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5.12.5 SPS Beam Effects on Communications

Effects of the SPS on communications is another "environmental" topic that has been studied by a number of professionals. Frequencies near the proposed 2.45 GHz SPS beam frequency are currently being used by some communications sectors, but they're a tiny segment of the communications services. Those operating at 2.45 GHz would probably want to switch frequencies, though ameliorative measures are feasible for frequencies close to 2.45 GHz in areas not near a rectenna. The SPS can be made to not interfere with other communications in general.

A major factor in setting the "baseline" SPS beam power density to peak at 23 milliwatts per square centimeter was its effects on the ionosphere, a layer of the upper atmosphere used to bounce some kinds of traditional communications, e.g., long range radio, TV (non-satellite and non-cable), and microwave relay of telephone calls. Only the spots of the ionosphere above rectennas would be affected, and those spots may not work as well in reflecting (actually, refracting) these kinds of communications back down to the Earth's surface if the beam intensity is pushed too high. Whether the SPS would significantly affect users of those kinds of communications in certain areas has yet to be determined, but it was later thought that the 23 mW/cm2 setting was considerably lower than the threshold for significant effects on the ionosphere. However, with fiber optics, larger satellites, and internet, those traditional ionosphere-reliant methods of communications are being phased out.

Experiments were conducted transmitting a 2.45 GHz beam up through the ionosphere (using the large Platteville, Colorado and the Arecibo, Puerto Rico high frequency transmission dishes) and taking many measurements over time. (171-177) "Experiments have shown that the limit is too low, and theory now suggests that the threshold is soft. The current consensus is that the limit of 23 mW/cm2 can be at least doubled, and perhaps more, pending further tests." (178) Further, it was concluded that atmospheric heatingcould be reduced by 80% by switching to the 5.8 GHz frequency.

There are 10 different bandwidths used for communications -- EHF, SHF, UHF, VHF, HF, MF, LF, VLF, VF and ELF. The SPS beam falls within the UHF band.

There would be interference with some communications operating at the 2.45 GHz frequency of the SPS beam and some of its harmonics. 2.45 GHz falls within the UHF part of the communications spectrum. The SPS beam of 2.45 GHz is located in the microwave area of the radio communications spectrum, near the TV and FM radio frequencies, and falls within the 2.3 GHz to 2.5 GHz bandwidth allocated for police, taxi, citizen's band, mobile, radiolocation, amateur, amateur-satellite and ISM (industrial, scientific and medical) applications -- the UHF band. The vast majority of these other applications do not use 2.45 GHz but use other frequencies near or somewhat near to 2.45 GHz. Many of these applications (except FM and UHF TV) have also been allocated frequencies in other bandwidths (outside UHF) by the authorities.

However, analytic studies concluded: "With the exception of sensitive military and research systems, equipment more than 100 km [60 miles] from a rectenna site should not require modification or special design to avoid degradation in performance," (157) and that conventional mitigative techniques would even permit operation of almost all devices at the rectenna boundary by filtering, nulling, minor circuit modifications and other mitigative techniques (158-162) which would cost between 0.1 to 5% of the unit cost to modify. Tests confirmed the effectiveness of these mitigative techniques. Sensitive military equipment would generally not be affected as long as the closest rectenna was more than 400 km (250 miles) away.

Pacemakers and other medical electronic devices would not be affected.

In summary, some kinds of communications would be adversely affected by the 2.45 GHz SPS beam, but the vast majority of today's communications would be unaffected.

However, the benefits to communications due to large scale space development would be immense.

Hence, the affects on birds flying over the rectenna will probably dictate the beam strength more than affects on communications.

Communications satellite benefits

Looking at the effects of SPSs on communications satellites is a waste of time unless we consider the revolutionary effects of large scale space infrastructure associated with SPSs. If SPSs are put into place, satellite communications will boom due to the related space-based manufacturing.

Currently, satellites are small with weak transmission powers and small reception antennas. Satellites are not constructed in space at all, but are small, compact objects built on Earth and deployed in space.

Space development will bring about large satellite platforms and "Orbiting Antenna Farms" (OAFs), allowing many times the number of satellites to be placed in geostationary orbit, and solving crowding problems. Bigger antennas in space mean smaller footprints on Earth, mitigating interference and allowing multiple use of each frequency. Larger power sources and larger antennas also make for clearer signals and smaller Earth ground stations. New frequencies currently not used due to partial atmospheric absorption will become usable. Linking satellites together on large platforms will lead to enhanced services. Fuel propellants from nonterrestrial materials will be used to ferry up satellites, provide stationkeeping in geostationary orbit, and extend the life of satellites (e.g., selling old satellite to less developed countries).

As for satellites in lower orbit passing through the beam on occasion, "Improved electromagnetic shielding and other minor modifications would be expected to eliminate or substantially reduce effects to allow normal performance." (169)






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