GPS Flashcards
1
Q
How does GPS work?
A
- 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
2
Q
How many satellites are required for position?
A
3 for 2D position
4 for 3D position
3
Q
What is HDOP?
A
Horizontal Dilution of Position
2 - good HDOP value, 6 - poor, 8 - very poor
Angle of cut is important
4
Q
What is GBAS?
A
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.
5
Q
What is SBAS?
A
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.
6
Q
What are the errors of GPS?
A
- 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
7
Q
What is DGPS?
A
Differential Ground Positioning System - Enhances GPS by improved accuracy from 15 metres to 10cm in best cases
8
Q
Which datum does GPS use?
A
WGS84
9
Q
What are the three segments of GPS?
A
- Ground Segment
- Space Segment
- User Segment
10
Q
What is the Ground segment?
A
- 1 master station in Colorado
- 5 monitoring stations near equator
- constantly monitor and maintain GPS system
11
Q
What is the Space Segment?
A
- 24 satellites on 6 orbit planes
- 4 satellites on each plane plus 1 spare
- Minimum for global coverage
12
Q
What is the User Segment?
A
Anyone with a GPS receiver
13
Q
How to monitor the accuracy of GPS underway?
A
- 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
