## 4 Potential Instrument, Suitable Platforms, and Technological Limitations
ELiTeBUS 1000 is a platform for Medium Earth Orbit (MEO) and Low Earth Orbit (LEO) orbit with 1000 to 1500 W of available payload power. * Scatterometers contribute to the Marine for Weather Forecast and Sea Ice Monitoring use cases. The instrument taken as a reference is the SCAT on board the CFOSAT mission [25; 82], the power consumption of this sensor is less than 200 W, and the mass less than 200 kg. According to the power consumption and mass requirements, this payload can be carried on board mini-platforms(Table 7). Scatterometers are valuable sensors for wind measurements. However, the main limitations are the coarse accuracy and spatial resolution of the data. However, their wide swath and the possibility of scatterometer constellations open the door to improve the accuracy and spatial resolution, combining the data from multiple passes of different satellites. * For radar altimeters, the accuracy of the measurements depends on the Pulse Repetition Frequency (PRF), which is directly driven by the power available on-board to the payload. Since the power available on-board decreases with solar panel size, the accuracy of the measurements on a small satellite is also expected to be degraded as compared to that of large satellites. For example, if the power consumption is reduced by a factor of 4, the PRF is reduced roughly by the same factor, and the Root-Mean-Square (RMS) error increases by a factor of 2. For the Jason-2 altimeter (power consumption \(\sim\)70 W), a reduction of its power consumption to 1 W, would increase the sensor error level from 2 cm to \(\sim\)16.7 cm, which is actually comparable to GNSS-R [55; 82]. It is easy to understand that the types of products that can be generated with this accuracy are different from the ones generated with an SRAL radar altimeter, but one must also consider that the number of radar altimeters with a transmitted power of 1 W that can be manufactured and launched at the same cost as for a high accuracy radar altimeter is much larger. These few examples illustrate the fact that the quality and frequency of the measurements have to be considered in the overall comparison process. In some cases, the concept of operations may partially be compensated by the degradation of the quality of the individual measurements (e.g., part-time measurement instead of systematic measurement if the power available on board is the main parameter driving the performance of the measurement). * SAR sensors are one of the most effective instruments for ocean, land, and ice observation. A good example of miniaturization of this technology is the Severjamin-M instrument (Meteor-M N missions) [83], an X-band SAR with power consumption of 1 kW and a mass of 150 kg, including the mass of the antenna of 40 kg. The main technological limitation is the narrow swath, but this could be compensated with a constellation of SAR satellites. * Optical payloads are characterized in terms of image quality such as the Ground Sampling Distance (GSD), the Modulation Transfer Function (MTF), and the Signal-to-Noise Ratio (SNR). To be able to interpret an image (e.g., in the maritime surveillance, the capability to estimate the type of a boat), the GSD is not sufficient, since a degraded MTF (i.e., blurred image) or a degraded SNR (noisier image) would prevent it. Ensuring a good MTF and SNR for a given GSD requires a minimum aperture for the optical instrument, and reducing it below this minimum value will limit the type of applications. Image quality is also limited by the platform's attitude control system, i.e, any jitter in the pointing will blur the image. This has also to be taken into account as smaller platforms exhibit poorer performances.