This module explains how GPS works, sources of error, and error correction using real time or post processing differential correction. Cost and accuracy of different grades of GPS units are also part of this module.

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The Global Positioning System (GPS) was first launched in 1978 by the Department of Defense. The system includes a constellation of satellites (NAVSTAR) and ground control stations. GPS can be used by anyone and for any application that requires location information.

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GPS requires three components: Direct connection to at least 4 NAVSTAR satellites, a GPS receiver, and location on or near (airplanes) the earth’s surface.

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GPS satellite constellation

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GPS satellites and orbits

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Locations of GPS ground monitoring stations. These stations controls and monitors the satellite orbits and function.

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GPS receivers calculate theirGPS receivers calculate their position by measuring the time it takes for the signal to travel from the satellites to the receiverthe satellites to the receiver. Satellite signals require a direct line to the GPS receiver. Signals can not go through water, soil, walls ornot go through water, soil, walls or others.

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The time it takes from the GPS signal to travel from the satellite to the receiver is measured and then converted to distance by multiplying with the speed of light. Distance measurements form 4 satellites is necessary to compute a 3-d position (including a reasonably accurate measure of elevation). A lower accuracy 2-D position can be computed from distance measurements from three satellites. Differential correction can be applied to the signal to correct for atmospheric and other errors. We will talk more about differential

i l i hi lcorrection later in this lecture.

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The basic concept behind GPS is very simple and is based on a process called triangulation. One distance measurement narrows down the position to the surface of a sphere.

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The second measurement narrows down the position to the intersection of two spheres.

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A third measurement narrows down the position to only two points. A fourth measurement narrows down the location to only one location. You can get a reasonable (less accurate) estimate of location with only three satellite measurements, however four satellite measurements are necessary for computation of altitude.

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GPS receivers and the NAVSTAR satellites are generating a synchronized code at exactly the same time. The receiver can back-calculate the time it took for the signal to travel from the satellite to the receiver by comparing the signal generated by the receiver and the signal sent from the satellite.

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Satellites transmit two signals L1 and L2. L1 - carries precision code (P-code) along with a coarse acquisition code (C/A-code), L2 - carries only the P-code. The P-code at a much higher frequency (10.23 MHz) - generates a much more accurate position compared to the C/A-code at 1.023 MHz.High end mapping units, survey units and military units can receive the more accurate P-code while recreational grade units only receives the C/A-code.

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Many contributing factors add to the error associated with acquisition of a GPS location. In the next few slides you will explore the sources and influence of the different errors on the total GPS error.The last bullet in the slide addresses the error introduced by ‘Selective Availability’. Selective Availability was a ~100 m error introduced to the GPS signal by the Department of Defense. Selective Availability was turned off in May 2000, and is currently not affecting the accuracy of GPS receivers.y y g y

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The ionosphere and troposphere both refract the GPS signals. This causes the speed of the GPS signal in the ionosphere and troposphere to be different from the speed of the GPS signal in space. Therefore, the distance calculated from "Signal Speed x Time" will be different for the portion of the GPS signal path that passes through the ionosphere and troposphere and for the portion that passes through space.

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A GPS signal bouncing off a reflective surface prior to reaching the GPS receiver antenna is referred to as multipath. Because it is difficult to completely correct multipath error, even in high precision GPS units, multipath error is a serious concern to the GPS user.

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PDOP is an indicator of the quality of the geometry of the satellite constellation. Your computed position can vary depending on which satellites are used for the measurement. Different satellite geometries affect the errors. A greater angle between the satellites lowers the PDOP, and provides a better measurement. A higher DOP indicates poor satellite geometry, and an inferior measurement configuration. PDOP under good conditions is below 6. The accuracy specification for a high g y p gend GPS unit is only valid when the PDOP is below a specified value (usually 6).

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The error levels for different sources of GPS error are listed in the table above. Selective Availability was an error introduced by the Department of Defense, however this error was turned off in May of year 2000.

Ephemeris data refers to imperfections in the satellite orbits.

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Differential GPS (DGPS) incorporates a series of techniques to account for some of the GPS errors. Differential GPS can be applied ‘on the fly’ (real-time differential GPS) or post-processing differential correction (correction applied to the GPS data after it has been collected).

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Both real-time and post processing differential correction involves two receivers, one that is stationary and one that is roving. The stationary unit collects data continuously and the location for this unit is therefore very accurate. The roving unit is carried by the navigator (mapper, surveyer, recreationist….).

As you learned in previous slides GPS errors are influenced by many factorsAs you learned in previous slides GPS errors are influenced by many factors, but receivers within 100 km usually have about the same errors. The stationary receiver ate a precisely known location compares the current reading with the known location and can tereby compute a correction factor.

