Challenges of Antenna in Radio-Astronomy

A major challenge for all high resolution low frequency radio astronomy is measuring and removing the effects of the ionosphere. The isoplanatic patch size for frequencies below a few hundred MHz is generally much smaller than the field of view. In addition, aperture arrays have beams on the sky which vary dramatically with observing geometry. These beams require careful calibration to be stable and known in order to allow imaging. It is noted that large radio telescope arrays such as the Very Large Array and Very Long Baseline Array at the National Radio Astronomy Observatory challenge the state of the art of microwave technology in many areas. Examples include the design of optimized sparse arrays and the design and the economic construction of high-performance, frequency-flexible antennas, reliable low-noise receivers, and highly phase stable local oscillator systems. Three-decade frequency range, greatly improved sensitivity and angular resolution that matches the capabilities of planned instruments in other frequency domains, are challenges that are now being investigated. Instruments for radio astronomical observations have come a long way. While the first telescopes were based on very large dishes and two-antenna interferometers, current Instruments consist of dozens of steerable dishes, whereas future instruments will be even larger distributed sensor arrays with a hierarchy of phased array elements. For such arrays to provide meaningful output (images), accurate calibration is of critical importance. Calibration must solve for the unknown antenna gains and phases as well as the unknown atmospheric and ionospheric disturbances. Future telescopes will have a large number of elements and a large field of view (FOV). In this case, the parameters are strongly direction-dependent, resulting in a large number of unknown parameters, even if appropriately constrained physical or phenomenological descriptions are used. This makes calibration a daunting parameter-estimation task.