GPS Solution - GPS can measure the moving direction of a rigid object

Январь 12th, 2016 by homesecuirtycameras and tagged , , ,

Currently, GPS ( tracking platform ) is the best known and most used satellite positioning system. Over the past 30 to 40 years, several other positioning solutions have been deployed for use in various scenarios, based on various approaches, using infrared, ultrasound, image processing or electromagnetic waves. Positioning or location can be understood as the unambiguous placement of a certain individual or object with respect to a known reference point. This reference point is often assumed to be the center of the Earth coordinate system. In practice, Mobile Positioning and Tracking João Figueiras and Simone Frattasi the reference point can be any point on the Earth that is known to the system and which all the coordinates can relate to. Although the position itself is obviously a very important source of information, this position must be related to a specific time to be even more useful.

In particular, when we consider tracking systems, time information is a key necessity, not only for knowing the position of a certain device at a specific time, but also for inferring higher-order derivatives of the position, i.e., speed and acceleration. Thus, “tracking” is a method for estimating, as a function of time, the current position of a specific target. “Navigation” is a tracking solution that aims primarily at using position information in order to help users to move towards a desired destination. The sever has associated a relational database to store all information about users, devices, and sessions, and a spatial database to support the storage of spatial information, such as the GPS magnetic tracker fixes from all the mobile devices. The location-based application here also responds to queries sent from the “control station,” or main monitoring station, to obtain the GPS fixes of a particular user or group of users and display their positions in a graphical user interface in real time.

GPS also employs TOA ranging and threedimensional positioning model to calculate user position. The measurement of vehicle GPS tracking device positioning model is similar to two dimensional positioning model. Assume that each satellite transmits a unique ranging signal in space. A satellite has a clocks synchronized with others clocks built on other satellites. The clock synchronization and orbit plane of these satellites are monitored and controlled by the control segment (MCS).

GPS also can measure the moving orientation (direction) of a rigid object, e.g., a vessel, an aircraft, etc. A GPS receiver also has a clock synchronized to GPS system time. Ranging signals transmitted from GPS satellites contain time information. It enables the receiver to measure the propagation time of the ranging signal left the satellite based on satellite clock time. We can find that the user position would be at two possible points, and only one of the points is the correct user position. For the user position, it is generally locates on the surface of the earth. Therefore, the lower one of two points will be the true position. In order to measure the more accurate user position, the fourth satellite will be applied to calculate and fix the user’s altitude. In addition GPS can determine the user velocity. The most popular used method is based on estimating the Doppler frequency of the received GPS signals.

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How GPS Tracking Device works

Январь 11th, 2016 by homesecuirtycameras and tagged , , ,

Navigation employing GPS and inertial sensors is a synergistic relationship. The integration of these two types of sensors not only overcomes performance issues found in each individual sensor, but also produces a system whose performance exceeds that of the individual sensors. portable GPS tracking provides bounded accuracy, while inertial system accuracy degrades with time. Not only does the GPS sensor bound the navigation errors, but the GPS sensor calibrates the inertial sensor. In navigation systems, GPS receiver performance issues include susceptibility to interference from external sources, time to first fix, interruption of the satellite signal due to blockage, integrity, and signal reacquisition capability. The issues related to inertial sensors are their poor long-term accuracy without calibration and cost.

Today digital advancements make it possible to pre-program asset GPS tracking device on common routes you usually tread on with the vehicle and the trackers will also bookmark your work and home GPS coordinates. This allows the tracker to automatically notice changes to common routes and begin emitting readings that raise alerts well in advance. vehicle GPS locator tend to be very discrete and will have unique and personalized beepers and sounds alerting the driver or owner regarding an unplanned reroute and they are required to respond by accepting the same by pressing a secret hidden button.

One primary concern with using GPS as a stand-alone source for navigation is signal interruption. Signal interruption can be caused by shading of the GPS antenna by terrain or manmade structures (e.g., buildings, vehicle structure, and tunnels) or by interference from an external source. Each vertical line in this figure indicates a period of shading while driving 460 Integration of GPS with Other Sensors and Network Assistance in an urban environment. The periods of shading (i.e., less than three-satellite availability) are caused by buildings and are denoted by the black lines in the lower portion. (This experiment was conducted when five to six satellites above a 5º mask angle were available for ranging.)

