To consider[s] the results of ITU R studies and spectrum identification for gateway links for high altitude platform stations (HAPS) in the range 5 850-7 075 MHz in order to support operations in the fixed and mobile services, in accordance with Resolution 734 (Rev.WRC 07).
RADIO ASTRONOMY SERVICE
The primary concern for Radio Astronomy is the presence of two important spectral lines within this frequency range: Methanol at 6668.518 MHz and OH at 6035.093 MHz. Additionally, there is concern that HAPS-like satellites will be line-of-sight to a very large area. For example at 22 km altitude the radius of the line of sight coverage is about 530 km in radius and consequently unwanted emissions need to be low enough to meet the levels of Recommendation ITU-R RA.769 if HAPS platforms are deployed within range of radio astronomy observatories.1
Observations of the spectral line of the methanol molecule at a rest frequency of 6668.518 MHz are critically important in the understanding of the structure of our galaxy. This structure is very difficult to determine because we live in the plane of our galaxy and view it edge on. The methanol line radiates as maser emission from the envelopes of newly formed massive stars throughout the galaxy. These maser stars act as precise beacons whose three dimensional coordinates and velocity vectors can be determined by high precision astrometry with very long baseline interferometry. Such precision astrometry cannot be achieved by optical space missions because of the effects of dust absorption in the galactic plane.
Another spectral line in this band is the OH transition at 6035.093 MHz. It also radiates as a maser in the envelopes of massive stars and is particularly useful in measuring the magnetic field strengths in the envelopes of these stars.
Recommendation: Unwanted emissions should be low enough to meet the levels of Recommendation ITU-R RA.769 in Radio Astronomy Service frequency allocations if high-altitude platform stations are deployed within range of radio astronomy observatories.
Earth Exploration-Satellite Service
The primary concern for EESS is Earth remote sensing satellites currently use, and will continue to use, spectrum near 6.8 GHz to measure soil moisture (SM) and sea surface temperature (SST).2 NASA’s EOS Aqua carries the Japanese AMSR-E radiometer that observes a bandwidth of 350 MHz at 6.925 GHz (soon to be accompanied by the Japanese GCOM-W1’s AMSR-2 that also observes a bandwidth of 350 MHz at 6.925) and the Navy’s WindSat radiometer that observes a bandwidth of 130 MHz at 6.8 GHz. The U.S. Joint Polar-orbiting Satellite System (JPSS) will use band(s) between 5-8 GHz to monitor SST and SM, both of which strongly affect the weather and climate. Because of the microwave physics this frequency range is the best for measuring SST and is very practical for measuring soil moisture. International Footnote 5.458 urges “Administrations [to] bear in mind the needs of the Earth exploration-satellite (passive) and space research (passive) services in their future planning” of this frequency range. Indeed, actions affecting this frequency range should be considered carefully so its use by EESS (passive) is preserved.
1 Rygl, K, Brunthaler, A., Reid, M.J., Menten, K, van Langelde, H.J., and Xu, Y., “Trigonometric Parallaxes of the 6.7 GHz Methanol Masers,” Astronomy and Astrophysics, 2010, 511, A2; and Menten, K., “The Discovery of a new, very strong and widespread methanol maser line,” Astrophysical Journal, 1991, 380, L75.
2 National Research Council, Spectrum Management for Science in the 21st Century, The National Academies Press, Washington, D.C., 2010.