## 5 Reference Instrument Selection  
The main requests of any satellite monitoring mission can be summarized as follows: (1) that observations are acquired with the required revisit time; (2) preferably in all weather conditions (clouds, rain, haze, and fog) and in all illumination conditions; (3) with a large swath to reduce the revisit time; (4) with the required radiometric and spatial resolutions; (5) with low manufacturing and launch costs, and with minimum deployment time in case of failure; and (6) keeping these parameters in mind, the reference instruments can be selected. In this way, the identification of instruments is based on the state-of-the-art at the payload level and the need to fulfill the gaps of the current Copernicus infrastructure. Reference instruments and small platforms have been selected in the previous chapter. In this way, it has as strategy been implemented a significant reduction of the development time and cost, thanks to the adoption of commercial technologies, but it requires that these have a good performance of the measurement capabilities. In this regard, the capability of the instrument technologies is evaluated according to the trends in the design for small satellites. For each instrument, the mass and power consumption constraints, and data quality (spatial resolution, swath, and accuracy) are taken as a reference. This chapter evaluates if the instruments selected to meet the requirements (defined in [6]) in terms of spatial resolution and accuracy. Table 8 summarizes the performance requirements over each instrument:  
* SGR-ReSI instrument presents a good performance for sea ice cover [99] because it satisfies the minimum requirement for spatial resolution and accuracy. For ocean surface currents, and significant wave height measurements satisfy the minimum requirement of spatial resolution at 25 km [100]. For other measurements, such as sea ice thickness [46], soil moisture [101], and wind speed [80] present worse performance than the minimum spatial resolution and accuracy requirements. * EON-MW is a satellite project under development and presents an approximate performance that the Advanced Technology Microwave Sounder (ATMS) [33], in this way, it will be expected that the instrument satisfies the minimum requirements for accuracy of 5% and spatial resolution at 23 km for atmospheric pressure over sea surface measurements (channel from 50 to 60 GHz). * MIRAS instrument presents a coarse spatial resolution \(\sim\)35 \(\times\) 50 km for horizontal- and vertical-polarization. This instrument has an accuracy of 0.04 m\({}^{3}\)/m\({}^{3}\) for soil moisture measurements [102] that is worse than 0.01 m\({}^{3}\)/m\({}^{3}\) required. For sea ice thickness, the accuracy is worse than the 1 cm required [103], but it can have an accuracy of 5% for sea ice cover. * SSM/I using an antenna (inflatable) of 2.2 m from 600 km orbit altitude can obtain a spatial resolution of 10 km and satisfy the minimal spatial resolution requirement for wind speed, and sea ice cover measurements. The accuracy for wind speed measurement can be until 1.5 m/s [104], and for sea ice data from 10% to 20% [105]. * TMI in order to meet the minimal spatial resolution requirement of 10 km (at 10.65 GHz) was proposed the modification of the aperture size of the antenna at 3.4 m (inflatable antenna). The accuracy for SST is of 0.5 K [104]. The accuracy is between 10% and 20% for sea ice data [105]. * AVHRR/3 presents a spatial resolution \(\sim\)1 km, and computes an accuracy better than 0.1 K [106]. * EON-IR is expected to be better than 0.25 K and present, with spatial resolution at 13.5 km. * SCAT--the accuracy for wind speed monitoring is 2 m/s, and for sea ice monitoring is 5%. * SRAL in SAR mode has a spatial resolution of 300 m, the accuracy for wind speed measurements is of 2 m/s [107]; for significant wave height, the accuracy is between 2 cm to 8 cm [108].