_________ data links are limited primarily by the curvature of the earth, but also by local obstructions.

The correct answer pertains to line of sight (LOS) data links, which function based on the direct visibility between the transmitter and receiver. The curvature of the Earth poses a significant limitation to these links, as the signal needs an unobstructed path to travel. If the terrain between the ground station and the UAS is uneven or has local obstructions, such as buildings or trees, the range and effectiveness of the LOS connection can be significantly reduced.

While radio frequency (RF) and satellite links can also be affected by similar factors, they operate differently. RF can penetrate some obstacles to a degree and can reflect off structures, whereas satellite links utilize satellites positioned in orbit to maintain communication over longer distances, reducing the impact of the earth's curvature in most scenarios. Ground-based links also might operate under LOS principles but are not distinguished in the same way as dedicated LOS links. This understanding clarifies why line of sight data links are specifically limited by the curvature of the earth and local obstructions.