Intelsat is hopeful it can recover use of a wayward communications satellite that had wreaked havoc with operators around the world until it was finally shut down just prior to Christmas.
Dubbed Zombie Sat,Galaxy 15 went out of control on April 5, sending operators scrambling to protect their spacecraft from interference. During the next six months, the spacecraft flew by a dozen satellites, in some cases coming to within a tenth of the distance normally maintained between geostationary satcoms (AW&ST May 24, 2010, p. 23).
However, thanks to complex maneuvers—many of them invented from scratch—serious interference and service interruption were avoided. A U.S. National Weather Service outage on Dec. 4-5 that some bloggers imputed to Galaxy 15 was caused by local mechanical damage to cables on the ground and was unrelated to interference issues, according to executives from Intelsat and SES, another operator affected by Zombie Sat.
Intelsat officials say Galaxy 15 lost Earth lock on Dec. 17, causing it to lose enough power to shut down its primary C- and L-band payload, which constituted the most serious threat to neighboring spacecraft. On Dec. 23, the battery completely drained, shutting off the radio beacons and removing the remaining interference risk.
Engineers had initially expected Galaxy 15 to power down over the summer; but in the absence of telemetry data, the estimate had to be based on experience with other orbiting spacecraft, which proved unreliable.
The power-down caused the baseband equipment command unit to reset automatically, allowing Galaxy 15 to once again begin accepting commands and sending telemetry—permitting Intelsat to regain control of the spacecraft and place it in safe mode.
The next step will be to run comprehensive diagnostic tests and upload new command software patches that should make it possible to immobilize the satellite, which has been drifting steadily along an eastward track, and keep it from going astray again. Engineers will then try to move the satellite to one of Intelsat’s orbital locations so the viability of the payload and the functionalities of the spacecraft can be determined.
In the second quarter of 2010, Intelsat took a $104-million impairment because of Galaxy 15, which was originally designed to operate through 2020. In the third quarter, the company said it might write down the residual value of the spacecraft, $34.2 million as of Sept. 30, if it is determined that it cannot be recovered.
Regardless of the outcome, operators say the six-month ordeal has given the industry a rich toolbox to protect against future spacecraft loss-of-control events. Intelsat says it pioneered the maneuvers used in the first two fly-bys and other operators—SES, Telesat and SatMex—then injected their own expertise. This joint effort minimized the potential negative impact of each subsequent event.
SES Chief Technical Officer Alan Young says a three-pronged strategy developed for its AMC-11—a C-band satellite located at 131 deg. W. Long. that was the first affected by Galaxy 15—served as a basis in dealing with the Zombie Sat threat. Engineers first temporarily uplinked AMC-11 customers from a very large antenna in Clarksburg, Md. The antenna’s narrow beam minimized the off-axis transmission to Galaxy 15 as it moved closer than the 2-deg. spacing normally allowed, limiting the interference that would otherwise occur. Engineers also changed the sensitivity setting on AMC-11 to minimize the amount of energy available to Galaxy 15 and so further reduce the interference risk.
In the final stages of the fly-by, when Galaxy 15 approached to within a dangerously low 0.3 deg. of AMC-11, engineers performed a “leap-frog” maneuver that permitted the two spacecraft to stay in sync and maintain a minimum spacing of at least 0.2 deg. until Galaxy 15 began moving away again. As an extra backup, engineers temporarily stopped another spacecraft, SES-1—which was passing through the 131 deg. W. Long.slot on the way to its 101 deg. W. position after an April 24 launch—to afford customers an alternate transmission path in the event the Clarksburg uplink failed.
The AMC-11 strategy proved useful in protecting other spacecraft from the Zombie Sat, including three heavily used “center of the arc” satellites that were approached by Galaxy 15 over a 21-day span in November-December, Young says. The “leapfrog” maneuver was carbon-copied on Nov. 24 for AMC-18, a C-band spacecraft at 105 deg. W., although this time no backup satellite was deemed necessary. Parts of the strategy were employed again on Dec. 4 and 15 for AMC-1, situated at 103 deg. W., and SES‑1, but with a variation required because of their C-/Ku-band payloads.
For these satellites, engineers set up temporary uplinks at SES’s Sunset Beach Earth station facility in Hawaii. At that point, Galaxy 15 had no coverage of Hawaii and thus could not interfere with signals uplinked from there. However, the antenna did not have sufficient power to handle all the traffic for AMC‑1, so some customers had to be linked to Clarksburg, where there was still coverage. Because the separation during the AMC-1 flyby was particularly low—0.1-0.2 deg.—engineers decided to move that spacecraft slightly outside its normal station-keeping box. Young says this provided enough separation for engineers at Clarksburg to off-point the antenna so that Galaxy 15 could pass through its “null.”
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