The Payload Assembly System is the instrument’s "cocoon", namely, a structural unit guaranteeing its thermal stability and mechanical resistance.
The Payload Assembly System is mechanically and thermally optimized to guarantee good structural resistance to launch vibrations and great thermal stability for the accelerometers inside the Sensor Unit (SU) and their Front End Electronics Units (FEEU), required for scientific measurements.
To guarantee the two required levels of thermal stability (± 1 mK for the SUs and ± 10 mK for the FEEUs), the Payload Assembly System must be isolated from the platform and the external environment, which constitute thermal sources of disturbances.
To this end, it is fixed to the most stable thermal wall of the satellite: the one that never sees the sun. Its structure is composed of two stages conductively decoupled by insulating titanium bars, and the unit is radiatively isolated from the remainder of the platform by a multilayer insulating blanket (MLI).
There is a radiator at the base of the Payload Assembly System to reject heat dissipated from the FEEU (12 W). A baffle is installed around this radiator to limit the inputs of variable heat fluxes coming from the Earth, which would adversely impact the FEEU’s thermal stability.
The Payload Assembly System weighs approximately 50 kg. Its diameter is 52 cm and its height 55 cm.
A mock-up was built in the first phase of the project to prove the feasibility of employing completely passive thermal control (i.e. without active regulation, which would disturb scientific measurements) to reach the particularly ambitious required thermal stability performance.
The mock-up was to be representative in terms of thermal inertia (mass and the nature of materials), geometry, and direct and radiative couplings. For example, the electric cables connecting the accelerometers to their front-end electronics were also modelled. Indeed, their particularly heat-conducting copper core plays a significant role in heat exchange between the various stages of the Payload Assembly System.
Thermal tests on this mock-up consisted in reproducing a certain number of thermal disturbances (representative of those expected in flight) at the interfaces of this unit and in measuring the effect on the temperature stability of the SU and FEEU mock-ups. This required a finely tuned methodology able to detect temperature variations lower than one mK.