MAR Y MUROS: LA FRONTERA NORTE

The whole range of services at which all kind of users involved in agriculture could be potentially interested has been identified and a certain level of confidence with actual technology has been acquired. Optical observation resulted to be the best solution for a space observation system devoted to provide a source of continuous, systematic, repetitive, reliable, complete and accurate information to support in particular agricultural monitoring and management practices. A set of information provided by multispectral and hyperspectral images in the VIS, NIR, SWIR and TIR offers an attractive opportunity to achieve all spatial and spectral details related to land and vegetation. Before starting with an accurate sizing of the platform, a preliminary analysis to identify potential mission architectures is required.

This analysis is intended to select a range of possible orbital solutions which allow the platforms to match the capabilities and the objectives of all different types of instruments carried on-board. Initially, a reference mission devoted to produce data for hypothetical users involved in agricultural practices in a small region like the Tuscany is set as target of the analysis. The principal objectives of the mission analysis is to design:

 a low-cost mission, considering the satellite total mass and the total number of satellites as main cost drivers.

 A very performant mission, able to satisfy every kind of

requirements in the frame of land and vegetation observation. The specific case of Tuscany will be used to derive general indications and relations that could be easily extended also to other different mission scenarios.

The mission analysis is based on three technical starting points:

 the maximum expected mass of the satellite: lower than 100 kg, in the frame of microsatellites [1].

 The presence of a micro-electric propulsion device on-board the microsatellite. In particular, the last version of the Alta SpA low power Hall thruster named HT100, the HT100D is expected to be carried on-board [6].

 The possibility to produce on large scale several microsatellites all based on a single standard platform.

Chapter 6 - Preliminary Mission Architecture Design

107 These technical starting points presents several interesting advantages:

 firstly, microsatellites are characterized by a lower development and production cost respect to larger satellites.

 Finally, the HT100D ensures the unique and very innovative possibility to create a very performant and versatile observation system. Electric propulsion provides interesting potentialities in terms of atmospheric drag compensation for long periods even at very low altitude and with minimum propellant mass expense and further costs reduction by means of orbit differentiation after a single shared launch of all eventual microsatellites.

Such a system could be able to respond to many different and general tasks, and hence to perform a very large gamma of possible missions. The potential production of several microsatellites based on a single recurrent and versatile platform suggests to design a constellation of satellites able to:

 achieve very good temporal performance, better respect to larger satellites.

 Exploit all required observation payload types (multispectral, hyperspectral and thermal IR) together.

Considering the very small dimensions expected for the platform, the mission analysis is based on the assumption that just one observation payload could be carried on-board each satellite. Therefore, the mission analysis will be based on several steps:

 definition of most relevant technical requirements, system drivers and orbit parameters to design a suitable and appropriate Earth Observation constellation.

 The conceptual division of the complete constellation in “mini- constellations”, each of one characterized by the presence of a specific payload and devoted to satisfy a particular portion of spatial and spectral requirements.

 Identification and analysis of several small existing instruments representative of each optical observation category.

 Separated design of each mini-constellation to find the best orbital solutions for each payload in terms of spatial and temporal

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6.1.1 A preliminary discrimination

When multiple satellites have to work together, depending on the characteristics of their configuration, they could compose a constellation, a formation or a swarm or cluster [61]:

Constellation: when several satellites flying in similar orbits without

control of relative position, are organized in time and space to coordinate ground coverage. They are controlled separately from ground control stations [61].

Formation: if multiple satellites with closed-loop control on-board provide

a coordinated motion control on basis of relative positions to preserve the topology. It is the collective use of several spacecraft to perform the function of a single, large, virtual instrument [61].

Swarm or cluster: if a distributed system of similar spacecraft is

cooperating to achieve a joint goal without fixed absolute or relative positions. Each member determines and controls relative positions to the other satellites [61]

6.1.2 Main technical requirements for an Earth Observation system

The main technical requirements for an Earth Observation system derive from the analysis of scientific requirements. In the particular case of land and vegetation observation the system performance which impact the orbit design process can be summarized in a limited number of peculiar figures of merit, such as [62]:

 Geographical coverage.

 Response time.

 Revisit time.

 On-ground resolution (spatial and spectral).

 Swath.

Linked to these fundamental requirements, there are a lot of other requirements that have to be provided contemporarily by the system, like for example [62]:

Chapter 6 - Preliminary Mission Architecture Design

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 Ground stations network available for launch, Early-Orbit-Phase and routine operations.

 Specific illuminations conditions, local time.

 Data availability and reliability.

 Volume of data produced by the payload.

 Data rate.

 Responsiveness, acquisition delay and latency.

6.1.3 System drivers

Strictly related to system technical requirements, there exist several technical parameters that drive the design of the system, like for example [62]:

 mass and volume of the instrument and spacecraft.

 Budget constraints for manufacturing and operations during system lifetime, which have to be carefully considered in order to design a low-cost mission together with the launch vehicle cost and

availability.

 System reliability and availability, needed to ensure the possibility of every target acquisition by all interested users.

 Timely continuity of data.

6.1.4 Main parameters to define for an Earth Observation constellation design

The system drivers identify the main mission parameters influencing the achievement of the required performance vs. cost, risk and schedule for both space and ground segment. The mission parameters are mainly related to orbital characteristics that are selected to allow the highest level possible of system operation. In the frame of a constellation design, it is very important to specify [3]:

 the orbital shape.

 The orbital altitude.

 The orbital inclination.

 The number of orbital planes.

 The between-plane phasing.

 The number of satellites.

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