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Loss of life, injury and huge economic losses are incurred annually due to irregular and insu¯cient sea-state information. Figures indicate that, each year, the marine insurance industry pays out over $2 billion in claims for weather-related accidents, while bad weather causes one ship of over 500 t to sink somewhere on the globe every week. Accurate knowledge of local ocean conditions is therefore crucial in providing forecasts and early warnings of severe weather conditions.Space-borne systems, particularly satellites, provide the ideal platform for global monitoring of sea conditions via altimetric measurements. As an alternative to active altimetry, another concept is passively receiving re®ected signals from the Global Positioning System (GPS) and other Global Navigation Satellite System (GNSS) satellites. This concept was rst developed by Dr Manuel Martin-Neira at ESA-ESTEC.ESA's Passive Re®ectometry and Interferometry System makes use of GPS/GNSS signals from satellites and their re®ection o¬ the ocean surface to derive oceanic properties such as surface height, signi cant wave height, wind speed and wind direction. Surrey Satellite Technology Ltd are proposing a nanosatellite demonstration mission to ascertain the feasibility of the GPS ocean re®ectometry concept.

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