Portagauge and Satellite Sea level monitoring system for the Southwest Indian Ocean – PASS-SWIO

Madagascar currently has very limited tidal prediction capability (based primarily on model data) and no national sea level monitoring capability. There is only one functioning tide gauge station, whilst an earlier tide gauge, in the cyclone-prone north of the island, was swept away several years ago. Working closely with its Madagascar partners, the Direction Générale de la Météorologie (DGM), PASS-SWIO aims to establish a sea level monitoring system for Madagascar based on the deployment of a low-cost relocatable tide gauge (Portagauge). Financial limitations make it impractical for Madagascar to install multiple tide gauge stations at all points of interest along their extensive coastlines, nor can they be maintained on the multi-decadal timescales that are required to derive robust estimates of long-term sea level trends. Yet, tidal information is vital for the safety of communities, infrastructure and commerce, and since short-term hazards can be exacerbated by long-term increases in sea level, knowledge of longer-term changes is also essential. Fortunately, longer-term sea level variability, including the trends associated with Climate Change, can be derived from satellite altimetry data, but these ‘absolute’ sea level measurements are calculated relative to an ellipsoidal reference frame and require both correction for vertical land motion as well as ‘ground-truthing’ to some known fixed point on land if they are to be meaningful for planning and mitigation purposes. Such vertical land motion is traditionally measured by GNSS receivers via the detection of positioning and timing information from a constellation of navigational satellites. The recent GNSS-IR technique exploiting the signal-to-noise ratio, allows sea level to be inferred relative to the same geodetic reference frame as satellite altimetry. The co-location of GNSS receivers with conventional tide gauge sensors (which measure relative to some fixed point on land), thus allows short-term tide gauge and GNSS-IR measurements to be connected to satellite altimetry, which can then substitute for long-term observations from tide gauge data. The NOC Portagauge is a low-cost system which uses GNSS interferometric reflectometry (GNSS-IR) technology alongside a conventional radar gauge. As part of the PASS-SWIO project a Portagauge will be deployed at Toamasina port on the NE coast of Madagascar, for a minimum period of 6 months. DGM will be trained to maintain and operate the Portagauge, and to carry out processing and analysis of Portagauge, tide gauge and satellite altimeter data (Jason-2, Jason-3, Sentinel-3A and 3B). They will cross validate portagauge data against satellite data and generate an analysis of tidal and non-tidal sea-level characteristics for the Madagascar coastal region, including seasonal signals, inter-annual variability and trends. Finally, the PASS-SWIO team will work with agencies and key users in Madagascar to define a road map to establish a long-term, sustainable, national sea-level monitoring system for the country. It will be important to ensure that the planned capacity development meets key requirements, complements existing capability and will be sustainable, considering resource requirements. The end goal of the project is to provide a model for a sea level monitoring system for developing island states and coastal nations, based on low-cost tide gauges and satellite data. The presentation will provide an overview of the project and present initial results from analysis of satellite altimeter data in the Madagascar region. PASS-SWIO is funded by ESA under the EO Science for Society Programme – Open Call.