Question 1

What is the difference between trilateration and triangulation?

Accepted Answer

Triangulation uses measured angles between the observer and reference points to determine position—a technique used in surveying with theodolites. Trilateration uses measured distances from known reference points. GPS is trilateration: satellites are the known reference points, and measured signal travel times converted to distances are the inputs. The distinction matters practically—GPS receivers measure time (converted to distance), not angles, so the correct term is trilateration. Both techniques require sufficient independent measurements (at least 3 in 2D, 4 in 3D) to uniquely determine position.

Question 2

Why does GPS need four satellites instead of three?

Accepted Answer

Three satellite distances define three spheres whose intersection gives two candidate points in 3D space (usually one underground and one in the air—the latter is chosen automatically). However, receiver clocks have timing errors of microseconds, which translate to hundreds of meters of range error. A fourth satellite adds a fourth equation to the system: the four range equations have four unknowns (x, y, z, and clock offset), allowing simultaneous solution for both position and clock correction. With the clock corrected, all four range measurements agree, giving accurate 3D position without requiring an atomic clock in the receiver.

Question 3

What is dilution of precision (DOP) and how does satellite geometry affect accuracy?

Accepted Answer

DOP quantifies how satellite geometry amplifies position errors. If satellites are clustered together in a small region of the sky, their range spheres intersect at a very shallow angle—a small error in any range shifts the intersection point dramatically. Well-spread satellites spanning the sky create range spheres that intersect nearly perpendicularly, so range errors cause much smaller position errors. PDOP (Position DOP) < 2 is excellent; PDOP > 6 significantly degrades accuracy. HDOP (horizontal) and VDOP (vertical) separately quantify horizontal and vertical accuracy. Urban canyons reduce visible sky, increasing DOP and degrading urban positioning.

🛰️ GPS Trilateration

What did you find?