1. Citizen Science Training Workshop
Lesson 2: Introduction to GPS
The Citizen Science Research Team
The National Institute of Invasive Species Science
Natural Resource Ecology Laboratory
Colorado State University
Content By: Alycia Crall (modified by Jim Graham)
May 30, 2007

2. Example: Universal Tranverse Mercator
Where does the point lie?
Here is an example of a grid using the UTM coordinate system. Using this grid, can you give the northing and easting of where the point lies? Remember that the northing represents the distance north or south of the equator in meters. The easting represents the distance east of a line representing the west edge of the zone in meters.
Answer:
4,696,750 meters north (Northing)
471,800 meters east (Easting)

3. Definitions Global Positioning System (GPS) hand-held receiver
determines position (geographic, UTM)
Considering that you may have many points over extensive areas that you are collecting location information for, using a paper map to determine location may not very realistic. That is why we have global positioning systems or GPS. GPS are digital hand-held receiver that can help you determine your location on the earth for any specified coordinate system. There are many makes and models, each having different features.

4. How does GPS work?
Utilizes a network of 24 satellites that transmit precise microwave signals
enables a GPS receiver to determine its location, speed and direction
How does a GPS unit work?
GPS utilizes a network of 24 electronic satellites placed into orbit by the US Department of Defense. This network of satellites is referred to as a satellite constellation. A satellite constellation uses a number of satellites with coordinated ground coverage, operating together under shared control, synchronised so that they overlap well in coverage and complement rather than interfere with other satellites' coverage. The cost of maintaining the GPS system is approximately US$750 million per year, but it is free for civilian use as a public good.

5. How do GPS units work? Satellite 1 Location
Receiver Location
A GPS receiver calculates its position by measuring the distance between itself and three or more GPS satellites. A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude). Once the user's position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more.
Measuring the time delay between transmission and reception of each GPS microwave signal gives the distance to each satellite, since the signal travels at a known speed. The signals also carry information about the satellites' location. By determining the position of, and distance to, at least three satellites, the receiver can compute its position using trilateration.
How does trilateration work? A signal is sent out from one satellite to the GPS receiver, which measures how long it took for the signal to reach it. It uses the length of travel time for the signal to calculate a circular range of possible locations.
Every point on circle is a possible receiver location
Receiver Location?

6. How do GPS units work? Satellite 1 Location Satellite 2 Location ? ?
Using the signal from a second satellite, possible locations of the receiver on the ground are narrowed to the two points where the arc intersect. ?

7. How do GPS units work? Satellite 1 Location Satellite 2 Location
Known Receiver Location
When a third satellite locates the receiver, the actual location can be determined. Most GPS receivers give a location within 100 meters using three satellites, but additional satellites will increase this accuracy.
Satellite 3 Location

8. GPS Data Uses Navigation Mapping Where am I?
How do I get from point x to point y?
Mapping
Define a point (e.g., campground, plot location)
Define a line (e.g., hiking trail, river)
Define a polygon (e.g., lake, area covered by weed)
So, now that we understand how GPS works, we’ll quickly review how this data can be used. As stated previously, one of the primary uses of GPS is for navigation…where am I? and how do I get from point x to point y? GPS data can also be used for mapping a point, line, or polygon. A point is a single geographic location using x, y coordinate pairs. A line is recorded as an ordered series of connected line segments, where each line segment has at least two x, y coordinates. A polygon is an ordered series of a minimum of three connected line segments, but the starting and ending x, y coordinates are at the same location.
Point
Line
Polygon