Presented By: Scott Rodgers UNC Chapel Hill, Engineering Information Services Types of GPS Receivers

Topics Accuracy vs. Precision Accuracy vs. Precision Grades of GPS Receivers Grades of GPS Receivers Field Experiment Field Experiment Sources of GPS Error Sources of GPS Error Planning a GPS Survey Planning a GPS Survey

Definitions Precision: Precision: Repeatability of a measurementRepeatability of a measurement Accuracy: Accuracy: The proximity to the TRUE value, or accepted value of a measurementThe proximity to the TRUE value, or accepted value of a measurement

Example In general, GPS receivers are precise but not accurate.

Grades of GPS Receivers Consumer Consumer Garmin Street PilotGarmin Street Pilot Garmin eTrexGarmin eTrex Mapping Mapping Trimble Pro- XR/XRSTrimble Pro- XR/XRS Trimble GeoXH/XTTrimble GeoXH/XT Geodetic/Survey Geodetic/Survey Trimble 5800Trimble 5800 Trimble R7/R8Trimble R7/R8

Consumer GPS Receivers Track the L1 code frequency only Track the L1 code frequency only L1 Frequency: MHz GPS carrier frequency which contains the C/A-Code, the encrypted P-Code (or Y- Code) and the Navigation Message.L1 Frequency: MHz GPS carrier frequency which contains the C/A-Code, the encrypted P-Code (or Y- Code) and the Navigation Message. 12 parallel channel GPS antenna 12 parallel channel GPS antenna Capable of simultaneously tracking 12 satellites.Capable of simultaneously tracking 12 satellites. Track all visible satellites Track all visible satellites Accuracy: Accuracy: meters15-30 meters

Mapping Grade GPS Receivers Track the L1 code and carrier frequencies Track the L1 code and carrier frequencies Some models capable of tracking L2 carrier frequency Some models capable of tracking L2 carrier frequency Built-in software to resist multi-path error Built-in software to resist multi-path error User definable PDOP and Elevation Masks. User definable PDOP and Elevation Masks.

Mapping Grade (cont.) Differential GPS (DGPS) Capable Differential GPS (DGPS) Capable WAAS – Wide Area Augmentation SystemWAAS – Wide Area Augmentation System CORS – Continuously Operating Reference StationCORS – Continuously Operating Reference Station OmniStar or other Satellite based DGPS sourceOmniStar or other Satellite based DGPS source 12 parallel channel GPS antenna 12 parallel channel GPS antenna Accuracy: Accuracy: Real – time: sub-meterReal – time: sub-meter Postprocessed: 1-50 cmPostprocessed: 1-50 cm

Geodetic GPS Receivers Track the L1 carrier/code frequencies and the L2 carrier frequency. Track the L1 carrier/code frequencies and the L2 carrier frequency. 24 parallel channel GPS antenna 24 parallel channel GPS antenna WAAS Support WAAS Support Built-in software to resist multi-path error Built-in software to resist multi-path error Accuracy: Accuracy: 5-30mm Horizontal/Vertical5-30mm Horizontal/Vertical

Differential GPS (DGPS) Definition: Definition: an enhancement to the Global Positioning System that uses a network of fixed ground based reference stations to broadcast the difference between the positions indicated by the satellite systems and the known fixed positions.an enhancement to the Global Positioning System that uses a network of fixed ground based reference stations to broadcast the difference between the positions indicated by the satellite systems and the known fixed positions.

Differential GPS Sources WAAS – Wide Area Augmentation System WAAS – Wide Area Augmentation System Developed by the FAADeveloped by the FAA Worst case accuracy is 7.6m at 95%Worst case accuracy is 7.6m at 95% This type of system is considered a Satellite Based Augmentation System (SBAS)This type of system is considered a Satellite Based Augmentation System (SBAS)

Differential GPS Sources

Other SBAS sources: Other SBAS sources: StarFire – developed by John Deere engineers to aid in crop yield calculationsStarFire – developed by John Deere engineers to aid in crop yield calculations OmniStar – Mapping and surveying applicationsOmniStar – Mapping and surveying applications EGNOS – European Geostationary Navigation Overlay ServiceEGNOS – European Geostationary Navigation Overlay Service

Differential GPS Sources CORS – Continually Operating Reference Station CORS – Continually Operating Reference Station National network of GPS sites that collect and distribute GPS dataNational network of GPS sites that collect and distribute GPS data Sampling rates vary; 1, 5, 10, 15 and 30-secondsSampling rates vary; 1, 5, 10, 15 and 30-seconds Data available in R IN Ex(RINEX) formatData available in Receiver INdependent Exchange (RINEX) format Some stations also broadcast a Real- time Differential correction signalSome stations also broadcast a Real- time Differential correction signal

Differential GPS Settings Auto Power: The receiver tracks and locks on to the most powerful radio beacon signal. Auto Power: The receiver tracks and locks on to the most powerful radio beacon signal. Auto Range: The receiver tracks and locks on to the nearest radio beacon signal. Auto Range: The receiver tracks and locks on to the nearest radio beacon signal. Manual: The receiver tracks only the frequency you specify in the Frequency field. Manual: The receiver tracks only the frequency you specify in the Frequency field.

