An optical communication system comprising: an optical transmission path; a plurality of repeaters (R1...Rn) coupled to the transmission path, each of the repeaters (R1...Rn) comprising a high loss loopback, HLLB, path (132-1 to132-n); first line monitoring equipment, LME, coupled to a first end of the transmission path, the first LME being configured to transmit first LME test signals on the transmission path and receive first LME loopback data from the transmission path in response to the LME test signals, the first LME loopback data comprising a peak associated with the location of each of the HLLB paths on the transmission path; second LME coupled to a second end of the transmission path, the second LME being configured to transmit second LME test signals on the transmission path and receive second LME loopback data from the transmission path in response to the second LME test signal, the second LME loopback data comprising a peak associated with the location of each of the HLLB paths on the transmission path; and a calibration processor configured to identify pairs of peaks in the first LME loopback data and the second LME loopback data to provide calibrated first LME loopback data and calibrated second LME loopback data, each pair of peaks including a peak of the first LME loopback data associated with one of the HLLB paths (132-1 to132-n) and a peak of the second LME loopback data associated the one of the HLLB paths (132-1 to132-n);, wherein the calibration processor is configured to omit any peak in the first LME loopback data from the calibrated first LME loopback data that does not have a pair in the second LME loopback data, and wherein the calibration processor is configured to omit any peak in the second LME loopback data from the calibrated second LME loopback data that does not have a pair in the first LME loopback data. an optical transmission path; a plurality of repeaters (R1...Rn) coupled to the transmission path, each of the repeaters (R1...Rn) comprising a high loss loopback, HLLB, path (132-1 to132-n); first line monitoring equipment, LME, coupled to a first end of the transmission path, the first LME being configured to transmit first LME test signals on the transmission path and receive first LME loopback data from the transmission path in response to the LME test signals, the first LME loopback data comprising a peak associated with the location of each of the HLLB paths on the transmission path; second LME coupled to a second end of the transmission path, the second LME being configured to transmit second LME test signals on the transmission path and receive second LME loopback data from the transmission path in response to the second LME test signal, the second LME loopback data comprising a peak associated with the location of each of the HLLB paths on the transmission path; and a calibration processor configured to identify pairs of peaks in the first LME loopback data and the second LME loopback data to provide calibrated first LME loopback data and calibrated second LME loopback data, each pair of peaks including a peak of the first LME loopback data associated with one of the HLLB paths (132-1 to132-n) and a peak of the second LME loopback data associated the one of the HLLB paths (132-1 to132-n);, wherein the calibration processor is configured to omit any peak in the first LME loopback data from the calibrated first LME loopback data that does not have a pair in the second LME loopback data, and wherein the calibration processor is configured to omit any peak in the second LME loopback data from the calibrated second LME loopback data that does not have a pair in the first LME loopback data.