Get GPS Data for this Lab

    Since the early 1980s, US global positioning system (GPS) satellites have provided a means of precise navigation. In the last ten years, small, inexpensive, high-precision GPS receivers has made this technology available to all, and the system has been adapted for a variety of novel, non-navigational uses. Geodetic and surveying application, including geologic mapping, rely on GPS receivers to report and store positional (e.g. latitude & longitude) rather than navigational (e.g. speed, heading, etc.) information. Transfer of GPS positions to topographic or other base maps, as is generally necessary for geologic mapping, requires additional considerations. Both map and GPS data must share a common datum (a specific model of the shape of the earth), and an assessment of true precision and accuracy for both GPS and map data is desirable. All GPS receivers are equipped with software that allows selection of a datum (e.g. NAD 27, WGS 84, etc.), and most can report geometric factors that influence theoretical precision. The true precision of a GPS reading, which is dependant on many factors besides satellite geometry (see lecture notes), is most easily determined by experiments. The experiment in lab this week tests the precision and accuracy of a variety of different receivers, operating in both stand-alone and differential (DGPS) mode. The objectives of the lab are:

• Learn how to operate a variety of different GPS receivers.

• Map the precision, as determined from 1 hour of observations, of several receiver.

• Compare the precision of positions determined from single receivers to that obtained from a pair of receivers (Differential GPS; DGPS), and to real-time DGPS data derived from receivers equipped with a Coast Guard beacon antenna.

• Gauge the accuracy of the GPS and DGPS data relative to a known benchmark.

I. The Experiment

    The Department owns a variety of GPS receivers, from inexpensive handheld devices that are not designed for surveying to top-of-the-line geodetic instruments capable of storing 10s of 1000s of measurements. The experiment consists of using this equipment to measure the position of 4 well-determined locations on the East Mall. Each location will be occupied by a group of students, a TA, and a receiver that will collect data for 1 hour, amassing up to 700 measurements at each location. These data will be used to construct graphs, one for each location/instrument, from which the precision and accuracy of the measurements can be determined. Data from three locations will provide more precise DGPS positions, one in real time, the other two after processing with another data set. DGPS measurements will be plotted with different symbols/colors on the same graphs as the "uncorrected" data from the same locations, allowing direct comparison of the two results. Accuracy will be determined by comparing the results with the know positions of each locality, as determined by taped distance from a benchmark (or by comparison with a DGPS carrier-phase-determined location for one of the stations).

II. The Equipment

A. Trimble 4000SSI receiver with free-standing, choke-ring antenna

• 12 channel, geodetic-quality, L1, L2 and P-code receiver. Great versatility in storing and processing signals - will store all satellite information with each measurement, not just a calculated position. This allows for precise corrections in calculating a DGPS solution. Capable of carrier- phase solutions. Can be equipped for real-time kinematic DGPS.

B. Trimble handheld Geoexplorer 3

• 12 channel, L1 receiver. Capable of highly precise, carrier-phase DGPS solutions with post-processing of downloaded data. Does not store satellite information in code-phase mode. Can be equipped for real-time kinematic DGPS with a beacon antenna installed.  Highly flexible data dictionary capabilities allows collection of feature and attribute information along with positions.

C. Trimble ProXR fanny-pack unit

• Near geodetic quality receiver with most of the same functionality of a Trimble 4000SSI and the data storage capabilities of the Geoexplorer 3.  Is equipped for real-time DGPS; antenna receives both GPS and beacon signals.

D. Garmin E-trex Summit

• 12 channel, L1, code-phase receiver.  Can  log "waypoints" manually, up to 500 positions that can be downloaded via inexpensive software. Can be equipped for DGPS using a separate beacon antenna.  Stores positions only - not capable of post-processing for DGPS solutions.

III. Collecting Data

    GPS measurements will be collected during lab. Data will be made available to you for analysis (see below) following the lab via the class web site. Three of the four receivers can continuously log data for several hours. These instruments will be set to record a measurement every 5 seconds. The fourth, the Garmin handheld, cannot automatically log data and data will be recorded in 30 second increments by hand. All data will be recorded relative to the WGS 84 datum in UTM Zone 14 coordinates.

IV. Data Analysis

1. Download an Excel workbook containing all data sets, graphs, a map of the stations, and a template for a table you will turn in onto a disk. Open the workbook in Excel.

2. Use Excel to calculate the average Northing, Easting and elevation for each station, for both GPS and DGPS measurements, where appropriate. Find twice the standard deviation of the measurements at each station. Record these values, for each station, in the table provided as the Excel worksheet "All data". Station 1 has already been done for you; use it as an example. The notes beneath the table provide additional information about each of the columns.

3. Graphs of the measurements at each station are provided as separate worksheets. Examine each of these. You may want to print them. The graphs are gridded in meters, though each has different cell sizes. The "nominal" (assigned) location of each station is shown with a filled square, and averages of the measured and corrected data are shown with different symbols. Also shown is a circle(s) that contain 95% of the measured and/or corrected locations. The centers and radii of these circles are another way of expressing the average location and precision ("error") of the measurements. Record these on the graphs in the spaces provided, for comparison with nominal station locations.

4. Use the information you record on the graphs to complete the "All Data" worksheet table. The information is shown on a map in the "East Mall Map" worksheet. Examine it.

5. Print the completed table and turn it in, along with answers to the questions below.

V. Questions

1. On the basis of the 95% confidence circles, among the non-DGPS data sets, which is most precise? Which is most accurate? Are the differences great enough to warrant a preference among receivers?
   
   

2. Again, On the basis of the 95% confidence circles, are the precisions of the non-DGPS data within an oft-cited manufacturer's nominal value of 25 meters? 
   
   

3. Among DGPS data sets which is most precise, that measured in real-time by the beacon antenna-equipped ProXR, or that derived from post-processing of data from one of the other Trimble receivers? What is the major assumption in interpreting the precision of "stationary" data from portable receivers like the ProXR, Geoexplorer and Magellan Pioneer?
   
   

4. On the basis of your calculated precisions, how much better is the precision of the DGPS data than the non-DGPS data?
   
   

5. How much better is the accuracy of the DGPS data or, stated another way, when is a DGPS solution necessary?
   
   

6. Based on your calculations, how much better is the horizontal (Easting and Northing) precision and accuracy than the vertical precision and accuracy?
   

END

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Geological Sciences
U. Texas at Austin