Post Tue Jul 19, 2011 9:46 am

How the spatial year 2010 ended-Part 4

In an article from April we mentioned an incident that affected one satellite of the Intelsat fleet- the Galaxy 15. Then , apparently following an intensification of solar activity on April 3 to 5, Intelsat completely lost control of the satellite -more precisely its transpoders continued to function but any attempt of controling it was unsuccesfull.
Galaxy 15, built by Orbital Sciences Corporation company-the same that provided another 5 satellites to Intelsat-was sent into orbit in October 2005 and had an estimated flight resource until 2022.
It's built like its predecessors on a STAR 2 platform that is considered by its manufacturer as cheaper, more compact and lighter than those offered by other competing companies and last but lot least- it's capable of hosting most of today's existing communication configurations.
The satellite weights 1892 kg, taking benefit of the two solar panels located on both sides, each consisting of four elements based on Ga-As tehnology. The energy generated is stored in two Li-ion battery systems with a capacity of 4840 Wh.
Three axes stabilization is achieved using a monopropellant propulsion system based on hydrazine and for orbital maneuvering a bipropelant system is used.
Equipped with 24 transporders in the C band and 1 in the L band, the satellite provides media services for operators in North America and respectively for the American Aviation Autorithy.
In April Intelsat announced that in order to prevent possible network disruptions, it was decided to temporarily move an older Galaxy 12 platform, launched in April 2003 and equipped almost identically with 24 transporders in the C band (without the L band transporder used for commertioal aviation)- from the 123 degrees West orbital slot at 133 degree West where the Galaxy 15 was operating.
Installed in the new position since April 14, the satellite took up duties of the defected platform until the investigations of the technical issue will povide a solution.
The failure of the Galaxy 15 is coming on top of the one of IS 4 in February and led to company finacial results below expectations for the fist months of the year but still increased by 2 percent over the same period of the last year. With a number of 2025 active transporders before the appearance of the problems at Galaxy 15 (of which 82% were used at maximum) Intelsat executives hope to see better results in the next period and thus to reassure shareholders about the company's future.
Returning to the initial tehnical problem, it has to be recalled that after intense testing and after sending of nearly 200,000 commands Intelsat failed to take control, then in a desperate attempt to resolve the situation, on May 3 the company decided to send powerfull radio pulses to force the satellite's power system to break and to close the active transporders- unfortunatly also without succes.
Without having all the details, we speculated on possible damage and technical solutions that engineers have to overcome, so we said that the fault coud be located either in the antenna's decoder or in the board processor that under the influence of a high energy flow could have corrupted the software (damage which can be solved by a classic software 'update' after the current board image is downloaded and compared with the right one). To reach that point, engineers needed to take control of the satellite by forcing the satelite's computer to execute commands sent by the ground. Because these situations happened many times in the spatial history, designers take care ever since the designing phase of a platform to provide a minimum of functionality, namely as we mentioned, command execution can be achieved either classical via software or in exceptional cases when the latter does not respond (as is the case here) via hardware interfaces that can directly reset the onboard circuits. These last-mentioned commands also called "high-priority" are executed quickly, along with their reception, and as we said do not have to follow the logic of the onboard algorithms.
At least theoretically, these commands should solve the classical satellite communication isues but it seems that in this case there was a more complex problem because after extensive testing and deployment of nearly 200,000 Intelsat commands, Intelsat failed to take control.
Without the reception of orders from the ground, the so-called "station keeping commands", the satellite has put itself into motion and began to move eastward. In nine months of uncontrolled flight, the satellite has crossed a geostationary satellites zone active in TV services for the American continent, satellites belonging to several operators: Intelsat, SES, Telesat of Canada and Satmex of Mexico.
At 131 degrees West orbital position, has met first on May 23 the SES Astra satellite AMC-11 which for some television programs uses the same frequency range. To avoid possible interferences, it has been decided to move AMC 11 from Galaxy 15's influence area.
Then Galaxy 15 came in the influence area of other Intelsat satellites-first time around Galaxy 13 in mid-July, then around Galaxy 14 and 18 in August.
Galaxy 23 (affected in late August) then Anik-F3 belonging to Telesat operator have also performed maneuvers to avoid the radio interference.
Problems caused by Galaxy 15 were supposed to come to an end in August because the command and attitude control system constantly degrades, and without the commands for maintaining attitude it was expected to lose the aligment towards the Sun-case in which the satellite would remain without power. Intelsat engineers' predictions proved to be innacurate because it had to wait until December for automatically reset of the board computer. More precisely, on December 23, the satellite has started to work again and was able to send telemetry and accept commands from ground operators. At least for the moment all the problems seem solved and Intelsat said in an official press release that the satellite is no longer a threat for the rest of the commercial operators and in short time it intends to put the satellite again in usage in its commercial fleet. A few weeks will be required for detailed investigation to understand what caused this anomaly and how such situations can be avoided in the future, to finaly stabilize its orbit and move it in an operational slot.
Until then, the satellite is kept in safe-mode position in the so-called 'sun-pointing'; that is aligned with the solar panels toward the Sun with the batteries charged at maximum. Current orbital position is 121 degrees West. Transporders are swiched off completely to avoid further complications and a mini-update of the software has already been done to improve the unit decoders that allow changing the antenna. A major SW update will come in place as soon as the satellite will be stabilized to the 'Earth-pointing' position.