GPS Global Positioning System

Transcription

1 GPS Global Positioning System

2 GPS Accuracy Error sources Differential correction GPS Accuracy levels TerraSync settings that affect accuracy of GPS measurements These topics will be covered in this slide set.

3 Atmospheric effects Multipath Satellite geometry Measurement noise Ephemeris data Satellite clock drift Selective availability (SA) GPS Error The accuracy of GPS is determined by the sum of several sources of error. The travel time of GPS satellite signals can be altered by the ionosphere and the troposphere. Multipath error occurs when GPS satellite signals bounce off other objects before reaching the receiver antenna. Satellite geometry affects the quality of GPS positions computed by the receiver. Measurement noise is distortion of the signal by electrical interference (occurring at the receiver antenna) or receiver error. Measurement noise is also called receiver error or receiver noise. Orbital position errors may be present in the ephemeris data. The ephemeris for a particular satellite is simply a list of the satellite's positions as a function of time. It tells where the satellite will be and when. Small variations in the satellite atomic clocks can translate to large position errors: a clock error of 1 nanosecond translates to 1 foot or.3 meters user error on the ground. Selective availability (SA) was intentional scrambling of the GPS satellite signals by the U.S. government. SA was turned off at midnight on May 1, Some of these error sources will be discussed in more detail in the following slides.

4 Atmospheric Effects The troposphere is the lower part of the earth's atmosphere where temperature decreases with an increase in altitude. It can be < 9 km thick over the poles and >16 km thick over the equator. This is the part of the atmosphere where our weather occurs. The presence of neutral atoms and molecules in the troposphere affects electromagnetic signal propagation. For mapping grade GPS data collection (10 cm to 5 meters) the troposphere has little effect. For survey grade (cm or better) tropospheric effects can be a consideration. The travel time of GPS satellite signals can be altered by the ionosphere and the troposphere. The ionosphere and troposphere both refract (or bend) GPS signals, causing the speed of the GPS signal to be different from speed of a GPS signal in space. In the ionosphere, which ranges from about 50 km above the earth's surface to about 1,000 km or more, ionizing radiation (principally from solar ultraviolet & x-ray emissions) causes electrons to exist in sufficient quantities to affect radio-wave propagation. The ionosphere contributes substantially to atmospheric error, which is the larges source of GPS error.

5 Earth s Atmosphere This diagram gives you and idea of the location and width of the various layers of the Earth s atmosphere.

6 Multipath Multipath is the phenomenon whereby a signal arrives at a receiver's antenna by way of two or more different paths. The difference in pathlengths causes the signals to interfere with each other at the antenna and to contribute an error to the pseudorange observable. This phenomenon is similar to a "ghost" or double image on a TV set. Multipath is usually noted when operating near large reflecting obstacles such as buildings and fences, but signals can also reflect off the ground and roofs.

7 GDOP can magnify or lessen other GPS errors Wider angles better measurements Components of GDOP HDOP (latitude/longitude) VDOP (altitude) TDOP (clock offset) Geometric Dilution of Precision (GDOP) Satellite Geometry Satellite geometry can affect the accuracy of GPS positioning. GDOP is a measure of the quality of the satellite configuration and refers to where the satellites are in relation to one another with respect to your position on Earth. More notes on satellite geometry for those who are interested in the difference between DOPs: HDOP (Horizontal Dilution of Precision) refers to horizontal measurements (lat, lon). VDOP (Vertical Dilution of Precision) refers to altitude. TDOP (Time Dilution of Precision) refers to clock offset. GDOP refers to three position coordinates plus clock offset in the solution. It is a measure of the quality of a geometric constellation for position and time solutions. GDOP 2 = PDOP 2 + TDOP 2

8 Dilution of Precision (DOP) The ideal orientation of four or more satellites is to have one satellite directly overhead and the other three evenly spaced above the receiver. In the diagram at the right, all the satellites are clustered together in one quadrant of the sky. This would result in a poor DOP value. A low DOP value represents a good satellite configuration, whereas a higher value represents a poor satellite configuration. The DOP changes with time as the satellites move along their orbits.

