The National Institute of Oceanography (NIO) in Goa has developed a realtime reporting and Internetaccessible coastal sealevelmonitoring system and it has been operational at Verem jetty in the Mandovi estuary in Goa since September 24, 2005. The gauge uses a cellular modem to put on the Internet realtime sealevel data, which can be accessed by authorised personnel. By using a cellular phone network, coastal sealevel changes are continuously updated on to a webserver. The sealevel gauge website can be made available to television channels to broadcast realtime visualisation of the coastal sea level, particularly during oceanogenic hazards such as storm surges or a tsunami. A network of such gauges along the coast and the islands that lie on either side of the mainland would provide data to disaster management agencies to disseminate warnings to coastal communities and beach tourism centres.

The gauge incorporates a bottom pressure transducer as the sensing element. The sea unit of the gauge, which houses the pressure transducer, is mounted within a cylindrical protective housing, which in turn is rigidly held within a mechanical structure. This structure is secured to a jetty. The gauge is powered by a battery, which is charged by solar panels. Battery, electronics, solar panels, and cellular modems are mounted on the top portion of this structure. The pressure sensor and the logger are continuously powered on, and their electrical current Consumption is 30 mA and 15 mA respectively. The cellular modem consumes 15 mA and 250 mA during standby and data transmission modes, respectively. The pressure sensor located below the lowtide level measures the hydrostatic pressure of the overlying water layer. An indigenously designed and developed microprocessor based data logger interrogates the pressure transducer and acquires the pressure data at the rate of two samples a second. The acquired pressure data is averaged over an interval of five minutes to remove highfrequency windwaves that are superimposed on the lower frequency tidal cycle. This averaged data is recorded in a multimedia card. The measured water pressure is converted to water level using sea water density and acceleration owing to the earth's gravity. The water level so estimated is then referenced to chart datum (CD), which is the internationally accepted reference level below which the sealevel will not, fall. The data received at the Internet server is presented in graphical format together with the predicted sealevel and the residual. The residual sea level (that is, the measured minus the predicted sea level) provides a clear indication of sealevel oscillation and a quantitative estimate of the anomalous behaviour, the driving force for which could be atmospheric forcing (storm) or physical (tsunami).

A network of sealevel gauges along the Indian coastline and islands would also provide useful information to mariners for safe navigation in shallow coastal waters and contribute to various engineering projects associated with coastal zone management, besides dredging operations, port operations and manwater treaties with greater transparency. Among the various communication technologies used for realtime transmission of sealevel data are the wired telephone connections, VHF/UHF transceivers, satellite transmit terminals and cellular connectivity. Wired telephone connections are severely susceptible to loss of connectivity during natural disasters such as storm surges, primarily because of telephone line breakage. Communication via VHF/UHF transceivers is limited by lineofsight distance between transceivers and normally offer only pointtopoint data transfer. Satellite communication via platform transmit terminals (PTTs) has wide coverages and, therefore, allows data reception from offshore platforms. However, data transfer speeds are limited. Further many satellites (for example, GOES, INSAT) permit data transfer only predefined timeslots, thereby inhibiting continuous data access. Technologies of data reporting via satellites have undergone a sea change recently in terms of frequency of reportage, data size, recurring costs and so forth. Broadband technology has been identified as one that can be used optimally for realtime reporting of data because of its inherent advantages such as a continuous twoday connection that allows highspeed data transfer and near realtime data reporting. While satellite communication is expensive, wireless communication infrastructure and the ubiquity of cellular phones have made cellular communication affordable. Low initial and recurring costs are an important advantage of cellular communication. A simple and costeffective methodology for realtime reporting of data is the cellularbased GPRS technology, with has been recently implemented at the NIO for realtime reporting of coastal sea level data.