Question

In the context of GNSS positioning, which of the following statements is/are INCORRECT?

• A. Centering and levelling are inconsequential for GNSS positioning as the GNSS antenna can receive signal from any direction
• B. Carrier phase measurements and code phase measurements can provide comparable positioning accuracy
• C. The satellite positions of the GNSS satellites are provided in terms of Kepler elements so that the orbit can be propagated in time
• D. An elevation mask is used in GNSS satellite data acquisition to avoid measurements from low elevation angles as they are affected by high atmospheric refraction and are susceptible to multipath effects

Hint:

Test each statement against GNSS fundamentals: antenna centering and levelling requirements, the accuracy gap between code and carrier phase, the broadcast ephemeris format, and the purpose of an elevation mask.

Answer 1

The Correct Option is A, B

Step 1: Restate the question.
We must identify which of the four statements about GNSS positioning are INCORRECT, false. The key idea is to test each statement against the actual physics and geometry of satellite based positioning.

Step 2: Evaluate statement (A).
Centering and levelling are inconsequential for GNSS positioning as the GNSS antenna can receive signal from any direction. It is true that a GNSS antenna can receive signals from satellites in almost any direction above the horizon, unlike an optical instrument that must be pointed at a target. However, the antenna phase centre still has to be placed precisely over the survey mark, centering, and its reference plane kept level, because any offset directly translates into a horizontal or vertical positioning error at the ground point, and antenna height and tilt corrections assume a level, correctly centred setup. So centering and levelling remain essential, not inconsequential. This statement is INCORRECT.

Step 3: Evaluate statement (B).
Carrier phase measurements and code phase measurements can provide comparable positioning accuracy. Code, pseudorange, measurements are derived from the C/A or P code chip length and typically give metre level accuracy, while carrier phase measurements use the much shorter carrier wavelength, about 19 cm for L1, and once integer ambiguities are resolved, give millimetre to centimetre accuracy. These differ by two to three orders of magnitude, so they do NOT provide comparable accuracy. This statement is INCORRECT.

Step 4: Evaluate statement (C).
The satellite positions of the GNSS satellites are provided in terms of Kepler elements so that the orbit can be propagated in time. Broadcast ephemeris messages do transmit a set of Keplerian orbital elements, semi major axis, eccentricity, inclination and so on, together with perturbation and correction terms, which a receiver uses to compute the satellite position at any epoch. This is factually correct, so statement (C) is CORRECT and should not be selected.

Step 5: Evaluate statement (D).
An elevation mask is used in GNSS satellite data acquisition to avoid measurements from low elevation angles as they are affected by high atmospheric refraction and are susceptible to multipath effects. This matches standard GNSS survey practice, satellites below a chosen elevation mask, commonly 10 to 15 degrees, are excluded because their signals travel a longer, more refractive path through the troposphere and ionosphere and are more prone to multipath reflection near the ground. This statement is CORRECT and should not be selected.

Step 6: Conclude.
The incorrect statements are (A) and (B).

\[ \boxed{\text{Options (A) and (B) are INCORRECT}} \]