Major phases of the project and models of the instrument
To ensure that PHEBUS will have the expected performance and verify that the instrument survives the launch, its interplanetary voyage and the environment close to Mercury, several "models" are manufactured, each for a specific purpose of technical validation.
How many models need to be built? Which ones? For what purposes? What tests and checks will be carried out on each model? This is called "The Philosophy of models". This is defined fairly early in the project: it is a key element that will have a significant impact on timing, costs and risk management.
Thus, several "PHEBUS" were built:
Objectives: To validate the performance of the optical concept of the instrument.
2010-Structural & Thermal model
Objectives: To test the mechanical and thermal design on a "macroscopic" scale. This model is representative of the flight model in terms of structural and thermal behaviour.
Objectives: To check the main features of the instrument's onboard software, operational procedures, all electrical interfaces with the satellite. This model is representative of the flight model in terms of Control/command of subsystems (mechanisms, detectors, DPU, etc.)
2013-Qualification Model 2013-Qualification model
Objectives: Fully validate the instrumental concept (features, performance, environment). It is similar in all respects to the flight model and is subjected to more tests than the latter at more stringent levels.
2015-Flight model 2015-Flight model
Objectives: This is the final flight model, installed on the satellite and that will carry out the scientific mission for which it was built.
2018-Replacement model 2018-replacement model
Objectives: This is the replacement flight model. In the event of a major problem or anomaly on the flight model, the latter is dismantled from the satellite and replaced by the FS.
Tests of the qualification campaign
These tests are performed to verify that the optical performance of the instrument satisfies the scientific objectives. The main test is to illuminate the instrument in the UV (55 to 425 nm) using various light sources and to characterize the instrument's response. This test is carried out in vacuum tank, in order to eliminate atmospheric absorption of the UV radiation at wavelengths less than 200 nm. The University of Padua in Italy provides the technical support on the optical characterization of PHEBUS.
Once the instrument is assembled each of its functions is tested. During functional tests, various electrical measurements are carried out: consumption in the various operating modes, current calls, etc.
The scope of these tests is to demonstrate that the instrument will survive the mechanical efforts it will encounter during its lifetime. The mechanical stresses occur mainly during the launch (vibrations) and during the separation of the stages of the launcher and the modules of the probe (shocks). These tests are carried out following 3 orthogonal axes on a vibrating pot for vibration tests or by means of a ram for impact tests.
Onboard software validation
The instrument's onboard software manages on one hand communications with the platform and on the other hand the control of the instrument. It sequences the onboard activities, while guaranteeing the integrity of the instrument by monitoring the internal parameters. In order to ensure a permanent operability of the instrument, the onboard software is qualified on several levels (processor simulator, EM, QM/FM/FS, on satellite).
The purpose of these tests is to ensure that the various equipment that make up the satellite will work properly together once integrated and won't disturb each other. It is to verify that the instrument does not emit electromagnetic disturbances too important for the other satellite equipment (so-called "emission" tests) and to verify that the instrument itself is insensitive to emissions produced by these other equipment (so-called "susceptibility" tests). Each type of test is carried out in "circuit" mode (cable interactions) and in "radiated" mode (wave interactions).
Thermal vacuum tests
Vacuum thermal tests reproduce the pressure and temperature conditions of the mission. They allow us to check, on one hand, that the instrument is capable of surviving the mission environment and, on the other hand, that the behaviour and performance of the instrument over the temperature range considered are compliant and known. The operating temperature of PHEBUS ranges from –20°C to +40°C.