Introduction to Global Positioning Systems (GPS) Mark Smith, Assistant Professor/Wildlife Extension Specialist

GPS Basics ► Satellite navigation system ► Maintained by the US Government  All weather  24 hour  Worldwide ► Available free to the public

Global Position “System” ► Space segment  24 satellites emitting radio signals ► Control Segment  US DOD ► User Segment  Your receiver

GPS Basics ► Radio signals sent from orbiting satellites to earth ► Measures receiver-to-satellite distance  How long the signal took to get to your receiver  Speed of light (186,000 m/s)  Atomic clocks ► GPS units receive and convert signals to position, velocity, and time information

GPS Basics ► What’s in a radio signal?  Psuedorandom code – identifies the satellite  Ephemeris data – tells where the satellite should be  Almanac data – tells the health of the satellite ► Constellation of 24 NAVSTAR earth-orbiting satellites ► Need to receive a signal from:  3 satellites for a 2D position  4 satellites for a 3D position

Sources of GPS Error ► Satellite availability ► Satellite geometry ► Ionosphere/troposphere conditions ► Signal blockage (i.e., forest canopy, tall buildings) ► Multi-path (signal bounce) ► Clock error (timing) ► Intentional degradation (the old Selective Availability)

Overcoming some forms of GPS error ► Differential correction  Real time  Post-processing  “Base” station at a known location recording satellite signals simultaneously  Differences in timing are then applied to the signals you recorded with your receiver ► WAAS enable units  Wide Area Augmentation System  Modified GPS signal---with error correction factor  North America only  25 ground reference stations that collect signals and then send correction info to satellites which gets sent to your receiver

Measures of Position Accuracry ► DOP – Dilution of Precision  Indicator of the quality of the satellite constellation  Lower is better  Types of DOP  PDOP – Position Dilution of Precision  VDOP – Vertical Dilution of Precision  HDOP – Horizontal Dilution of Precision

Coordinate Systems ► Latitude/Longitude  Degrees, minutes, seconds (33 o 14’ 32”)  Decimal degrees ( o ) ► Projections  How “3D” objects are converted to “2D” objects  Cylindrical, Conical, and Planar ► Datums  How the earth is shaped  Based on mathematical models of the earth’s shape and dimensions ► Universal Transverse Mercator (UTM)  Cylindrical  60 6-degree slices  Meters ► Example: UTM, Zone 16N, NAD-27

UTM Zones

Which GPS Receiver is Best? ► Deluxe  Survey grade units ► Used for highway and engineering projects ► Horizontal accuracy within a centimeter ► Costs up to $40,000  Mapping/resource grade units ► Used for mapping roads, stand boundaries, and point data ► Relocating data that is not flagged or marked ► 1-5 meter accuracy with good signal ► Usually requires formal training to use ► Download data to Geographic Information System ► Costs begin around $2,000

Which GPS Receiver is Best? ► Economical  Recreational grade units ► General navigation ► Relocating plots, trees, etc. that have been flagged ► Estimating acreage of forest stands ► 3-5 meter accuracy with good signal ► Easy to learn using manual ► Costs $100 - $500

Which GPS Receiver is Best? ► Grade GPS you choose depends on field work and data collection needs  Recreational grade – most forest inventory tasks  Resource/mapping grade- determining harvest boundaries  Recreational grade are good for first-time GPS users

GPS in Forestry ► Navigation ► Pre-harvest and post-harvest stand delineation ► Road and landing layout ► Acre control ► Equipment tracking ► Wildlife uses

Stand Delineation ► Estimating acreages  Tree planting  Herbicide application  Slope/aspect

Wildlife ► GPS collars  Large mammals  Waterfowl 155 gm 760 gm 350 gm 1,100 gm

Questions/Discussion