, the MG4100 chip has the IF Front End for the receiver that actually receives the GPS signals coming from the satellite. This front end is identical in function to the receiver in your car radio. However, GPS does use a different frequency band than standard broadcast radio. The MG4100 chip contains the GAM or GPS Acquisition Module. This section of the MG4100 is used to locate and receive the GPS data coming from the satellites. The MG4100 device also is a single chip solution. With an external antennae and a few external filter components, the MG4100 locates the satellites and determines its location.
Question Are you familiar with some of the GPS applications? Complete the following sentences by dragging the letters on the left to their corresponding sentences on the right. Click Done when you are finished. A B C D Tracking E911 Two-Way Radios Location Based Services C B A D for consumer and industrial uses; provide heading or pointing functions. requires very fast times to first fix with less than a six-second cold start. is used mainly for assets, vehicles, people, and pets. are now emerging in Europe and Asia. Done Reset Show Solution Let s review some of the GPS applications. Correct. GPS can accurately track assets, vehicles, people, and pets. With E911, the emergency use of a cellular phone or system provides a caller s position and time to Emergency Service Providers. This requires very fast times to first fix,. GPS used in two-way radios provide real-time tracking capability for industrial uses. It is also used by consumers. GPS for Location Based Services (LBS) are emerging in Korea, Japan, and the UK.
How Does GPS Work? There is a constellation of satellites orbiting the Earth. There is a system of transmitting stations around the globe. The satellites locate themselves with respect to the ground-based stations. GPS receivers locate themselves with respect to the satellites. Let s examine in greater detail how GPS works. There is a constellation of 24 satellites that orbit the Earth. These satellites are not in geo-synchronous orbit. However, their positions at any given time are very accurately known. The satellites communicate with ground-based stations to locate themselves in space. Then, GPS receivers monitor signals sent out by the satellites and use these signals to locate themselves. All of this relies on the accuracy of some tremendously complex electronics.
Constellation of Satellites Space Segment Constellation consists of 24 satellites Six orbits Four satellites in each orbit Orbit altitude is 10,898 miles One orbit around Earth takes 12 hours 5-11 satellites are always visible from any place on earth Control Segment Master Control Station Schriever AFB, Colorado Springs, CO Monitor Stations and Ground Antennas Schriever AFB (Just MS) Hawaii (Just MS) Kwajalein Diego Garcia Ascension Island User Segment Now, let s discuss the constellation of satellites. As mentioned previously, the GPS constellation consists of 24 satellites. The GPS satellites orbit the Earth in six equally spaced rings. Each ring contains four satellites. Each satellite orbits the Earth twice per 24 hours. At any time, there will be at least five visible satellites from any place on Earth. Four satellites are required to give an unambiguous location. With a little mathematical slight of hand, three satellites can be used for a location fix. The problem with using three satellites is that there will be two possible locations generated by the GPS receiver. However, one of the answers will be obviously incorrect. That is, one of the answers will put the location in an unrealistic position, such as the far side of the moon. Ridiculous answers can simply be discarded. The location of a satellite in the sky is important for getting really good accuracy. Satellites that are very low on the horizon will have lower accuracy due to the fact that their radio signals will have to travel further through the atmosphere. Satellites that are high in the sky give better positional accuracy for the opposite reason, that is, the signals coming from these satellites travel a shorter distance through the atmosphere. Most GPS receivers will pick and choose the satellites that will give the best results. Here, you can see a list of master control stations, monitor stations, and ground antennas. The user segment consists of the ground-based GPS unit that is used to track the location of a person or object. Issues associated with the User Segment are receiver noise, antennae quality and general design of the portable GPS unit.
Ground Stations Navstar Global Positioning System (GPS) Control Segment Hawaii, USA (Pacific Ocean) Monitor Station Colorado Springs, CO, USA (Central United States) Monitor Station, Upload Capability, Master Control Station Gaithersburg, MD, USA (Eastern United States) Monitor Station, Upload Capability Cape Canaveral, FL, USA (Eastern United States) Upload Capability Ascension Islands (Atlantic Ocean) Monitor Station, Upload Capability Diego Garcia (Indian Ocean) Monitor Station, Upload Capability Kwajalein (Pacific Ocean near Australia) Monitor Station, Upload Capability Here, you can see a list of the various GPS ground stations. The GPS ground stations are located around the Earth. There are seven groundbased GPS transmitters. Some have upload capability while other stations only monitor the satellites, that is, they check on the position and condition of each satellite. Along with the ground station in Colorado Springs, CO, USA, the ground stations in the Ascension Islands, Diego Garcia, and Kwajalein are the stations that the satellites actually use to find their own locations.
