GPS

Question 1

How does GPS work?

Answer 1

• Transmits a signal with time and position
• Receiver recieves signal
• Receiver knows when signal was sent
• Receiver works out the difference and calculates the range from the satellite

Question 2

How many satellites are required for position?

Answer 2

3 for 2D position

4 for 3D position

Question 3

What is HDOP?

Answer 3

Horizontal Dilution of Position

2 - good HDOP value, 6 - poor, 8 - very poor

Angle of cut is important

Question 4

What is GBAS?

Answer 4

Ground Based Augmentation System

GBAS is a local service that covers a small area like a port

• It uses a single reference station that tracks the GNSS signals and calculates the corrections for a specific location
• The corrections are then transmitted by VHF radio to the GBAS users.

Question 5

What is SBAS?

Answer 5

Satellite Based Augmentation System

• Regional service that covers a large area and supports multiple GNSS constellations
• Consists of a network of reference stations that monitor the GNSS signals and detect any errors or anomalies.
• GNSS receivers use them to improve their position, velocity, and time estimation and to check the validity of the GNSS signals.

Question 6

What are the errors of GPS?

Answer 6

• Satellite Clock Error
• Satellite Orbit Error - satellites drift from orbit
• Ionosphere Delay Error
• Troposphere Delay Error
• Receiver Noise Error - interference from GPS on similar frequency
• Multi-path Error - bounces off mountains or buildings

Question 7

What is DGPS?

Answer 7

Differential Ground Positioning System - Enhances GPS by improved accuracy from 15 metres to 10cm in best cases

Question 8

Which datum does GPS use?

Answer 8

WGS84

Question 9

What are the three segments of GPS?

Answer 9

• Ground Segment
• Space Segment
• User Segment

Question 10

What is the Ground segment?

Answer 10

• 1 master station in Colorado
• 5 monitoring stations near equator
• constantly monitor and maintain GPS system

Question 11

What is the Space Segment?

Answer 11

• 24 satellites on 6 orbit planes
• 4 satellites on each plane plus 1 spare
• Minimum for global coverage

Question 12

What is the User Segment?

Answer 12

Anyone with a GPS receiver

Question 13

How to monitor the accuracy of GPS underway?

Answer 13

• Verify the number of satellites
• Check DOP values
• Cross reference with other navigation methods
• Observe sudden accuracy changes
• Use a GPS with WAAS/EGNOS support
• Keep the GPS updated
• Understand GPS limitations