Activity: Location-Finding Using GPS

Your Name _________________________ Your Teacher _________________________

Location-Finding
Using Global Positioning System (GPS)

by Phil Wherry & Walter Sanford

Teaching Notes
& Technical Tips

Grade Level:
6-12

Ties to Curriculum:
This activity is aligned with national teaching standards for the following subjects:
Earth Science
Geography
Mathematics
Technology

PURPOSE & OBJECTIVES

The Global Positioning System (GPS) is a satellite-based navigation system that can be used to find your location quickly and accurately anywhere in the world. In this activity, you will learn how to use a GPS receiver to locate your exact position on Earth (latitude and longitude). You will then locate a mystery position (somewhere in the vicinity of your school) based on information collected from the GPS system and a local map. In the process, you will discover how GPS works to determine your exact position.

TIME REQUIREMENT

Approximately two- to three class periods (assuming 60- or 90-minute periods).

Teaching Notes:
The directions for this activity were written specifically for the Garmin GPS 40 receiver (which is very similar to the Garmin GPS 12 receiver). This activity may be used with other GPS receivers; refer to your GPS receiver owner's manual for equivalent technical information. Garmin GPS receiver owner's manuals are available online in Adobe Acrobat PDF format.

Hyperlinks to some teaching notes are password-protected and are accessible to teachers only.

Technical Tip:
External links will open automatically in a second browser window; subsequent clicks on external links will replace the contents of the second window. This enables the user to keep both the activity and related text and graphics on the desktop.

EQUIPMENT & SUPPLIES

GPS receiver (e.g., Garmin GPS 40 Personal Navigator)
Large-scale map of your school, such as the maps provided by DeLorme's Street Atlas USA 6.0 software
Drawing compass (such as the Safe-T Compass)
Protractor (ideally, a 360-degree, full-circle protractor)
Laptop computer or Palm Computing Organizer with GPS receiver interface cable (optional, but required for activity extensions)

Technical Tips:

After extended power-off or memory loss, the GPS receiver must be given an opportunity to collect satellite almanac data and its current position. This process can take 7.5-15 minutes.
If the receiver has trouble acquiring the satellites, you may need to power-off the receiver and try again at a new location. Press and hold the red key for three seconds until the power turns off.
When satellites are being acquired, the round part of the Satellite Status Page represents the sky. North is at the top of the display. The outer ring represents the horizon; the inner ring represents 45 degrees above the horizon; and the center point directly overhead (the zenith). Each satellite has its own number; these are the numbers shown on the display. Though this page disappears once the satellites have been acquired, you can return to it by cycling through the pages (refer to Technical Tip #4, below).
You can cycle through the various pages displayed by the receiver using the PAGE key to move forward and/or the QUIT key to move backward. The pages are interconnected, so pressing either key repeatedly will cycle through the five main pages.
Under some circumstances, you may see a flashing box in the middle of the display instructing you to press PAGE to view a message. This generally happens when the receiver loses its lock on the satellites. Use the PAGE key to move to the Satellite Status Page (sky view). You'll know the receiver has locked on again when the "POOR CVG" (poor coverage) message at the top left of the display changes to "2D NAV" or "3D NAV".

PROCEDURE

Part I. Using a GPS Receiver to Find Your Location

Proceed to an outdoor location with a clear view of the sky from horizon to horizon. You should stand well away from the school building, trees, etc., so that you have an unobstructed view of the sky. Hold the receiver at arm's length from your body so the built-in antenna (the flat area above the display) is parallel to the ground. Power-on the GPS receiver by pressing the red key. After the Welcome Page, by default the receiver displays the Satellite Status Page (sky view) and begins searching for satellite signals. GPS receivers get their information from a system of 24 orbiting satellites located approximately 18,300 kilometers (11,000 miles) above the Earth's surface. To provide accurate position information, the receiver must be able to "see" three or four satellites.

As satellites are acquired, you will see bars appear on the graph at the bottom of the display; these bars indicate the strength of the satellite signal. Once enough satellites have been acquired, the Satellite Status Page will disappear automatically and be replaced with the Position Page (graphic compass).

Your position (latitude and longitude) should be shown on the Position Page. Record your current position in the following data table:

Your Position
Latitude
(deg. N) Longitude
(deg. W)

Teaching Note:
Programming the Mystery Location.

