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Flying solo: ATV in pursuit of the ISS

Airbus Defence and Space’s ATV Programme Director Wolfgang Paetsch explains the voyage of the ATV to reach the ISS.

The towering Ariane 5 ES rocket, 50 metres tall and specially adapted for this massive but infinitely precious passenger, blasts off from its launch base in Kourou on the Amazonian equator, heading in a north-easterly direction. Its trajectory takes it above the Atlantic Ocean towards Europe, over France, Luxemburg, Germany, Poland and then Ukraine and Russia.

ATV attached to the upper stage of Ariane 5 ESAt around three minutes after lift-off, the fairing is jettisoned; some minutes later the main cryogenic stage separates, leaving the ATV attached to the upper stage. The upper stage carries the ATV to a circular orbit at an altitude of about 260 kilometres, aligned with the plane of the ISS’ orbit, inclined at 51.6 degrees to the equator. About one hour after lift-off, the ATV detaches from the upper stage. The ATV is now completely autonomous, and the real journey begins.

While still in Earth-pointing mode, the ATV goes about setting up its in-orbit configuration; it must first establish a communication link with the US Tracking & Data Relay Satellite System (TDRSS) in geostationary orbit. Then it unfurls its solar panels, followed by the antenna for communications with the ISS, and checks its systems, starting with the priming of its propulsion subsystem. Managing its own attitude, the ATV keeps its solar panels tracking the sun to supply the electrical power needed by the on-board equipment and its antenna pointing to the relay satellites to communicate with the ground and to the GPS constellation.

Getting into step with the ISS

Now the ATV commences a series of ‘phasing’ manoeuvres to align its orbit with that of the ISS, using a combination of natural orbital drift and powered movement. These manoeuvres must bring the ATV to a point about 5 km below the orbit of the ISS and 30 km behind the station itself.

The ATV approaches a point situated on the ISS’ orbit at a distance of 3.5 km behind the station From this point, a direct line of communication is established between the ATV and the ISS, and the ATV navigates by correlating the GPS data from its own on-board receivers with those of the receivers installed on the ISS. The ATV steers its own course autonomously, with authorisation from the ATV Control Centre (ATV-CC) in Toulouse and under the supervision of the mission control centres in Moscow and Houston. In less than one revolution around the Earth, the ATV approaches a point situated on the ISS’ orbit at a distance of 3.5 km behind the station and 40 minutes later reaches another point, a mere 249 metres behind the ISS. Here, the ATV changes attitude in order to bring its optical rendezvous sensors into line with the laser retro-reflectors installed on the aft of the Russian Zvezda module, where it will dock. At a few hundred metres, a laser pulse is sent to the retro-reflectors on the ISS and the beam transit time is measured to calculate the distance between the ATV and the docking point.

Smooth approach and contact

Due to the mass of the ATV, which is three times greater than that of the Russian Progress freighters, contact with the ISS not only requires ultra-high precision, but also extreme smoothness (at a relative velocity of less than 7 cm/s).

The final approach is therefore conducted in three stages: closing in from 249 metres to 19 metres distance from the station; closing in to 11 metres; final phase – and docking!

Holds are scheduled at the 19-metre position, where the ATV controls its attitude relative to the ISS docking port, and at the nearly-there 11-metre point, to make sure that the ISS crew has good visibility for monitoring the final phase. All of these manoeuvres are carried out autonomously by the ATV, under the joint supervision of the ISS crew, the ATV-CC and an independent system on board the ATV which uses its own measuring instruments to ensure that the primary system (which is itself fault-tolerant) does not in any way endanger the mission.

In the unlikely event of a crisis

Of course, the unexpected has been considered. In case of single anomaly, the ATV’s Mission & Vehicle Management (MVM) software detects and isolates the anomaly, then reconfigures the systems to continue the mission. In the unlikely event of more severe anomalies, the MVM software switches to the ‘escape’ mode, which commands the ATV to accelerate at 5 m/s in the opposite direction to the ISS so as to move away from the vicinity of the Station.

Should the anomalies be extreme, beyond the MVM’s fault management capabilities, as a last resort an emergency collision-avoidance manoeuvre (CAM) can also be triggered, either by the ATV itself, the ISS crew, or the Toulouse control centre, transferring the ATV to a lower orbit at a safe distance. This CAM uses dedicated sensors and thrusters that are independent from the primary guidance and navigation system.

The ATV really is the most intelligent space robot flying today.