When two satellites approach within a few hundred meters, they typically execute collision avoidance maneuvers. However, ESA's Proba-3 mission involves a controlled formation flight, with the two satellites approaching as close as 25 meters.

Aligning with the Sun
When positioned around 150 meters apart, the satellites will align with the Sun, casting a shadow from one to the other to create artificial solar eclipses. This alignment will enable the observation of the Sun's faint outer atmosphere without the interference of its brightness.

To achieve this, the Proba-3 satellites utilize multiple sensors, similar to autonomous cars on Earth, to determine their positions relative to each other. These systems were tested together with the spacecraft software for the first time in the Redwire cleanroom in Kruibeke, Belgium.

"Proba-3 is a formation flying demonstration mission, and these systems really form the mission's core," explained ESA software and systems engineer Teodor Bozhanov. "To maintain the position of the two satellites to the necessary precision, we need to perform specific steps in sequence. If one of these steps is not successful, we cannot move on to the next."

Satellites face to face
The two Proba-3 satellites were placed over 15 meters apart in the Redwire cleanroom, the maximum distance available within the confined setting. In space, the pair will be flying around ten times further apart during the mission's formation flying operations.

Personnel from Proba-3 prime contractor Sener in Spain joined the Redwire and ESA teams for the week-long test campaign, along with experts from Danish space research institute DTU Space, which manufactured Proba-3's Vision Based Sensor system.

Radio, satnav, and stellar navigation
The satellites will maintain a connection using a radio-based inter-satellite link system from Tekever in Portugal, continuously updating their distance from each other. They will also determine their absolute positions in space using specially designed satellite navigation receivers, accounting for Proba-3's highly elliptical 60,000 km altitude orbit, which intersects the orbits of satnav constellations and extends high into space. Additionally, the satellites are equipped with star trackers, computer-linked cameras that recognize constellations to reveal each satellite's current pointing direction in space.

LEDs, cameras, laser - getting into position
Once the satellites are within 250 meters of each other, the relative navigation systems for precise formation flying are activated, trialed sequentially during the test campaign.

The first step involves the Vision Based Sensor system. A wide-angle camera tracks an LED pattern on the other satellite, providing coarse information on their distance and attitude. This is supplemented by a narrow-angle camera that locks onto a smaller LED pattern, offering relative positioning information down to about a centimeter.

Millimeter accuracy achieved
Next, the Fine Lateral and Longitudinal Sensor (FLLS) on the 'Occulter' spacecraft shines a laser towards a corner cube retro-reflector on the 'Coronagraph' spacecraft, reflecting back to the Occulter. This system provides relative positioning accuracy down to a millimeter.

The final positioning technology is the Shadow Positioning Sensor system, which uses photo detectors arranged around the coronagraph lens to monitor the Sun's corona. If the shadow is correctly centered, it should be equally illuminated on all sides; any discrepancy would trigger a correction.

First-time combined testing
"All these systems have been tested before at unit level and in simulations," said ESA guidance, navigation, and control engineer Jonathan Grzymisch. "But this was the first time all our hardware and software operated together as they will in space. The navigation system processed actual hardware inputs, progressing from coarser measurements to finer metrology.

"The confined test setup was complex because we were tricking the guidance, navigation, and control software to work outside its operational domain. We used a robot from our Guidance, Navigation, and Control Lab at ESTEC in the Netherlands to hold a laser retroreflector, as the layout didn't allow alignment with the real one on the Coronagraph satellite.

"However, the testing went smoothly, bringing us closer to launch readiness. The next time all these systems operate together will be in space, after the mission begins."

Proba-3 is scheduled to launch this autumn on an Indian PSLV-XL launcher.

Related Links
Proba Missions at ESA
Space Tourism, Space Transport and Space Exploration News