GPS Data & Correction Services Flashcards
Differential Techniques to Accuracy
- GPS users employ various techniques to improve positional accuracy.
Two Key Methods:
- Differential GPS (DGPS): Requires a base and rover; can be costly.
- Accessible GPS services: Use one receiver, varying accuracy and costs.
GPS Data & Product Services
Many organizations have set up precise GPS reference networks for geodetic purposes.
Globally, the IGS (International GNSS Service) is a key player.
GPS Data & Product Services
Regionally, reference stations are positioned close together.
U.S.: CORS (Continuously Operation Reference Station)
Canada: CACS (Canadian Active Control System).
Europe: EPN(European Reference Frame Permanent
Network).
GPS data from these stations supports high-precision positioning.
Global Correction Service - IGS
Operates a global network of over 512 tracking stations.
A voluntary group of more than 350 agencies producing precise GPS/GLONASS products.
Committed to top-quality data, setting GNSS standards.
Rinex (Receiver Independent Exchange Data Format - IGS)
Refer to Chapter 8.1 for information about the RINEX format.
A standardized format by IGS.
Used for archiving and online access at the global data center.
With accuracy level depending on availability.
Regional Correction Services
CORS (Continuously Operating Reference Station)
United States
Over 2,000 continuously operating stations.
Providing GPS measurements for 3D positioning.
Regional Correction Services
CACS (Canadian Active Control System)
Canada
Operates continuously.
With over 135 stations.
Offers real-time positioning accuracy within one meter and post-
processing accuracy within three centimeters.
Both CORS and CACS stations are closer to the reference
stations than the IGS stations.
Minnesota CORS (2023)
A collaborative effort involving MnDOT and various state agencies and institutions.
Providing statewide GPS
corrections
DGPS Radio Beacon System
Marine radio beacons at lighthouses and coastal locations are electronic
navigation aids operating in the low-to-medium frequency band (283.5-325
kHz).
DGPS Radio Beacon System
A reference station (RS) generates real-time DGPS corrections in the RTCM (Radio Technical Commission for Maritime Services) format.
With an integrity-monitoring (IM) unit overseeing its performance.
Free for all users.
RTCM (Radio Technical Commission for Maritime Services) format
Refer to Chapter 8.3 for information about the RTCM format.
Coastal networks of reference stations.
Continuously transmits real-time DGPS corrections with the
RTCM format.
Enhancing marine navigation safety.
A beacon receiver connected to a GPS receiver that accepts
RTCM corrections is needed to use this service.
RTCM (Radio Technical Commission for Maritime Services) format
RTCM – Continued
GPS receivers that accept RTCM corrections are known as
differential-ready GPS receivers.
Offering accuracy from sub-meter to a few meters.
Free for the general public.
Radio Beacon Receiver
Combination of Beacon/GPS
Receiver:
Micro-Trak T100
Single Unit
DGPS Radio Beacon Receiver:
Trimble beacon-on-a-belt (bob)
DGPS Radio Beacon System
These receivers pick up the transmitted DGPS corrections and come in single-
or dual-channel options, with dual-channel being more reliable but pricier.
The official range is 150 miles, as per the Coast Guard.
Coverage depends on factors like transmitter power output, atmospheric
noise, receiver sensitivity, and propagation characteristics, which are better
over water than inland areas.
Beacon locations are strategically selected for overlapping coverage to
enhance accuracy.
DGPS Radio Beacon System
Differential corrections come from the NAD 83 position of the
reference station (REFSTA) antenna, so DGPS positions should
align with the NAD 83 coordinate system.
WADGPS (Wide-Area DGPS) Systems
A satellite-based differential correction service.
Using widely separated reference stations.
Providing sub-meter accuracy.
Utilizing the RTCA (Radio Technical Commission for Aeronautics)
format for aviation telecommunications.
WADGPS (Wide-Area DGPS) Systems
Real-time DGPS with a single reference station faces the
challenge of declining accuracy:
(1) Users moves farther from the reference station.