GPS tracker works by the process of triangulation. (Technically, it is called trilateration because it calculates your position using distances rather than angles, but the concept is similar, and the terms are often used interchangeably.) When only three usable satellite signals are available, most receivers revert to a two-dimensional navigation mode by utilizing either the last known height or a height obtained from an external source. If the number of usable satellites is less than three, some receivers have the option of not producing a solution or extrapolating the last position and velocity solution forward in what is called dead-reckoning (DR) navigation. Inertial navigation systems (INSs) can be used as a flywheel to provide navigation during shading outages. The discrete-time nature of the GPS 3G in some equipment is also of concern in real-time applications, especially those related to vehicle control. if a vehicle’s path changes between updates, the extrapolation of the last GPS measurement produces an error in the estimated and true position. This is particularly true for high-dynamic platforms, such as fighter aircraft. In applications where continuous precision navigation is required, inertial sensors can be employed. An alternative solution is the use of a GPS receiver that provides higher rate measurement outputs.

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GPS applications - personal vehicle tracking devices

Январь 8th, 2016 by homesecuirtycameras and tagged , , ,

Understanding the right positions and directions of people and objects is a significant issue from the ancient eras to the present. In the past, people often launched a war in order to satisfy the craving for the dominating powers and spread their realms. In the recent, Global Satellite Positioning System (GPS) has become the one of most popular positioning technologies. GPS can provide users precise positioning information, no matter wherever that may present their own positions. The early GPS positioning technology has been widely used in military, marine use, until recently gradually applied into our daily life, e.g. automotive navigation, geodesy surveying, etc. In this chapter, the authors will briefly introduce some GPS issues including the origins of GPS, GPS system architecture, and related vehicle GPS tracking device applications.

The most driving purpose is to traverse to the destination safely, efficiently, and comfortably. Two types of approaches could achieve the goals, including the static and dynamic approaches. In the static aspect, vehicles use the static road and traffic information to navigate. Conversely, in the dynamic aspect, vehicles adopt the dynamic information instead. However, both of the two approaches first require getting the vehicle’s location and then map the position on an e-map. Thus, this chapter first introduces some important vehicle location determination algorithms: the dead reckoning and global position system algorithms, in which the precision of location technologies are compared. Then, the map-matching algorithm is described in detail. Finally, various vehicle navigation approaches are detailed, in which the important topics include: the navigation architecture, the navigation routing algorithm, and navigation applications.

The aim of tracking platform is to guide the vehicle along the optimal path from the starting point to destination. A reliable vehicle navigation system can reduce the traffic chaos in the city and improve the transportation delay. In order to achieve reliable vehicle navigation system, the detail system requirements, system analysis, and system architecture are shown in the chapter. Each component of vehicle navigation system is briefly illustrated, and the system communication is also described. They also present the architecture of the proposed vehicle navigation system, and show the difference of these systems. Therefore this chapter helps understanding the architecture of vehicle navigation system.

A smart GPS tracking device will tell you everything you want to know about the location of the vehicle, the speed at which it is traveling as well as the other conditions in that locality. It will give you a peace of mind as you now know clearly where the vehicle is. Moreover, you don’t have to worry constantly about calling up the driver every other minute to find out the status of the travel. There are some tracking systems that will also keep sending constant updates to your Smartphone. So, even if you are on the move and away from your laptop or desktop, you can still keep track of the vehicle.

Most electronic tracker work based on GPS, known as Global Positioning System. Hi-end systems even give you real time data imports. A good tracking system can keep track of multiple vehicles at the same time. So, if you are running a taxi service or is a college administrator keeping tracks of the many buses you have in your college fleet, you can keep track of all the vehicles using one system. When you are installing an online vehicle tracking system, you can be assured of the safety of your loved ones. Keep an eye them from afar with these high-tech tracking systems.

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GPS tracking device are available as handheld portables

Январь 7th, 2016 by homesecuirtycameras and tagged , , ,

GPS tracker mini are available as handheld portables, as stand- alone bracket mounted or in - dash models, and as “ black boxes ” combined with sonar devices. Sportfi shing boaters use GPS to get to and from selected locations and to determine the exact location of important fi shing grounds. The additional value to anglers is in pinpointing places to fish, schools of fish, or significant underwater structures, and being able to return to them unerringly. In some instances there is great value to pinpointing the specifi c part of an area to fish such as the riprap near a submerged wreck. The locations saved in GPS receivers are called waypoints. Once a waypoint has been saved, an angler can return to it at any time. Saving is most commonly done by pressing a button while the unit is located at the waypoint ’ s position; the position can also be saved by entering its latitude and longitude coordinates through the keypad.

