Differential Gps (a Method Of Processing The Gps Data)

Differential GPS (A method of processing the GPS Data) Ashutosh Bhardwaj ([email protected]) IIRS

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

DGPS: What it serves?

High Accuracy

Steps for Data Preparation & Integration • Geocoding: assigning geographic coord. to points – Perhaps the most basic form of spatial data entry • data media conversion : scanning/digitizing • data format conversion-raster & vector • data reduction • Topology, error detection and topological editing • rectification and registration (one on top of the other) – overlaying sheets and referencing to the real world • edge matching & image adjustment (side by side) – linking & balancing adjacent sheets • interpolation • GIS Application

How Well Does It Work? Navigation Accuracy Comparisons DGPS gives cm level accuracy GPS- 15m LORAN C- 180m Transit- 200m TACAN- 400m

Inertial- 1km

Omega- 2km

DGPS ¾ Method of removing errors that affect GPS measurements ¾ Uses a GPS receiver at a fixed, surveyed location i.e. base station, to measure error in pseudo range signals from satellites ¾ Base station receives the same GPS signals as the roving receiver and instead of using timing signal to calculate its position, it uses its known position to calculate timing. ¾ Pseudorange error for each satellite is subtracted from rover before calculating position during: a) post processing of data or b) real time i.e. kinematic mode.

DGPS ¾ Signal time at reference location is compared to time at remote location ¾ Time difference represents error in satellite’s signal; clock bias

Differential GPS Correction

Real - Time

Differential GPS Correction

Post - Processed

Differential GPS

Example: Field Work- Static

R1

R2

R1/3 Base R2/4

The Technique

Differential Correction

• In the post processing, some of the errors between the base 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.

Differential Correction Signal • Uses standardized output: RTCM-104 • Binary code, not directly readable • Receivers often provide RTCM104 on a RS232 serial output port

Sources of error 1. [Selective availability] 2. Clock errors 3. Ephemeris errors 4. Atmospheric delays 5. Multipath effects 6. Receiver errors

GPS Errors Diff. Corrected Atmospheric

partial

Clock and Ephemeris

yes

Multipath

no

Receiver noise

no

Typical error “budget” (m)* Source: Satellite clocks Ephemeris errors Ionosphere delays Troposphere delays Multi-path (reflections) Receiver errors Typical totals

Standard GPS

Differential GPS

1.5 2.5 5.0 0.5 0.6

~0 ~0 0.4 0.2 0.6

0.3 10 – 15

0.3 1-2

Differential GPS Explained. J. Hurn. Trimble Navigation

GPS Data Post Processing Software • Commercial Software – SKIPRO from LEICA – GPSURVEY from TRIMBLE – GPSS from ASHTECH – SPECTRUM SURVEY from SOKKIA

Post Processing Software • 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

Use of GPS/DGPS Power Grid Interfaces

Personal Navigation Trucking & Shipping

Precision farming

Surveying & Mapping

Aviation Railroads

Recreation

Off shore Drilling

Communications

Fishing & Boating

GPS Opportunities in India • Indian GPS market can be broadly divided in following segments • National Survey Agencies(NRSA, SOI,Mil. Survey,NHO,) • Land Records (Kerala,TN,J&K, Rajasthan,Gujrat, etc.) • Oil and Energy( OIL, ONGC, BPCL, HPCL) • Power(NHPC Assam, Himachal, J&K,Arunachal, Kaveri Basin) • Defense • Scientific Institution(IIGM,CMMAC,CESS,WIHG, NGRI, SAC,GSI)

GPS Opportunities in India • Mining and Consultancy Engineering /Infrastructure(IRCON,RITES,Delhi Metro, NHAI) • Private Surveying Companies(ICT,Theowell, Secon, Shiv Vani) • Marine and Maritime Boards(DGLL,Port Trust,MMB, ICMAM, NIO) • Remote Sensing and Forestry • Education • GIS

Advantage of Differential GPS ?

improved accuracy has a profound effect on the importance of GPS as a resource.

?

It becomes more and more useful for applications where cm accuracies are required, like:

1. Urban mapping. 2. Alignment: Road, Railways, canal, 3. Cartography,etc.

References • Gunter Seeber: Satellite Geodesy, 2nd Edition, Walter de Gruyter, Germany • Internet Resources • http://www.aero.org/education/primers/gps/index.html • www.garmin.com/aboutGPS/waas.html • http://gipsy.jpl.nasa.gov/igdg/system/index.html