## 1 Introduction  
The emergence of small satellites, and in particular the CubeSat standard [1], has opened up new ways of exploiting space [2]. Future projections foresee that in the next 5-10 years most satellites will be small satellites of less than 50 kg (nano and microsatellites) [3]. The inherent strategy behind the small satellites approach has allowed for ES (Earth Science) missions, such as \({}^{3}\)Cat-3/MOTS (3: Cube, Cat: Catalunya, 3: 3 rd CubeSat mission/Missio Observacio Terra Satel-lit), at an affordable risk and cost [4]. However, small satellites present limitations and vulnerabilities associated with their low cost philosophy. In order to overcome a hostile space environment, both the behavior of the different subsystems and that of the components need to be accurately studied [5].  
The mission statement of \({}^{3}\)Cat-3/MOTS is based on three pillars: viability (budget, human resources, and know-how), feasibility (technology readiness), and desirability (usefulness of the results). These three concepts converge in \({}^{3}\)Cat-3/MOTS with an innovative combined optical/ Global Navigation Satellite Systems Reflectometry (GNSS-R) [6, 7] payload and data fusion solution for high-resolution soil moisture mapping. Since the concept of innovation is linked to a limited time window, the combination of the knowledge provided by the Institut Cartografic i Geologic de Catalunya (ICGC) in the field of satellite imagery applied to the management of the Catalan territory, and the experience of the Universitat Politecnica de Catalunya-BarcelonaTech (UPC) in the field of Remote Sensing, in the development of previous small satellite missions (\({}^{3}\)Cat-1 [8], \({}^{3}\)Cat-2 [9], and \({}^{3}\)Cat-4 and \({}^{3}\)Cat-5 as part of the FYS (Fly Your Satellite) program [10] and FSSCat (Federated Satellite System Catalunya) [11] respectively, both in the design phase), and in the access to facilities to test and qualify space hardware [12] define an optimum environment for \({}^{3}\)Cat-3/MOTS.  
The main goal of \({}^{3}\)Cat-3/MOTS is to acquire multispectral imagery of the Earth in conjunction with GNSS-R data. The regions of interest (ROIs) for this mission will be land cover areas with high vulnerability and impact by climate change. The ICGC has defined a set of design requirements (Table 1) where the specific criteria for attitude control, exploitation, and data acquisition are detailed. In parallel to this main goal, the mission objectives can be listed as:  
1. To identify the limit of current CubeSat technology in terms of spatial resolution and required power to accomplish a multispectral optical and GNSS-R space mission.
2. To evaluate the feasibility of using Commercial off the Shelf (COTS) optical equipment in space to achieve \({}^{3}\)Cat-3/MOTS's mission requirements.
3. To acquire multispectral images from the visible to the near infrared (400 nm to 870 nm) with a spatial resolution better than 30 m and swath wider than 30 km with a signal-to-noise ratio (SNR) better than 30 dB in each band.
4. To achieve a revisit time of less than 10 days over the Catalan territory to properly respond to territorial changes.
5. To perform data fusion of the observables acquired by both payloads: multispectral imagery from the optical sensor and L-band reflectometry data from the GNSS-R soil moisture mapping at 30 m resolution.