Using RTK GNSS Wisely

Transcription

1 Using RTK GNSS Wisely February 017 Autonomous Positioning Differential Positioning Concept: Detect and cancel identical errors with simultaneous observation. F + E = G + E 1

2 Static & RTK Computations Static survey: RTK survey: post process data in office process data immediately at rover Key Element = Simultaneous Observation Ionospheric Effects Ionospheric effects cancel when receivers are within 10 km (6.1 mi). Real-Time Kinematic Positioning Base receives and rebroadcasts GPS data as RTK message. RTK message Rover receives GPS data and RTK message to compute vector.

3 Single Base RTK Corrections message Correction Correction Known Position Corrected Position Single Base RTK Corrections Single base unit broadcasts uniform RTK corrections in all directions. RTK Positional Error RTK positional error increases with distance from the base. 3

4 Trimble R8 Accuracy Specs Constant error Scalar error RTK Vector Errors Vector is the line from base to rover. 10 mm + 1 ppm horizontal error for RTK vectors is typical. 10 mm = ft = constant error 1 ppm = scalar error (distance dependent) 1 part error to 1,000,000 parts measurement 1 mm error / 1 km RTK vector ft error / 1 mile RTK vector RTK Vector Errors 1 ppm = scalar error (distance dependent) 1 part error to 1,000,000 parts measurement 1 mm error / 1 km RTK vector ft error / 1 mile RTK vector This error approximates the misfit between a uniform RTK correction and reality. RTN eliminates most of this. 4

5 Differential Position Errors RTK vector (miles) E const (feet) E scalar (feet) E dist (feet) E dist ( E const E scalar ) RTK relative accuracy Base can occupy a known point or an unknown (assumed) point. Absolute & relative accuracy Absolute accuracy Accuracy relative to map surface Relative accuracy Accuracy relative to adjacent positions Position = coordinate 5

6 Absolute & relative accuracy A: known B: unknown Relative accuracy = ±0.05 Poor absolute accuracy Good relative accuracy Absolute accuracy = ±5 Absolute accuracy = ±5 A: known B: unknown Relative accuracy = ±0.05 Absolute accuracy = ±0.05 Absolute accuracy = ±0.07 Good absolute accuracy Good relative accuracy C O R S Continuously Operating s CORS is a permanently mounted survey-grade GPS receiver which broadcasts RTK corrections. Each CORS collects GPS data 4 hours per day, 7 days per week, 365 days per year. Stored data from each CORS is available for download to enable survey post-processing. Continuously Operating s The CORS network is the anchor of our National Spatial System. Much higher accuracy than first order monuments. Actively monitored. Tracks crustal motion. Data is used for research. 6

7 Correction Correction 3 Correction 1 Using RTK GNSS Wisely Regional CORS Network Single Base RTK Solutions 1 3 Uniform RTK correction within each zone Potential for mismatched corrections Single Base RTK Solutions 1 Uniform correction within each zone 3 7

8 Correction Correction 3 Correction 1 Using RTK GNSS Wisely Network RTK (RTN) Solutions 1 3 Interpolated RTK correction within polygon Network RTK (RTN) Solutions RTK correction interpolated for rover location 1 3 RTN Flavors All RTN solutions provide a single vector solution from a physical reference station. Common RTN solution methods: Non-physical (VRS) Master Auxiliary Corrections (MAC) 8

9 RTN Solution 1. Data from the reference stations streams to the processing and control center.. The RTK rover sends its approximate position. 3. The processing center selects reference stations forming a polygon around the rover. 4. The processing center transmits correction data to the rover. Master Auxiliary Corrections (MAC) Master Auxiliary Rover position Correction data Streaming GPS data Auxiliary Master Auxiliary Corrections (MAC) Auxiliary Auxiliary Rover computes vector from nearest reference station. Streaming GPS data Master 9

10 MAC & VRS MAC networks make the rover compute RTN corrections. VRS networks compute RTN corrections at the control center. What is a VRS? A virtual reference station is the center of an area for which an RTN correction applies. First VRS is your first initialization point. Control center creates new VRS points as you move. Virtual (VRS) Rover position Virtual VRS correction Streaming GPS data 10

11 Virtual (VRS) Virtual 1 Virtual Streaming GPS data Rover computes vector from nearest reference station. RTN Positional Error RTK constant error still applies. PPM error (distance dependent) approaches zero. Trust but Verify 11

