Failure
On April 5, 2010, Galaxy 15 ceased responding to commands sent to it by controllers on the ground.[12] However, it is unclear when the actual initial failure date was because commands are often sent by satellite operators days or even weeks apart. Without commands necessary for stationkeeping it began to drift east away from its allotted orbital slot. All television signals were transferred to other satellites and all intentional transmissions were ceased. The Galaxy 12 satellite was removed from on-orbit storage at 123° West[13] to replace Galaxy 15 at the 133° West orbital slot. Galaxy 12 was originally slated to replace another satellite Galaxy 27 which had to be relocated to 45.1° East for the start of a mission critical service for the United States government. Due to the necessity of relocating Galaxy 12 to 133° West, Intelsat was forced to leave 129° West vacant.[14] On 20 April, Orbital Sciences theorized that they believed solar activity was responsible for the satellite malfunctioning,[15] although a later statement by the company said that the company had been unable to actually settle on "a single root cause."[16] On May 3, an attempt at a very momentary series of strong pulses intended to cause a power system malfunction were sent to Galaxy 15. Unfortunately, this did not have the desired effect of causing a power system overload and subsequent shut down of the active transponders.[17] It was the concern of neighboring satellite operators that this type of recovery attempt would have had the potential to permanently damage sensitive hardware on board a satellite and could have contributed to placing a satellite into a permanent unrecoverable control state.
The failure was most likely caused by an electrostatic discharge that knocked out the satellite's communications systems, as a side-effect of spacecraft charging from solar radiation.[18]
Reported passes and interference
On May 12, 2010, Intelsat and SES World Skies confirmed that Galaxy 15 would pass close to the latter's AMC-11 satellite, potentially causing interference with cable programming over the United States because the two satellites broadcast on similar frequencies.[19] Between May 23 and June 7, 2010, Galaxy 15 passed within half a degree of AMC-11.[20] As the two satellites passed close to each other, particularly during closest approach on May 31 and June 1, signals from Galaxy 15's still-active transponders could have interfered with signals being broadcast by AMC-11.[20] SES maneuvered the AMC-11 satellite to reduce the possibility of interference. The SES-1 satellite followed behind Galaxy 15 during the pass to provide a backup to AMC-11 should it be needed.[21]
On June 2, Intelsat and SES reported that no interference had occurred during closest approach, with the satellites passing within 0.2 degrees of each other.
Uplink signal avoidance and mitigation
Galaxy 15, like many communications satellites rebroadcasts the signals it receives on its uplink frequencies to the corresponding C-Band downlink frequencies. Therefore, the potential for interference only existed when Galaxy 15 drifted into the line of sight of a ground uplink segment intended for another communications satellite operating on the same uplink frequency range in the path of Galaxy 15. The magnitude of the interference risk depended on a large number of variables, including the size of the uplink antenna used (the larger the antenna, the lower the risk to hit Galaxy 15), the location of the uplink and whether inside the Galaxy 15 footprint, and the onboard gain capabilities of the satellite proximate to Galaxy 15. Satellite operators devised a variety of strategies to mitigate the potential interference to their birds' downlink transmissions including, temporarily transferring uplinks to larger uplink antenna, deliberately off-pointing uplink antennas to put Galaxy 15 into the null, maneuvering the satellite being passed to maintain a minimum angular separation, increasing the satellite's gain sensitivity settings allowing a lowering of the uplink power needed and latterly moving the uplink to a location where Galaxy 15 had no uplink coverage (i.e. Hawaii). These techniques were successfully able to reduce, mitigate and minimize for commercial satellite operations most interference potential during a Galaxy 15 fly by. Smaller sized stationary receive only sites, like those used by amateurs or SMATV which have a wider beam width reception potential, may have experienced interference, while for the most part commercial operators experienced no adverse effects from Galaxy 15 during its failure period thanks to the execution of the mitigation practices devised in the wake of the crisis.
Power loss predictions and criticism
The satellite was theorized to lose attitude control when its reaction wheels became saturated. When this event occurred, it would prevent the spacecraft's solar panels from tracking the sun, and it would shut down. After the power reset event, further attempts would be made to recover control of the satellite.[24] This event was originally predicted for late August or early September 2010,[29] then Intelsat revised its estimate to sometime between 28 November and 29 December 2010. After subsequent criticism of the actual science behind Orbital Sciences' predictions Intelsat completely abandoned the published timeline method. Instead for a period Intelsat stated Based on the revised analysis, the estimated window for a Galaxy 15 off-point and loss of power could occur as early as this month without clarifying the month indicated.[1] The hypothesis used to calculate the timeline of any potential occurrence of loss of power was imprecise, due to the fact the satellite has experienced erratic loading conditions of radio frequency signals since it began to drift.[30] Six months after the initial failure date published timelines for shut down proved to be more hopes of a public relations campaign based in theory, and in an interview conducted in October 2010 during the Satcon Conference, both Intelsat & Orbital admitted that the hoped timeline scenario was, in theory and this has been an unprecedented situation, and we are learning as we go.
Recovery
On 23 December 2010, Intelsat successfully regained control over the satellite after the Baseband Equipment Command Unit reset following a loss of lock and full discharge of the batteries, reportedly the most critical phases of the recovery of Galaxy 15 have been completed. The emergency command patch which would allow ground controllers to gain access to redundant BBEs in the event of a similar failure in the future had also successfully been applied to Galaxy 15 according to Intelsat.[2] Galaxy 15 was relocated at 93° West[32][33] in order to conduct further in orbit testing of the viability of the payload and return to service of the satellite. Once Galaxy 15 was fully recovered it was moved back to 133° West and then the Galaxy 12 spacecraft was relocated for its intended mission at the 129° West slot.[14]
On 18 October 2011, Intelsat transitioned all of its 133° West customers back from Galaxy 12 to Galaxy 15.
