DGPS 28oct,2010 Edusat

8/10/2019 DGPS 28oct,2010 Edusat

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Principles of Differential GPS,

Surveying methods andintegration with GIS themes

Ashutosh Bhardwaj([email protected] )Photogrammetry and Remote Sensing Division

Indian Institute of Remote Sensing


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Remote Sensing

Acquiring Information from remotely sensed data

GIS

Data management, manipulation, analysis and visualization in

computer environment

GPS

Accurate positional information

Integration of disciplines dealing with spatial information

Photogrammetry

Science/technology of obtainingspatial measurements/information

from photographs.


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Position and coordinates of GCPs(known location).

The distance and direction between your positionand a destination/waypoint/GCP.

Velocity and travel progress reports.

Time measurements.

Serves as data input to GIS besides navigation.

Integration with aerial photos or satellite images.

Basic Functions of GPS


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Features Point Line

Area

Features can have different attributes and theirvalues.

Trimble Pathfinder Office Software. Leica GeoOffice / SkiPro. Spectrum, Sokkia.


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String of positions converted into a line / area Mapping Grade Receivers : Collected directly in the

field as a line / area feature Recreational Grade Receivers : Collected as a

TrackLog then converted back in the office

Point features are one or more positionsaveraged into a point

Mapping Grade Receivers : Called Points Recreational Grade Receivers : Called Waypoints

PointLine Area

A GPS View of GIS Features


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http://www.aero.org/education/primers/gps/index.html


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How Well Does It Work?

Navigation Accuracy Comparisons

Omega- 2km

Inertial- 1km

TACAN- 400m

Transit- 200m

LORAN C- 180m

GPS- 15mDGPS gives cm/mm level accuracy


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Method to remove errors from GPS measurements

Uses a GPS receiver at a fixed, surveyed location i.e. base

station, to measure error in pseudo range

Base station receives the same GPS signals as the rovingreceiver and instead of using timing signal to calculate itsposition, it uses its known position to calculate timing.

Pseudorange error for each satellite is subtracted fromrover before calculating position during RTK mode or postprocessing

Signal time at reference location is compared to time at

remote location. Time difference represents error insatellites signal; clock bias

DGPS


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Differential GPS Correction

Post - Processed


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Differential GPS Correction Signal-RTK

Real - Time

Uses standardized output: RTCM-104 Binary code, not directly readable Receivers often provide RTCM-104 on a RS-232 serial port


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Example: Field Work- Static

Base

R1 R2

R1/3

R2/4


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In the post processing, some of the errors between thebase station and rover data can be recognized, & corrected.

Requires post-processing of field data.

(1) download the rover data

(2) download the base station data

(3) use software to make the corrections

Remove or reduce all errors except multipath.

The Technique


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Differencing Techniques

Single Differencing: Eliminate satellite clock bias.

Double Differencing:

Eliminate receiver clock bias. Triple Differencing:

Eliminate integer ambiguity. Identify and remove the cycle slips.


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Sources of error

1. [Selective availability]

2. Clock errors

3. Ephemeris errors

4. Atmospheric delays

5. Multipath effects6. Receiver errors


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Sources Of Error

Geometric Degree of Precision(GDOP)

Selective Availability Discontinued from 1 st May 2000

Atmospheric Effects

Ionospheric Tropospheric Multipath Ephemeris Error

(satellite position data) Satellite Clock Error Receiver Clock Error


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GPS Errors Diff. Corrected

noReceiver noise

noMultipath

yesClock and Ephemeris

partialAtmospheric


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Typical Error Budget (m)

~ 0#Choke Ring Antenna, openarea

0.6Multi-path (reflections)

0.30.3Receiver errorsSub-meter accuracy~10Typical totals

0.20.5Troposphere delays~ 05.0Ionosphere delays~ 02.5Ephemeris errors~ 01.5Satellite clocks

Differential GPSStandard

GPSSource:


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Error Mitigation Techniques Carriers at L1 and L2 frequencies

Ionospheric error is frequency dependent sousing two frequencies helps to limit error

Differential GPS Post-Process user measurements usingmeasured error values

Space Based Augmentation Systems(SBAS) SBAS provides atmospheric, ephemeris and

satellite clock error correction values in realtime

Examples are U.S. Wide Area AugmentationSystem (WAAS), European GeostationaryNavigational Overlay Service (EGNOS)

GAGAN, India


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GPS Data Post Processing Software

Commercial Software SKIPRO from LEICA GPSURVEY from TRIMBLE GPSS from ASHTECH SPECTRUM SURVEY from SOKKIA


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Scientific Post Processing BERNESE from University of Berne,

Switzerland GAMIT from MIT, USA GIPSY from Jet Propulsion Laboratory, USA DIPOP from University of Brunswick, Canada

GEONAP from University of Hannover,Germany

GPS Data Post Processing Software


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Juno ST GPS HandheldStylus holder Headset connector

Reset

button

GPSantennaconnector

Powerbutton

Lockswitch

Power/USB socket

SD cardslot

Directionalbutton

Contacts

MessagingTodayscreen

Calendar

Integration with latest technologies GPS Mobile Camera GIS s/w TS + GPS

MIO


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GIS/GPS Field Mapping Tools

GPS unit + Handheld PC + ArcPad:

Realtime display of vector /raster data Hook to any GPS Unlimited Storage (dependent on storage

capacity of device) Instant shapefile creation

Garmin and IPAQrunning ArcPad


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128128128512512512Logging Memory(MB)

NoNoNoNoNoYesH-Star Technology

NoNoNoNoYesYesEverest MultipathRejection Technology

Yes

Yes

Yes

2-5 m

XC Edition

2-5 m2-5 m1-3 mSubmeter


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Communications

Surveying &Mapping

Fishing &Boating

Off shoreDrilling

Recreation

Trucking &Shipping

Personal Navigation

Aviation

Railroads

Power GridInterfaces

Use of GPS/DGPS Precision farming


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References Gunter Seeber: Satellite Geodesy, 2 nd Edition, Walter de Gruyter,

Germany www.garmin.com/aboutGPS/waas.html

http://www.gdgps.net/ http://gipsy.jpl.nasa.gov/igdg/system/index.html http://www.glonass-ianc.rsa.ru/pls/htmldb/f?p=202:1:15000421459964108253

http://igscb.jpl.nasa.gov/ http://www.navcen.uscg.gov/gps/precise/default.htm http://www.aero.org/education/primers/gps/index.html

Interface Control Documents: http://www.navcen.uscg.gov http://www.Glonass-ianc.ras.ru http://www.Galileoju.com