# _remote sensing_
## 5 Implementation, Technical Challenges and Extensions to Multiplatform Missions  
The development and qualification of the described 6U CubeSat can be scheduled to last for two years, with particular effort on the payload prototyping, flight model development and qualification. The orbital bus is based on commercial components and will present shorter times of development, potentially with a PFM philosophy (probably limited to the bus only, with an engineering model development for the payload). Indeed, all the bus components present a TRL between 8 and 9, with the large majority of the platform components presenting flight heritage and maturity in all the development and qualification processes [60; 61; 62; 63; 64; 65; 66]. On the other hand, the experimental payload presents a lower TRL (between 5 and 6, with a technology which is operations-ready but with the need to be space-ready) and requires a more thorough qualification for launch on-board a nano-satellite mission like HORUS. During the development and qualification of the payload, special calibration campaigns for the sensors shall be taken into account. As an example, the fixed pattern noise of the imagers shall be evaluated through a calibration test with a blank screen. This campaign would significantly improve the impact of the mission and of the acquired data, yet potentially requiring post-processing techniques aimed at filtering out such noise while on-orbit.  
The scientific challenges of HORUS can be related to the exploitation of a different sensor with respect to the MISR, which can offer interesting opportunities in terms of acquisition strategy. Indeed, the HORUS mission uses a CMOS sensor instead of a push broom sensor (which is implemented on MISR and other similar missions). Therefore, the CMOS may be used to perform image acquisitions of the target to be combined with the push broom-like radiometric measurements. This could be also extended to a full imaging observation campaign. The acquired data can be complementary to both imaging and radiometric missions, extending ground coverage and revisit time. An in-orbit calibration campaign shall be performed at the beginning of the operations over known targets. These tests will allow to evaluate the real performances of the developed sensor. Such calibration can be repeated at regular intervals to evaluate the degradation of the sensors over time, representing another technical challenge of the mission, as not many multispectral payloads have been flown on small satellite missions and their performances in longer term missions can be studied for further extensions of the mission concept. Although a low degradation due to radiation is expected, thus allowing a mission of several years, alternative procedures in case of higher degradation (e.g., adjustment of the sensor integration times) can be considered for managing the later operations of the CubeSat and for maintaining the significance of the acquired data at the requirement levels.  
Finally, the HORUS payload implementation in 6U CubeSat allows for effective and inexpensive multiplication of the sensors on a multiplatform system, such as multiple satellites at appropriate distance along the same orbit, with the aim of reaching a daily or two-days revisit time, potentially useful in complementing the measurements of larger Earth Observation satellite platforms.  
The simplest configuration of a HORUS multiplatform-based mission includes a number of identical satellites, all at the same orbital height in a 10:30 am SSO. The number of satellites necessary in this configuration for daily revisit depends on the HORUS cross track VFOV of 33.4 degrees and on the orbital height. As an example, for full daily revisit at the equator with 500 km orbital height satellites, nine spacecraft in the same orbital plane are necessary.