K2 : Nanosatellite student project IGOsat UnivEarthS

When talking about space or satellites, what usually first springs to mind is the bipping noise of the Sputnik, a striking image from the Hubble Space Telescope (HST), or Chris Hadfield singing ``Space Oddity'' aboard the International Space Station (ISS). Indeed, we tend to block out the idea that much smaller satellites could be up to the task, while there  actually are many of them orbiting the Earth right now and some already have completed their mission.

 

So, it's cute and tiny but what can you achieve with it ?

 

Back in 1999, Caltech and Stanford designed the first models of nanosatellites (or CubeSat) and developped the present standards. A Cubesat correspond to a miniaturised satellite : the base unit is a cube, which sides are 10 centimeters long and that can transport a mass of about 1 kilogram. What Caltech and Standford were aiming for was to create a type of satellite that would place aerospace within reach of a greater number of public and private institutions.

The first nanosatellites were launched in 2003 and while some projects tested then, and still test now, the feasability of developped concepts (such as using only off-the-shelf components, like a smartphone, or making nanosatellites orbit in a tight flight formation), other projects focused essentially on scientific experiments.

In 2003, the QUAKEsat mission seeked to detect variations in the magnetic field due to earthquakes ; the AAUSAT-II project (University of Aalborg) , launched in 2008, was able to perform space mecanics experiments and used a gamma-ray bursts detector. In 2009, the EPFL's SwissCube successfully performed nightglow measurements; the PharmaSat project, born from the joint work of the NASA Ames Research Center and the University of Santa Clara, was able to carry out biomicrology experiments in microgravity.

The list goes on and on, and today, all across the globe, in Equador, Japan, Hungary, India or Vietnam, universities and space research institutions are designing tools taking the space adventure to a brand new level.

At the Paris-Denis Diderot University, as a part of the Laboratoire d'Excellence (LabEx) UnivEarthS, the AIM, APC and IPGP laboratories have teamed up to launch a nanosatellite project which will be entirely conceived and developped by students.

Alrigth then, I've got myself some ducktape, matches and a Swiss army-knife, what's the plan ?

In order to survive the launch and be able to function under the harsh conditions of space, a satellite must be designed as a self-sufficient, reliable and robust system capable of dealing with almost anything that space would throw at it. That's the reason why a satellite is actually a complexe assembly of interdependant sub-parts.

Thus, regardless of the mission type, a satellite must necessarily come with the following equipments :

- a complete and self-sufficient electrical architecture, capable of distributing the electrical production of the solar panels towards sub-systems and able to manage the use of batteries energy.

- a perfectly reliable radio-frequency telecommunication system which must function during the entire lifespan of the satellite and allow data transmission and commands reception with ground control stations.

- altitude control captors and actuators allowing for the determination and control of the relative position and direction of motion of the satellite.

- an onboard computer controlling other sub-systems and programmed with enough failsafes and redundancies so as to be geared to meet any eventuality.

Furthermore, every satellite is designed with one or several functions in mind, those functions corresponding to the purpose of the payload. There exist as many satellites as payload purposes, whether it is to allow for information broadcasting (telecommunication satellites), Earth observation (civilian or military), worldwide localisation services (GPS, GLONASS, GALILEO), or scientific research (XMM, Plank, AKARI, GOCE, THEMIS satellites).

I was always told to aim for the stars...

The Paris-Diderot University IGOsat will take onboard two scientific payloads. It will be following a polar heliosynchronous orbit at an altitude around 700km, and in so doing IGOsat will extend our knowledge of high-energy particles trapped in radiation belts. It will also allow in situ measurements of the electrons density in the ionosphere (the upper layer of the atmosphere consitute a diluted ionized gas) and thus help us have a better understanding of the correlation of these phenomenons with solar activity.

More specifically, those two payloads will be a GPS receiver and a charged particles and photon detector.

The GPS receiver will measure phase differences between two carrier signals of 1,2 GHz (L1) and 1,6 GHz (L2). The ionosphere is a dispersive medium which introduces a propagation delay in a signal of a defined frequency. The phase difference between the two carrier signals will give us information about the cumulative electronic density on the aiming line between one of the 31 GPS satellites on the MEO (Medium Earth Orbit), at about 20 000 kilometers of altitude, and the nanosatellite's receiver, on the near-polar LEO (Low Earth Orbit) at about 700 kilometers of altitude.

The relative movements of these two satellites regularly cause an occultation : there is no direct line of sight between them. During this occultation, the aiming line crosses the ionosphere and an estimate of the total electron content in this area can be calculated.

A new geometry

The charged particles and photon detector consists of three parts : a cubic organic scintillator (also known as a plastic scintillator) with an inorganic scintillator (a LaBr3 crystal) encased in it. The pulses of light are read with a photomultiplier. When a particle (a photon or a charged particle) interacts with a scintillator, a fluorescence flash is emitted, and its caracteristics give information about the energy deposited and about the nature of the particle.

Therefore, the plastic scintillator will essentially be sensitive to electrons and protons, whereas the LaBr3 crystal also detects gamma photons. Combining these two sets of data allows for the precise calculation of the population density for these two particle types. The photomultiplier transforms the optic signal in an electrical signal and amplifies it, making the measurement possible.

These instruments will allow us to build a set of data on the nature and energy of the intercepted particles, and to gather measurements of the electron content in the ionosphere.

Scientifically speaking, these measurements will allow us to track the evolution of these populations in time, and to quantify the correlations and interactions they have with solar activity. These in situ measurements will complete and refine ongoing ground and space studies led by other scientific teams.

Moreover, the low cost of a nanosatellite mission allows a higher technical risk taking, along with the testing of innovative equipments in space.

This will be the case here for all the detection chain of the scintillator - including the LaBr3 crystal, the electronic aquisition (that was initially developped for particle accelerators), and the optical signal detection (achieved by avalanche photodiodes). None of these new technologies have been directly tested on a satellite yet, and their space qualification will be a significant breakthrough for future space instruments.

Contact us

If you have any question or suggestion, or if you are interested in joining the project, please don't hesitate to contact us at [email protected]

 

 Jessica Tanon  & Clément Feller