High capacity directional wireless communications networks are an active research area because of their Gb/s or greater data rates over link lengths of many kilometers, providing fiber-like networks through the air. Their high data rates arise partly from their very high carrier frequencies (<60GHz for RF, and ~1550nm for free-space optical (FSO)) and partly because of their very narrow beamwidths. This second characteristic requires that transceivers be pointed precisely to their counterparts. In almost all cases this means that the transceiver aperture is mechanically pointed by a rotation stage, commonly known as a gimbal. How these platforms initially point at a target, acquire the signal, and then stay locked on the signal is known as pointing, acquisition, and tracking (PAT). Approaches for PAT in both RF and FSO have some similarities, but require overall divergent solutions, especially if the platforms are moving. This paper elaborates on the various considerations required for designing and implementing a successful PAT system for both directional RF and FSO systems. Approaches for GPS or beacon based pointing, types of acquisition scans, and the effects of platform vibration are analyzed. The acquisition time for a spiral scan of a given radius with an initial pointing error has been measured experimentally for a gimbal pointing system.
Flight tests were conducted at Greenbrier Valley Airport (KLWB) and Easton Municipal Airport / Newnam Field
(KESN) in a Cessna 402B aircraft using a head-up display (HUD) and a Norris Electro Optical Systems Corporation
(NEOC) developmental ultraviolet (UV) sensor. These flights were sponsored by NEOC under a Federal Aviation
Administration program, and the ultraviolet concepts, technology, system mechanization, and hardware for landing
during low visibility landing conditions have been patented by NEOC. Imagery from the UV sensor, HUD guidance
cues, and out-the-window videos were separately recorded at the engineering workstation for each approach. Inertial
flight path data were also recorded. Various configurations of portable UV emitters were positioned along the runway
edge and threshold. The UV imagery of the runway outline was displayed on the HUD along with guidance generated
from the mission computer. Enhanced Flight Vision System (EFVS) approaches with the UV sensor were conducted
from the initial approach fix to the ILS decision height in both VMC and IMC. Although the availability of low
visibility conditions during the flight test period was limited, results from previous fog range testing concluded that UV
EFVS has the performance capability to penetrate CAT II runway visual range obscuration. Furthermore, independent
analysis has shown that existing runway light emit sufficient UV radiation without the need for augmentation other than
lens replacement with UV transmissive quartz lenses. Consequently, UV sensors should qualify as conforming to FAA
requirements for EFVS approaches. Combined with Synthetic Vision System (SVS), UV EFVS would function as both
a precision landing aid, as well as an integrity monitor for the GPS and SVS database.
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