GNSS

Question 1

What does GNSS stand for? And what is it?

Answer 1

Global Navigation Satellite System. Any satellite constellation that provides positioning, navigation, and timing services on a global or regional basis.

Question 2

List the global and regional systems of GNSS.

Answer 2

NAVSTAR GPS - US - Worldwide
GLONASS - Russia - Worldwide
Galileo - Europe - Worldwide
BeiDou - China - Worldwide
IRNSS - India - Regional
QZSS - Japan - Regional

Question 3

What are the three types of coded information used by the GNSS to calculate position?

Answer 3

• Pseudorandom Code
• Ephemeris Data
• Almanac Data

Question 4

What are the carriage requirements for the GNSS?

Answer 4

Under SOLAS Ch.V, Reg.19: All vessels of international voyages regardless of size.

Question 5

What information is taken from the GNSS?

Answer 5

• Position in Lat & Long
• Course & Speed Over Ground
• Time

Question 6

How many satellites should the GNSS receive data from for it to be accurate information?

Answer 6

4

Question 7

What is DOP?

Answer 7

Dilution of Precision is used to quantify the accuracy of GNSS-derived position & time data.

Question 8

What does a higher/lower value of DOP mean?

Answer 8

Higher - Worse Position Accuracy

Lower - Better Position Accuracy

Question 9

What is RAIM?

Answer 9

Received Autonomous Integrity Monitoring. Works only on NAVSTAR GPS. It uses other additional satellites within range to cross-check the integrity of position data.

Question 10

What are some augmentation methods for the GNSS used for and how do they work?

Answer 10

Augmentation methods are used for improving the accuracy of GNSS devices. By comparing a known fixed location on earth and compare it to where the GNSS unit thinks it is.

Question 11

What are the two augmentation systems for the GNSS?

Answer 11

Augmentation Systems can be ground based (GBAS) or space based (SBAS).

Question 12

According to MGN 379, what is the approximate accuracy of GNSS-derived positions?

Answer 12

5-25 meters 95% of the time.

Question 13

What are some limitations of the GNSS?

Answer 13

Limitations of the GNSS include:
- Over-reliance
- Solar Interference (Scintillation)
- Jamming
- Spoofing

Question 14

What is GNSS scintillation, jamming and spoofing?

Answer 14

GNSS Scintillation: Rapid variations of amplitude & phase as signals pass through the ionosphere.

Jamming: Overpowering satellite signals so that a receiver can not receive genuine ones

Spoofing: Replacing the genuine satellite signal with a fake so that the receiver calculates a fake position.

Question 15

What are some errors of the GNSS?

Answer 15

• Ionosphere/troposphere Error: As the signals pass through them, it decreases in speed.
• Multipath Error: When the signal reflects off an object before reaching the receiver.
• Orbital Error
• Timing Errors: When there is a timing difference between the atomic clock and a satellite in orbit.