GNSS Positioning: Incorrect Statement Analysis

The question asks to identify the INCORRECT statements about GNSS positioning. Based on standard GNSS principles, statements A and B are incorrect.

Statement A Analysis: Centering and Levelling

Statement A claims that centering and levelling are inconsequential for GNSS positioning. This is INCORRECT.

• Centering: Precise centering of the GNSS antenna over the survey control point is crucial. Any offset (error) in centering introduces a lever arm error, directly impacting the calculated position. This is especially important for high-accuracy work.
• Levelling: While GNSS receivers are less sensitive to levelling than traditional theodolites, maintaining a consistent and accurately measured antenna height is vital. The antenna's phase center offset must be known, and deviations from the assumed height introduce errors.

Therefore, centering and levelling are NOT inconsequential; they are important for accurate GNSS measurements.

Statement B Analysis: Measurement Accuracy Comparison

Statement B suggests that carrier phase and code phase measurements offer comparable positioning accuracy. This is INCORRECT.

• Code Phase Measurements: These measurements (e.g., C/A code, P(Y) code) determine position based on the time it takes for the code signal to travel from the satellite to the receiver. Accuracy is typically limited to meters or sub-meters, influenced by noise and receiver clock errors.
• Carrier Phase Measurements: These track the phase of the satellite's carrier wave. By counting the whole wavelengths and fractional parts, carrier phase measurements can achieve much higher precision, often reaching centimeter-level accuracy when combined with techniques like RTK or PPP and resolved ambiguities.

Carrier phase measurements are significantly more accurate than code phase measurements, not comparable.

Statements C and D Analysis

Statements C and D describe standard practices and data formats in GNSS:

• Statement C is correct: Satellite positions (ephemeris) are indeed often provided using Keplerian elements, allowing orbit prediction.
• Statement D is correct: An elevation mask is used to filter out low-elevation satellites, mitigating atmospheric refraction and multipath effects, thereby improving accuracy.

Thus, the incorrect statements are A and B.