Surveying. Session GPS Surveying 1. GPS Surveying. Carrier-Phase (RTK) Pseudo-Range (DGPS) Slide 1

Transcription

1 GPS Surveying Slide 1 GPS Surveying Surveying Mapping Standalone Relative Relative Standalone Post-Processed Real-Time Static / Fast Static Kinematic Stop & Go Rapid-Static Carrier-Phase (RTK) Pseudo-Range (DGPS) Session GPS Surveying 1

2 Pseudo-Ranges (Code) Measurement measure time difference between same part of code from satellite from ground receiver Slide 3 Carrier Phase Measurement Used in high-precision survey work Can generate sub-centimeter accuracy The ~20 cm carrier is tracked by a reference receiver and a remote (user) receiver simultaneously The carrier is not subject to S/A (due to double differencing) and is a much more precise measurement than pseudo-ranges (code). Requires bookeeping of cycles ( subject to slips ): Ionospheric delay differences must be small enough to prevent full slips Requires remote receiver be within ~30km from the base From post-processed mode to Real Time Kinematic (RTK) Slide 4 Session GPS Surveying 2

3 Carrier Phase Differencing Slide 5 Ambiguity Resolution λ = First Partial Wavelength N = Integer Ambiguity Solving for the Integer Ambiguity yields centimeter precision Slide 6 Session GPS Surveying 3

4 The Integer Ambiguity λ λ = First Partial Wavelength N = Integer Ambiguity Solving for the Integer Ambiguity yields centimeter precision Slide 7 How to Resolve The Integer Ambiguity Float Integer ambiguity not resolved. Fix Integer ambiguity resolved, RTK system initialized Initialization is the process of resolving integer ambiguities. Three initialization methods: - Known Point - New Point - On-the-Fly (OTF) Slide 8 Session GPS Surveying 4

5 Results Baseline or Vector (cm precision) Azi = 212 o Dist = m OR DElev = m X = m Y = m Z = m Slide 9 ECEF Coordinate System ECEF X = m Y = m Z = m +Z Z X Y -Y +X Slide 10 Session GPS Surveying 5

6 Reference Ellipsoid a = semi-major axis b = semi-minor axis (a b) Flattening f = a H φ latitude λ longitude H ellipsoidal height b a φ WGS-84 Ellipsoid a = m b = m 1/f = Slide 11 ECEF and WGS-84 ECEF X = m Y = m Z = m +Z WGS-84 φ = 37 o N λ = 122 o W h = m h Z φ Y X b -Y +X Slide 12 Session GPS Surveying 6

7 GPS Heights vs Elevations MSL height was derived from the ellipsoidal heights using the following formula: H = h N accurate to ± 5cm Where H = Orthometric / MSL height derived from GPS/EGM96 h = Ellipsoidal (GPS) height N = Geoid height (based on MyGeoid) Slide 13 GNSS/GPS Surveying Conventional GPS Surveying Techniques (requires two or more GPS receivers) - Static Technique (min 1 hour observation, station separation km, single or dual frequency receiver) - Rapid Static Technique (min minutes, Station separation <10km, dual frequency receiver) - Kinematic Technique (normal kinematic or RTK) Modern GPS Surveying Techniques - MyRTKnet (requires only ONE GPS receiver) - VRS-RTK Technique (few minutes) Slide 14 Session GPS Surveying 7

8 GNSS/GPS Surveying Cont Network Adjustment Plan Project Assemble Equipment Slide 15 Troubleshoot Evaluate Results Process Data Perform Survey Import Data GNSS/GPS Surveying Cont Elements of the GPS survey task: Definition of the task: How many points? Accuracy required? Horizontal & Vertical? Connection to datum? Distribution of points? Resources available? etc. Slide 16 Session GPS Surveying 8

9 GNSS/GPS Surveying Cont Planning: logistical considerations, connection to control, standards & specs for GPS surveys, number of receivers/parties, site selection, observation schedule, etc. Reconnaissance: satellite visibility & availability, site conditions & access, station marking, etc. Field procedures: equipment checklist, on-site procedures. Office procedures: (Post-processing & result presentation) baseline processing, minimally constrained solutions, fitting GPS network results to geodetic control, QC, heights, etc. Slide 17 Project Area W 122 o N 37 o Slide 18 Session GPS Surveying 9

10 Good Satellite Geometry Slide 19 Poor Satellite Geometry Slide 20 Session GPS Surveying 10

11 Obstructions Slide 21 Obstructions - Cycle Slips Slide 22 Session GPS Surveying 11

12 Obstructions - Multipath Slide 23 Elevation Mask 15 Degrees above horizon Atmosphere Slide 24 Session GPS Surveying 12

13 Atmospheric Effects < 10 km > 10 km Slide 25 Network Design Acquire control within project area. Use good network geometry. Incorporate independent baselines. Build network redundancy. Require two independent occupations per station. Use stations with low multipath. Do not let logistical constraints degrade network design. Slide 26 Session GPS Surveying 13

14 Project Control Slide 27 Project Control Good Network Geometry Slide 28 Session GPS Surveying 14

15 Project Control Poor Network Geometry Slide 29 Project Control Bad Network Geometry Slide 30 Session GPS Surveying 15

16 End of Session Slide 31 Session GPS Surveying 16