Execs Say Interference is Mostly Unintentional, but Often Hard to Resolve
CANNES, France — Satellite fleet and ground-network operators say the types of radio frequency interference they encounter are as broad and varied as the applications for satellite telecommunications. Almost all of it is unintentional, much of it is hard to locate and some of it is apparently impossible to resolve.
Here are case histories that industry officials spoke about.
STL Ghana, an operator of two-way very small aperture terminal (VSAT) satellite networks, is used to dealing with regular power outages and with damage caused by rats eating into VSAT cabling. But it was stymied earlier this year when some of its customers reported transmission interference.
In a Sept. 17 presentation to the VSAT 2009 conference in London organized by the Comsys consultancy, STL Ghana Chief Technical Officer Guy Shmuel said interference to his company’s Intelsat Ku-band satellite service occurred like clockwork every Tuesday, from 9 a.m. to 4 p.m.
The interference was on the in-bound transmission, meaning STL could not simply increase its transmitting power to overcome it.
It continued for weeks, forcing STL to shut down some operations between Monday evening and Tuesday evening. “To us it was obvious that someone doing training, and we had some suspicions about who it was. We told Intelsat about it, and they spent four months trying to find the source. They finally moved us to another satellite transponder.”
Intelsat spokeswoman Dianne J. VanBeber declined to comment on the specific STL Ghana incident, but acknowledged that “just because you know what is causing the interference, it doesn’t mean you will get that party to admit that they are interfering or to change their actions. I am not saying that is the case here. We spend hundreds of thousands, if not millions, in salary time in our operations center trying to identify the source, and clearing the interference. Sometimes it works. Many times it doesn’t.”
VanBeber said incidents like this are why Intelsat is leading an effort to train satellite Earth station operators and to get satellite fleet operators to coordinate their activities by sharing data.
Satellite fleet operator Satmex of Mexico believes it has identified the source of brief periods of interference with its C-band satellite links but is unsure what to do about it. Cesar Lopez of Satmex, in an Oct. 27 presentation here to the Satellite Users Interference Reduction Group (SUIRG) annual conference, said the interfering signals likely are coming from radio altimeters on aircraft that fly across the line-of-sight beam of a large Satmex Earth station on their final airport approaches, causing several seconds of interference each time.
The affected antenna is 64 kilometers from the airport, he said.
Satellite operator PT Pasifik Satelit Nusantara (PSN) of Indonesia spent years trying to persuade government authorities that broadband wireless access systems like WiMax were overwhelming their satellite transmissions in C-band. It was an issue that received international attention in 2007 during a global conference of International Telecommunication Union (ITU) regulatory agencies. The conference concluded with a resolution that, where satellite systems are in place in certain portions of C-band, the burden is on WiMax operators not to interfere.
Erwin Pardamean of PSN told the SUIRG meeting that Indonesian authorities agreed to move the wireless services out of the satellite bands of 3.4-3.6 GHz. But the replacement frequency found for the terrestrial broadband systems are at 3.4 GHz — right next door to the satellite frequencies.
“Interference in-band is now taken care of, but out-of-band interference is now the problem because there is no guard band” at the frontier of where satellite and terrestrial wireless systems are licensed to operate. Pardamean said PSN is now concerned that the solution will only reduce, and not eliminate, the interference. What is more, he said, the government decision to force a relocation of terrestrial broadband expires in 2010, and no one knows what will happen after that.
“This has resulted in a loss of trust, in losses of opportunity and in continued uncertainty” for PSN, he said. “If this persists, the satellite business in my country will no longer be able to attract investment.”
Pardamean said the repeated interference has cost the company a substantial sum of money but that, despite having proven its case to government authorities, “we have been given no dispensation from the annual spectrum fee.”