Gps and Common Errors - Essay Example

GPS and Errors No: GPS and Errors Global Position System for land surveyors and engineering demands high degree of accuracy to find accurate and error free locations. Accordingly, GPS receivers have been tested, upgraded and modernized to meet this requirement. Despite all the efforts, certain factors like, weather, dense forest, terrain, location of satellite, position of user in relation to satellite are the factors which still require some attention to enhance the performance. GPS used for land surveyors passed through a period of revolution to eradicate the problems faced in accuracy. High cost receivers produce accuracy of 1 centimetre under dense clouds and forests. Still GPS receiver produce problems of signal generation, satellite communication and certain multiple problems still require solution. There are many features, which may be used to increase accuracy, but some times, it is more time consuming for the users. Now there is a point of concern that, it would be appropriate for the users to work with less stringent settings to save time and money and similarly gaining better accuracy. All the GPS related problems and solution to those problems are discussed in ensuing paragraphs. Errors GPS works out position with the help of 24 satellites orbiting the earth. These satellites are operated by US department of Defence. These 24 satellites orbit earth twice a day and organized at six orbital planes that are inclined at 55 degree. Now there are clocks in receiver and satellite as well, used for calculating the distance and time that a signal takes to reach receiver. Different enhanced settings used by user help in increasing the time of a signal to reach receiver. Limited settings are the remedy to counter this problem for saving time in position calculation (Frank, 2011, 83) GPS faces two types of interference in the open; these are faced in ionosphere and troposphere. In this type of error, signals take long time to reach receiver because they have to pass through the charged particles and in troposphere, signals have to pass through moisture that is time taking. Signals travel at the speed of light in upper space but they get slower the moment they pass through ionosphere and troposphere. Force of sun creates the positive charged particles at the height of 80 to 400 Kilometres. These ions form layers in the ionosphere, which disturbs the flow of electromagnetic waves generated by satellites (Kennedy, 2010). Multiple errors occur when some amount of signals transmitted by satellite bounce back in the atmosphere before reaching a receiver. This can result in variation because signals get divided in two part and reach receiver at two different times (Kennedy 2010). Such incidents are likely to occur in desert area where the delayed signals reaching receiver can cause error. These can be faced in large building areas, which can interfere in signal reception or in high elevated areas. Few decades ago, television antennas were used for signal reception instead of satellite dish, this used to cause ghost image at the television. In such cases, the difference of indirect signals reaching the receiver can be up to few meters (Van Sickle, 2008). Furthermore, trees in forested areas can reduce the exposure of receiver to multiple satellites. Same disturbance may be felt while travelling in a vehicle where GPS takes long time to locate the satellites. This obstruction to satellites can enlarge the horizontal error as data received from less number of satellites may be rendered as inaccurate (Kennedy, 2010). There are certain orbital errors associated with the position of satellites at a given time. Despite knowing the satellite position very precisely, there are still chances of minor shifting due to gravitational force. For example, sun and moon also have less control on their orbital position. Besides all the efforts and updating of satellite positions, inaccuracies still occur. Both satellites and receivers require very accurate clocks to measure signal timings. The smallest errors in range measurement can cause large inaccuracies which my go up to thousand of meters. For example, a 10 nanosecond error in clock timings may result into inaccuracy of 3 meters. Modern GPS are provided with the feature of selective availability (SA). Use of this optional causes intentional error of approximately 100 meters in navigational systems. This is done with a clear intention of denying civil users the precise location of weapons (Van Sickle, 2008). SA was turned off in March 2000 which caused vertical error of 50 meters and horizontal error of 100 meters. When SA is turned off, it affects every GPS receiver equally in a specific area. Ephemeris data transmitted usually have low accuracy to show false and inaccurate position of satellite at a particular time. Inaccuracy of about 50 to 150 meters can be achieved for few hours. Safety was the only concern behind the reason of SA. Terrorists in the world could not get precise locations of large buildings and weapon installations for destroying them by using different means (Van Sickle, 2008). Another example is Gulf War where American troops did not have many military GPS and SA was deactivated for some time. American troops and families had to purchase and rely on the civil GPS. More than 10000 civil GPS were acquired to navigate in deserts with great accuracy (Frank, 2011). Another problem associated with satellite and GPS inaccuracies is relativity effect. Time is the most important factor in navigation. Time should be accurate to near 20 to 30 nanoseconds to get the acceptable accuracy in navigation. In this connection the fast pace of satellites in the space must be kept in mind. According to theory of relativity, time