n instrument for-accurate sea level and wave measurement

Measurements of sea level in remote areas such as the Arctic, Antarctic and at oceanic islands are the responsibility of research organisations such as the Proudman Oceanographic Laboratory in the UK. As part of its South Atlantic network a newly developed instrument was installed at St Helena in the South Atlantic during April 1997 to improve the operational characteristics of this station which was first installed in 1984. Data are measured processed and stored locally and are available worldwide in near real-time. The sea unit measures sub-surface pressure using a pair of quartz crystal sensors. The primary sensor measures the full range of sea level whereas the second sensor is attached to a datum tube fixed at a known position relative to some local benchmarks. The latter is used to fit the full measurements accurately (2 mm) to the local levelling network. One of the main functions is to allow remote access by the operator in the UK to reprogram the system or to investigate fault conditions. The basic sampling scheme of the station consists of a sea level measurement made every 15 minutes, from both the full and half tide sensors together with barometric pressure, air and sea temperature. Once each day the device switches to a rapid sampling mode and records wave amplitude for 1 hour.