## 2 Requirements Specifications and Measurements Gaps
The marine weather forecast use case has six measurements with gaps; sea ice monitoring (extent and thickness) involves 13 measurements with gaps; fishing pressure and fish stock assessment has five measurements with gaps; land for infrastructure status assessment use case will be met by future Copernicus infrastructure and contributing missions (the measurements require high spatial resolution with a long revisit time); the agriculture and forestry (hydric stress) use case implies four measurements with gaps; the land for mapping (risk assessment) use case presents only one measurement with observation gaps (this measurement is associated with the agriculture and forestry (hydric stress) use case); the sea ice melting emission use case involves six measurements with gaps (these measurements are associated with sea ice monitoring (extent and thickness) use case); the atmosphere for weather forecast use case has only one measurement with gaps, and this measurement is associated with the marine weather forecast use case; the climate for ozone layer and UV use case will be satisfied by future Copernicus infrastructure and contributing missions; and natural habitat and protected species monitoring has only one variable with an observation gap (this measurement is associated with the agriculture and forestry (hydric stress) use case). In most cases, each measurement is associated with different use cases. There are four use cases that involve all the measurements with gaps (marine weather forecast, sea ice monitoring (extent and thickness), fishing pressure and fish stock assessment and agriculture and forestry (hydric stress)). In this way, if only four use cases are addressed, they cover all the 20 measurements with gaps. These four use cases that involve all the measurements are congruent with respect to a recent survey into the state and the health of the European EO services industry [3], where the results indicated that the Copernicus data and services do not fully respond to the customer needs. Principally, it reported that the agriculture, maritime and fishery domains are of major importance for the European EO market. For these reasons, the next sections focus on the analysis of these four use cases. Marine weather forecast covers measurements such as wave and wind parameters. This information is of predominant importance to a wide variety of activities, from tourism to fishing, oil and gas exploration and exploitation. Results of the measurement gap analysis focused on \"marine weather forecast\" showed that revisit time gap to be reduced to 3 h and data latency \(<\)1 h. Table 2 shows the Copernicus and contributing missions in the time frame from 2020-2030, which are capable of measuring the variables defined for the marine weather forecast use case. These future missions will provide high horizontal spatial resolution for ocean measurements, but the provision of appropriate sea-state products and the adequacy of EO observations in near- or real-time (\(<\)1 h) are not satisfied. This translates into a system mission that shall support existing and planned EU infrastructure to reduce the revisit time for \"marine weather forecast\" use case to 3 h and data latency to less than 1 h.  
The use case \"sea ice monitoring\" covers a wide range of measurements that are of high relevance to marine operations and to understand global climate change. This use case requires providing real-time sea ice data and improving the precision of ice thickness measurements, as well as increasing the operational monitoring capability of polar regions. On the one hand, the Arctic and Antarctic are the parts of the globe with better revisit time statistics as most EO missions are in polar Low Earth Orbits (LEO) and fly over the poles 14-15 times per day. On the other hand, polar regions represent a blind zone for instruments flying in a geostationary orbit and are poorly covered by some narrow-swath nadir-pointing instruments such as radar altimeters (e.g., SRAL/Sentinel-3 and Poseidon-4/Sentinel-6).