9 Uncertainty in GPS Measurements Trees, buildings, bridges, mountain ranges, your hand (over the receiver antenna) or your body can all block the satellite signals. Heavy forest canopy causes interference, making it difficult to compute positions. In canyons (and "urban canyons" in cities) GPS signals are blocked by mountains or buildings. The width of the ring in this diagram represents the error or uncertainty in GPS distance measurements

10 Uncertainty in GPS Measurements When the satellites are close together in the sky, the angle is narrow. And, when the rings are close together, the box representing the area of uncertainty becomes long and pointed: there is greater uncertainty in the position calculation. Most receivers select the satellite constellation that gives the least uncertainty.

11 Vertical Accuracy The vertical component of a GPS measurement is typically two to five times less accurate than the horizontal component. Why is this? It's strictly a matter of geometry: it's a function of where the satellites are with respect to your position. Here is an analogy from the rodeo circuit: Imagine a bull jumping up and down in the rodeo arena. Four cowboys and cowgirls can restrict the movement of the bull in a horizontal direction, but it can still jump up and down in the vertical direction, because there is no cowboy/cowgirl roping it from beneath the arena grounds (the earth is beneath us). (The cowboys and cowgirls represent satellites and the bull represents your GPS position on the earth). The vertical component is difficult to calculate because satellites have a limited perspective from which to measure height. If the receiver could use signals from underneath it, the vertical component could be accurately measured, but the Earth blocks these signals. The arcs (representing satellite measurements) are more vertical than horizontal (they are almost vertical if the satellite is on the horizon), so there is more error in the vertical component. You can strengthen the geometry by putting a satellite directly overhead (this is usually not the case). VDOP becomes larger with fewer satellites overhead.

12 PDOP refers to horizontal (HDOP) and vertical (VDOP) measurements (latitude, longitude and altitude) PDOP values <=4 excellent 5-8 acceptable >=9 poor PDOP 2 = HDOP 2 + VDOP 2 Satellite Geometry Position Dilution of Precision (PDOP) is the most commonly used measure of satellite geometry. It is a unitless measure that refers to the quality of horizontal (HDOP) and vertical (VDOP) measurements (lat/lon/alt). PDOP predicts the accuracy of positions relative to satellite geometry. A low PDOP indicates a higher probability of position accuracy. A high PDOP indicates a lower probability of accuracy. You can set a parameter known as the PDOP mask on your receiver to ignore constellations that have a PDOP higher than the limit you specify. However, with the new Trimble postprocessing engine, we no longer use masks to restrict GPS settings in the field. We use wide-open settings and the processing software filters out bad measurements to give you the best accuracy possible. However, when mapping using real-time DGPS, you should use masks to restrict GPS settings in the field, providing you are NOT planning to post-process your data.

13 Ephemeris Data A list of the satellite s positions as a function of time Each satellite broadcasts its individual ephemeris Almanac vs. ephemeris Almanac = predicted orbit data for all satellites Ephemeris = precise orbit data for an individual satellite The ephemeris for a particular satellite is simply a list of the satellite's positions as a function of time. It tells where the satellite will be and when. Each satellite broadcasts an individual ephemeris that is updated continuously. The ephemeris contains satellite locations that have been computed from orbit measurements, along with corrections that are transmitted to the satellites by the DoD. The ephemeris is used by receivers, along with almanac data, to establish precisely the position of each satellite being tracked. The ephemeris message contains orbit data for an individual satellite, whereas the almanac contains orbit data for all the GPS satellites. Remember that the almanac is a prediction of the orbits of all satellites and can be obtained from any GPS satellite. A GPS receiver automatically collects an almanac each time it is in operation for about 12.5 minutes. The almanac is also used for quick acquisition of satellite positions by the receiver. Orbital position errors may be present in the ephemeris data, causing errors in the positions a GPS receiver calculates.

