To consider[s] the requirements for new applications in the radiolocation service and reviews allocations or regulatory provisions for implementation of the radiolocation service in the range 30-300 MHz, in accordance with Resolution 611 (WRC-07).
The primary concern for radio astronomy is that additional radiolocation service allocations being considered by Agenda Item 1.14 should avoid the 37.50-38.25 MHz, 73.00-74.60 MHz and 150.05-153.0 MHz passive Radio Astronomy Service (RAS) bands, and those new allocations should provide suppression of unwanted emissions in these RAS bands to meet the requirements of ITU-R RA Rec.769 in these bands at registered RAS low-frequency observing sites. Signals in the 30-300 MHz band can propagate up to 2000 km by sporadic E and meteor scatter and up to about 800 km by tropospheric scattering and refraction.
Radio astronomy observations in this frequency range include the study of the early universe for which new arrays are currently under construction. For example, the Murchison Widefield Array (MWA) is being built in the radio quiet zone of the Western Australian outback. One of the major goals of the MWA is to study the hydrogen in the cosmic background for redshifts from about 6 to 17 (frequency range of 80-200 MHz) to study the structure of the early universe through the period known as the “Epoch of Reionization” when the primordial hydrogen becomes ionized as star formation starts.1
PROPAGATION BY SPORADIC E AND METEOR SCATTER
Sporadic E propagation is the occasional reflection of radio waves at frequencies up to about 200 MHz by thin clouds of intense ionization in the “E” layer of the ionosphere at an altitude of 90 km to 120 km. Sporadic E events may last for just a few minutes to several hours. The causes of the ionization enhancement is an active area of study. Recent research indicates that high altitude lightning, wind-shear and gravity waves may be involved in the concentration of metal ions thought to dominate the thin sporadic E clouds. Research also shows a correlation with the deposition of metal ions from meteor sources in the ecliptic plane which might explain the increased frequency of sporadic E during the summer months. Typical distances of propagation range from about 900 km up to about 2500 km. For correlation with deposition of metal ions from meteor sources, see C. Haldoupis, D. Pancheva, W. Singer, C. Meek, and J. MacDougall, 2007, An explanation for the seasonal dependence of midlatitude sporadic E layers, J. Geophys. Res. 112:A06315, doi:10.1029/2007JA012322. For wind shear gravity waves, see J.D. Mathews, 1998, Sporadic E: Current views and recent progress, simultaneous measurements of meteoric influx and Es, J. Atmos. Sol. Terr. Phys. 60:413. For lightning, see C.J. Davis and C.G. Johnson 2005, Lightning-induced intensification of the ionospheric sporadic E layer, Nature 435:799-801.
Meteor scatter is a mode of propagation used for low data rate communications up to distances of 2000 km. It is routinely the mode of reception of distant FM stations in remote areas and is characterized by its “bursty” nature. Billions of meteors enter Earth’s atmosphere each day and produce ionization at altitudes from about 80 to 100 km. The region of mutual visibility between two Earth stations extends out to about 2000 km. At this distance the typical path loss due to scattering is about 195 dB at 73 MHz.
Recommendation: Additional radiolocation service allocations in the 30-300 MHz band being considered should avoid the 37.50-38.25 MHz, 73.00-74.60 MHz and 150.05-153.0 MHz passive bands, and should provide suppression of unwanted emissions in these radio astronomy service bands to meet Recommendation ITU-R RA Rec.769.
1 National Research Council, Spectrum Management for Science in the 21st Century, The National Academies Press, Washington, D.C., 2010, pp. 112-113.