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Projecte llegit

Títol: Design of a Femtosatellite for High-Altitude Thermospheric Studies

Estudiants que han llegit aquest projecte:

Director/a: GUTIÉRREZ CABELLO, JORDI

Departament: FIS

Títol: Design of a Femtosatellite for High-Altitude Thermospheric Studies

Data inici oferta: 28-07-2020     Data finalització oferta: 28-03-2021


Estudis d'assignació del projecte:
    MU AEROSPACE S&T 15
Tipus: Individual
 
Lloc de realització: EETAC
 
Segon director/a (UPC): GIL PONS, PILAR
 
Paraules clau:
Thermosphere, small satellites, femtosatellite, PocketQube, COTS, MEMS
 
Descripció del contingut i pla d'activitats:
The thermosphere extends from 100 to 500-600 km (depending on solar activity state) above sea level, so encompassing a significant part of LEO satellites. Despite its extremely low
density, it controls the re-entry of satellites and provides the neutral species that, after becoming ionized by solar UV, constitute the ionosphere.

In this project, we propose the design of a femtosatellite (or picosatellite, if the mass cannot be kept under 100 grams with feasible devices) to determine the average density of the
thermosphere at altitudes up to 500 km. The proposed strategy for retrieving the density is the orbit reconstruction using the state vector (position and velocity) and time gathered by means
of a suitable GNSS receiver.

In order to eliminate any attitude dependency, the satellite will be spherical, with a diameter in the range of 7-10 cm. The internal mass distribution should be such that the inertia tensor is diagonal and with equal inertia moments, to preclude the existence of a preferred leading face.

DRAMA simulations indicate that, for masses between 0.1 and 1 kg, the orbital lifetime of a spherical satellite of 10 cm will be around 20 months, with an uncertainty of plus/minus 5 months due to solar activity state differences.

This long lifetime requires the use of solar cells to provide energy to the satellite. As there are no spherical solar cells, and the sphere is too small to allow it covered by flat cells and still be close to a spherical shape, a different strategy must be followed: a PocketQube satellite embedded in a suitable transparent plastic. In this way, we can keep the spherical shape and, at the same time, allow the use of solar cells. The plastic must be space qualified and avoid, at all costs, any blackening caused by solar UV radiation or atomic oxygen effects.
The payload of the satellite will be a space-qualified, MEMS GNSS receiver. It will determine the position and velocity of the satellite every five seconds. The data, around 250 bytes per
determination (composed of the state vector and the time, plus extra bytes for EDAC purposes), shall be downlinked. For this, we will consider two possible strategies: the use of a MEMS transmitter and an omnidirectional antenna, or by means of a suitable LoRa system.

Provided that the spherical shape and mass distribution are as required, we will need no attitude determination and control subsystem. Nevertheless, if the mass distribution could not
be made in such a way that complies with the desired inertia tensor stated above, we would undertake a secondary analysis of interest. In this case, the satellite would experience an
aerodynamic torque that depends on the position of the centre of mass of the satellite and its size. A simple analysis of the power generated by the solar cells would give a rough attitude
determination (as well as an accurate rotational velocity), and this would allow us to determine the effect of the aerodynamic torque on the attitude.

Thermal control will be completely passive, and we will analyse it by means of standard software (perhaps ESATAN). The rest of the satellite bus will comprise a standard PocketQube
structure, a set of six solar cells, a simple on-board computer with a flash memory for data storage, a secondary battery, and the required support electronic devices.

The proposed femtosatellite is envisioned as a secondary payload to be released from a proposed 6U CubeSat, but it could fly as a secondary payload in a more standard launch. A final, but secondary, goal of this project is to design a container which can hold and release the satellite once in orbit. The container itself should have a configuration compatible with a launch with an existing POD (like PPOD), and then its external shape should follow the CubeSat standards.
 
Overview (resum en anglès):
This work deals with the design of a femto-satellite for scientific studies. The goal of the femto-satellite is to model the density of the thermosphere by measuring the orbital decay caused by atmospheric drag. The proposed femto-satellite, having a mass of maximum 100 g by definition, is embedded in a PocketQube structure and equipped with solar panels in order to guarantee operability during the entire expected lifetime of approximately three years. In order to ensure a constant drag coefficient regardless of the attitude, the femto-satellite is to be covered by a spherical structure, which moreover allows for solar illumination of the solar panels.

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