Intelsat, SES Safely Negotiate Passage of Wayward Craft
PARIS — The world’s two largest commercial satellite fleet operators have negotiated the passage of an out-of-control satellite owned by one of them though the orbital neighborhood of the other with no loss of signals to the latter company’s customers.
In what they described as perhaps their finest hour of engineering,and successfully performed a series of hide-and-seek maneuvers 36,000 kilometers above the equator the weekend of May 30-June 1 to avoid what could have been major interference for the cable television customers of SES’s AMC-11 satellite caused by the wayward Galaxy 15 spacecraft.
In June 2 interviews, officials from the two companies attributed their success in avoiding a disaster to the pinpoint precision of a large Intelsat antenna and some deft satellite positioning by SES.
“We observed exactly what we expected to observe, when we expected to observe it,” said Alan Young, chief technology officer for SES World Skies, the division of Luxembourg-based SES that manages AMC-11.
During the peak interference threat period starting late in the evening Eastern time May 30, SES guided AMC-11 to within 0.2 degrees of Intelsat’s encroaching Galaxy 15 satellite — 10 times closer than what is considered optimal to avoid interference — as the two satellites crossed paths.
Compounding the challenge, SES had asked its customers to dial down the power of their uplink signals as low as 5 watts per channel, compared with the normal 50-watt level, after having tuned AMC-11’s transponders to capture the fainter-than-usual signals.
“We had some tricky conversations with customers,” Young said of the SES order to AMC-11 users to reduce signal strength. “Some of the things we were doing seemed counterintuitive. One of our customers told me, ‘I can’t believe I am still getting a signal’” at such low power levels.
The idea was to tiptoe around Galaxy 15 without, in effect, attracting its attention. The satellite, for reasons perhaps due to solar storms, stopped responding to commands April 5 and began an eastward drift along the geostationary arc, its C-band payload still active. That means the satellite can wreak havoc with others transmitting in the same frequencies.
SES previously had rerouted many of its customers’ uplink signals to other SES satellites and then down to a 19-meter-diameter antenna operated at Intelsat’s Clarksburg, Md., teleport, which in the days leading up to the U.S. Memorial Day weekend was turned into command central for the SES and Intelsat teams dealing with the problem.
During the maneuver, the Intelsat antenna was so precise in pointing its signal to AMC-11 and avoiding Galaxy 15 that SES decided not to make use of the newly launched SES-1 satellite, which was stationed on the western end of the AMC-11 orbital box at 131 degrees east longitude in the event customers had to be off-loaded from AMC-11.
To avoid the Galaxy 15 satellite arriving from the west, AMC-11 had been moved to the extreme eastern end of its orbital box — 0.3 degrees to the east of its normal operating location — as the Intelsat craft entered its territory. SES officials had calculated that a 0.3 degree move east or west was as far as they could take AMC-11 without risking loss of signals to its customers.
Once Galaxy 15 had drifted to the center of the SES orbital box, AMC-11 was then maneuvered to the western end of the orbital slot, staying within the 0.3-degree limit but this time on the western end and passing within 0.2 degrees of Galaxy 15 in the process.
Unfortunately for SES, the optimal period for this “leapfrog” maneuver started around 11 p.m. EDT Sunday May 30, still prime-time in the United States, especially given that Monday was a holiday for most Americans.
The three-pronged strategy — minimizing the energy being sent to AMC-11 by its customers, using the Intelsat antenna to maintain customer links and moving AMC-11 to the eastern limit of its permissible coverage area before reversing direction to the western end of the box — worked.
Young, who with his counterpart, Intelsat Chief Technology Officer Thierry Guillemin, was on site at the Clarksburg teleport for a largely sleepless night May 30, said the two companies had rigged screens to allow constant monitoring of the several dozen cable channels carried by AMC-11.
“We were taking readings every 15 minutes,” Young said. “We saw no signal disruptions.”
In a separate interview, Guillemin said the two companies had converted the Clarksburg site into a media hub for the occasion. “We did not see even a pixilation [image distortion] on the video monitors,” Guillemin said. “It’s a fantastic outcome, a result of exemplary cooperation between our two companies.”
AMC-11 was slowly making its way back to the center of its orbital slot the week of June 2 as Galaxy 15 continues to drift east. It is expected to exit the AMC-11 location by June 6.
Young said SES is moving its AMC-11 customers, one by one, back to their usual transmission routes, disconnecting from the Clarksburg antenna and then reconnecting through the customers’ own smaller antennas in maneuvers that result in a second or two of program outages normally planned to occur in the middle of the night. For SES, the drama is over.
For Washington- and Luxembourg-based Intelsat it is not.
Guillemin said Intelsat’s latest assessment is that the satellite will remain active, its C-band payload still waiting for signals to capture and rebroadcast to Earth, until mid-August.
Until mid-July, Galaxy 15’s path will not cross that of any C-band satellite. Guillemin said Intelsat may take advantage of the relative absence of nearby satellites to send a high-power signal to try and force Galaxy 15 to shut down. A
similar effort in early May did not work, and Guillemin said Intelsat is assuming that it will fail again this time.
That means Galaxy 15 threatens to interfere with three Intelsat-owned satellites in its path between mid-July and mid-August. First up is Galaxy 13 at 127 degrees west. Galaxy 15 is expected to arrive there around July 13. Two weeks later, Galaxy 14 at 125 degrees west will have to contend with the same issues. Finally, Galaxy 18 at 123 degrees west awaits Galaxy 15’s arrival in mid-August.
Guillemin said the experience with AMC-11 will make the task of avoiding interference with these three satellites much easier. Also making things easier will be that, as it continues its drift east, Galaxy 15’s orbital inclination will degrade a bit, making it easier for Intelsat to perform leapfrog maneuvers without fearing interference.
Finally, as Galaxy 15 heads “downhill” toward the 105 degrees west libration point — a kind of valley in the geostationary orbit arc resulting from variations in the Earth’s gravity field — it is picking up speed. Guillemin said the satellite was traveling at a rate of 0.06 degrees per day as it passed through the AMC-11 position. By the time it enters the Galaxy 13 slot, it will be moving nearly twice as fast, at 0.11 degrees per day.
Sometime in mid-August, Galaxy 15 will lose its Earth pointing ability, which will cause its solar arrays to lose their lock on the sun, draining the satellite’s power until full shutdown.