The ATV-3 as it approached ISS on March 28. The vessel is scheduled to remain docked until September 25 (Credits: NASA).

Source: ESA

One ATV function is to maintain the ISS altitude by performing regular reboosts, raising the Station’s altitude. This manoeuvre uses the ATV propulsion system, and is commanded by ISS software.

The regularly planned ISS reboost conducted by ATV-3 on 15 August stopped prematurely. The thruster burn was set to run for 1876 seconds to increase orbital speed by 4.4 m/s and raise the Station’s orbit by 7.7 km to 414.4 km altitude.

The boost began as scheduled at 18:00 CEST using the No. 1 & 3 thrusters of the ATV vessel’s Orbital Control System (OCS).

However, after achieving a 2.9 m/s speed increase, the boost was stopped by the ISS. A temperature alarm had been triggered by ATV on a thruster of the propulsion system (which was not used for this boost), but that is nevertheless permanently monitored.

This (correctly) generated an isolation of the suspect thruster from the rest of the vessel.

A signal was sent from ATV to the ISS to highlight this ‘partial reconfiguration’ (with ATV continuing to be available for the boost). But the ISS software stopped the boost, which was not the excepted reaction to this kind of anomaly.

Engineers at ATV-CC and their counterparts in Moscow and Houston are studying the cause of the initial ATV issue and of the unforeseen ISS reaction.

In order to give time to the ground experts to complete their investigation, it has been jointly agreed with the ISS control centres to perform the remainder of the yesterday’s reboost during the next scheduled reboost slot, set for 22 August; this manoeuvre is compatible with the next vehicle (HTV and Soyuz) movements.

ATV remains in a safe configuration and is fully capable of performing any necessary operations, such as reboosting the Station or performing a debris avoidance manoeuvre if so commanded.

Below, animation of the reboost maneuver: