Sistema de apoio articulado para antenas rádio receptoras, aplicado na medição do Runup na praia de Jacarecica, Maceió/AL

Abstract

Geographical satellite positioning defines the geometrical location of the focus in phase in a parabolic reflector, whose exact placement is specified by the manufacturer. Precisely locating any point throughout a terrain requires adjustments that involve the distance of such point to antenna phase center location and the verticality deviation of the pole to which the antenna is fixated. In studies that apply kinematic positioning techniques, this precision becomes unattainable due to the instability to which the receiving antenna is submitted during transportation. This instability is translated into inconsistent verticality of the pole and into oscillations on the distance between the receiving antenna and the ground, since the transportation is usually manual. The present study tests an articulated support prototype composed by a tripod trolley into which the fixation pole of a GNSS antenna is settled but still able to move as a pendulum 360º. A heavy weight is attached to the base of the pole so it will always be in upright position and the use of a vehicle minimizes the oscillations on the distance between the antenna and the ground. The prototype testing consisted on measuring the waves run-up at the beach Jacarecica, Maceió – AL, using satellite geographical positioning. The coordinates to the most elevated points of the swash line were obtained by transporting the GNSS receiving antenna to the furthest point of it, and the static sea level at that very moment was estimated based upon the tide information of that day. The many vertical references (Vdata) of those variables were correlated through a very simple algebraic operation, where one adds or subtracts a constant corresponding to the difference in height among the Vdata measured in an ordinary place, i.e. as the ellipsoid height were measured by the GNSS at the shore, the corresponding sea level was verified at the tide table. The most significant differences between the furthest wave spreading points and the static sea level for the same moment, t(run-up), were around 60cm, which can result in a displacement of dozens of meters in the high tide line, depending on the slope of the beach.