These coordinates are important because with the lat/lon coordinates you can fi nd a place that you have never been to; the coordinates can be given to you, or you can take them off a good navigational chart. It’s a good idea to record waypoint information in a logbook as a backup, in case the unit is lost, stolen, or breaks down. You can reenter the waypoints manually after fixing or replacing your unit or acquiring another. Integrating with a personal computer will allow you to store appropriate information as well as plan trips.

No detailed study of vehicle GPS locator is complete without a study of the most commonly used protocol for communication between the GPS and other devices. Even if you have no intentions of writing applications that process or work with GPS data, and instead plan on using only readymade applications. Therefore, anglers who troll use various other instruments, referred to as speed gauges, speed sensors, or speed indicators, to accomplish this. These include electronic instruments fi tted with a paddlewheel, which is secured to the boat transom flush to the hull; GPS navigational devices; and non - electronic drag - weight gauges, which measure speed relative to markings on a plate. Additionally, there are devices, attached to downrigger weights, that measure via paddlewheel the speed of the lure at the depth of a downrigger weight. This is sometimes different than surface speed. The most accurate speed gauge is a GPS unit, which measures distance traveled over time. If you ’ ve used an automobile navigator you know how reliable this system is at determining speed. Most larger sportfishing boats are equipped with some type of vehicle GPS tracking systems, often in combination with a sonar device, and when trolling, the speed is always known and displayed.

Dustproof and Waterproof Car tracking GPS is a powerful and easy-to-use GPS tracker specially designed for motorcycle, electromobile and common vehicles. Operating voltage is from 7.5-90 VDC, which would be suitable for every vehicle on the road.  With GSM+GPRS+GPS technology, the device can easily achieve fast and accurate positioning. IP65 standard protects the device from rain and dust. Vibration detect, SOS button, geo-fence alarm and remote cut off power protects the vehicle in 360 degree.

Remote control ( Remote cut Off power/electricity )

Send SMS to the device to cut off the vehicle engine and circuit when someone startup the vehicle illegally in remotely.

Low voltage protection for battery

When your vehicle is laid-up, it will be in low battery to protect your vehicle.

Dustproof and Waterproof

It is waterproof and dustproof so that it can be protected well for a long time.

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Electronic tracking devices - about Multipath error

Январь 5th, 2016 by homesecuirtycameras and tagged , , ,

Multipath is a major error source for both the carrier-phase and pseudorange measurements. Multipath error occurs when the GPS ( personal GPS tracking devices ) signal arrives at the receiver antenna through different paths. These paths can be the direct line of sight signal and reflected signals from objects surrounding the receiver antenna. Multipath distorts the original signal through interference with the reflected signals at the GPS antenna. It affects both the carrier-phase and pseudorange measurements; however, its size is much larger in the pseudorange measurements. The size of the carrier-phase multipath can reach a maximum value of a quarter of a cycle. The pseudorange multipath can theoretically reach several tens of meters for the C/A-code measurements. However, with new advances in 32 Introduction to GPS receiver technology, actual pseudorange multipath is reduced dramatically. With these multipath-mitigation techniques, the pseudorange multipath error is reduced to several meters, even in a highly reflective environment.

Under the same environment, the presence of multipath errors can be verified using a day-to-day correlation of the estimated residuals. This is because the satellite-reflector-antenna geometry repeats every sidereal day. However, multipath errors in the undifferenced pseudorange measurements can be identified if dual-frequency observations are available. A good general multipath model is still not available, mainly because of the variant satellite-reflector-antenna geometry. There are, however, several options to reduce the effect of multipath. The straightforward option is to select an observation site with no reflecting objects in the vicinity of the receiver antenna. Another option to reduce the effect of multipath is to use a chock ring antenna (a chock ring device is a ground plane that has several concentric metal hoops, which attenuate the reflected signals). As the GPS signal is right-handed circularly polarized while the reflected signal is lefthanded, reducing the effect of multipath may also be achieved by using an antenna with a matching polarization to the GPS signal. The disadvantage of this option, however, is that the polarization of the multipath signal becomes right-handed again if it is reflected twice.

When a cell phone operator calls 911 to report emergency situation, it is desirable to locate the caller automatically. Often the caller is inside a building. If the building is equipped with a geolocation system, the caller’s location inside the building can be identified. The location of the building must also be identified. This combination of information is sent to the emergency unit receiving the call. Another approach is to use the GPS signal to locate the caller. However, inside a building the GPS ( GPS Tracking Device ) signal strength may be very weak.

