GPS error source

• GPS Error Sources


  Accuracy | Precision
• Errors in GPS and Applications range from:
• GPS Accuracy


  Prior to the de-activation of the Selective Availability accuracies to within around 100 metres could be obtained. Afterwards, the systems accuracies to within 15 metres could typically be obtained.
• Factors affecting GPS accuracy


  • Number and position of the satellites
  • Design of the receiver - parallel multi-channel receivers are able to provide significant improvements over earlier systems
  • Effectiveness of the Differential GPS methodology
• Sources of error in GPS


  • Ionospheric effects (±5 meters)
  • Shifts in the satellite orbits (±2.5 meter)
  • Clock errors of the satellites' clocks (±2 meter)
  • Multipath effect (±1 meter)
  • Tropospheric effects (±0.5 meter)
  • Calculation- and rounding errors (±1 meter)
• Altogether these sums up to an error of ±15 meters.
• Corrections by systems like WAAS and EGNOS, which mainly reduce ionospheric effects, but also improve orbits and clock errors, the overall error is reduced to approximately ±3-5 meters.

  
• Ionospheric Error


  The biggest error in the GPS system after deactivation of the Selective availability. It causes the delay in the signal transmission, from between 80-640km radius of signal entry from space to earth.

  
• Ionosphere and Other Gasses the Signal Passes Through


  Ionosphere starts at about 70-80 km high and continues for 640 km. It contains many ions and free electrons (plasma). Ions are created when sunlight hits atoms and tears off some electrons. Auroras occur in the ionosphere and also have an effect on the ionosphere. The density of the Ionosphere is affected by the sun. At night, there is very little ionospheric influence. In the day, the sun increases the effect of the ionosphere and slows down the signal. The amount by which the density of the ionosphere is increased varies with solar cycles (sunspot activity). Sunspot activity peaks approximately every 11 years. In addition to this, solar flares can also randomly occur.

  
• Exosphere


  The outermost layer of the Earth's atmosphere, going from about 640 km high to about 1,280 km. The lower boundary of the exosphere is called the critical level of escape, where atmospheric pressure is very low and the temperature is very low.

  
• Thermosphere


  A thermal classification of the atmosphere. In the thermosphere, temperature increases with altitude. The thermosphere includes the exosphere and part of the ionosphere.
• Troposphere


  The lowest region in the Earth's atmosphere, going from ground (or water) level up to about 17 kilometers high. The clouds occur in the troposphere. The temperature in that region generally decreases as altitude increases. The total stations on the earth's surface also has effect from the Tropospheric gas. From the lowland to the highlands the pressure of millibars on total stations are applied appropriately in order to compute and measure distances accurately as one goes from the sea level to higher levels of the topography and atmosphere.

  
• Mesosphere


  Characterized by temperatures that quickly decrease as height increases. The mesosphere extends from between 31 and 50 miles (17 to 80 kilometers) above the earth's surface.

  
• Stratosphere


  Characterized by a slight temperature increase with altitude and the absence of clouds. The stratosphere extends between 11 and 31 miles (17 to 50 kilometers) above the earth's surface. The earth's ozone layer is located in the stratosphere. Ozone, a form of oxygen, is crucial to our survival; this layer absorbs a lot of ultraviolet solar energy. Only the highest clouds (cirrus, cirrostratus, and cirrocumulus) are in the lower stratosphere.

  
• Tropopause


  The boundary zone (or transition layer) between the troposphere and the stratosphere, characterized by little or no change in temperature.

  
• Propagation errors


  Errors caused by the Atmospheric pressure of which the GPS signal passes through, the ionosphere and troposphere.

  
• It is only possible to estimate the average errors that are likely to be encountered. Only with precise GNSS research/technical software such as Garmit, and do corrections for these corrections. Local conditions may also alter the validity of these calculations.
• Satellite and Receiver clock errors


  Even the clocks in the satellite are very accurate (to about 3 nanoseconds), they do sometimes drift slightly and cause small errors, affecting the accuracy of the position. The US Department of Defense monitors the satellite clocks using the Control Segment and can correct any drift that is found. The clock inside the receiver will be nowhere near as accurate as the four atomic clocks on board the satellite, which can introduce some small errors.
• 1,000,000 nanoseconds = 0.001 second
• Multipath Errors


  Multipath occurs when the receiver antenna is positioned close to a large reflecting surface such as a lake or building. The satellite signal does not travel directly to the antenna but hits the nearby object first and is reflected into the antenna creating a false measurement.
• Reducing Multipath Errors


  • Use of special GPS antennas that incorporate a ground plane (a circular, metallic disk about 50cm in diameter) that prevent low elevation signals reaching the antenna (the Chockering Antenna). None of the handheld receivers have chokering antenna built into the handheld.
• The time taken for the deviated signal is longer to register on the receiver while the straight signals travel directly to the receiver.
• Clock inaccuracies and rounding errors


  Despite the synchronization of the receiver clock with the satellite time during the position determination, the remaining inaccuracy of the time still leads to an error of about 2m in the position determination. Rounding and calculation errors of the receiver sum up approximately to 1m.
• Satellites Visible


  • More satellites a GPS receiver can "see," the better the accuracy. Buildings, the natural terrain, electronic interference, or sometimes even dense foliage can block satellite signals. These cause positional error or possibly no position reading at all.
• Satellite Geometry/Shading


  Ideal satellite geometry exists when the satellites are located at wide angles relative to each other. Poor geometry is when the satellites are clustered in one area of the sky.
• Dilution of Position (DOP)


  Describes the likely accuracy of position by the GPS receiver according to the satellite geometry in space. Different variants are GDOP (Geometric Dilution Of Precision), PDOP (Positional Dilution Of Precision), HDOP (Horizontal Dilution Of Precision), VDOP (Vertical Dilution Of Precision), and TDOP (Time Dilution Of Precision). HDOP values below 4 are good, above 8 bad. HDOP values become worse if the received satellites are high on the firmament. VDOP values on the other hand become worse the closer the satellites are to the horizon and PDOP values are best if one satellite is positioned vertically above and three are evenly distributed close to the horizon. For an accurate position determination, the GDOP value should not be smaller than 5.
• DOP Value Rating


  • Ideal
  • Excellent
  • Good
  • Moderate
• GPS accuracy is far greater than anything that was previously available, and it is sufficiently accurate for most applications. However there are GPS errors that have been significant for some applications, and much work has been undertaken to reduce the level of GPS errors to a level where they are insignificant.
• Scatter plot


  The plot of the dispersion of the indicated GPS positions over time, which manufacturers use to determine the accuracy of the GPS equipment.
• GPS accuracy


  The degree of closeness the indicated readings are to the actual position.
• GPS precision


  The degree to which the readings can be made. The smaller the circle of unknown the higher the precision.
• Sometimes other abbreviations may be seen: HDOP, VDOP, PDOP, and TDOP are abbreviations for Horizontal, Vertical, Positional (3D), and Time Dilution of Precision.