Question Identify the correct statements by dragging the letters on the left to the corresponding statements on the right. Each letter may be used more than once. Click Done when you are finished. A True A Each satellite orbits the Earth twice per 24 hours. B False B There is a constellation of 12 satellites that orbit the Earth. B Satellites that are high in the sky will have lower accuracy due to the fact that their radio signals will have to travel further through the atmosphere. A Cape Canaveral, Florida is a GPS ground station location. Done Reset Show Solution Let s take a moment to review what you have learned so far. Correct. The GPS constellation consists of 24 satellites. The GPS satellites orbit the Earth in six equally spaced rings, with each ring containing four satellites. Each satellite orbits the Earth twice per 24 hours. At any time, there will be at least five visible satellites from any place on Earth. However, satellites that are high in the sky give better positional accuracy as the signals coming from these satellites travel a shorter distance through the atmosphere. There are seven ground-based GPS transmitters located around the world, including Hawaii, Cape Canaveral, and Diego Garcia.
GPS Satellite Signals Each of the 24 satellites must determine its own location. This is accomplished by comparing a pattern that is generated by the satellite with the same pattern that is being transmitted from each Earth-based station. Distance = speed of light x time delay between signals The Earth stations send a unique pattern to each of the 24 satellites. Let s continue to examine GPS satellite signals. GPS is based on all of the 24 satellites knowing exactly where they are in space. This is accomplished as follows: the ground-based transmitters send a signal to the satellites, this signal is a precalculated random digital signal; the signal is then repeated over and over. A unique pattern is used for each of the 24 satellites. The satellite knows what the pattern is and the satellite is generating the same pattern at exactly the same time that the ground station is generating the pattern. The electronics in the satellite measure the time delay between when the signal is received from the groundbased stations and the same signal generated in the satellite. The approximate distance from the satellite to the ground station is calculated as the speed of light times the time delay between the two signals. Each satellite is given a different pattern. The GPS receivers must know the pattern of each satellite. The pattern is a pseudo-random string of pulses generated by a computer. All of the satellites are loaded with their respective pattern. This pattern does not change. Since all of the portable GPS units must know this pattern to work, changing the pattern would create tremendous logistical problems for GPS users.
Pseudo-random Signal Signal Generated by Satellite Time Delay Between Signals Signal Received by Satellite From Ground Station Here, you can see a signal generated by satellite and a signal received by satellite from the ground station. In reality, the pseudo-random signal patterns are much longer than what is shown in the diagram and there are no repeating parts of the pattern. A satellite generates the pattern internally and also receives the pattern from a ground-based station. Remember that both the satellite and ground transmitter are synchronized by their respective atomic clocks. Once again, both the transmitting ground station and the receiving satellite know exactly what the pseudo-random pattern is and they both know the exact tick of the atomic clock that each and every edge will occur. Once the receiving satellite finds the incoming pattern and matches that to the pattern it is generating, the time differential can be established. Remember, the pseudo-random pattern is calculated by a computer. Once the pseudo-random code is generated, it is fixed and then sent to the ground station and the satellite. This pseudo-random code is repeated continuously in a never ending loop.
Pseudo-random Signal Pseudo-random signals are generated by a computer. One pseudo-random signal is used for each satellite. The pseudo-random signal is loaded into a satellite with the exact time that each edge in the signal will occur. Let s continue to look at pseudo-random signals. You now know that each satellite has a different pseudo-random pattern. The GPS receivers must know what these patterns are; otherwise, it would be impossible to come up with a time delay. The pseudo-random pattern is used so that there can be no doubt about aligning the patterns. If the satellite was sending a simple square wave, it might be possible for the pattern being generated in the satellite to be off by more than one clock period. Then, it would not be possible for the GPS receiver to determine whether the time differential was greater than one clock period, greater than two clock periods, etc.
