PS GNSS & RTK Survey Technology 3 — Questions and Answers
Question 1: The NMEA 0183 GGA sentence provides which of the following fields critical for RTK status monitoring?
- Satellite PRN numbers and elevation angles
- Fix quality indicator, number of satellites, and HDOP (Correct answer)
- Baseline vector in local ENU components
- Carrier-to-noise density ratio per satellite
Correct answer: Fix quality indicator, number of satellites, and HDOP
The GGA sentence includes fix quality (0=invalid, 1=GPS, 2=DGPS, 4=RTK fixed, 5=RTK float), satellite count, HDOP, and antenna height.
Question 2: What is the typical horizontal accuracy specification for a high-quality RTK GNSS receiver under good conditions?
- ±1–2 meters
- ±10–30 cm
- ±1–2 cm + 1 ppm (Correct answer)
- ±0.1 mm
Correct answer: ±1–2 cm + 1 ppm
RTK systems commonly achieve ±1–2 cm horizontal accuracy with an additional parts-per-million component that grows with baseline length.
Question 3: A surveyor observes that satellite signals arrive at the antenna via reflected paths off nearby buildings. This error source is called:
- Ionospheric scintillation
- Cycle slip
- Multipath (Correct answer)
- Signal attenuation
Correct answer: Multipath
Multipath occurs when GNSS signals reflect off surfaces and arrive at the antenna via multiple paths, distorting code and carrier phase measurements.
Question 4: In GNSS carrier-phase surveying, an abrupt jump in the measured phase due to brief loss of signal lock is called a:
- Phase ambiguity
- Cycle slip (Correct answer)
- Code bias
- Multipath spike
Correct answer: Cycle slip
A cycle slip is a discontinuity in the carrier-phase observable caused by momentary loss of lock, resulting in an integer jump in the ambiguity.
Question 5: When initializing an RTK rover, what is the minimum recommended number of common satellites between base and rover for a reliable fixed solution?
- 3
- 4
- 5 (Correct answer)
- 8
Correct answer: 5
At least 5 common satellites are generally required for reliable RTK initialization, providing sufficient redundancy for ambiguity resolution.
Question 6: The RTCM SC-104 standard is used in RTK surveying primarily to:
- Define coordinate reference frames for RTK output
- Standardize the format of differential correction messages transmitted from base to rover (Correct answer)
- Specify antenna calibration procedures
- Govern the legal use of RTK data in boundary surveys
Correct answer: Standardize the format of differential correction messages transmitted from base to rover
RTCM SC-104 defines the message format for transmitting differential GNSS corrections from a reference station to a rover receiver.
Question 7: What effect does a large antenna phase center offset (PCO) have on GNSS measurements if left uncorrected?
- It only affects code measurements, not carrier phase
- It introduces a systematic position bias proportional to the offset and satellite geometry (Correct answer)
- It causes cycle slips during initialization
- It prevents ambiguity resolution in RTK mode
Correct answer: It introduces a systematic position bias proportional to the offset and satellite geometry
An uncorrected phase center offset shifts the effective measurement point from the mechanical antenna center, causing systematic position errors that vary with satellite geometry.
PS GNSS & RTK Survey Technology 3
The NMEA 0183 GGA sentence provides which of the following fields critical for RTK status monitoring?