Interference, Orbital Spacing Key Issues for New Satellite Service
July 5, 2006
As reported in last week's RF Report, the FCC has proposed processing and service rules for a new 17/24 GHz Broadcasting Satellite Service (BSS). The Notice of Proposed Rulemaking (FCC 06-90) posted last week devotes substantial space to technical issues such as orbital spacing and interference protection.
The proposed BSS 17 GHz band covers 17.3-17.7 GHz (space-to-Earth) and 24.75-25.25 GHz (Earth-to-Space). In addition, the NPRM considers use of 17.7-17.8 GHz for international services outside of the U.S. Under FCC rules and the ITU Region 2 allocation, the allocation of these frequencies for BSS will take effect April 1, 2007. After that date, the mobile service allocation in this band will revert to secondary status and the radiolocation service allocation will remain secondary. Also, after April 1, 2007 FSS downlink services may not claim protection from and may not cause harmful interference to BSS operations. In the 24 GHz band, the FCC previously allocated 24.75-25.05 GHz on a primary basis for FSS (uplink) and limited FSS use for BSS feeder links. The band 25.05-25.25 GHz was allocated for co-primary use between the 24 GHz Fixed Service and BSS feeder links.
When the BSS allocation at 17.3-17.8 GHz becomes effective, it will be shared with current DBS feeder links operating in the same band (earth-to-space). Frequency sharing between services is typically handled by using antenna off-axis discrimination at each end of the interference path and limits on spatial proximity (orbital separation) and transmission power. The reverse-band sharing scenario between BSS and DBS is different because, according to the NPRM, two new and distinct interference paths occur: "(1) between the earth stations of different systems; and (2) between the space stations of different systems." These paths could result in 17 GHz DBS uplinks interfering with terrestrial BSS receivers or with BSS satellite transmitters causing interference to receivers in DBS satellites. The NPRM notes that for DBS operators proposing new 17 GHz uplinks to protect BSS receivers, they would need to know the location of consumer receivers and that BSS operators may not want to reveal that data.
Interference could also occur between DBS feeder-links and BSS TT&C earth stations, but the NPRM notes that the choice of facility sites is a system design parameter that under the control of the operator and as such does not necessarily require FCC action to remedy. The issue of satellite-to-satellite interference is highly dependent on orbital spacing.
There are other issues with the 17 GHz allocation. U.S. military services have a considerable investment in radiolocation operations in the 15.7-17.3 GHz band, including airborne high-powered synthetic aperture radars, which could pose a significant harmful interference threat to 17/24 GHz BSS subscriber earth stations. The NPRM discusses some of the options for mitigating this interference and asks 17/24 GHz BSS operators to comment on their systems' sensitivity to adjacent-band emissions and the level of protection they may require.
Sharing in the 24 GHz band is not as complex as it does not involve reverse-band sharing. In the NPRM, the FCC's major concern was with Fixed Service (FS) (formerly known as the Digital Electronic Messaging Service or DEMS) and BSS uplinks. Current FCC rules provide a coordination mechanism between satellite earth stations and FS stations in bands where both services have equal rights. The NPRM requested comment on whether the current rules provide sufficient protection-to-hub and user stations of 24 GHz FS systems. The FCC said that BSS feeder-link facilities will typically use "relatively large diameter antennas with antenna off-axis rejection characteristics sufficient to minimize interference into 24 GHz FS systems."
Orbital spacing options for BSS satellites are covered in detail using tables and graphs in the NPRM. The trade-offs should be obvious to anyone experienced with satellites. Closer orbital spacing allows more satellites, but requires narrower beam-width subscriber receiver antennas (i.e. larger dishes). Assuming the 17 GHz reverse-band interference issues are resolved, BSS satellites could be located in the same clusters as DBS satellites, which would suggest either maintaining the 9 degrees DBS spacing or reducing it to 4.5 degrees to allow interleaving and reduce interference between BSS and DBS satellites. In their BSS applications, DirecTV and EchoStar proposed satellite fleets located at orbital separations of 4.5 degrees. Intelsat's filing asserted that to provide service to receiving earth station antennas larger than 18 inches, orbital spacing should be at least 4 degrees.
In the NPRM, the FCC recognizes that using the FSS spacing of 2 degrees would require receive antennas that "may be unacceptably large for the direct-to-home market," but offers the option of 3-degree, 4-degree or 4.5-degree orbital spacing. Since many applicants want to co-locate or interleave with DBS satellites operating between 101 degrees WL and 119 degrees WL, the FCC said it could adopt a 3 degree or 4.5 degree spacing policy in that part of the arc and a different spacing policy, such as 4 degrees, in other portions of the arc. It asked for comments on this and also what orbital spacing regime, if any, might be best for the eastern or far-western regions of the arc, where DBS channels are allotted to the U.S. in the ITU Region 2 BSS and Feeder Link Plans. The FCC said it has received proposals to modify the spacing for DBS satellites and cautioned parties to consider all possible outcomes when formulating comments in the proceeding.
Other portions of the NPRM deal with minimum receive antenna diameter and performance standards as well as licensing and processing issues. If you have an interest in BSS, I encourage you to download and read the 65 page Notice of Proposed Rulemaking (FCC 06-90).