One of the approaches is to use a GPS base station in the neighborhood to receive signals at normal strength. The GPS signals of normal strength are acquired and tracked. In other words, the initial phase of the C/A code, the carrier frequency, and the navigation data can all be obtained from the signals of normal strength. This information can be transmitted through the cell system to the cell phone users in the vicinity. The cell phone unit will include a GPS receiver. This GPS tracker can perform acquisition based on the received GPS information. Since the carrier frequency of the GPS is known only a very narrow frequency range need to be searched.

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The accuracy of a electronic tracker

Декабрь 30th, 2015 by homesecuirtycameras and tagged , ,

GPS Sources of accuracy

With satellites, the electronic signals travel about 300,000,000 meters per second (the speed of light). So the errors in the satellite clock and the receiver clock contribute profoundly to errors in distance measurements. One billionth of a second (one nanosecond) of inaccuracy in a satellite clock results in about 30 centimeters (one foot) of error in measuring the distance to that satellite. For this reason, the satellites are equipped with very accurate (Cesium) atomic clocks. Even these very accurate clocks accumulate an error of 1 billionth of a second every three hours. To resolve the satellite clock drifts, they are continuously monitored by ground stations and compared with the master control clock systems that are combinations of more than 10 very accurate atomic clocks. The errors and drifts of the satellites’ clock are calculated and included in the messages that are transmitted by the satellites. In computing the distance to the satellites, GPS 3G receivers subtract the satellite clock errors from the reported transmit time to come up with the true signal travel time. Even with the best efforts of the control centers in monitoring the behavior of each satellite clock, their errors cannot be precisely determined. Any remaining satellite clock errors accumulate typically to about a few nanoseconds, which cause a distance error of about one meter.

the accuracy of our computed position also depends on how accurately we know the location of the satellites (the points of references). The orbits of satellites are monitored continuously from several monitoring stations around the earth and their predicted orbital information is transmitted to the satellites, which they in turn transmit to the receivers. The history of GPS has shown, thus far, that the accuracy of the orbital prediction is in the order of a few meters. This will create about a few meters of error in computing our position.

GPS receiver

A fair number of geocachers use only their GPS tracking device for cars to get them to a cache, but a good local map of the area can be very helpful. Although a receiver can lead you directly in a straight line to cache, it’s probably not going to tell you about the river, deep canyon, or cliffs between you and the cache. Even GPS receivers that display topographic maps often won’t show enough detail that can help or hinder you on your way to a cache. Additionally, a map and compass serve as a backup just in case something goes wrong with your GPS.

You can certainly find caches by using only a map and compass (my adventure racing team does this to practice our navigation skills), but it’s sure a lot easier when using a GPS receiver. You don’t need an expensive GPS unit with lots of whistles and bells to geocache; a basic model around or under $100 will work just fine; receivers that support WAAS (Wide Area Augmentation Service) usually are more accurate than those that don’t. Don’t forget to bring the GPS receiver user manual, especially if you just purchased your receiver and are still trying to figure out how to use it.

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Using GPS Tracking Device will enhance your basic navigational skills

Декабрь 29th, 2015 by homesecuirtycameras and tagged , , ,

Most GPS receiver cases have a loop on the back to hold the case on a belt. If you’re wearing shorts or tights and don’t have a belt, waist packs for carrying a radio or a cassette/CD player are an option. These packs can accommodate larger GPS receivers. They’re not very noticeable when you’re working out because they’re designed not to bounce much. Small fanny packs and waist packs that carry water bottles also carry a GPS receiver. If the GPS receiver is small, try carrying it in your front pants pocket. I’ve carried a GPS while running and cross-country skiing in trail-running shorts and tights with zipper pockets. Although satellite reception is sometimes lost while under heavy tree cover, the GPS receiver records track data as long as I have a mostly clear view of the sky.

Using GPS will greatly enhance your understanding of the map-and-compass technique, but it’s wise to hone basic navigational skills before or in conjunction with use of a GPS tracking device. When out in the field, you will develop your own style of GPS use. You may want to plot a course ahead of time, upload it to your GPS, and follow that. You may want to upload only key points of your journey such as crucial trail intersections, waterfalls, high points, or shelter locations. You may want to carry the GPS along without uploaded data and navigate without the aid of pre-plotted track points or waypoints. Or you may just want to have the GPS to turn on once or twice a day to check your position against a map.