12 Truth versus Address Physical monument Truth Relatively stable (in most of CONUS) Point coordinate Merely an address Contains error Subject to change New Accuracy Standards for NGS Datasheets ( Local Accuracy: adjacent points Network Accuracy: relative to CORS Numeric quantities, units in cm (or mm) Both are relative accuracy measures Will not use distance dependent expression Order/Class codes will no longer be used OLD: Order & Class Codes LC1766 *********************************************************************** LC1766 CBN - This is a Cooperative Base Network Control. LC1766 DESIGNATION - ALEXANDER LC1766 PID - LC1766 LC1766 STATE/COUNTY- IL/PIATT LC1766 USGS QUAD - SEYMOUR (1970) LC1766 LC1766 *CURRENT SURVEY CONTROL LC1766 LC1766* NAD 83(1997) (N) (W) ADJUSTED LC1766* NAVD (meters) 759. (feet) GPS OBS LC1766 LC1766 X - 18, (meters) COMP LC1766 Y - -4,883, (meters) COMP LC1766 Z - 4,087, (meters) COMP LC1766 LAPLACE CORR (seconds) DEFLEC99 LC1766 ELLIP HEIGHT (meters) GPS OBS LC1766 GEOID HEIGHT (meters) GEOID99 LC1766 LC1766 HORZ ORDER - B LC1766 ELLP ORDER - FOURTH CLASS I 1

13 NEW: Relative Accuracy LC1766 *********************************************************************** LC1766 CBN - This is a Cooperative Base Network Control. LC1766 DESIGNATION - ALEXANDER LC1766 PID - LC1766 LC1766 STATE/COUNTY- IL/PIATT LC1766 COUNTRY - US LC1766 USGS QUAD - SEYMOUR (1970) LC1766 LC1766 *CURRENT SURVEY CONTROL LC1766 LC1766* NAD 83(011) POSITION (N) (W) ADJUSTED LC1766* NAD 83(011) ELLIP HT (meters) (06/7/1) ADJUSTED LC1766* NAD 83(011) EPOCH LC1766* NAVD 88 ORTHO HEIGHT (meters) 759. (feet) GPS OBS LC1766 LC1766 LC1766 FGDC Geospatial Positioning Accuracy Standards (95% confidence, cm) LC1766 Type Horiz Ellip Dist(km) LC LC1766 NETWORK LC LC1766 MEDIAN LOCAL ACCURACY AND DIST (039 points) LC Observed positions and errors Positions lose their credibility without error estimates. Mean = distance measurements Mean = feet Standard deviation = ±0.10 feet 13

14 Mean = distance measurements Mean = feet Standard deviation = ±0.04 feet Standard Deviation 99% 95% 68% 14

15 D Position Quality 65% confidence 39% confidence Observed Position (address) DRMS ( E 1 E ) Commonly Known As HRMS Trimble Topcon DCQ Leica Observed Position (address) DRMS ( E 1 E ) 15

16 Positional Errors HRMS = 0.03 ft HRMS = 0.03 ft ft calculated ft ft Effects of Positional Errors HRMS = 0.03 ft HRMS = 0.03 ft ft calculated Any of these lines are possible. Weakness of GNSS HRMS = 0.03 ft HRMS = 0.03 ft ft calculated ft ft 16

17 Match the tool to the task 80.00ft 80.00ft 80.00ft 80.00ft 80.00ft 80.00ft Can you stake this straight boundary line accurately with GNSS? Match the tool to the task GPS methods give greater accuracy over long distances. Total station methods give greater accuracy over shorter distances. Confidence Levels Ground Truth 99% 95% Observed Position 65% 65% confidence = 65% probability that the TRUTH falls within 1 HRMS of the ADDRESS (mean). 95% confidence = 95% probability that the TRUTH falls within HRMS of the ADDRESS (mean). 17

18 Reported Precision Displayed at data collector Specified in contracts Reported in NGS datasheets Reported in OPUS solutions Comparing Results Previous Observation Current Observation Difference Northing ft Easting ft HRMS ft ft 65% confidence overlap Radius = 1 X HRMS Observed Coordinate Published Coordinate 18

19 95% confidence overlap Observed Coordinate Published Coordinate Radius = X HRMS Strengthening Field Practices DOP is always changing PDOP < 3 gives best results 19

20 Adding Redundancy Repeat the observation with different satellite geometry. Re-measure control points before, during, and after survey session. Satellite geometry changes. 9:00 AM 11:00 AM 0

21 MOLA to RV 10.8 Km Day 64 dh (m) Day 65 Importance of dh (m) Mean dh (m) 14:00-14: :00-17: :30-15: :30-18: :00-15: :00-18: :30-16:00 Redundancy :30-19: :00-16: :00-19: :30-17: :30-0: :00-17: :00-0: :30-18: :30-1: :00-18: :00-15: :30-19: :30-16: :00-19: :00-16: :30-0: :30-17: :00-0: :00-14: :30-1: :30-15: Mean 14:00-1: :00-1: Two Days / Same Time > Spread = m Mean = Difference = 0.03 m Two Days/ Different Times > Spread = m Mean = Difference = m Repeat with a new base location Measuring points in the network that have already been located creates redundancy. Avoiding Multi-Path Error Survey Manual Appendix M 1

22 Avoiding Multi-Path Error Set the base at a wide open site. Expect multi-path near buildings. Chain link fences reflect GPS signals. Mesh openings are too small to allow signals to pass. Set offset points with GPS from which to measure by other means. Raise antenna. Wrap Up What was the most valuable thing you learned today? Contact Information Todd.Horton.PE.PLS@gmail.com (17) mobile thorton@parkland.edu (17)