PROBA-3

PROBA-3 Mission

PROBA-3 is devoted to the demonstration of technologies and techniques for highly-precise satellite formation flying. It consists of two small satellites launched together that will separate apart to fly in tandem, to prepare for future multi-satellite missions flying as one virtual structure and characterise sensors and other related technologies.

Current scientific and application challenges call for the detection of ever fainter signals and smaller features. Larger apertures, longer focal lengths and baselines that are beyond what can be achieved with a single spacecraft will be required to meet these goals. The solution is satellite formation flying.

Achieving precise formation flying opens up a whole new era for science and applications. Future missions could be assembled on a much larger scale. Applications of interest include Earth observation as well as in-orbit satellite servicing.

In recent years, Europe has made significant progress in the field of multi-satellite missions. ESA’s Automated Transfer Vehicle has demonstrated centimetre-scale accuracy when docking with the ISS while Sweden’s Prisma mission has demonstrated formation flying for brief periods, maintaining cm-level accuracy across tens of metres.

PROBA-3 marks the next step in formation flying. Its two satellites will maintain formation to millimetre and arc second precision at distances of 150 m or more. In effect the pair will be flying as a virtual giant satellite. And this will be achieved autonomously, without relying on guidance from the ground.

Two small satellites will be launched together into a highly elliptical orbit (600 x 60530 km at around 60 degree inclination) and will then separate. After a short preparatory period the two satellites will be separated and injected into a safe relative tandem orbit. The commissioning period will include demonstration of the mission’s Collision Avoidance Manoeuvre, ensuring they can be left safely in an orbit with no chance of collision or running away from each other. Normal operations will then include both formation flying manoeuvres and coronagraph observations.

The cost in fuel of maintaining formation throughout each orbit would be too high, so each orbit will be divided between six hours of formation flying manoeuvres at apogee and the rest of the orbit in passive drifting. The PROBA-3 satellites will repetitively demonstrate acquisition, rendezvous, proximity operations, formation flying, coronagraph operations, separation and convoy flying every orbit.

The orbit, selected after a thorough trade-off, has several significant and attractive features. It is similar to the orbits of navigation satellites – and Molniya and Tundra orbits that are gaining importance in view of the evolution of the polar regions.

PROBA-3 will be a laboratory in space to validate strategies, guidance, navigation and control and other algorithms previously tried in ground simulators. These techniques and simulators developed in the frame of PROBA-3 will then be available more widely, becoming instrumental in the preparation of future missions.

Source and credits: www.esa.int, ESA, PMOD/WRC

PMOD/WRC Instrument: DARA

PMOD/WRC constructed DARA (Digital Absolute Radiometer), a new type of absolute radiometer, designed to measure Total Solar Irradiance (TSI). Absolute radiometers are based on electrically calibrated heat flux transducers and use multiple blackbody cavities as sensing elements. The DARA instrument will host three such cavities.

During normal operation, an electronic circuit keeps the heat flow between all three cavities and a common heat-sink constant. One of the cavities will be kept shut at all time and will be considered as the reference cavity. The measurement principle is based on the difference in electrical power required to keep a constant heat flux with the cavity shutter open or closed. The difference in power between these two phases is then considered to be total solar input absorbed by the exposed cavity. Each cavity is designed to ensure a very high absorption over the spectral range of interest.

Scientific Objectives

DARA will be traceable to the WRR and SI-units (through the NIST-traceable TSI Radiometer Facility at LASP in Boulder, USA) and will eliminate the usual problem of stray-light using a special design (inverted viewing geometry). PMOD/WRC expects to confirm the WRR-to-SI discrepancy of ~0.3% and the TSI measurements to agree with the TIM (SI) scale. A confirmation or disagreement of the TIM scale using a WRR-traceable radiometer is a key aspect in order to assess the absolute level of TSI. There is a fundamental difference in the measurement principle of TIM and PMO-6: TIM measures the irradiance in a phase sensitive mode of operation whereas PMO-6 awaits a thermal equilibrium state. DARA on PROBA-3 is the first instrument that will offer the option of both modes of operation, allowing us to determine whether the instrument’s behavior depends on the operation mode. The DARA measurements will lead to a fundamental improvement of the uncertainty of the solar constant.

Design and Heritage

A first prototype instrument of DARA was built during project phase B of PROBA-3. This model underwent intensive testing concerning its radiometric functionality. The principle of a common heatsink and the symmetric design for all sensor cavities has been successfully proven. In addition, the instrument control electronics and on-board S/W were specifically developed for DARA and gave good results.
The DARA design concept was applied to the development of the CLARA space instrument. CLARA is a payload on the Norwegian NORSAT-1 microsatellite, which was launched in July 2017.

Source and credits: www.esa.int, ESA, PMOD/WRC

Mission Facts

Source and credits: www.esa.int, ESA, PMOD/WRC