The Timex Speed + Distance system is a cigarette pack-size GPS magnetic tracker that straps on your arm and transmits data to a special wristwatch. You can look at your watch and see how far you’ve gone and how fast you’re going. Timex updated the system in 2003, adding more components and calling it Bodylink. In addition to the GPS receiver and watch, the Bodylink includes a heart-rate monitor (the GPS receiver, watch, and heart-rate monitor ) and a data recorder that collects data from both the GPS receiver and the heart-rate monitor. You can connect the data recorder to your personal computer to upload your workout data and analyze it with the included software.

Even with vehicle GPS tracking device technology becoming better every day, it is still a good idea to have backup navigation. Having a paper map, a simple compass,and knowledge of manual navigation is a good, safe practice of prudent navigators! Remember, GPS is a complement to navigation and should not be the only navigational tool you use.

The main disadvantage to carrying your GPS receiver on or below your waist is that it’s not the best place for satellite reception. There’s a good chance that you’ll lose the signal in areas with reduced satellite coverage. If you use a hydration pack or a lightweight backpack, you can get your GPS higher for better satellite reception by mounting the case either on one of the front shoulder straps or putting the GPS receiver in the upper, top pocket of your backpack. (It isn’t as accessible in the backpack pocket but should get good satellite reception.

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GPS tracking solutions —— basic knowledge about GPS system

Декабрь 28th, 2015 by homesecuirtycameras and tagged , , ,

Today, GPS (  GPS for vehicle ) is causing a renaissance of the navigation, surveying and mapping professions and may, within only a few years, completely replace conventional methods of transportation navigation and land surveying. The uses and implications of the GPS system are yet to be fully realized, and new applications are being found at an ever-increasing rate. There are three parts to the GPS system: the satellite segment, the user segment, and the control segment.

Control segment

This part keeps the whole system running smoothly. Satellites need to be kept in their proper orbits and their signal transmissions kept up-to-date. The Air Force operates a series of five ground stations around the globe, typically at exotic tropical locations: Hawaii, Ascension Island, Diego Garcia, Kwajalein, and the decidedly nontropical master station in Colorado Springs. You’ll probably never think about the control segment, but without it, the entire system would quickly fall into disrepair.

Satellite segment

Satellites are the heart of the Global Positioning System. They broadcast the signals your receiver uses to determine your position. At least 24 satellites are in operation at all times, each orbiting the earth every 12 hours (or 11 hours and 58 minutes, if you want to be precise). Their orbits are designed so that, theoretically, at least 6 and as many as 12 satellites are above the horizon virtually all the time, regardless of where you are. “Theoretically” is the key word here—the satellite signals don’t travel through mountains, buildings, people, or heavy tree cover, so unless you’re on a flat plain or body of water, some signals probably will be blocked. Since your receiver must lock onto at least four satellites to accurately determine its position, you may have to move around to get better reception. (By the way, it’s a little-known fact that all GPS satellites perform a second duty: Each includes an X-ray detector that lets the U.S. government monitor nuclear explosions anywhere in the world.)

User segment

Your handheld receiver makes up the user segment. There’s a lot of power inside that little package. Not only does it contain a sensitive receiver capable of detecting signals less than a quadrillionth the power of a light bulb, it also includes a powerful computer that converts the raw data into such useful information as your position and speed. A GPS magnetic tracker doesn’t include any kind of transmitter, meaning it is a passive positioning system—you can determine your own position, but there’s no way for anyone else to use it to track you.

However, accuracy can be improved by combining the electronic tracker with a Differential GPS (or DGPS) receiver, which can operate from several possible sources to help reduce some of the sources of errors described above. Differential GPS works by placing a GPS receiver (called a reference station) at a known location. Since the reference station knows its exact location, it can detennine the errors in the satellite signals. It does this by measuring the ranges to each satellite using the signals received and comparing these measured ranges to the actual ranges calculated from its known position. The difference between the measured and calculated range.

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How GPS tracker work - Time is of the Essence

Декабрь 25th, 2015 by homesecuirtycameras and tagged , ,

With progressive development in technology in recent years, one can relish the prime advantages of the various innovative software solutions such as field service tracking software. A lot of operational costs can be reduced using this software to improve the ROI. From various records, it is also proven that many industries have saved up to 30% using the field service tracking system. The best thing about using this software is that, it can be accessed from computers, smart phones or tablets. In addition to these types of GPS receivers, larger and less portable models are used in airplanes, boats, and commercial vehicles. The U.S. government and military uses restricted-use GPS units, and expensive receivers are used for surveying. Although some of these vehicle GPS tracking devices are discussed briefly, don’t expect to find out as much about them as the land and auto consumer models. I tell you what questions to ask when selecting a GPS receiver and give you some hints on which features are best for different activities. You can apply these questions and selection criteria to pretty much any GPS 3G tracker for Car to pick the right model for you.

