GPS Assignment

August 30, 2017 | Author: Christine M M Mariwo | Category: Global Positioning System, Satellite, Navigation, Code Division Multiple Access, Telecommunications Engineering
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1. Notes from GPS tutorial The Global Positioning System (GPS) is a system that allows us to precisely identify locations on the earth's surface. It was originally developed for the military by the United States Department of Defense (DoD). The GPS system has 3 parts :The Space segment: This is a network of 24 satellites placed into orbit. The first GPS satellite was launched in 1978 and a full constellation of 24 satellites was achieved in 1994. Each satellite is built to last about 10 years. Replacements are constantly being built and launched into orbit. The Control segment : These are ground stations which are located around the world that make sure the satellites are working properly. The User segment: It is made up of the GPS receivers used by the community.

Figure 1 : The 3 GPS segments Using this satellite-based system enables highly precise location data for any point on the planet, in any weather conditions, 24 hours a day. It’s main use is for navigation, positioning and other research applications. GPS operation principle GPS satellites circle the Earth twice a day in a very precise orbit at an altitude of around 19 000 Kilometers (Figure 2). They are separated by angles of 55 degrees allowing any user to access between four and eight satellites from any point on the Earth.

Figure 2: The GPS Satellite network The user’s exact location is calculated using each satellite’s radio transmission signal information tracked and used by the GPS receiver. The receiver measures the travelling time of the signal between the satellites and itself and transform it into a distance so as to determine the precise latitude and longitude/position. The satellite signal: This constantly transmitted radio signal passes through clouds, glass and plastic but not through most solid objects such as buildings and mountains. There are three kinds of coded information essential for determining a position in a GPS signal:  An I.D. that identifies each of the 24 satellites of the network  The almanac data that contains the orbital information for all satellites in the system.  The ephemeris data, which contains important information about the status of the satellite (healthy or unhealthy), and the current date and time. The receiver:Calculates the distance to the first satellite detected. If the receiver calculates a distance of 17'000 km between this first satellite and the receiver it will mean that the receiver is located somewhere on a sphere that is centered on this first satellite and that has a radius of 17'000 km. The distance to a second satellite for which it has detected a signal (e.g.19'000 km )is then calculated. This tells the receiver that it is not only located on the first sphere but also on a

second sphere centered on the second satellite with a radius of 19'000 km. This implies that the receiver is somewhere on the point that these two spheres intersect. This calculation is repeated with a third satellite. In our example the receiver finds out that it is 20,000 km from the third satellite which narrows its position down even further, to the two points where the 20,000 km sphere cuts through the circle. A fourth satellite calculation is necessary to determine which of the two points is the correct position as well as in order to increase the accuracy and/or reduce the sources of error. GPS signal errors Though today's GPS receivers are extremely accurate, certain atmospheric factors and other sources of error can affect the accuracy of GPS receivers. Some sources of the errors are unavoidable, thus it is important for the user to be aware of which ones can be influenced so as to take steps to reduce their impact. The position of the satellite in the sky when taking the measurement can be the greatest source of error. The spread of the satellites in the sky is called the Positional Dilution of Precision (PDOP). When the satellites are located at wide angles relative to each other they have a good PDOP but if satellites are located in a line or in a tight grouping the PDOP is poor. The infrastructure (buildings, bridges) and particular landform (mountains) that are located around the receiver can be a source of error as they can block signal reception, thus causing position errors or possibly no position reading at all. GPS units typically will not work indoors, underwater or underground. Signal multipath may also occur when the GPS signal is reflected off these objects before reaching the receiver. Errors of distance estimation between the satellite and the receiver result due to the increase in the travel time of the signal. Sources of error over which the user does not have control include: Atmosphere delays — satellite signals slow down as they pass through the atmosphere. A model that calculates an average amount of delay to correct this type of error is imbedded in the GPS system.  Receiver clock errors — since a receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites there may be very slight timing errors.  Orbital errors — also known as ephemeris errors are inaccuracies of the satellite's reported location.