runs slower during very fast movement. If satellite is moving at the speed of 3874 m/s then clock moves slower on the earth. This relativistic point results in accuracy of 7.2 microseconds per day (Kennedy, 2010). According to theory time moves slower when there is strong gravitational force. In this case, the satellite clock seems to be faster to an observer on the earth. There is another relativistic effect known as Signac- effect, which is related to movement of an observer on the ground. Who moves with the speed of 500 m/s due to rotation of earth. This effect is very small in nature and difficult to calculate as it depends on the direction in which movement has taken place. Hence, this is only considered in cases of delicate nature (Kennedy, 2010). Solution to Errors Differential correction is a method used for eradicating atmospheric and other related errors by using a ground based receiver. These corrections are applicable during real time data collection and in post time as well. Ground station uses two ways to correct roving receiver. First method is known as real time differential correction and second method is post-processed differential correction. In the first case the position is read from the GPS position but that position is received from ground station after corrections. In second case, corrections are performed on a computer after collecting data from GPS. Therefore, differential correction software is used to correct the data received from the GPS. Timing errors are worked out by base station and than incorporated in computer for finding out accurate position. The main aim of keeping a base station is to calculate timing error. Timing error is difference of seconds, in receiving satellite signals in roving receiver and base station receiver (Van Sickle, 2008). The ground-based receiver is used to improve performance of other receiver to get accurate results. Ground receiver acts as a “static reference point” that gets more accurate data than roving receiver. Ground receiver continuously keeps record of location that is influenced by the atmosphere. Difference between the present location and perceived location are calculated and ground receiver for accuracy applies corrections. Now it is important that ground and roving radars receive data from same set of radars (Frank 2011, 83-84). Receivers carry out different calculations to correct atmospheric errors. The variations faced by the signals while passing through ionosphere for low and high frequencies are already known for standard conditions. All such variations are catered for when calculations for working out positions are in progress. However, military receivers are much capable of performing such tasks and civil receivers cannot correct unpredicted runtime changes, for example by strong solar winds (Grewal et al, 2001). It is a well known fact that electromagnetic waves get slowed down and are inversely proportional to the square of their frequency (1/f2) when signals pass through the ionosphere. It means frequency of waves decides strength of signals. Lower frequency electromagnetic waves suffer reduction in signal strength while passing through ionosphere. Similarly, electromagnetic waves with higher frequency travel at fast speed than low frequency waves. Ionosphere runtime of signals can easily be calculated if signals of higher and lower frequencies are calculated as per their time interval of reaching receivers. Military GPS receivers are comparatively more accurate as they use both frequencies (L1 and L2) which are influenced in different ways by the ionosphere. Therefore inaccuracies are easy to eliminate from such systems (Van Sickle, 2008). Multiple errors can only be eliminated if high grade and costly survey receiver are used. Only the strong receiver having the capability to attract desired frequency of signals can avoid multiple errors. Orbital data of satellites is being continuously controlled and corrected time to time. This data is sent to receivers in the form of ephemeris data. Such errors are small in number and normally do not exceed more than 2 meters. Keeping in view the time errors caused by the satellite and receiver clocks, it is imperative to synchronize both the clocks before generating any data (Grewal et al, 2001). A fixed station which precisely knows its own position, can help other GPS receiver in the area to correct SA error. This method is known as differential GPS that has already been discussed in this essay. Besides eradicating the error of SA, it also corrects certain other multiple errors. President Clinton finally gave order of turning off SA because of the fact that inaccuracies caused by SA can easily be countered by using differential GPS method (Grewal et al, 2001). SA was permanently deactivated after the Gulf War and GPS systems got more fame due to their accuracy and speed. WAAS and EGNOS systems can be used to counter relativistic effects. Besides reducing ionospheric effects, it also counters clock and orbit errors. This system reduces the overall error from 3 to 5 meters (Kennedy, 2010). GPS is a very reliable source of navigation for land survey. Despite all the errors it is yet accurate and efficient system which has man tasks to perform. If solutions given above are catered for than it would be possible to make GPS more reliable medium of navigation in future. Bibliography Frank Jereme M (2011) Mapping-Grade GPS Accuracy in Second Growth Douglas-fir Forest. Oregon State University. Grewal MS, Weill, LR and Andrews AP (2001). Global positioning systems, inertial navigation, and integration. London: John Wiley and Sons. Kennedy M (2010) The global positioning system. 3rd Ed. Boca Raton: CRC Press. Van Sickle J (2008) GPS for Land Surveyor. 3rd Ed. London: CRC Press. Read More