In a conventional Personal GPS tracker the information, such as carrier frequency and initial C/A code phase obtained from the acquisition, can be used to track a signal. The navigation data phase transition can be found through the tracking program. For a weak signal it is difficult to determine the exact location of the navigation data transition from the tracking program. In order to process the data efficiently, the exact location of the navigation data phase transition is needed such that one can start to process the data every 20 ms. Thus in this section the determination of the location of the navigation phase transition is discussed. In general, it is easier to determine whether there is an existence of data transition than to determine the exact location of a data transition.

Even though the vehicle GPS tracking device knows the precise location of the satellites in space, it still needs to know how far away the satellites are (the distance)so it can determine its position on Earth. There is a simple formula that tells the receiver how far the satellites are (the distance)so it can determine its position on Earth.  There is a simple formula that tells the receiver how far it is from each satellite:Your distance from a given satellite object equals the velocity of the transmitted signal multiplied by the time it takes the signal to reach you (Velocity x Travel Time = Distance).

The satellites orbit at an altitude of approximately 20,200 kilometers, or about half the altitude of a geostationary satellite. A geostationary satellite, orbiting at about 40,000 kilometers altitude, circles the Earth every 24 hours, the same time period that the Earth takes to complete one full rotation (one day). Therefore, a geostationary satellite always remains over the same spot on the Earth (thus “geostationary”), essentially following that “spot” on the surface as the Earth rotates. The GPS satellites, at one-half that altitude, complete one orbit every 11 hours, 58 minutes (its “orbital period”). Since the Earth is rotating underneath the orbiting satellites, any given satellite’s orbit slowly “moves” slightly westward with each rotation.

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Causes of electronic tracking devices position errors

Декабрь 24th, 2015 by homesecuirtycameras

Technology has drastically advanced and unlike the past when people needed a special satellite GPS receiver to track movements, today the 3G GPS are found in smart phones and other small gadgets. They work by connecting to the internet connection as well as receiving a signal directly from the GPS satellite, which helps pinpoint the exact location of the GPS signal. This has proven to be a truly important tool for automobile owners since the Car GPS tracker can be installed in a secret location on a vehicle allowing security personnel to track the signal from the vehicle and locate the vehicle.

Now the GPS receiver needs to determine the time part of the formula. The answer lies in the coded signals the satellites transmit. The transmitted code is called “pseudo-random code” because it looks like a noise signal. When a satellite is generating the pseudo-random code, the vehicle GPS tracking devices is generating the same code and tries to match it up to the satellite’s code. The receiver then compares the two codes to determine how much it needs todelay (or shift) its code to match the satellite code. The accuracy of our computed position also depends on how accurately we know the location of the satellites (the points of references). The orbits of satellites are monitored continuously from several monitoring stations around the earth and their predicted orbital information is transmitted to the satellites, which they in turn transmit to the receivers. The history of GPS has shown, thus far, that the accuracy of the orbital prediction is in the order of a few meters. This will create about a few meters of error in computing our position.

Causes of GPS position errors

The global positioning system is afflicted by many technical problems and uncontrollable natural phenomena that affect the accuracy of a GPS position fix. A worst-case scenario would be if your Personal GPS tracker broke down or the whole GPS system stopped working. Some less severe but common causes of position error are discussed below.

1. Deliberate signal degradation

The people who operate the global positioning system are able to deliberately degrade the timing of the signature signals to selected parts of the world. A less accurate position reading in that region would result. The position error can be made arbitrarily large.

2. Satellite distribution

Because of the inherent constraints of geometry, the distribution of the visible satellites affects the accuracy of your position. Ideally, all the visible satellites should be evenly spaced across the sky. In practice, the satellites are never perfectly distributed. The position accuracy degrades in proportion to how close the visible satellites are clustered together.

3. Signal obstructions

Obstructions such as trees or adjoining terrain that block the satellite signals are the most common cause of position errors or inability to get a position fix. The greater the number of satellite signals that reach your receiver and the stronger the satellite signals, the better the accuracy. Most receivers attempt to calculate a two dimensional position fix after obtaining signals from just three satellites. The receiver does this by assuming that it is at a known elevation, usually sea level or the elevation of its last position fix. If the assumed elevation is significantly wrong, the calculated two-dimensional position could be off by a kilometer or more. Manually inserting the correct elevation into the receiver will alleviate this problem

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