In real-time differential GPS this correction factor is transmitted to the roving i Th f h i i d h d GPS di di lunit. The user of the roving unit reads the corrected GPS coordinates directly

on the GPS display.

In post-processing differential correction the correction factors are posted in a database, many are available via the Internet. The GPS data from the roving unit is downloaded to a computer and corrected using the information from the

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p gstationary unit (base-station). Post-processing can currently not be applied to recreational grade units.

Base station are located all over the country, commonly managed by the Forest Service, Bureau of Land Management, US Geological Survey, Department of Transportation or the US Coast Guard. Base station data used in post-processing differential correction can accessed at: http://www.fs.fed.us/database/gps/welcome.htmWAAS is a system of 2 satellites and 25 ground stations that transmit a real-time differential correction signal to GPS units that are WAAS enabled.gThere are also two commercial satellite differential service providers, Thales Survey LandStar (formerly Racal LandStar) and OmniSTAR Inc.. These companies use a control hub where reference station data is checked, formatted, and uploaded to a geostationary satellite for broadcasting to subscribers for an annual fee.A beacon can be attached to many GPS units for reception of the real-timeA beacon can be attached to many GPS units for reception of the real-time differential signal. One example is the ‘Beacon on a Belt’ from Trimble: http://www.neigps.com/products_tp_bob.phpIf you cannot receive a real-time differential correction signal, post-processing is another way to achieve differentially corrected data for higher end mapping units (not available for recreational units).

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WAAS is a network of 25 ground stations and 2 geostationary satellites put up by the Federal Aviation Administration. The system broadcasts a real-time differential correction signal at no annual fee or registration requirements – all you need is a WAAS enabled GPS unit. WAAS is not received everywhere or all the time. For GPS you can expect an improved accuracy of ~7 meters during the times the GPS unit receives the WAAS signal.

Read more about WAAS: http://www.trimble.com/products/pdf/waas_cfaq.pdf

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The WAAS system includes two geostationary satellites, one over the Atlantic and one over the Pacific, and 25 ground control stations.

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Base station data for post-processing differential correction can be accessed at: http://www.fs.fed.us/database/gps/welcome.htmRead more about differential correction at: http://www.esri.com/news/arcuser/0103/differential2of2.html

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The table gives a few examples of GPS systems, cost, and accuracy. Data storage capabilities is another important aspect of GPS data collection. Mapping grade and survey grade units (Trimble units for example) have data storage capabilities and gives the user the ability to create a custom made database form directly on the GPS unit. In the data dictionary the GPS user can store attribute information associated with the location recorded with GPS. The Pocket PC with a GPS card can also store extended attribute data. Recreational units can store waypoints, but are very limited when it comes to storing attribute data.

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New GPS technology allows the user to load aerial photographs or topographic maps into the GPS unit for display during the GPS data collection. The current GPS locations is displayed on top of this backdrop of photo or map. Software also exists (for example ArcPad from ESRI) that allows the user to create features via on-screen digitizing on the GPS unit or Pocket PC. Read more at: http://www.esri.com/software/arcgis/arcpad/index.html

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In summary, GPS is commonly used in natural resource and land management applications. It is important to be aware of GPS accuracy, sources of GPS errors, options for differential correction and what unit to use for what application.

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If you are going to import your collected GPS data to a GIS it is advantageous to record the GPS locations in the same coordinate system as your GIS layers are in. The default setting on most GPS units is to collect in Geographic coordinates (Degrees, Minutes and Seconds) and the WGS84 datum. If you want to record GPS locations in a different coordinate system and datum you can adjust the settings under ‘SETUP’ on the GPS unit. It is important to coordinate data collection within a field crew such that all members of the

d d i h di d dcrew records data in the same coordinate system and datum . As we learned in the “Map Projection” section of this course the map projection tools in ArcToolbox can be applied to convert data from one coordinate system and datum to another. This is however contingent upon the fact that the original coordinate system and datum for the collected data is known!! Units can be set to collect in Degrees, Minutes, Seconds or degrees and decimal minutes or in decimal degrees along with other coordinate systems such as UTM (Universal Transverse Mercator).

Format for degrees, minutes and seconds: DD MM SS.ssf d d d i l i

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Format for degrees and decimal minutes: DD MM.mmmFormat for decimal degrees: DD.ddddddd

If data is collected in decimal degrees it is important to record at least 6 decimals.

Map coordinates are present on most maps. This 7.5 minute USGS quadrangle map shows coordinates in both degrees&minutes and in the UTM coordinate system. The map datum is also available in the map information.

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A few GPS references on the Internet.

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