PARIS — The world’s two largest commercial satellite fleet operators are about to find out whether their unusually close collaboration over the past two months to head off potentially disastrous frequency interference caused by an uncontrolled satellite has been equal to the task.

Starting May 23, Intelsat’s Galaxy 15 satellite, out of control since April 5 but with its electronics payload still active, was expected to close to within 0.5 degrees of SES’s AMC-11 satellite at 131 degrees east longitude in geostationary orbit some 36,000 kilometers over the equator.

Since AMC-11 and Galaxy 15 are both C-band satellites, Galaxy 15 threatens to pick up signals intended for AMC-11 and rebroadcast them in a way that is as uncontrollable as the wayward satellite’s drift along the geostationary arc.

Dozens of U.S. television programmers are at risk of a service outage as a result. Galaxy 15 will pose a frequency-interference threat for AMC-11 over a nearly two-week period starting around May 25. As luck would have it, the peak threat is over the long Memorial Day weekend in the United States, which runs through Monday, May 31.

Washington-based, Luxembourg-headquartered Intelsat will face a repeat of the AMC-11 issue starting in late July, when Galaxy 15, continuing its eastward drift, is scheduled to enter the neighborhood of Intelsat’s own C-band satellites. But for the moment, Intelsat and Luxembourg-based SES are focused on minimizing the risk for AMC-11’s customers.

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Both companies have put personnel on 24/7 alert and deployed engineers to Intelsat’s Clarksburg, Md., teleport, where a 19-meter-diameter Intelsat antenna will be used to spoon-feed programming to AMC-11 in such a way as to avoid Galaxy 15 as it passes through the heart of AMC-11’s orbital territory. Use of the Clarksburg antenna for AMC-11 was to begin May 23.

To offer further backup, SES has begun moving its just-launched SES-1 C-band telecommunications satellite toward the AMC-11 orbital position to take over a portion of AMC-11’s television broadcasts. Some AMC-11 customers will transfer to SES-1 during the height of the threat period, while others will count on the Intelsat antenna and other SES maneuvers to keep their broadcasts from going dark.

Alan Young, chief technology officer for SES World Skies, the SES division that handles AMC-11, described what SES plans to do to protect its customers during a 10-day period starting May 25 that, were it not real, could serve as a war-games-type test for aerospace engineering students.

Young said many AMC-11 customers have been rerouted to other SES satellites — AMC-9, AMC-1 and AMC-2 — so that their uplink signals to AMC-11 are sent through Intelsat’s Clarksburg installation. This antenna’s size and power allow it to form what Intelsat referred to as a “pencil beam” to direct the traffic to AMC-11 more precisely than the 9-meter and 11-meter antennas normally used.

In another key preparatory maneuver, SES on May 19 reset AMC-11’s transponders to dramatically reduce the uplink power customers normally use in transmitting to the satellite. This has the effect of reducing the radiofrequency “noise” of the uplink, and will make it more difficult for Galaxy 15 to capture these signals as it drifts into AMC-11’s neighborhood.

Galaxy 15, like AMC-11, is a so-called bent-pipe satellite that receives signals, amplifies them on board and then beams them to customers. “If nothing is coming into Galaxy 15, then nothing is coming out of it,” Young said May 21 in explaining the decision to dial down the power of the AMC-11 transponders.

The goal, he said, is to make AMC-11 as signal-sensitive as possible relative to Galaxy 15.

A second goal was to optimize the separation between Galaxy 15 and AMC-11. Starting May 25, as Galaxy 15 approaches the center of AMC-11’s orbital “box” from the west, AMC-11 will be drifted eastward to maintain its distance.

Young said SES has determined that it can move AMC-11 up to 0.3 degrees east or west of its assigned orbital slot of 131 degrees east without losing its communications links. The maneuver will drift AMC-11 as far east as possible, to 130.7 degrees east. The drift should last for about five days.

Meanwhile, the SES-1 satellite, which when launched was not intended to be involved in this drama, will be moved to the western edge of the AMC-11 orbital slot to take over some AMC-11 customers’ programming. SES-1, which was launched April 24, is expected to arrive at the western edge of the 131-degree slot May 29.

SES and its customers, which in the case of AMC-11 are mainly media companies, have agreed that some customers will remain on AMC-11 while others will be switched to SES-1.

The customers themselves will decide the exact timing of their transfer starting May 29. Once they do, SES will switch off the AMC-11 transponder associated with their programming and switch on equivalent capacity on SES-1. This kind of transfer is a routine occurrence for satellite fleet operators and their customers and results in a program outage of no more than a few minutes.

With Galaxy 15 continuing to move eastward, and with AMC-11 now having nowhere else to go if it wants to maintain its links with customers, SES will perform what it calls its leapfrog maneuver, rushing AMC-11 back to the western limit of its operating ability, at 131.3 degrees.

That leapfrog maneuver will begin around midnight May 30 U.S. Eastern Daylight Time and last for 24 hours — through Memorial Day.

SES will time the maneuver to take maximum advantage of the fact that Galaxy 15’s orientation, because it no longer responds to ground commands, has eroded somewhat since it went out of control April 5. In particular, it is no longer fully stable on its north-south axis. As it oscillates, it traces a parabolic arc along the geostationary line every 24 hours.

Young said SES is confident that it can use Galaxy 15’s north-south oscillation to reduce the likelihood of interference as the leapfrog maneuver occurs.

Once Galaxy 15 has moved to the eastern end of the 131-degree slot, AMC-11 will be allowed to drift back toward the center of its operating position. Galaxy 15 is expected to exit the slot on the east June 7.

Peter B. de Selding was the Paris bureau chief for SpaceNews.