Differential GPS Settings Web Resources for Radio Beacon Frequencies Web Resources for Radio Beacon Frequencies

Comparison of different GPS receivers and processing methods Trimble Pro-XR Trimble Pro-XR Terra Sync set to “Production”Terra Sync set to “Production” Terra Sync set to “Precision”Terra Sync set to “Precision” Real Time DGPSReal Time DGPS Post Processed DGPSPost Processed DGPS

‘Production’ vs. ‘Precision’

Real-time vs. Post-Processed

All positions Post-Processed

GeoXH and GeoXT GeoXH GeoXH H-Star TechnologyH-Star Technology GeoXT GeoXT Sub-Meter GPSSub-Meter GPS Both units: Both units: Sub-Meter GPSSub-Meter GPS Post-Process FilesPost-Process Files External DGPS BeaconExternal DGPS Beacon ArcMap or TerraSyncArcMap or TerraSync Microsoft Window MobileMicrosoft Window Mobile

GeoXH and GeoXT Uncorrected Positions

GeoXH and GeoXT Corrected Positions

Comparison of different GPS receivers and processing methods Trimble 5700 Geodetic Receiver Trimble 5700 Geodetic Receiver GPS Data Collected for 15 minutesGPS Data Collected for 15 minutes Comparison of ‘Raw’ and Post-processed positionsComparison of ‘Raw’ and Post-processed positions

Geodetic vs. Mapping Grade

Conclusions?

GPS Error Sources Satellite Geometry Satellite Geometry Measures of satellite geometry are called Dilutions of Precision or DOP’sMeasures of satellite geometry are called Dilutions of Precision or DOP’s GDOP –Overall Accuracy (3-D, Time) GDOP –Overall Accuracy (3-D, Time) PDOP – Positional Accuracy (3D) PDOP – Positional Accuracy (3D) HDOP – Positional Accuracy (2D) HDOP – Positional Accuracy (2D) VDOP – Vertical Accuracy (Height) VDOP – Vertical Accuracy (Height) TDOP – Time accuracy TDOP – Time accuracy

GPS Error Sources Multi-path Multi-path The reflection of GPS signals off buildings, cars, trees, etc.The reflection of GPS signals off buildings, cars, trees, etc.

GPS Error Sources Atmospheric Effects Atmospheric Effects IonosphereIonosphere Electromagnetic particles slow and distort the GPS signal Electromagnetic particles slow and distort the GPS signal TroposphereTroposphere Water vapor slows and distorts the GPS signal Water vapor slows and distorts the GPS signal

GPS Error Sources Clock Inaccuracies and Rounding Errors Clock Inaccuracies and Rounding Errors Relativistic Effects Relativistic Effects Errors in Satellite Orbits Errors in Satellite Orbits

GPS Error Sources Summary: Summary: Ionospheric/Tropospheric Effects: +/- 5.5mIonospheric/Tropospheric Effects: +/- 5.5m Shifts in Satellite Orbits: +/- 2.5mShifts in Satellite Orbits: +/- 2.5m Clock Errors: +/- 2mClock Errors: +/- 2m Multipath Effects: +/- 1mMultipath Effects: +/- 1m Calculation and Rounding Errors: +/- 1mCalculation and Rounding Errors: +/- 1m

Methods to Eliminate Error DGPS – Real-time broadcast solutions DGPS – Real-time broadcast solutions PDOP – Setting receivers so data cannot be collected when PDOP is greater than 6. PDOP – Setting receivers so data cannot be collected when PDOP is greater than 6. SNR – Signal to Noise Ratio; set to minimum of 4. SNR – Signal to Noise Ratio; set to minimum of 4. Elevation Mask – Setting receivers to track only satellites 15 degrees above the horizon. Elevation Mask – Setting receivers to track only satellites 15 degrees above the horizon.

Methods to Eliminate Error Software and antennas designed to resist multi-path interference Software and antennas designed to resist multi-path interference Avoid using high-powered CB radios Avoid using high-powered CB radios

Planning a GPS Survey Tools Tools Trimble’s ‘QuickPlan’ UtilityTrimble’s ‘QuickPlan’ Utility Satellite Visibility Satellite Visibility PDOP Chart PDOP Chart Site Investigation Site Investigation Site VisitSite Visit Getting the lay of the land Getting the lay of the land Google EarthGoogle Earth

Planning a GPS Survey 15 Degree Elevation Mask

Planning a GPS Survey 5 Degree Elevation Mask

Planning a GPS Survey 15 Degree Elevation Mask

Planning a GPS Survey 5 Degree Elevation Mask

Five Degrees or Fifteen degrees?

Questions?