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Session 03 01 GPS Surveying 1
GPS Surveying
Slide 1
GPS Surveying
Surveying Mapping
Standalone Relative
Post-Processed Real-Time
Static / Fast Static Stop & Go
Kinematic Rapid-Static
StandaloneRelative
Carrier-Phase(RTK)
Pseudo-Range(DGPS)
Session 03 01 GPS Surveying 2
Pseudo-Ranges (Code)Measurement
from satellite
from ground receiver
measure time difference between same part of code
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 03 01 GPS Surveying 3
Carrier Phase Differencing
Slide 5
Ambiguity Resolution
∆λ∆λ∆λ∆λ = First Partial Wavelength
N = Integer Ambiguity
Solving for the IntegerAmbiguity yields
centimeter precision
Slide 6
Session 03 01 GPS Surveying 4
The Integer Ambiguity
∆λ∆λ∆λ∆λ = First Partial Wavelength
N = Integer Ambiguity
Solving for the IntegerAmbiguity 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 03 01 GPS Surveying 5
Results
Baseline or Vector (cm precision)
Azi = 212o 42’ 49.8244”Dist = 557.05307 m
DElev = 4 .8751 m
∆X = -408.251 m
∆Y = -84.830 m
∆Z = -369.413 mOR
Slide 9
ECEF Coordinate System
+Z
-Y
+X
X
Y
Z
ECEFX = -2691542.5437 mY = -4301026.4260 mZ = 3851926.3688 m
Slide 10
Session 03 01 GPS Surveying 6
Reference Ellipsoid
a
b
a = semi-major axisb = semi-minor axis
Flattening f(a b)
a=
−
b
φ
Hheight lellipsoida H
longitude
latitude
≡
≡
≡
λ
φ
WGS-84 Ellipsoid
a = 6378137.000000 mb = 6356752.314245 m1/f = 298.2572235630
Slide 11
ECEF and WGS-84
ECEFX = -2691542.5437 mY = -4301026.4260 mZ = 3851926.3688 m
WGS-84φ = 37o 23’ 26.38035” Nλ = 122o 02’ 16.62574” Wh = -5.4083 m
+Z
-Y
+X
X
Y
Z
b
φ
h
Slide 12
Session 03 01 GPS Surveying 7
GPS Heights vs Elevations
MSL height was derived from the ellipsoidal heights using the following formula:
H = h – N accurate to ± 5cmWhere
H = Orthometric / MSL height derived from GPS/EGM96h = Ellipsoidal (GPS) heightN = 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 20-30 km, single or dual frequency receiver)
- Rapid Static Technique (min 10-15 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 03 01 GPS Surveying 8
Network
Adjustment
Troubleshoot
Evaluate
Results
Process
Data
Import
Data
GNSS/GPS Surveying Cont…
Perform
Survey
Assemble
Equipment
Plan Project
Slide 15
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 03 01 GPS Surveying 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
280
W 1
22o
02
’ 0
0”
N 37o 23’ 00”
Slide 18
Session 03 01 GPS Surveying 10
Good Satellite Geometry
Slide 19
Poor Satellite Geometry
Slide 20
Session 03 01 GPS Surveying 11
Obstructions
Slide 21
Obstructions - Cycle Slips
Slide 22
Session 03 01 GPS Surveying 12
Obstructions - Multipath
Slide 23
Elevation Mask
15 Degrees above horizon
Atmosphere
Slide 24
Session 03 01 GPS Surveying 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 03 01 GPS Surveying 14
Project Control
Slide 27
Project ControlGood Network Geometry
Slide 28
Session 03 01 GPS Surveying 15
Project ControlPoor Network Geometry
Slide 29
Project ControlBad Network Geometry
Slide 30
Session 03 01 GPS Surveying 16
End of Session
Slide 31
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