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