4. What is GPS
Space Segment
Control Segment
User Segment
User Segment
The Global Positioning System (GPS) is a Constellation of
Earth-Orbiting Satellites for the Purpose of Defining
Geographic Positions On and Above the Surface of the
Earth.
5. Space Segment
•Very high orbit
20,200 km
1 revolution in
approximately 12
hrs
• For accuracy
• Survivability
• Coverage
It uses range measurements based on radio signals
• 24 satellites
• Constellation has
Spares 8 Satellites
• 6 planes with 55°
Inclination
• Each plane has 4 or 5
satellites
• Broadcasting position
and time
information on 2
frequencies
L1 and L2
6. GPS ACCURACY
Depends on some variables
Can be more than 15 meters with non-differential GPS
Sub-meter to 5 meters with differential GPS (DGPS)
Sub-centimeter accuracy from survey products (GNSS)
7. Selective Availability
U.S Government can (and does!) degrade the
accuracy
Largest source of error in GPS
To prevent hostile forces from using GPS to full
accuracy
By introducing intentional errors in timing signals and/or
satellite ephemeris
S/A was removed on 02.05.2000
15. SBAS (Satellite Based Augmentation System) SYSTEMS
1.WAAS-Wide Area Augmentation System (US)
2.EGNOS- European Geostationary Navigation Overlay Service
3. GAGAN- GPS Aided Geoaugmented Navigation (India)
4.MSAS- The Multi-functional Satellite Augmentation System, Japan
5. SDCM- The System for Differential Corrections and Monitoring, Russia
6.SNAS- Satellite Navigation Augmentation System, China
SBAS is the improvement of the positioning Service of the GNSS
17. RTK – Real Time Kinematic
First introduced in early 1990s
Differential correction source
Requires a local base station
Originally radio based, also
available via cellular/IP (NTRIP
or IBSS)
Capable of some of the highest
accuracy measurements
possible
18. OmniSTAR is a satellite-based augmentation system (SBAS) service provider. OmniSTAR correction
signals are proprietary, and a subscription must be bought from the OmniSTAR corporation to receive
a subscription authorization. OmniSTAR uses geostationary Satellites in eight regions covering most of
the landmass of each inhabited continent on Earth.
I. MSV-E, MSV-C, MSV-W (North America)
II. AMSAT (Central and South America and the Caribbean)
III. AORWH (Atlantic Ocean East Coast)
IV. AOREH (Atlantic Ocean Europe/Africa)
V. EUSAT (Europe and Africa)
VI. IORHN (Indian Ocean Region)
VII. APSAT (Asia, Australasia, Western Pacific, Eastern Africa, Southern Africa)
VIII.OCSAT (Australia, New Zealand)
To access the OmniSTAR solution the user must have an OmniSTAR-capable receiver.
OmniSTAR capable receivers are available from a number of GPS manufacturers
such as Trimble, Topcon, Autofarm, Geneq, Hemisphere GPS, NovAtel, and Raven.
19. RTK – How it Works
GNSS receiver “base station” at a
known location observes the
same satellites as the “rover”
Removes errors by canceling
their effects rather than modeling
Transmits these corrections via
radio or cellular connection to any
rovers connected to base station
21. Advantage of CORS Infrastructure
No local Reference Station required
Seamless and accurate positioning
Consistent coordinate frame
Higher Productivity, Reliability and Accuracy
Quick and accurate.
Economical.
Makes Surveying simple and uniform.