(2) With the highest accuracy limited to a small area around the
reference station.
Fix: WADGPS is employed.
WADGPS Systems
Steps outlining how the WADGPS system
operates:
1.Reference stations
Gather the GPS data and transmit it to the
master station.
2.Master station
Analyzes correction data and uploads it to a
geostationary satellite.
3.Geostationary satellites
Transmit the data to a local GPS receiver.
4.GPS receiver
Applies the necessary corrections.
WADGPS Systems
WADGPS includes four satellite-based augmentation systems:
1. WAAS (Wide Area Augmentation System) in North America
2. EGNOS (European Geostationary Navigation Overlay System)
in Europe
3. MSAS (Multi-Functional Satellite Augmentation System) in Asia
– Japan
4. GAGAN (GPS and GEO Augmented Navigation) in Asia - India
Geostationary Satellite
A geostationary satellite, or GOES (Geostationary Operational
Environmental Satellite).
Orbits the earth at the same rate as its rotation.
Maintaining a fixed positioning over the equator.
Geostationary Satellite
Often called TV satellite.
Completes a 24-hour orbit that matches the Earth’s rotation.
In contrast, GPS satellites orbit twice a day, finishing their orbit in 12 hours.
Satellite Orbit
Low earth orbit: sun-synchronous
Crosses the equator multiple times daily in relation
to the sun.
Useful for capturing images of the Earth’s surface.
Used by Earth observation and spy satellites.
Medium earth orbit: semi-synchronous
Orbits the Earth twice a day, completing an orbit in
12 hours.
Utilized by GPS satellites.
High earth orbit: geo-synchronous
Rotates within the Earth’s rotation
Stays fixed from the surface.
Employed by weather satellites.
WAAS vs. LAAS
Both WAAS and LAAS are GPS augmentation systems that
enhance accuracy, availability, and integrity.
WAAS (wide-area augmentation systems) is satellite-based.
LAAS (local-area augmentation systems) is ground-based.
WAAS (Wide-Area Augmentation Systems)
A Space Base Augmentation System (SBAS) supported by the FAA and DOT.
Specifically implementing WADGPS.
Enchains GPS signal accuracy.
Initially designed for civil aviation, its coverage now extends to inland and
offshore areas, making it suitable for land and marine applications.
Continental DGPS systems are limited to North America due to no ground
reference stations elsewhere.
WAAS (Wide-Area Augmentation Systems)
Pros: WAAS requires no additional receiving equipment and offers broader coverage, including inland and offshore areas, compared to land-based DGPS.
Cons: Signal reception can be hindered by obstructions like trees or mountains due to the satellite positions over the equator.
WAAS
As of today:
38 ground reference stations collect GPS data.
3 master stations (East Coast, West Coast)
generating the WAAS augmentation message.
6 ground uplink stations upload the WAAS
message to geostationary satellites.
2 operational control centers monitor the
system’s performance
3 GEO satellites involved in the system.
WAAS - How Does It Work?
Similar to WADGPS on slide no. 26:
1. Ground stations: Collect GPS data and send it to the master station.
2. Master station: Analyzes correction data and uploads it to a geostationary satellite
3. Geostationary satellites: Transmits data to a local GPS receiver.
4. GPS receiver: Apply the appropriate correction.
WAAS - Accuracy
Differential corrections provided by WAAS increases the accuracy of C/A signals.
Regular GPS with SA: 100m
Regular GPS w/o SA: 15m
DGPS: 3-5m
WAAS: <3m
LAAS (Local Area Augmentation System)
Achieves higher accuracy through local-area base stations
Operates on a smaller scale.
Reference receivers near runways provide significantly more
accurate correction data to incoming planes.
LAAS (Local Area Augmentation System)
Are located near airports.
Broadcast correction messages within a limited range of 20-30
miles.
Uses a VHF (very high frequency) radio data link.
LAAS accuracy is less than one meter.
