GPS Receivers: Basics and Selection T.S. Stombaugh, J.D. Luck and S.A. Shearer Biosystems and Agricultural Engineering University of Kentucky

Global Positioning System  Space-based radio navigation facility  System of 24 Rockwell satellite vehicles (plus a few obsolete) maintained by the DoD  Altitude is 12,500 miles  Radio signals broadcast from satellites allow users to calculate position

Distance to Satellite  Lightening vs. Thunder  Radio signals travel at the speed of light  Let’s put some numbers on it  Altitude = 12,500mi  Speed of light = 186,000 mi/s  Transmission Time = s  What happens if we are off a little?  s * 186,000 mi/s = 186 mi

Triangulation

Satellite Position  Satellites broadcast several pieces of information including  Atomic clock corrections  Ephemerides - parameters to describe position and velocity of each satellite within its orbit

GPS Errors  Clock  Ephemeris  Satellite Configuration  Atmospheric interference  Multipath  Selective Availability (SA)

Dilution of Precision Low DOPHigh DOP

Multipath Error

Atmospheric Interference

Selective Availability  The intentional dithering of timing and ephemeris information by the DoD intended to prevent hostile use of GPS technology.

Local Area DGPS  Establish your own base station  Radiobeacon (USCG)  National DGPS (NDGPS)  Local Area Augmentation System (LAAS)  RTK Arrays  RTK Networks More on these later….

Roving Receiver Correction Signal Base Station

USCG Radiobeacon Correction

Wide Area DGPS Sources  Commercial satellite providers  Omnistar  Racal  Deere  Wide Area Augmentation System (WAAS)  Global DGPS (GDPS)

Roving Receiver Correction Signal Base Station

RTK DGPS  Real Time Kinematic or, Carrier Phase DGPS  Accuracy of Mobile Rovers: under 2.0 cm horiz. and 5.0 cm vert.  Accuracy of Static Surveying Units: under 1.0 cm horiz. and 2.5 cm vert.

RTK Correction Signal RTK Roving Receiver RTK Base Station GPS Satellites

Static Accuracy 1 m

Terms  Error – difference between measurement and actual value  Accuracy – lack of error  Precision – random errors  Bias – systematic errors

Reporting Methods  Visualize circles that contain the majority of data points:  CEP – 50% of measurements  95% CEP - 95% of measurements  2DRMS – 95.4 to 98.2% of measurements  1 Sigma – 66.7% of measurements  2 Sigma – 95% of measurements  Be certain that you compare apples to apples -- be aware of accuracies reported as CEP or 1 Sigma!

Static Accuracy – 95% CEP 0.83 m

Dynamic Testing

Ideal Path 15 m

Test Results

Filter Evaluation No Filter Filter

Purchase Considerations  Update rate (1, 5 or 10 Hz)?  Accuracy?  Correction signal capabilities (Radiobeacon, commercial satellites, or WAAS)?  Compatibility (RS-232 or CAN; data format)?  Reliability?  Service after the sale?  Expected life?

Interfacing  User Interface  On-unit programming  External programming  Data Interface – Software  NMEA 0183  Data Interface – Hardware  RS-232  CAN

Satellite Positioning Systems  GPS (United States)  GLONASS (Russia)  EGNOS (European) - GALILEO

Summary  Update rate  Accuracy  Correction Method  Compatibility  Reliability  Service  Useful Life