Defects in a pipeline may be detected by an in-line inspection tool passing therethrough. However, as the tool travels through the pipeline, errors associated with certain onboard components may accumulate. These errors may reduce the accuracy with which the locations of detected defects can be determined. Accordingly, markers may be positioned at various locations along the pipeline. Each marker may include a radio receiver to receive signal transmitted by an in-line tool passing thereby and one or more magnetic flux detection systems that may detect a magnetic field emanating from the in-line tool. The radio receiver may include an antenna comprising two or more coils connected in series and positioned side-by-side. The flux detection system may include one or more flux concentrators to amplify the strength of the magnetic field. Signals received through the antenna or flux detection system may be used to correct any errors associated with the onboard components charting the progression of the in-line tool through the pipeline.