PS GNSS & RTK Survey Technology 2 — Questions and Answers
Question 1: What is the primary purpose of the ionospheric-free (L3) linear combination in GNSS processing?
- Improve signal strength
- Eliminate first-order ionospheric delay (Correct answer)
- Reduce multipath errors
- Increase code pseudorange accuracy
Correct answer: Eliminate first-order ionospheric delay
The L3 ionospheric-free combination uses a weighted difference of L1 and L2 measurements to cancel first-order ionospheric delay.
Question 2: In an RTK survey, a 'float' solution indicates that:
- The base station is floating on water
- Integer ambiguities have been fully resolved
- Ambiguity resolution is incomplete and decimal values remain (Correct answer)
- The rover antenna is moving too fast
Correct answer: Ambiguity resolution is incomplete and decimal values remain
A float solution means the carrier-phase integer ambiguities have not yet been fixed and the solution retains non-integer (decimal) ambiguity estimates.
Question 3: Which GNSS constellation operates primarily in the L1, L2, and L5 frequency bands and is operated by the United States Department of Defense?
- GLONASS
- Galileo
- BeiDou
- GPS (Correct answer)
Correct answer: GPS
GPS (Global Positioning System) is the U.S. DoD-operated constellation transmitting on L1 (1575.42 MHz), L2 (1227.60 MHz), and L5 (1176.45 MHz).
Question 4: A surveyor notices the RTK baseline length is 45 km. What is the most likely accuracy concern?
- Satellite clock errors dominate
- Tropospheric and ionospheric errors become difficult to model and cancel (Correct answer)
- The data link will fail
- PDOP degrades beyond 4.0
Correct answer: Tropospheric and ionospheric errors become difficult to model and cancel
Over long baselines, atmospheric errors (troposphere and ionosphere) do not cancel between rover and base, degrading RTK accuracy significantly.
Question 5: What does PDOP stand for and what does it measure?
- Positional Dilution of Precision — geometric strength of satellite configuration in 3D (Correct answer)
- Phase Dilution of Precision — carrier phase noise level
- Potential Delay of Position — ionospheric delay factor
- Precision Delay of Output — data latency index
Correct answer: Positional Dilution of Precision — geometric strength of satellite configuration in 3D
PDOP (Position Dilution of Precision) quantifies how the geometric arrangement of satellites magnifies ranging errors into position errors in three dimensions.
Question 6: In Network RTK (NRTK), what does a Virtual Reference Station (VRS) provide to the rover?
- A physical antenna installed near the rover
- Synthesized corrections as if a reference station were located near the rover (Correct answer)
- Raw satellite data from the nearest physical CORS station only
- Almanac updates for satellite acquisition
Correct answer: Synthesized corrections as if a reference station were located near the rover
A VRS generates synthetic correction data interpolated from a network of reference stations, simulating a nearby base station for the rover.
Question 7: Which atmospheric layer causes the most significant signal delay for GNSS and is non-dispersive (frequency-independent) in the radio frequency range?
- Ionosphere
- Troposphere (Correct answer)
- Stratosphere
- Mesosphere
Correct answer: Troposphere
The troposphere causes a frequency-independent (non-dispersive) delay that cannot be eliminated using dual-frequency combinations and must be modeled.
PS GNSS & RTK Survey Technology 2
What is the primary purpose of the ionospheric-free (L3) linear combination in GNSS processing?