BASIC CONCEPTS OF GLOBAL POSITIONING SYSTEMS (GPS) Introduction to GPS Merritt College Marc Epstein, Instructor

History of Navigation ► ► Early sailors navigated by sun and stars ► ► Until the late 1700’s sailors could only determine latitude ► ► Development of accurate seaworthy chronometers by the British enabled calculation of longitude

Early Electronic Navigation ► ► Radio based system ► ► LORAN-C and DECCA ► ► Triangulation of ground based radio signals ► ► Accurate to 900 – 1000 feet

What is GPS? ► ► The Global Positioning System (GPS) is a satellite based navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense beginning in 1976.

How is GPS Used in Society? ► GPS technology is used by any individual or group which needs accurate location data. ► Aeronautic Navigation ► First Responder Emergency Management ► Ground Transportation Management ► Land Use Management ► Law Enforcement ► Marine Navigation ► Natural Resource Management ► Utility Management

Why GPS? ► ► GPS was originally intended for military applications, but in the late 1980’s the government made the system available for civilian use.

Is GPS Restricted? ► ► GPS works in any weather conditions, anywhere in the world. 24 hours a day. There is no subscription fee or setup charges to use GPS.

The Satellites ► ► The 24 GPS satellites circle the earth twice each day in a very precise orbit and transmit signal information to the earth.

About the Satellites ► ► The GPS satellites are orbiting the earth about 12,000 miles above the surface. ► ► The GPS satellites are constantly moving, making two orbits in 24 hours at speeds roughly 7,000 miles per hour. ► ► The GPS satellites are powered with solar energy with backup batteries.

The GPS Unit ► ► The GPS Unit receives the signals from the satellites to determine location and conduct other functions programmed into the unit.

Determining Location ► ► The GPS Unit calculates location based upon geometry and time. ► ► GPS triangulation really is trilateration since it uses geometry rather than angles. ► ► The GPS unit measures distance through the time it takes for the radio signals to be received from the satellites. ► ► The radio waves from the satellites travel at the speed of light (186,000 miles/second)

Calculation of Location ► ► The GPS unit is programmed with information on the orbits of the satellites. ► ► The GPS unit calculates the time it takes for each of the satellite signals to reach the surface to determine distance. ► ► The location on the surface is the point where the GPS unit calculates where all the distances intercept.

The GPS Signal ► ► GPS satellites transmit two low power radio signals designated L1 and L2. Civilian GPS uses the L1 frequency of MHz in the UHF band. The signals travel by line of sight, meaning they pass through clouds, glass, and plastic but will not pass through most solid objects such as buildings and mountains.

What’s in the GPS Signal ► ► Pseudorandom Code ► ► Ephemeris Data ► ► Almanac Data

Pseudorandom Code ► ► ID code that identifies which satellite is transmitting information. Many GPS units will provide information on which satellites it is receiving.

Ephemeris Data ► ► Provides the GPS unit with the information on where each GPS satellite should be at any time throughout the day. Each satellite transmits ephemeris data showing orbital information for that satellite and every other satellite in the system.

Almanac Data ► ► Contains information about the status of the satellite, current date and time. This part of the signal is essential for determining position.

Signal Error ► ► Atmospheric conditions ► ► Signal multi-path ► ► Receiver clock errors ► ► Orbital errors ► ► Number of satellites visible ► ► Satellite geometry ► ► Intentional signal degradation

Atmospheric Conditions ► ► The satellite signal slows as it passes through the ionosphere and troposphere layers of the atmosphere. The GPS system uses a built in model that calculates an average amount of delay to partially correct for this type of error.

Signal Multi-path ► ► This occurs when the GPS signal is reflected off of objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases travel time of the signal, thereby causing errors.

Receiver Clock Error ► ► A receiver’s built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors.

Orbital Errors ► ► Also known as ephemeris errors, these are inaccuracies of the satellite’s reported location.

Number of Satellites Visible ► ► The more satellites the GPS unit can receive, the better the accuracy. Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no reading at all.

Intentional Signal Degradation ► ► Selective availability (SA) is an intentional degradation of the signal once imposed by the Department of Defense. SA was intended to prevent military adversaries from using the highly accurate GPS signals. The government turned off SA in May 2000.

DGPS ► ► Differential GPS ► ► Ground stations which transmit true position ► ► WAAS – Wide Area Augmentation System ► ► DGPS increases accuracy ► ► Non WAAS enabled units accurate to 30 feet ► ► WAAS enabled units accurate to 15 feet

The GPS Unit Functions ► ► Display of Position ► ► Marking Waypoints ► ► Tracks ► ► Compass

Display of Position ► ► The GPS Unit can display position in several coordinate systems ► ► In order to make position data consistent with GIS software set the units to display in latitude and longitude ► ► Use the degrees – decimal option

Waypoints ► ► A waypoint is any point that you store in memory ► ► Waypoints are identified with unique numbers ► ► Prior to recording a waypoint allow the unit time to average the position for greater accuracy ► ► The time you use to average your position is dependent on the estimated accuracy

Tracks ► ► The tracks feature creates an electronic trail as you move ► ► It records and saves your movements ► ► It allows you to retrace your path

Compass ► ► The compass page uses traditional navigation to display your current direction of travel ► ► This is not a magnetic compass ► ► It only provides readings with motion ► ► Most GPS compass pages will provide bearing and current speed

Accuracy, Precision, and Resolution ► ► Accuracy – Relationship to True Value ► ► Precision – Relationship Between Data Points ► ► Resolution - Smallest increment the GPS unit can measure or ability to distinguish between different points

Recording GPS Data Points When recording GPS data points you want to use a spread sheet format

The Data Sheet Columns ► ► Waypoint or identification number ► ► Latitude ► ► Longitude ► ► Descriptive data

Improving Accuracy ► ► Check satellite geometry ► ► Further apart the satellites the greater the accuracy ► ► Increase height ► ► If you are in a canyon or under canopy cover increasing height may increase signal or increase number of satellites being received ► ► Use multiple GPS units and average readings

Field Collection Tips ► ► Object which you can not reach the center – Collect data points at corners and average the readings to get center point ► ► When collecting data points where the dots will be connected by GIS software – Collect data points for corners and in between corners ► ► Pay attention to waypoint numbers if you are recording the waypoints in the GPS unit

Support the Data Inputter ► ► Write neatly ► ► Sketch maps ► ► Provide unit number ► ► Provide names of data collectors ► ► Make notes of any special features or problems