Global Navigation Satellite Systems (GNSS) Self-Test
1. What is the minimum number of satellites required for a GNSS receiver to determine its 3D position (latitude, longitude, and altitude)?
a) 2
b) 3
c) 4
d) 5
Correct Answer: c) 4
A GNSS receiver needs signals from at least 4 satellites to calculate its 3D position. Three satellites are sufficient for 2D positioning (latitude and longitude), but a fourth is needed to solve for altitude and to correct for receiver clock errors.
2. Which of the following is NOT a global GNSS system?
a) GPS
b) GLONASS
c) Galileo
d) IRNSS
Correct Answer: d) IRNSS
IRNSS (Indian Regional Navigation Satellite System), now called NavIC, is a regional system covering India and surrounding areas. GPS (USA), GLONASS (Russia), and Galileo (EU) are global systems.
3. What is the primary frequency band used by GNSS systems?
a) L-band (1-2 GHz)
b) C-band (4-8 GHz)
c) Ku-band (12-18 GHz)
d) S-band (2-4 GHz)
Correct Answer: a) L-band (1-2 GHz)
GNSS systems primarily operate in the L-band (1-2 GHz) because these frequencies can penetrate the ionosphere with relatively low attenuation while still providing good positioning accuracy.
4. What is the purpose of the atomic clocks on GNSS satellites?
a) To maintain precise timing for position calculations
b) To control the satellite's orbit
c) To communicate with ground stations
d) To power the satellite's systems
Correct Answer: a) To maintain precise timing for position calculations
GNSS positioning is based on precise timing of signal transmission. Atomic clocks (typically rubidium or cesium) on satellites maintain extremely accurate time (nanosecond precision), which is crucial because an error of just 1 microsecond would result in a positioning error of about 300 meters.
5. What is the main source of error in GNSS positioning?
a) Satellite clock errors
b) Ionospheric delay
c) Multipath interference
d) All of the above
Correct Answer: d) All of the above
GNSS errors come from multiple sources: satellite clock errors (despite atomic clocks), ionospheric delay (signal slowing in the ionosphere), multipath interference (signals reflecting off surfaces), orbital errors, tropospheric delay, and receiver noise. Differential GNSS and augmentation systems help mitigate these errors.
6. What is the typical orbital altitude of GPS satellites?
a) 2,000 km
b) 10,000 km
c) 20,200 km
d) 35,786 km
Correct Answer: c) 20,200 km
GPS satellites orbit at medium Earth orbit (MEO) with an altitude of approximately 20,200 km (12,550 miles) and an orbital period of about 12 hours. This is different from geostationary orbits (35,786 km) used for communication satellites.
7. What is Selective Availability (SA) in the context of GPS?
a) A technique to improve signal strength
b) Intentional degradation of civilian GPS signals
c) A method for satellite communication
d) A frequency hopping technique
Correct Answer: b) Intentional degradation of civilian GPS signals
Selective Availability was a program where the U.S. Department of Defense intentionally degraded the accuracy of civilian GPS signals (to about 100m) while maintaining precise accuracy for military users. SA was turned off in May 2000, significantly improving civilian GPS accuracy.
8. What is the main advantage of using multiple GNSS constellations (GPS + GLONASS + Galileo)?
a) Increased signal strength
b) Better accuracy and availability
c) Lower power consumption
d) Reduced cost
Correct Answer: b) Better accuracy and availability
Using multiple GNSS constellations provides more visible satellites, especially in urban canyons or mountainous areas where signals might be blocked. This improves both accuracy (through better geometry) and availability (more signals to work with).
9. What is Differential GPS (DGPS) used for?
a) To increase signal strength
b) To correct atmospheric and other errors
c) To communicate between receivers
d) To reduce power consumption
Correct Answer: b) To correct atmospheric and other errors
DGPS uses a fixed, surveyed reference station to calculate errors in the GNSS signals (mainly caused by ionospheric delay) and broadcasts correction data to nearby receivers. This can improve accuracy from meters to centimeters.
10. What is the purpose of the almanac data broadcast by GNSS satellites?
a) To provide precise orbital parameters
b) To help receivers locate satellites faster
c) To authenticate the satellite signal
d) To correct atmospheric errors
Correct Answer: b) To help receivers locate satellites faster
The almanac contains approximate orbital information for all satellites in the constellation. It helps receivers determine which satellites should be visible, reducing the time needed for first fix (TTFF). More precise ephemeris data is used for actual positioning calculations.