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NGSO satellites Creating new spectrum capacity

Sun, 02/28/1999 - 7:00pm
Jeffrey Krauss, Satellite Orbiter and and President of Telecommunications and Technology Policy
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By Jeffrey Krauss, Satellite Orbiter and and President of Telecommunications and Technology Policy

Radio spectrum has become so valuable, that heroic efforts are underway to create new over-the-air spectrum capacity. For example, the Ku-band satellite spectrum is all used up, with satellites spaced every two degrees along the geostationary (GSO) orbital arc. The latest technology will allow non-geostationary (NGSO) satellites to share the Ku-band, with hardly any additional interference.

Interference

The Ku-band spectrum (generally from 10.7 GHz up to 14.5 GHz) is used today by point-to-point and CARS microwave, VSAT satellite systems, and broadcast satellite downlinks for DirecTV and Echostar. All the frequencies are pretty heavily used. But last year, a new company, SkyBridge (a client of mine), submitted a design for a satellite system that can operate on these frequencies and share them without causing interference. NGSO satellites go around and around—they don't hang motionless at a fixed spot in the sky like the geostationary satellites do. This means that an NGSO satellite will sometimes come in between the geostationary orbital arc and GSO earth stations. The NGSO satellite is not allowed to cause interference when that happens. Because a single NGSO satellite might be anywhere over the earth, it's an international problem, and GSO satellite operators all around the world are worried about interference.

The NGSOs plan to avoid interference by "arc avoidance;" that means they shut down their transmissions when the NGSO satellite comes close to the line between the GSO earth station and the GSO satellite. Yes, these shut-down commands require complicated computer programming to track the orbits, but it's all quite within the state-of-the-art.

Because of the international implications, the interference issues are being analyzed by the International Telecommunications Union (ITU), an international agency like the UN. The ITU is examining power levels, antenna patterns and other technical matters. It hopes to adopt standards in about a year, just in time for the FCC to grant NGSO licenses to SkyBridge, Boeing, Hughes and some of the others that recently submitted applications.

But it isn't easy. The existing GSO operators are being very conservative. They want to protect their most sensitive links against the most unlikely cases of interference. They insist on using worst-case assumptions in their calculations, and some of them refuse to compromise.

While nobody disputes the technical feasibility of GSO/NGSO spectrum sharing, the economic feasibility depends on the details. For example, if the NGSO satellite must shut down whenever it comes within 20 degrees of the GSO orbital arc, this results in lower NGSO system capacity than if 10 degrees defines the shutdown range. The tradeoff is that the likelihood of interference would be much lower, which would give the GSO operators more comfort. But to fully satisfy the GSO operators and protect against highly unlikely interference cases, the engineering tradeoffs would result in lower capacity and higher costs for NGSO systems. For GSOs, it's the difference between four seconds of interference per month and two seconds; for NGSOs, it's millions or billions of dollars. These negotiations between GSO and NGSO operators, going on at the ITU, are a long way from reaching an acceptable compromise. Sensitive links

The sharing situation regarding microwave users is far more promising. NGSO operators only want to put a small number of earth stations, known as gateway stations, in bands shared with microwave services such as CARS. And even though the calculations show that no interference will be caused by satellite downlink transmissions into microwave receivers, the microwave operators have accepted the principle that the most sensitive microwave links might suffer occasional but very short term interference. For microwave, the links that are most sensitive to interference are the shortest ones. Because short links are less likely to be hit by thunderstorms, they are designed without very much excess power (also called "margin" by engineers) to counteract rain attenuation. It is this margin, whose primary purpose is to protect against rain, that also protects against interference.

For GSO satellites, the most sensitive links are also those with the smallest margin. The GSO operators claim that these are links to earth stations located in areas with low rain probability. In fact, these are earth stations located in deserts. Never mind that there aren't many earth stations located in deserts—it's just another example of the GSO satellite operators being overly conservative.

The basic dispute between the incumbent GSO and new NGSO operators is sharing the burden. In order to shoehorn in these new and expensive NGSO systems, the incumbent users of the spectrum have to agree to accept some part of the burden. So far, the GSO satellite operators have been unwilling to share the burden. But there is still another year or so before the ITU and the FCC make their final decisions, plenty of time to work it out.

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