Technical Tips:

Ensure that "MYSLOC" appears on the display at this point. If you press the GOTO key twice by mistake, this will activate the receiver's "man overboard" function (which automatically stores the current position as a destination). If this happens, the destination point will be titled "MOB" rather than "MYSLOC"--just press GOTO again (once) and select "MYSLOC" from the list using the arrow keys.
It's important that the receiver display distance information in metric units and bearing information for true North. If this isn't the case, then push the PAGE key until the MENU Page appears. Then use the arrow keys to highlight the "NAVIGATION SETUP" field and press ENTER. Highlight "UNITS", press ENTER, use the arrow keys to select "Metric", and press ENTER again. Next, highlight "HEADING", press ENTER, use the arrow keys to select "True", and press ENTER again. Then use the PAGE key to move back to the Navigation Page (graphic highway).

Part II. Finding the Mystery Location

Part A. Collecting Field Data

Your GPS receiver has been pre-programmed (by your instructor) with a mystery location. Now let's explore how the GPS receiver can be used to navigate to an unknown location.

Randomly choose three-to-five different locations on the school grounds. These locations should be fairly distant from each other (at least 500 feet apart). Remember to choose locations where the GPS receiver will have a good view of the sky. Proceed to Point No. 1. Record the following information in the data table below:

Use the Position Page (graphic compass) to acquire your current position. Record your latitude and longitude.
Press the GOTO key. The Navigation Page (graphic highway) will appear with the waypoint field highlighted. Press the up or down arrow keys to scroll through the available waypoints until "MYSLOC" (short for "mystery location") is displayed. Press the ENTER key to confirm that you want to navigate to "MYSLOC". Record the bearing (in degrees) and distance (in kilometers) to the mystery location.
Briefly describe the location.

Repeat Steps 1-3 until you have visited at least three different locations on the school grounds. Do not actually go to the mystery location!

Field Data
Point
No. Latitude
(deg. N) Longitude
(deg. W) Bearing
(deg.) Distance
(km) Brief
Description

1

2

3

4

5

Power-off the receiver by holding down the red key for three seconds until the display goes blank.

Teaching Note:
Suggested strategies for guiding student analysis of Field Data.

Part B. Analyzing the Field Data to Find the Mystery Location

Imagine that the only information provided by the GPS receiver is your position (latitude and longitude) and the distance from your current position to other locations, or waypoints. From how many different positions would you need to measure the distance to the mystery location to actually find it?
1. What if you make only one distance measurement--will you be able to find the mystery location? No! The mystery location could lie anywhere along a circle whose radius is equal to the distance between your location and the mystery location (refer to Figure 1a). Using your Field Data for Point No. 1 (latitude, longitude, and distance), draw Circle 1. Technique Hint: Use latitude and longitude to locate Point 1 on the map; use the map scale to measure the radius of Circle 1; draw the circle.
2. If you make a second distance measurement from a different position, then the mystery location could lie anywhere along another circle whose radius is equal to the distance between Point No. 2 and the mystery location. Refer to Figure 1b. Notice that the two circles intersect at two points--the mystery location could be either point of intersection. Using your Field Data for Point No. 2, draw Circle 2.
3. If you make a third distance measurement and draw a third circle whose radius is equal to the distance between Point No. 3 and the mystery location, then you should discover that there is one and only one point where all three circles intersect (refer to Figure 1c); this is the mystery location! Using your Field Data for Point No. 3, draw Circle 3. In relation to major map features, where is the mystery location?
4. Explain how finding the mystery location by drawing three intersecting circles ("triangulation") is similar to how GPS works to determine your exact position on Earth.
Now imagine that the only information provided by the GPS receiver is your current position (latitude and longitude) and the bearing (direction) from that position to other locations, or waypoints. From how many different positions would you need to measure the bearing to the mystery location to actually find it? Illustrate your answer by drawing lines on a hardcopy map of the area surrounding the school.
What specific information would the GPS receiver have to provide to enable you find the mystery location from a single random position?

If you make only one distance measurement between your current position and the mystery location, then the mystery location could lie anywhere along a circle whose radius is equal to the distance between your location and the mystery location.

Figure 1a. (Click on graphic to view larger image.)

If you make a second distance measurement from a different position, then the mystery location could lie anywhere along another circle whose radius is equal to the distance between Point No. 2 and the mystery location. Notice that the two circles intersect at two points--the mystery location could be either point of intersection.

Figure 1b. (Click on graphic to view larger image.)

If you make a third distance measurement and draw a third circle whose radius is equal to the distance between Point No. 3 and the mystery location, then you should discover that there is one and only one point where all three circles intersect; this is the mystery location!

Figure 1c. (Click on graphic to view larger image.)

Part III. Activity Extensions

Satellite-linked navigation systems are standard equipment on many luxury automobiles. In the near future, satellite-linked navigation systems are likely to be standard equipment on all automobiles. In this activity extension, you will use a laptop computer (running DeLorme's Street Atlas USA 6.0 software) interfaced with a GPS receiver to simulate an automobile satellite-linked navigation system.