Which of the following is NOT a source of GPS error among common factors?

• A. Satellite geometry
• B. Atmospheric effects
• C. Anti-Spoofing
• D. Multipath

Answer: (C)

GPS measurements can be biased by how satellites are arranged in the sky, the atmosphere the signal travels through, and how signals bounce off surfaces near the receiver. Poor satellite geometry leads to dilution of precision: when satellites crowd into a small region of the sky, the intersection of their pseudorange measurements becomes less well-defined, increasing the potential error. Atmospheric effects, including ionospheric and tropospheric delays, slow and bend the radio waves, introducing travel-time biases that must be corrected for accurate range estimates. Multipath happens when signals reflect off buildings, water, or terrain before reaching the antenna, causing the measured distance to be longer than the direct path and biasing the result. Anti-spoofing, however, is a security feature meant to prevent spoofed signals from misleading the receiver; it does not inherently create a measurement error, so it is not a source of GPS error.