Using Trimble GEO 3 GPS Unit

Stage 1: PreProcessing: Mission Planning and Almanac Transfer

In the space below, summarize what you feel are the advantages and limitations of the GEO 3 unit compared to the Lowrance 100 unit. In a couple of weeks we will revisit this list and revise what is written today.

Advantages / Limitations

Before using the GEO 3 we’ll take a look at the best times to collect GPS data.

Background: Mission planning is the process of discovering the optimal times for collecting quality GPS positional data. As you learned last week, data quality depends, in part, on the configuration of satellites. Poor quality GPS data would occur if satellites are bunched in one spot or aligned only along the horizon. Better quality would occur if satellites are distributed evenly across the sky both by height and azimuth. The quality of satellite geometry across the sky is measured by position dilution of precision (PDOP), which is recorded on higher end GPS units. A PDOP of 4 or less, for example, yields desirable precision. A PDOP between 5 and 7 is alright; a PDOP of 7 or more is poor. Consider discontinuing data collection when values reach such levels.

An almanac is used in mission planning.

Source: GeoExplorer 3 System, 1999

Basically, the almanac helps the GEO 3 determine which satellites will be sending usable signals at any point in time and where satellites will be located in the sky. Even though the satellite constellation of 24 SV’s (satellite vehicles) is complete, some satellites “go down” for servicing and testing; therefore, it is important to get up to date information on satellite availability.

The Quick Plan utility, which comes with Pathfinder Office software, can predict PDOP for a selected day and identify the optimal times for collecting data. When using GPS professionally, don’t use an almanac file if it is greater than 30 days old.

Get the most current almanac file by simply allowing the GEO 3 to acquire a signal outside and then uploading the revised almanac to Pathfinder Office. Allow at least 15 minutes to acquire a new almanac.

Transferring the Almanac to the GeoExplorer 3

1. After taking the GEO3 unit outside for 15 minutes to acquire an almanac, you must make sure that the GPS unit you are using is configured to transfer data through either the black serial clip or the yellow support module cradle. To do this turn on the Trimble Geo 3 unit and Press the "SYS" button until you get to the "SYS Setup"tab, highlight "Configuration" and press "ENTER" Scroll down and highlight "COMMS" Press "ENTER" The top line of the "COMMS" window says "Data transfer" Select "Serial clip" or “Support Module”, whatever is at your workstation. The unit is now ready to transfer data.
2. Gently place the GeoExplorer 3 in the yellow and black support module or serial clip sitting next to select computers in Dow 274. Be sure that the support module is securely cabled to the office computer and that you have the proper data transfer mode (support module or serial clip) highlighted on the GeoExplorer 3.
3. Open Pathfinder Office software from the Start Menu. Start the “Data Transfer” utility by selecting the “Utilities” menu in Pathfinder Office. A pull down menu appears. Choose “Data Transfer.” A “Data Transfer” menu appears.
4. The almanac file that you created appears. Add and then transfer the almanac to the PC. Click the “Transfer” button. The almanac will be loaded to the PC.

Procedure for Mission Planning with “Quick Plan”

1. Open Pathfinder Office from the Start Menu. From the “Utilities” menu in Pathfinder Office, select “Quick Plan.” You can also select the icon. A new window “Quick Plan” window will appear.

2. A “Select Date” window will appear. Select today’s date.

3. After selecting the proper date, an “Edit Point” window appears. With Pathfinder Office you can key in the exact coordinates of your sampling site or select a more general location from a menu. Click on the “World Map” button and find the three cities that are closest to Mt. Pleasant. Magnify to Michigan using the world map options.

2. Use information from the Mount Pleasant topographic map, found in the computer lab, to discover the latitude and longitude of the Mount Pleasant, Michigan. You only need to record to the nearest whole minute. Record the elevation of Dow Science Building using the provided topographic map. Convert this figure to meters.

3. Return to the “Edit Point” window and press the “keyboard” key. Key in the appropriate information only for these lines: Name, latitude, longitude, (don’t key in seconds value), height, and choose “Estimated Coordinates” from the “Position Quality” pull down menu. Select “OK” to dismiss the “Edit Point” window. You have just created a new profile for the status screen that reflects your current position in Mt. Pleasant. The threshold elevation indicates that any satellite lower than 15º from the horizon will not be used in mission planning. There may be occasions, if, for example, you are working in mountainous terrain or within urban canyons, that you might want to set this threshold elevation mask to 40º or more. Assuming an unlimited horizon, what are some of the problems associated with receiving signals from satellites that are 15º-25º?

4. From the “Quick Plan” menu select “Graphs” and then “number SV’s (satellite vehicles) and PDOP” What is the correlation between the number of SV’s and PDOP?

5. Quick Plan can also be used to identify optimal mapping periods based on your window of preferred sampling time, which for this class is between 0900 and 1700. Select “Options” from the main “Plan” menu and then “Auto Time’. An “Auto View Time Selection” window will appear. You want a minimum of 6 satellites observable, a PDOP of less than 5, and the need to work at least in 70 minute uninterrupted intervals. Use the example below to help you configure this window (be sure to check Auto Recalc).

4. Based on these parameters, at what time could you conduct this fieldwork? Use your exploratory skills and print out a PDOP graph showing the optimal times for conducting your field work. Once you create the graph use the the “M” and “D” keys on your keyboard to zoom in and out on the graph. The “P” button allows you to pan to different portions of the screen. The example pictured below is for illustrative purposes only and does not represent the “answer.”

Questions to Complete for 9/13. Type all answers.

1. Using the “World Map” button on “Edit Points” menu, name the three cities that are closest to Mt. Pleasant.

2. Discover the latitude and longitude of the Mount Pleasant, Michigan. Record the elevation of Dow Science Building using the provided topographic map. Convert this figure to meters.

3. Assuming an unlimited horizon, what are some of the problems associated with receiving signals from satellites that are 15º-25º?

4. What is the correlation between the number of SV’s and PDOP?

5. Based on these parameters, at what time could you conduct this fieldwork? Use your exploratory skills and print out a PDOP graph showing the optimal times for conducting your field work.

6. What satellites reach the highest elevation during the sampling window? What time did this occur?

7. What is the general direction of satellite travel across the sky?

8. During the sampling window(s), what is the maximum number of satellites ever available?