Radio Astronomers Raise Ch.37 Spectrum Protection Concerns

The deadline for comments in the FCC Incentive Auction NPRM is today. Several of the comments on the FCC Electronic Comment Filing System are from users of wireless microphones and LPTV station licensees concerned about loss of the spectrum they depend on for their businesses. However, one of the filings that I found interesting was submitted two weeks ago from the National Radio Astronomy Observatory. The comments were written not by an attorney, but rather by Dr. Harvey S. Liszt, astronomer and spectrum manager at the NRAO.

Liszt explained how Ch. 37, the 608-614 MHz band, is used not only at multiple locations in the United States, but around the world. Simultaneous observations from multiple locations such as the Very Long Baseline Array, a series of 10 telescopes spanning a 5,351-mile area, are combined to provide higher resolution data. As a result, there is a need for a common UHF band for radio-astronomy. According to Dr. Liszt, there many historical measurements have been conducted in this band.

NRAO stated that it was not opposed to changing frequencies within the band, stating that “As far as the science is concerned, the Ch. 37 band is fungible within certain limits. Moving the frequency ± 20 percent would preserve the science and could be accommodated, at some expense and with some loss of efficiency (a loss of angular resolution at lower frequency), by the current VLBA optics.”

However, moving the observation frequency spectrum close to the existing 1.4 GHz radio astronomy service band would be pointless, and moving it to half the frequency would cause reduce resolution by a factor of two. (Radio astronomy service already has a co-primary allocation at 322 MHz.)

Dr. Liszt summarized the comments noting, “The allocation at 608-614 MHz is a valuable asset that permits astronomers to use very powerful U.S. instruments in a protected band which is widely available internationally. It is the hope of NRAO that this allocation, or one relatively close in frequency, will be available in the future.”

NRAO is also concerned about the repacking, noting that “Even if protected RAS use of the Ch. 37 band is preserved, conditions will change drastically after the UHF rebanding plan is implemented, as adjacent spectrum is re-purposed and/or re-packed. Even continued use of the same Ch. 37 allocation may not be a zero-cost option for astronomy.”

NRAO works with very weak signals. Dr. Liszt provided an example using an isotropic 1 Watt transmission on Ch. 37. 100 km away, in free space, the power flux is −111 dB(W/m2). Table 1 of ITU-R Recommendation RA.769 shows the threshold for detrimental interference to single dish continuum operations at 611 MHz (applicable to the NRAO at Green Bank, W.Va.) is −185 dB(W/m2). The NRAO comments note that while it may be possible to obtain the additional 74 dB of attenuation for some VLBA stations far from major population centers, “it is manifestly impractical for the VLBA antenna on Kitt Peak that is within line sight to metropolitan Phoenix.”

At the end of the NRAO comments, Dr. Liszt says, “The worst possible outcome for all concerned would be to create or preserve an allocation to RAS that is rendered unusable by RAS because it is not adequately supported. This will require strong coordination of a variety of emissions that may occur in adjacent spectrum, and a comprehensive, if not absolute, prohibition on emissions in-band.”

Next week, after the comment deadline, we'll see how the broadcasters and wireless carriers feel about the FCC's proposed rules and band plans.

Doug Lung

Doug Lung is one of America's foremost authorities on broadcast RF technology. He has been with NBC since 1985 and is currently vice president of broadcast technology for NBC/Telemundo stations.