Treasure Hunt Using a Magellan GPS Receiver
Created By: Leigh Truong
Grade Level: High School, AP Biology
Project Number: unknown
Objective
The aim of today’s activity is to acquaint you with use of the Magellan GPS unit. You
will be asked to find several locations that have been programmed into your GPS receiver
using the navigational tools of your system. As you find each point, you must answer a
series of questions to evaluate the efficacy of your global positioning system.
Background Information
What is GPS?
The global positioning system (GPS) was conceived and constructed by the United States
Department of Defense in the 1970s as a means of allowing its military forces to navigate
and track their position on the Earth’s surface; hence, providing them with an advantage
over their adversaries. While the Department of Defense provided for civilian use of their
GPS satellites, the intention was to limit the accuracy of those GPS receivers and throw
them off position by 100 meters. Despite their attempt at relaying modified signals
(termed “selective availability”), many civilians and navigational companies have been
able to circumvent these altered signals and provide their customers with positioning
accuracy to within a few centimeters. Because of the ineffectiveness of selective
availability, the military has discontinued its modification of these signals. Today, GPS
is used for a variety of purposes from helping drivers navigate roads to tracking
movements in the Earth’s tectonic plates.
How does GPS work?
GPS relies on 24 satellites launched into an orbit 11,000 miles above Earth. Those
satellites travel in a geosynchronous (“geo” – earth; “synchronous” – moving at the same
rate) orbit, and at any one time there are at least four satellites overhead. Each satellite
transmits a unique radio signals called a pseudo-random sequence that relays its own
position and time back to Earth’s surface. When a GPS receiver picks up the signal, it
calculates the distance from the satellite to the receiver’s position with the following
equation.
d = rt
Where d denotes the distance, r is the rate, and t represents time. The rate is the speed of
light and is given by the value 186,000 miles per second. The time is calculated by taking
the difference between the satellite’s clock (sent) and the time of the receiver’s clock
(received).
The distance from the satellite to the receiver is important because this is what allows you
to determine your location (i.e. latitude, longitude, and altitude) on the planet with a
concept called ranging or triangulation. To simplify the concept, let’s imagine the
1
following situation in two-dimensions. You are a lost hiker in the woods. Suppose you
know that you are 10 miles from a very prominent mountain. Given this information, you
could be in any direction within a 10-mile radius of the mountain. So, this information
(by itself) does not give you a position. You also know that you are 5 miles from a fire
tower. Drawing a circle around each of your reference points with a radius of 10- and 5miles for the mountain and fire tower, respectively, you have narrowed down your
location to a relatively small area (see Figure 1).
rR==55
r = 10
Figure 1. Two reference points allow you to limit the possible area that you could be in
(blue), but does not fix your position.
If you know your distance from a third reference point, you can pinpoint your position
where the three circles intersect (see Figure 2). GPS works under the same principle
except in three-dimensions. Three satellites are used to locate your position on Earth’s
surface, and a fourth satellite is used to correct the error in the receiver’s clock.
2
rR==55r = 5
r=4
r = 10
Figure 2. A third reference pinpoints your position.
Figure 2. Three reference points allow you to pinpoint your position (in red).
Treasure Hunt
After you have reviewed the Magellan GPS user’s manual, you will each be given a GPS
receiver. Your goal is to locate the programmed destination or waypoint in your receiver.
You may use any of the navigational tools in your receiver. At each location, you must
retrieve the object in order to receive a prize at the end. (Note: There is some error to
pinpointing each waypoint. Be sure to look around for the object specified)
Discussion Questions
1. What navigational window or display did you use to locate each waypoint? Why did
you use this window?
2. Does the number of satellites orbiting above influence the accuracy of your signal in
locating each waypoint?
3. What might affect the accuracy of locating your position?
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Data Sheet
In the data sheet below, the waypoint is the destination that your teacher has programmed
into your GPS receiver. The latitude and longitude will be measured in degrees, minutes,
and direction (e.g. north, south, west, and east). The number of satellites refers to how
many satellites are orbiting above your GPS receiver as you are tracking each waypoint.
Under the column “Description of location”, simply describe the area around the
waypoint (e.g. trees, shrubs, buildings, etc.).
Waypoint
Latitude
Deg.
Min.
N/S
Longitude
Deg.
Time
Min. W/E
HH:MM:SS
1
2
3
4
5
6
7
8
9
4
Number of
Satellites
Description
of location