Location verification is often accomplished by surface instruments that record the pig's passage by either audible, magnetic, radio-transmission or other means. The sensors record when they detect the passage of the pig (time-of-arrival); this is then compared to the internal record for verification or adjustment. The external sensors may have Global Positioning System capability to assist in their location. A few pig passage indicators transmit the pig's passage, time and location, via satellite uplink. The pig itself cannot use GPS as the metal pipe blocks satellite signals.