GPS Data & Correction Services

Question 1

Differential Techniques to Accuracy

Answer 1

1. 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.

Question 2

GPS Data & Product Services

Answer 2

 Many organizations have set up precise GPS reference networks for geodetic purposes.
 Globally, the IGS (International GNSS Service) is a key player.

Question 3

GPS Data & Product Services

Answer 3

 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.

Question 4

Global Correction Service - IGS

Answer 4

 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.

Question 5

Rinex (Receiver Independent Exchange Data Format - IGS)

Answer 5

 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.

Question 6

Regional Correction Services

Answer 6

CORS (Continuously Operating Reference Station)
 United States
 Over 2,000 continuously operating stations.
 Providing GPS measurements for 3D positioning.

Question 7

Regional Correction Services

Answer 7

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.

Question 8

Minnesota CORS (2023)

Answer 8

 A collaborative effort involving MnDOT and various state agencies and institutions.
 Providing statewide GPS
corrections

Question 9

DGPS Radio Beacon System

Answer 9

Marine radio beacons at lighthouses and coastal locations are electronic
navigation aids operating in the low-to-medium frequency band (283.5-325
kHz).

Question 10

DGPS Radio Beacon System

Answer 10

 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.

Question 11

RTCM (Radio Technical Commission for Maritime Services) format

Answer 11

 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.

Question 12

RTCM (Radio Technical Commission for Maritime Services) format

Answer 12

 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.

Question 13

Radio Beacon Receiver

Answer 13

Combination of Beacon/GPS
Receiver:
 Micro-Trak T100
 Single Unit
 DGPS Radio Beacon Receiver:
 Trimble beacon-on-a-belt (bob)

Question 14

DGPS Radio Beacon System

Answer 14

 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.

Question 15

DGPS Radio Beacon System

Answer 15

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.

Question 16

WADGPS (Wide-Area DGPS) Systems

Answer 16

 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.

Question 17

WADGPS (Wide-Area DGPS) Systems

Answer 17

 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.

Question 18

WADGPS Systems

Answer 18

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.

Question 19

WADGPS Systems

Answer 19

 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

Question 20

Geostationary Satellite

Answer 20

 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.

Question 21

Geostationary Satellite

Answer 21

 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.

Question 22

Satellite Orbit

Answer 22

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.

Question 23

WAAS vs. LAAS

Answer 23

 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.

Question 24

WAAS (Wide-Area Augmentation Systems)

Answer 24

 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.

Question 25

WAAS (Wide-Area Augmentation Systems)

Answer 25

 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.

Question 26

WAAS

Answer 26

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.

Question 27

WAAS - How Does It Work?

Answer 27

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.

Question 28

WAAS - Accuracy

Answer 28

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

Question 29

LAAS (Local Area Augmentation System)

Answer 29

 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.

Question 30

LAAS (Local Area Augmentation System)

Answer 30

 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.