2. Notes from wikipedia The Global Positioning System(GPS) is a space based satellite navigation system that provides location and time information in all weather anywhere on or near earth, where there is an obstruct line of sight to four or more GPS satellites. The GPS program provides critical capabilities to military ,civil, commercial users around the world .In addition GPS is the backbone for modernizing the global air traffic system .The GPS project was developed to overcome the limitations of previous navigation systems integrating ideas from several predecessors, including a number of classified engineering design studies .Advances in technology and new demands on the existing system have now led to efforts to modernize the GPS system and implement the next generation of GPS 3 satellites and Next Generation Operational Control System. In addition to GPS ,other systems are in use or under development. The design of GPS is based partly on similar ground –based radio –navigation systems .GPS modernization has now become an ongoing initiative to upgrade the GPS system with new capabilities to meet growing military, civil and commercial needs. The program is being implemented through a series of satellite calculates its position by precisely timing the signals sent by sent by GPS satellites. Each satellite continually transmits messages that include : The time the message was transmitted  Satellite position at time of message transmission The receiver uses the messages it receives to determine the transit time of each message and computes the distance to each satellite using speed of light. These distances along with the satellites location s are used with the possible aid of trilateration,depending on which algorithm is used ,to compute the position of the receiver .This position is then displayed perhaps with a moving map display or latitude and longitude; evaluation information may be included .Many GPS units show derived information such as direction and speed ,calculated from position changes .Three satellites might seem enough to solve for position since space has three dimensions and a position near the Earth's surface can be assumed. This is indeed possible if the GPS receiver has its own exceedingly accurate clock (good to a nanosecond or so). This super accurate clock is required since any clock error will be multiplied by the very large speed of light and result in large position errors. However, clocks this good are not portable, cheap, or low power. To get around this problem, receivers do not count on their internal clocks being accurate, but instead use four or more satellites to solve for four variables - the receiver's location and the exact time. The very accurately computed time is effectively hidden by most GPS applications, which use only the location. A few specialized GPS applications do however

use the time; these include time transfer, traffic signal timing, and synchronization of cell

phone base stations. Although four satellites are required for normal operation, fewer apply in special cases. If one variable is already known, a receiver can determine its position using only three satellites. Some GPS receivers may use additional clues or assumptions such as reusing the last known altitude, dead reckoning, inertial navigation, or including information from the vehicle computer, to give a (possibly degraded) position when fewer than four satellites are visible .The current GPS consists of three major segments. These are the space segment (SS), a control segment (CS), and a user segment (US). Many GPS receivers can relay position data to a PC or other device using the NMEA 0183 protocol. The navigational signals transmitted by GPS satellites encode a variety of information including satellite positions, the state of the internal clocks, and the health of the network. These signals are transmitted on two separate carrier frequencies that are common to all satellites in the network. Two different encodings are used: a public encoding that enables lower resolution navigation, and an encrypted encoding. All satellites broadcast at the same frequencies. Signals are encoded using code division multiple access (CDMA) allowing messages from individual satellites to be distinguished from each other based on unique encodings for each satellite (that the receiver must be aware of). Two distinct types of CDMA encodings are used: the coarse/acquisition (C/A) code, which is accessible by the general public, and the precise (P) code, that is encrypted so that only the U.S. military can access. Because all of the satellite signals are modulated onto the same L1 carrier frequency, the signals must be separated after demodulation. This is done by assigning each satellite a unique binary sequence known as a Gold code. The signals are decoded after demodulation using addition of the Gold codes corresponding to the satellites monitored by the receiver. 3. Garmin, Trimble, Magellan, TomTom are all manufacturers of GPS receivers. These are all made in consideration of the following:Software-whether you want to plan a trip or save favorite locations Mounts-can be useful to keep the hands free while navigating the bike, train or aeroplane Price-an inexpensive entry level can be a good way to enter the GPS world Antenna configuration-where the antenna is to be used and whether it requires the use of only the built in antenna or addition of an external antenna Product level-whether you require the basics only or u also require the bells and whistles

