A system for determining position of a distal end of a tubular string (15; 315) in a wellbore (10; 310) formed in a subsurface formation, the system comprising: casing lining the wellbore (10; 310); a tubular string (15; 315) disposed in the wellbore (10; 310) and carrying a radiation sensor on a length thereof; characterised in that the system further comprises a processor configured to receive measurements from the at least one sensor and further configured to determine the position of the distal end of the tubular string (15; 315) by correlating a plurality of received measurements, wherein the casing carries a plurality of radioactive sources; the tubular string (15, 315) also carries a sensor configured to measure a formation or wellbore fluid property that is other than radiation and varies as a function of position within the wellbore (10; 310); and the processor is configured to determine the position of the distal end of the tubular string (15; 315) by correlating a plurality of parameters which comprise (i) a formation or wellbore fluid property that varies as a function of position within the wellbore (10; 310) and (ii) the measured intensity of the radiation received by the radiation sensor from the plurality of radioactive sources; and further wherein the plurality of radioactive sources are disposed at differing longitudinal positions along the casing (12;312) with the differing longitudinal positions including at least two positions at a known distance from one another and at least one position spaced apart from one of the at least two positions by approximately a multiple of the known distance between the at least two positions and the processor is configured to recognize a pattern, formed by peaks in the measured radiation intensity at the differing longitudinal positions. casing lining the wellbore (10; 310); a tubular string (15; 315) disposed in the wellbore (10; 310) and carrying a radiation sensor on a length thereof; characterised in that the system further comprises a processor configured to receive measurements from the at least one sensor and further configured to determine the position of the distal end of the tubular string (15; 315) by correlating a plurality of received measurements, wherein the casing carries a plurality of radioactive sources; the tubular string (15, 315) also carries a sensor configured to measure a formation or wellbore fluid property that is other than radiation and varies as a function of position within the wellbore (10; 310); and the processor is configured to determine the position of the distal end of the tubular string (15; 315) by correlating a plurality of parameters which comprise (i) a formation or wellbore fluid property that varies as a function of position within the wellbore (10; 310) and (ii) the measured intensity of the radiation received by the radiation sensor from the plurality of radioactive sources; the casing carries a plurality of radioactive sources; the tubular string (15, 315) also carries a sensor configured to measure a formation or wellbore fluid property that is other than radiation and varies as a function of position within the wellbore (10; 310); and the processor is configured to determine the position of the distal end of the tubular string (15; 315) by correlating a plurality of parameters which comprise (i) a formation or wellbore fluid property that varies as a function of position within the wellbore (10; 310) and (ii) the measured intensity of the radiation received by the radiation sensor from the plurality of radioactive sources; and further wherein the plurality of radioactive sources are disposed at differing longitudinal positions along the casing (12;312) with the differing longitudinal positions including at least two positions at a known distance from one another and at least one position spaced apart from one of the at least two positions by approximately a multiple of the known distance between the at least two positions and the processor is configured to recognize a pattern, formed by peaks in the measured radiation intensity at the differing longitudinal positions.