AROSAT

The project was initiated by Rheinmetall Italia at the end of 2009 having perceived the high interest of our Defense to own and operate a very high resolution (say: at the state-of-the-art level) optical satellite as a complement to the  observation resources already existing in Italy. Spacesys was called to provide ideas and a possible solution to the problem. Based on the work independently developed in house ( see the box 'VHR Optical satellite') we initially proposed a triplet of LEO minisatellites flying at 400 km, each carrying an electric propulsion system for drag compensation.For the camera it was unavoidable to recourse to an external buy due to the peculiar features required for such a mission. Initially,the camera was conceived as providing both panchro high resolution  and coarser resolution multispectral (MS) images. However, budget reasons suggested -later- to renounce to the multispectral capability. Accordingly, a minimum mission with one satellite only, and  carrying only a panchromatic camera,  became the baseline  

Since the very beginning the satellite design was biased by the choice - for cost reasons- of Falcon-1 by  SpaceX. This launcher cannot accommodate 'fat' satellites, accordingly AROSAT  shape is long and quite thin, with the solar panels folded onto the outer spacecraft surface. The minisatellite height  is around 230 cm and is  mainly due to the accommodation of a telescope having a 1 m diameter and a lenght of about  1.8 m.

By mid 2010 the general spacecraft configuration was defined as an hexagonal body carrying, on its interior, a large telescope ( 1 m diameter for 1.8 m lenght) providing less than 0.5 m resolution panchro imaging. The body carried two in orbit deployable solar wings and a platform on the opposite side to the telescope opening, carryng the xenon-based electric propulsion subsystem. Two dynamically repointable antennas serve to relay the satellite gathered images to ground.

The satellite is injected in a sunsynchronous orbit plane making an angle of 30° with  respect to the terminator: a solution which provides a good illumination of the solar panels thus eliminating the need for yokes. Indeed, see the figure below, the yearly evolution of the mean Sun illumination of the solar panels for the selected orbit does not differ much from the Sun illumination of a spacecraft injected in a dawn-dusk orbit.

The 2nd half of 2010 focused on the pre-definition of the subsystems, also via a survey of prospective Suppliers which were used to refine the ptogramme cost estimates. The performance and operation of the Camera which has strong impacts on the structure, thermal control,  attitude control, data handling and storage,  and transmission system was the subject of interface meetings with the candidate Supplier.

Several trade-offs and analyses were carried-out for  the attitude determination and control subsystem which plays a key role in achieving the required performance in both a synchronous and asynchonous pushbroom mode of the high resolution camera. The impact of drag, lunisolar effect and Earth gravity anomalies were analyzed for impact on the spacecraft orbital motion and its compensation via a xenon-based  propulsion system.

The data storage demand, stemming from the assumed number of hi-res images to be daily taken and stored waiting for being downloaded towards a Data Receiving station, put severe requirements on the hardware which might require a dedicated R&D phase. Also the transmission system is a critical subsystem, considering both the datarate , coding, encryption and  multirate modulation capability requirements.

Combining the DC power requirements of both the bus and payload, and the expected operating duties, it turns out that the mean power demand is close to 1 kW, implying a solar plant capable of abour 1.4  kW considering all inefficiency  sources. A Li-ion battery pack was accordingly sized providing some margin too. The estimated overall satellite mass is around 450 kg , see preliminary budget below:

Optical payload	120  kg
Structure, mechanisms, thermal control	90    "
power and DC harness	55    "
AOC	40    "
Data handling	15    "
Proplulsion ( incl . propellant)	40    "
TTC and antennas	10    "
Data storage and Transmission	55   "
margin	25   "
total	450   "