Power source- whether you will be using the unit from an auxiliary power source or not Portability- whether there is preference between a portable or built -in unit Ease of use- whether the unit provides a tutorial or an easy to use touchscreen interface Mapping capability- whether you want to know the general direction or the street-level details of your chosen path. 4. a) Two other current and future satellite navigation systems are GLONASS and Galileo. Other satellite navigation systems are Wide-Area Augmentation System (WAAS), the Chinese Compass/BeiDou Navigation Satellite System, Japan’s Multi-functional Transport Satellite (MTSAT) and the Indian Regional Navigation Satellite System (IRNSS). b) 10 specific uses of GPS are:Agriculture in precision farming , used to monitor application of fertilizers and pesticides , locate plough harvest map fields and mark areas of disease and weed infestation Aviation pilots use GPS for en route navigation and using satellite navigation provides accurate aircraft location anywhere on earth Marine GPS technology helps commercial fishing fleets to navigate to optimum fishing locations and track fish migrations Rail GPS technology is used to acquire precise knowledge of train location to avoid collisions Space GPS is used to track and control satellites in orbit Timing GPS helps synchronize clocks and events around the world Recreation GPS technology is used by people to accurately mark and record any location and return to the exact same spot Environment GPS technology is used to locate areas on the ground that have been altered already by natural disasters Military GPS technology is used for target designation and weapon technology Ground transportation GPS helps with automatic vehicle location and navigation Surveying GPS technology is used to locate a precise point of reference Public safety GPS technology is used for status information and location in case of emergency c) GPS is used in cell phones as a way of enhancing and improving receiver performance and becomes known as Assisted GPS(A-GPS). The mobile network helps obtain a faster "time to first fix" (TTFF). A-GPS acquires and stores information about the location of satellites via the cellular network so the information does not need to be downloaded via satellite. It also helps

position a phone or mobile device when GPS signals are weak or not available. GPS satellite signals may be impeded by tall buildings, and do not penetrate building interiors well. A-GPS uses proximity to cellular towers to calculate position when GPS signals are not available. d) It is true that many GPS users fear that by having a GPS receiver others can track them as it can pin point their location in 3D space with an accuracy of just a few feet or meters away. A cell phone can be GPS tracked as it permits GPS data to be collected by the cell phone and transmitted via the chosen network. The installed technology will communicate via web –based services to maps and the software will report the user’s location to the online service. In online mode, their personal web page will be updated every 15 seconds displaying current position, speed and heading. When in offline mode, locations will be stored for later submissions, loggin routes abroad when an internet connection is unavailable. Since the GPS tracking system is designed with 100% stealth technology, it is made invisible to the user who will never know that their GPS device is constantly sending signals to the satelites.At the same time, no changes will be noticed on the device such as loading periods or icon changes, thus owners will never know that they are being tracked. e) Geocaching is an outdoor sporting activity in which the participants use a GPS receiver or mobile device and other navigational techniques to hide and seek containers, called "geocaches" or "caches", anywhere in the world. f) Assisted GPS or A-GPS uses the mobile phone network to assist the GPS receiver in

the mobile phone to overcome the problems associated with TTFF and the low signal levels that are encountered under some situations. The network provides the Ephemeris data to the cell phone GPS receiver and this improves the TTFF. This can be achieved by incorporating a GPS receiver into the base station itself, and as this is sufficiently close in position to the mobile the data received by the base station is sufficiently accurate to be transmitted on to the mobiles. The base station receiver is obviously on all the time, and will be located in a position where it can "see" the satellites. Assisted GPS or A-GPS is also used to improve the performance within buildings where the GPS signals are by 20 dB or possibly more. Again by providing information to the GPS receiver in the mobile it is able to better correlate the signal being received from the satellite when the signal is low in strength.

g) GNSS-satellite navigation system with global coverage may be termed a global navigation satellite system or GNSS GLONASS- is a radio-based satellite navigation system operated for the Russian government by the Russian Aerospace Defence Forces. It both complements and provides an alternative to the United States' Global Positioning System (GPS) Galileo- Europe’s global navigation satellite system which provides a highly accurate, guaranteed global positioning service under civilian control. It is inter-operable with GPS and GLONASS, the two other global satellite navigation systems.

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