A position detection system includes a GNSS sensor terminal that receives a satellite signal from a GNSS satellite as a snapshot, at least one relay device that receives the snapshot GNSS signal transmitted by the GNSS sensor terminal, and a calculation device that measures a position of the GNSS sensor terminal by using a code phase and a Doppler frequency of the GNSS satellite obtained from the snapshot GNSS signal, wherein the calculation device estimates an initial position of the GNSS sensor terminal by using position information of the relay device that has received the snapshot GNSS signal transmitted by the GNSS sensor terminal.