GPS Signals Consists of 2 parts: 1. Repetitive 1023 bit (Gold Code) PRN Can be detected to 153 dbm (typical in building power level) 2. Modulated by background data @ 50 Bits per second data in 6s pages Can be decoded to ~ -143 dbm (typical weak outdoor power level) 50 BPS Modulation 1 ms repetitive 20 ms per bit 1.57542 GHz carrier 1.023 MHz chip rate PRN 1023 bits long 1 data bit = 20mS = 20 PRN repetitions 50 BPS Data 6 second page 6 second page Background data page = 6s = 300 bits = 6000 PRNs A GPS signal has two basic parts. One is the pseudo-random number (PRN) or Gold Code. The satellites also transmit a tremendous amount of data, such as satellite location, correction factors due to atmospheric conditions, correction factors due to errors in the satellite s predicted orbits as opposed to its actual orbit, and almanac data about the satellite s orbit. These GPS receivers need this data to find their position accurately. This data is sent at a rate of 50 bits per second. When a GPS unit is initially powered up, it can take several minutes to receive this data. Therefore, being able to get all of the necessary data for accurately calculating positions from a local computer network or a local phone network can greatly speed up the GPS units time to first fix.
Atomic Clocks Each satellite and each ground station has three atomic clocks that have incredible accuracy. Each ground station sends a pseudo-random pulse train over and over. Each ground station sends a different pseudo-random signal. How do the satellites generate the pseudo-random signal at exactly the same time as the ground station? Let s say that two people stand at opposite ends of a football field. Both people have a trumpet and they want to start playing a song at exactly the same time. Now, let s add the restriction that there is no band leader and the two people cannot see each other. How do they start playing at the same time? To solve this problem, we can give both players clocks and set them to exactly the same time. Then, we tell the players to start playing at a certain time. We have to assume that the players can keep one eye on the clock and that there is zero delay between when the start time occurs and when the players start to play. This is exactly the principle that GPS uses. Each satellite has three atomic clocks that are synchronized with the ground-based stations. They are very accurate. The satellite not only knows the pseudo-random pattern to transmit, but it also knows the time for each edge down to 0.1 nanoseconds. (The cesium clock frequency is 9,192,631,770 cycles per second.) Light travels about 1 inch in this amount of time. In other words, this would mean that the trumpet players could start within 0.1 nanoseconds of each other if they used a cesium atomic clock. Translating this back to GPS, this would mean a 1- inch error. Remember, the satellites both receive and internally generate the pseudo-random signals to locate themselves. Ground-based GPS receivers use a similar procedure to locate themselves. However, there is a problem with clock accuracy. It is not even remotely practical for a handheld GPS receiver to have the accuracy of an atomic clock. However, mathematical magic can be employed to compensate for timing differences between the clocks in the satellites and the clocks in the GPS units.
How Does GPS Work? Using the accuracy of the atomic clocks, the ground stations and the satellites generate the same pseudo-random signals at exactly the same time. The satellite compares the signal it generates with the incoming signal. There will be a time delay between the signal generated by the satellite and the signal received from the ground station. The satellite computer measures this time delay. With some correction factors, the distance is the time differential between the two signals multiplied by the speed of light. This is all possible because of the extreme accuracy of the atomic clocks on the satellites and in the ground stations. What have we learned so far about how the satellites locate themselves? Both the satellite and the ground station are generating the exact same pattern at exactly the same time (well, within 0.1 nanoseconds) of each other. The satellite shifts the received pseudo-random pattern in 0.1 nanosecond increments until it matches the pattern being generated by the satellite. The computer on the satellite, along with its very high resolution atomic clock, compares the pseudo-random signal it is internally generating to the same pseudo-random it receives from one of the ground-based stations. The satellite s distance from the ground station is somewhat close to the speed of light multiplied by the time it took for the signal to travel from the ground station to the satellite. It is important to note that all of this is possible because of the extreme accuracy of cesium atomic clocks. The resolution with which a GPS receiver can locate itself is basically a function of the timing interval of the basic clocking structure.
Question How do the satellites generate the pseudo-random signal at exactly the same time as the ground station? Select all that apply and then click Done. Satellites have three atomic clocks that are synchronized with ground-based stations. The satellites know the pseudo-random pattern to transmit and the time for each edge down to 0.5 seconds. Every satellite sends the same pseudo-random pattern to all ground-based stations. Both the satellite and the ground station are generating the exact same pattern at almost exactly the same time of each other. Let s take a moment to review some of the details of pseudo-random signals. Correct. Each satellite has three atomic clocks that are synchronized with the ground-based stations. These are very accurate. The satellites know the pseudorandom pattern to generate and compare to the incoming signal and they also know the time for each edge down to 0.1 nanoseconds. The satellite then finds the time delay between the pattern it generates and the same pattern received from the Earth-based transmitters in increments of 0.1 nanosecond.
Module Summary Global Positioning System (GPS) GPS Applications GPS Satellite Signals In this module, we have outlined the basics of the Global Positioning System. We have described the various applications for GPS and examined the relationship between ground stations, satellites, and the pseudo-random signals used to communicate between them.