14 The accuracy of GPS signals was intentionally degraded by the DoD SA was the largest component of GPS error SA was turned off on May 1, 2000 Selective Availability (SA) SA, implemented for national defense reasons, introduced artificial clock and ephemeris errors. This caused position errors up to 70 or even 100 meters. (SA was activated on July 4, 1991). On May 1, 2000, President Clinton announced that SA would be discontinued. According to the President, "the decision to discontinue Selective Availability is the latest measure in an ongoing effort to make GPS more responsive to civil and commercial users worldwide. This increase in accuracy will allow new GPS applications to emerge and continue to enhance the lives of people around the world." New technologies developed by the military enable the U.S. to degrade the GPS signal on a regional basis for national security purposes, making the worldwide degradation unnecessary. According to the White House Office of the Press Secretary, "GPS users worldwide would not be affected by regional, security-motivated GPS degradations, and businesses reliant on GPS could continue to operate at peak efficiency."

15 Ionosphere meters Troposphere meters Ephemeris data meters Satellite clock drift meters Multipath meters Measurement noise meters Selective availability meters Numbers indicate possible error in meters US Government accuracy specification: 7.8 meters (95%) GPS Error Budget Ionospheric and tropospheric error are referred to collectively as atmospheric error. These numbers indicate possible error in meters. Remember that satellite geometry, an additional source of GPS error, can magnify or reduce the effects of other GPS errors. The official government horizontal accuracy specification for GPS is <=7.8 m (95% confidence level) during normal operations. Remember, vertical accuracy is always worse than horizontal accuracy. How can we cancel out some of this error and achieve better accuracy with GPS receivers? The solution is explained in the next slide.

16 GPS receiver on the ground in a known location (base station) Acts as a static reference point Transmits error correction messages to other GPS receivers in the local area (realtime), or Differential correction can be done on computer after GPS data are collected (post-processed) Differential Correction Differential correction reduces the effects of some GPS errors. It cannot correct for multipath or receiver error because it counteracts only errors that are common to both reference and roving receivers. With real-time differential correction the base receiver computes timing errors and transmits error correction messages to other GPS receivers in the local area. With post-processed differential correction, the base receiver computes timing errors and stores the error data to a file. The rover file is later processed using differential correction software which uses error data from the base file to correct the rover data. The base and rover receivers have to "see" the same set of satellites at the same time, so the base file has to start before the rover file starts and end after the rover file ends.

17 Recreational and mapping grade m (95%) C/A code Autonomous Recreational and mapping grade m C/A code With differential correction Submeter mapping grade...10 cm to 1 m C/A code & carrier With differential correction Survey grade...1 cm Dual frequency Advanced survey methods How Accurate is GPS? GPS accuracy depends on many factors including type of equipment, time of observation, and position of satellites. With autonomous data collection, no differential method is applied. The largest source of error with uncorrected positions is atmospheric delay. Remember, the government specification for GPS accuracy is 7.8 meters (95% confidence level). Recreational and mapping grade receivers using real-time or post-processed differential correction can achieve from 1-5 meter accuracy. Real-time correction is usually less accurate than postprocessing. The accuracy specification for real-time WAAS is 1.6 m (95% confidence level). Carrier phase receivers measure the distance from the receiver to the satellites by counting the number of waves that carry the C/A code signal; some receivers use what is called "carrier-smoothed code" to increase the accuracy of the C/A code. The carrier wave is a much finer measuring tool than the superimposed code (19 cm vs m), so it yields more accurate satellite ranges. Dual frequency survey grade receivers can provide cm or even mm accuracy using network surveys or real-time kinematic (RTK) methods.

