Preliminary results from the Arecibo 430 MHz spatial interferometry system

The 430 MHz radar system of the Arecibo Observatory (AO) possesses one of the largest power-aperture products in the world. The aperture is effectively 200-300 m and the peak power is approximately 2.5 MW. Designed originally for ionospheric and radioastronomy, the AO is built into a natural bowl in the Puerto Rican terrain. The feed system is directed downward, toward the dish, and then reflected into the atmosphere. As a result, the platform which houses the feed systems must be very stable. The structure is suspended by a network of steel cables strung from large concrete towers. In total, the feed platform weighs approximately 600 tons and cannot be moved rapidly. This results in an enormous amount of wasted time during the steering of the beam, which can take up to 30 min. Doppler beam swinging (DBS) techniques have been attempted with the system but adequate temporal resolution is difficult to obtain. The DBS technique determines the wind velocity by obtaining the Doppler shift of the scattered signal from several beam directions, which are converted into radial velocity estimates. Subsequently, the radial velocity estimate can be used to derive the overall wind field. DBS techniques have been attempted with the AO system but adequate temporal resolution is difficult to obtain because of the slow beam steering. Multi-receiver techniques, which use only a single beam direction, can alleviate the temporal resolution concern since no beam steering is needed. In a collaborative effort a spatial interferometric (SI) system, i.e., multireceiver, has been built and will be installed at the AO. Preliminary tests have been performed in 1995 with final installation planned for the spring of 1996. The results of the tests are discussed