Which of the following is necessary to compute 3D positioning in GPS?

To compute a precise three-dimensional position in GPS, at least four signals are necessary. This is because GPS positioning involves determining the receiver's location in three-dimensional space, which consists of latitude, longitude, and altitude.

Each satellite provides a signal that contains a time stamp, allowing the receiver to calculate the distance to that satellite using the time it takes for the signal to arrive. To accurately determine its three-dimensional position, the GPS receiver needs to calculate its position in relation to multiple satellites.

With three signals, the receiver could compute a position in two dimensions (latitude and longitude), but it wouldn't be able to accurately determine altitude because there would be an additional unknown (the height above the Earth's surface). Four signals allow the receiver to solve for three dimensions and also to correct for timing errors in the receiver's clock.

Thus, at least four signals are necessary to ensure that the computation accounts for all dimensions and provides a fully accurate three-dimensional position.