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: Set up a quick checklist of GNSS facts to test each option against.
Fact 1: antenna phase centre placement, centering and height, directly enters the position solution, so poor centering biases results even though reception is omnidirectional. Fact 2: code based ranging resolution is tied to the chip length, about 300 m for C/A code, giving metre level range precision after correlation, while carrier based ranging is tied to a roughly 19 cm wavelength, giving sub wavelength, mm to cm, precision once cycle ambiguities are fixed, so these two are not in the same accuracy class. Fact 3: broadcast ephemerides are packaged as Keplerian elements plus small correction terms specifically because a Keplerian orbit model can be propagated forward in time with simple formulas. Fact 4: signals from low elevation satellites pass through a much longer slant path of atmosphere and reflect more easily off nearby surfaces, multipath, so an elevation cutoff or mask is standard practice.

Step 2: Score option (A) against Fact 1.
The claim is that centering and levelling are inconsequential. Fact 1 says the opposite, any centering error shows up directly as a horizontal position error, and any levelling or tilt error corrupts the antenna height reduction. So (A) is FALSE as written.

Step 3: Score option (B) against Fact 2.
The claim is that code and carrier phase measurements give comparable accuracy. Fact 2 shows a two to three order of magnitude accuracy gap between them, metres versus millimetres to centimetres. Comparable is not an accurate description, so (B) is FALSE as written.

Step 4: Score option (C) against Fact 3.
The claim matches Fact 3 exactly, broadcast ephemeris is indeed given as Kepler elements, plus corrections, precisely so the orbit can be propagated. So (C) is TRUE and not one of the incorrect statements.

Step 5: Score option (D) against Fact 4.
The claim matches Fact 4 exactly, low elevation signals are masked out because of high atmospheric refraction and multipath susceptibility. So (D) is TRUE and not one of the incorrect statements.

Step 6: Collect the false statements.
Only (A) and (B) fail the fact check, so these are the INCORRECT statements asked for.

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