18 Type of receivers used at rover and base Internal components (chipset, antenna, multipath rejection technology, ) Grade (mapping, sub-meter mapping, survey grade) Number of visible satellites (influences PDOP) Occupation time at a point Additional Factors Affecting Accuracy Additional factors affecting accuracy of positions collected using GPS are listed here. It is important to research the type of equipment and receiver settings needed to achieve a specified level of accuracy. Be sure you know the accuracy specifications for receivers that you use, and be careful to correctly report the accuracy of your data. Theoretically, a longer occupation time, yielding more positions to average, should result in better accuracy. That said, there is a point at which extending the occupation time yields little increase in accuracy.

19 Autonomous m Real-time differential 1.. < 1 m + 1 ppm Real-time differential cm + 1 ppm Post-processed differential. 50 cm + 1 ppm H-Star postprocessed cm + 1 ppm GeoXH Horizontal Accuracy All accuracies specified are expressed as RMS error (63-68% confidence level), except autonomous which is expressed as 2D RMS error (95% confidence). 1 Code corrections, such as from WAAS 2 Carrier corrections, such as from MSU GPS base station (with the GeoXH, these must be accessed via the Internet or using an external radio we don t use that equipment in this class) The following factors increase the availability of decimeter (10 cm / 4 inch) accuracy after H-Star postprocessing: longer elapsed time tracking uninterrupted L1/L2 carrier phase data, use of the optional external Tornado or Zephyr antenna, tracking of more satellites with L2 measurements, shorter distance to the base station(s), and use of more (than one) base stations for postprocessing.

20 Autonomous m Real-time differential (code corrections) m Post-processed code differential.. 50 cm 1.5 m H-Star postprocessed (internal antenna) cm H-Star postprocessed (external antenna) cm GeoXHVertical Accuracy All accuracies listed are 1.5 times worse than horizontal, except H-Star postprocessed with an appropriate external antenna 1 Zephyr or Tornado antenna

21 Setting Real-Time Post-Processed Logging interval 1 second or 5 seconds, user preference Elevation mask 15 5 SNR mask* * Typical values range from 33 to 43 dbhz PDOP mask** 6 20 **Typical values range from 1 to 30 These settings can be configured automatically by moving the productivity/precision slider bar as follows: Real-Time: move the slider bar to the middle position Post-Processed: move the slider bar to the far left (maximum productivity) 1 1 This causes the receiver to collect all possible data records that don t contribute to a high quality position solution are filtered out during postprocessing TerraSync Settings Several TerraSync software settings affect the GeoXH receiver operation, and the resulting accuracy of collected positions. Recommended settings for real-time and post-processed differential correction are listed here. A logging interval defines the frequency at which positions are stored. Generally, positions for point features are logged at 1- second intervals and positions for line and area features are logged at 5 second intervals. If you are driving, line and area features can be logged at 1-second intervals. An option called Log between features stores positions even when you are not collecting features. Elevation mask restricts the receiver to using only those satellites that are above a certain elevation in the sky. There is an increased chance of encountering atmospheric error and multipath effects when tracking satellites low on the horizon. SNR stands for signal-to-noise ratio and is a measure of a satellite s signal strength. When SNR is low, the quality of computed position is degraded. Causes of weak signals include local environment such as canopy penetration, multipath and satellites at a low elevation. A PDOP mask prevents the receiver from using satellites whose PDOP is too high. Trimble recommends using a PDOP mask of 6 for real-time differential correction.

22 Atmospheric effects Multipath effects Satellite geometry Measurement noise Ephemeris data Satellite clock drift 6 Main Sources of GPS Error Here are the six main sources of GPS error (SA is no longer a factor). Which two of the 6 GPS errors cannot be corrected by differential correction? Hint: Only errors common to the base and rover can be corrected.

23 5 Things to Take Away Today 1. 6 sources of error (and additional factors) affect the accuracy of GPS positions 2. Atmospheric error is the largest source of GPS error (and the ionosphere is the largest contributor to atmospheric error) 3. Almanac and ephemeris data are different know the purpose of each 4. Vertical accuracy is usually worse than horizontal accuracy 5. Know the TerraSync data collection settings for real-time vs. post-processed differential correction