Horizontal control of the GSI vessel Eugene McDermott for the 1980 Seismic Survey was provided by a GSI operated Syledis chain. The syledis system performed to its expectations and some remarkable accuracies have been achieved. Signal instabilities beyond 130 km were known prior to commencement of operations but due to much lower operating cost of Syledis compared to a long range system (Argo) and with about 90% of the programme with ranges under 130 km, Syledis was the most suitable system for the survey.