ALTAIR is an experimental radar installation. Its 150-ft-diam antenna dish is supported by a four-legged frame that rotates on a 118-ft circular track. The track is centrally located on a 9-ft-wide ring-beam foundation. Four radial ribs and a cable tunnel connect to the beam and act as stiffeners. The subsurface materials consist of coral sand and rock. The antenna dish operates in two modes: elevation (vertical) and azimuth (horizontal). The elevation is changed by rotating the dish about a horizontal axis passing through the base of the dish, and the azimuth is varied by rotating the entire support frame. The transient (elastic) deflection of the beam due to rotation of the superstructure ranged from 0.0044 to 0.0088 in., depending on the beam stiffness at the monitoring station. The beam twisted outward as any of the bogies passed any of the monitored points. The angle of this twist was a maximum of about 8 sec. The foundation vibration was monitored radially, tangentially, and vertically to the circular foundation. The main frequency of response in all three modes was 21 Hz. Short-duration frequencies as low as 10 Hz were recorded in the tangential mode. High-frequency vibrations (130 to 170 Hz) of relatively minor amplitude were superimposed on the lower frequencies at azimuth rotation rates greater than about half the design rate. The maximum recorded vibration was 0.0007 in. peak to peak vertically, and was caused by maximum azimuth deceleration from a rotation rate slightly greater than design rate.

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