Use established mobile wireless infrastructure
Does not require radio frequency license
29. ACCURACY (RMS ERROR)
SOI (Survey of India) LOCAL REFRENCE STATION
STATIC METHOD OF OBSERVATIONS (1Base+ 1Rover) : 3 mm +0.1
PPM
RTK METHOD OF OBSERVATIONS (1Base+ 1Rover) : 8 mm +1PPM
(Stand alone RTK)
Omni STAR Positioning with RTK
(SBAS Corrections: OMNISTAR & RTX)
Omni STAR XP/HP Service accuracy: 10cm (0.1m or 0.66
feet)
Omni STAR G2 service combines GPS plus GLONASS
based corrections to provide decimeter level positioning
Omni STAR VBS offering sub-meter positioning
30. ACCURACY (RMS ERROR)
Centre Point RTX Subscription accuracy : 1-2 cm Hz
(Satellite/Cellular/Internet) <5 cm V
ACCURACY - 1cm, 10 cm,30cm, 50 cm
Network RTK (CORS Stations) : <1 cm accuracy
31. RECOMMENDATIONS
Observation time in Static method can be reduced from 15 minutes to 10 minutes
depending on the field conditions.
Ambiguity will be resolved with 6 constellations with 10 minutes of observation.
And RMS error can be reduced from 20 cm to 5 cm in Static method.
RMS error in RTK method can be reduced from 20 cm to 15cm depending on the instrument
used by the User agency, canopy, hill slope regions, highly undulated areas, inaccessible
areas in the Forest.
Above parameters can be fixed after detailed study in the field by GIS team.
33. A.P.Survey & Land Records established CORS (Continuously Operating Reference Stations)
Network throughout Andhra Pradesh.
A total of 72 CORS Base stations with 25 km x25 km buffer covering every corner in Andhra Pradesh
With MOU/ Nominal fees, our Department can utilize the CORS Base Stations with a single ROVER
in Static and RTK methods with 1 cm accuracy.
No need of using Local Reference stations (SOI-GCP)
No need of using Centre Point RTX Positioning
T. S. Survey & Land Records Under SOI (SWAMITWA) Project establishing CORS
Network throughout Telangana.
A total of 35 CORS Base stations with 25 km x25 km buffer covering every corner in Telangana.
CORS NETWORK
It is important to explain the different levels of accuracy that can be obtained. Remember that without differential correction accuracy is 100 meters 95% of the time (2DRMS).
The accuracy of a GPS depends on the following:
1. Time Spent on Measurements - Logging a number of positions and averaging them is more accurate than recording a single position.
2. Receiver type - Different receivers are designed to obtain different levels of accuracy. Survey products are designed to achieve sub-centimeter accuracy, while mapping receivers are designed to achieve either submeter accuracy or 1-to 5-meter accuracy with differential correction.
3. Relative positions of the satellites - The actual geometry of the satellites affects accuracy. Satellites which are well spread out in the sky are better than satellites clustered together, however you do not want the satellites to be too low in the sky.
Government degradation.
The Government can and does degrade the accuracy through Selective Availability (S/A). You can use differential correction to remove the error.
Selective Availability (S/A) is the artificial degradation of the satellite signal by the Department of Defense. Errors can be introduced into the navigation data (ephemeris) transmitted by the satellite. Misinformation is transmitted as to the exact location of the satellite. The other form of S/A is the satellites internal clock being offset slightly. Slowly varying errors are created in the position solutions.
The error in position caused by S/A can be up to 100 meters.
S/A is the worst source of error BUT can be reduced using differential correction techniques.
With autonomous GPS you can expect to get less than 50 meters RMS.
With P(Y) code GPS you can expect to get less than 12 meters RMS.
Only show this slide if those attending the presentation are able to use P(Y) code GPS. Typically only approved countries armed forces and US federal agencies are allowed to use P(Y) code receivers.
10
With a mapping grade GPS receiver logging C/A code, you can typically obtain less than 2 meters RMS with differential correction.
With high quality mapping GPS receiver logging C/A code, you can achieve sub-meter accuracy (RMS) with differential correction.
With a survey grade receiver it is possible to achieve accuracy better than half a centimeter (RMS). This GPS receiver processes carrier phase data in postprocessed mode.
14
With real-time kinematic survey techniques it is possible to achieve sub-centimeter accuracy (RMS). The GPS receiver processes carrier phase data in real-time.
