The Sky Station System will be a system of geostationary platforms located in the stratosphere for up to ten or more years. The stationary platforms will be the backbone of a global Internet system which uniquely offers variable broadband data rates at a nominal data rate of 1.5 Mnps to portable terminals, and up to 155 Mbps (OC3) to fixed terminals.
There will be at least 250 Sky Stations at 20 - 30 kilometers altitude positioned over areas of greatest population. More Sky Stations can be added at any time.
Stratospheric Telecommunications Service will commence with the first Sky Station deployment in 2000. After that, additional Sky Stations will be launched one per week until all populous parts of the world enjoy Stratospheric Telecommunications Service. Each Sky Station provides Stratospheric Telecommunications Service to an area of approximately 750,000 square kilometers. Sky Stations will be implemented in accordance with user demand as expressed by responsible organizations in each country.
The Sky Station System will operate in the 47 GHz frequency band (47.2-47.5 GHz stratosphere-to-Earth and 47.9-48.2 GHz Earth-to-stratosphere). It motivates the band selection as follows:
This band was selected for several reasons: Use the highest and least occupied frequency bands consistent with their necessary operating characteristics and table of frequency allocation classifications. Also, new technologies should use the minimum amount of bandwidth necessary to provide the required service. The required services require a paired band of 300 MHz. The stratospheric telecommunications service can operate with up to approximately 1 dB per kilometer of atmospheric attenuation, but not much more. This level is reached between approximately 40 and 50 GHz. The Sky Station System will operate in the Fixed Service. There are two Fixed Service bands above 40 GHz, one near the lower edge and one near the upper edge. Spectrum management principles dictate a preference for the upper edge, especially since the lower band is already the subject of networks in an advanced stage of planning. At the upper portion of the 40-50 GHz range, there are other spectrum occupants in place or in an advanced stage of planning for all sub-bands allocated to the Fixed Service except for 47.2-47.5 GHz and 47.9-48.2 GHz. Hence, according to established principles of spectrum management, the stratospheric telecommunications service should occupy the 47.2-47.5 and 47.9-48.2 GHz band. Directionally within the preferred band pair should be 47.2-47.5 GHz (stratosphere-to-Earth) to minimize a first harmonic interfering signal with space-based sensors monitoring the earth in the infrared transparency window at 94 GHz.
Sky Stations can be of variable size depending on market demand. Normally, a Sky Station will be approximately 50m by 50m by 140m and have a total mass of about 10,540 kg. Each Sky Station will generate 157 kW of power from solar panel arrays. After subtracting power needed for station-keeping and other functions, there is 15 kW of RF power for telecommunications. This is adequate to support 400,000 simultaneous 64 kbps and 1,000 multi-megabit transmissions.
Harmful Interference is anticipated since the Sky Station System shares its downlink frequencies with the Radio Astronomy Service in the 48.94-49.04 GHz band in which the Radio Astronomy Service has a primary allocation (see FN 5.555). Radio astronomy is extremely vulnerable for interference downward from the sky (see FN 5.149).
From the system information already available an initial analysis for the 47.9-48.2 GHz downlink is as follows:
We consider a radio astronomy observatory located in a rural area cell which sees a Sky Station 30o above the horizon (50 km from the nadir). This rural cell has a 6250 m radius, a surface area of 1.34x108 m2, and a normal area of (surface area) sin (30o) = 6.68x107 m2.
The maximum transmitter output power illuminating this cell is 76W across 1.4 MHz across 1.4 MHz of bandwidth.
The power flux density illuminating this cell is 76 / 6.68x107 = 1.143x10-6 W/m2 = -59.4 dB(W/m2).
The spectral power flux density (spfd) illuminating the cell is 1.143x10-6 / 1.4x106 = 8.16x10-13 W/m2 = -121 dB(W/m2/Hz) which is well above the level of harmful interference in this band for the Radio Astronomy Service which is -209 dB(W/m2/Hz) (ITU-R RA769).
To avoid interference harmful to the Radio Astronomy Service, Sky Station's downlink modulation and transmitter filtering will need to guarantee a ratio of 88 dB (nearly 9 orders of magnitude) between the wanted transmissions in the downlink band and the unwanted transmissions only 0.74 GHz away in the radio astronomy band.
Filtering will be a substantial technical challenge and accordingly should be addressed as soon as possible. There is a potential for interference to the radio astronomy bands 42.5 - 43.5 GHz and 48.94 - 49.04 GHz, and in the harmonically related radio astronomy bands 97.88 - 98.08 GHz, 144.68 - 144.98 GHz, 145.45 - 145.75 GHz and 146.82 - 147.12 GHz.
Last modified: June 25, 2004