Global Positioning System (GPS)

Introduction

Until John Harrison developed his chronometer in the middle 1700's, hundreds of sailors of English, Spanish, Portuguese, and French heritage died each year because the connection between vitamin C and scurvy prevention had not been made. However, the real reason for their deaths was not the lack of available vitamin C but the fact that the captains of their ships were simply lost at sea. The captains had no idea in which direction they should sail to get to port. Eventually men realized that eating sauerkraut, lemons, or limes (hence "limey" as nickname for English sailors) prevented scurvy. They also came to realize that the "sea-clocks" or chronometers build based on John Harrison's design allowed them to find out where in the world they were as long as they could see the sun at noon and the stars or sun to determine their latitude.

Today, few of us have the skill to use a sextant to establish our latitude or to sight the sun at noontime. However, there is a relatively inexpensive device that is easy to use and that allows you to know your precise position on earth at any moment. That inexpensive device is a handheld GPS receiver that costs under $200. In conjunction with a cellular phone, the instantaneous position that a GPS receiver provides you can call a rescue vehicle to your car, your boat, or to your high-mountain campsite. It can be used to find your way on a hike, on a trip to a new vacation destination, or to find an open gas station when you are about to run out of gas.

Background Information

Before you tackle our lesson plan, you may want to explore some of the following links to help improve your understanding of how GPS works.

As an introduction to the methods of navigation, you may first want to read Historical Methods and Tools of Navigation.
Trimble Navigation Limited has prepared an on-line GPS tutorial that can be found HERE.
We have prepared a thorough GPS Tutorial to help you understand some of the basics.
Once you have reviewed these GPS materials, you can try our GPS Test, complete with answers.

Lesson Plan

Here is a brief lesson plan that we have put together to help pull together some of the ideas related to GPS and the operations at Haystack.

Day One
The first day can be spent introducing your students to what GPS is. Depending on the level of detail you want to go into, you can present some or all of the information that is found in our Background Information, above. There are a few key points that you need to make sure the students understand:

GPS relies on VERY precise clocks in the satellites and the receivers.

GPS receivers take signals from several different satellites to "triangulate" their position.

GPS satellite signals are no longer scrambled, allowing all hand held GPS units access to the same signal quality that the Department of Defense uses

Day Two
Day two simply involves monitoring the performance of a GPS unit. This can be done be placing a GPS receiver in a fixed location (usually by a window is best) and recording the position indicated by the receiver.

It is best to do this using "meters" for units.

Over the course of a period (or, ideally, several periods with several classes participating) watch what happens to the readings. You should see a considerable drift if the unit's position. A large table can be posted in the front of the class showing the coordinates of the GPS unit at different times. It is also very effective to plot the units "location" on a large graph, where (0,0) should correspond to the first readings that the GPS unit gave you.

By the end of the data acquisition period, you should have a long data table, a graph which may seem to have a snake crawling around on it, and a confused group of students.

Day Three
This final day should start with you presenting the data to the kids and asking them, "What's up with this?" Give the students plenty of time to explore this on there own, either generating a class list of ideas or letting the students break into groups. Once the students are done, you can explain that this wandering is due to changes in the amount of time it takes the GPS signal to get from the satellite to the receiver. (Remember, GPS is VERY dependent on slight time variations.) There are three main reasons for this time variation:

Inaccuracies in the atomic clocks on the satellites. (This is a very SMALL source of error.)

Changes in the composition of the atmosphere.

Activity levels in the ionosphere.

The last reason is another direct link between the Sun and the Earth, through the Sun's effects on
Space Weather.

On-Line Resources

Instead of simply placing a GPS receiver on a table for the lesson plan, there are several pieces of software available that will allow you to connect your GPS unit to a PC or Mac. This will allow you to do your data gathering directly to the computer. Here is a brief list of some of the software vendors:

GPSy - Macintosh GPS Communications

http://www.gpsy.com/

GehPehEs - Alpha 03

http://www.medienkonzepte.de/heise/crash/htd/gps/gps01.html

MacGPS Pro 2.7

http://www.macgpspro.com/GPSPRO.html

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