The electromagnetic spectrum is a valued shared resource. Its scientific use allows us to learn about our universe, measure and monitor our planet, and communicate scientific data. The use of the spectrum is managed by national, regional, and global regulatory frameworks. There are increasing demands for new or extended allocations because of vast technological advances in the past few years. Understanding spectrum management is important in the successful planning and execution of missions and instruments, as well as in determining the potential source of radio frequency interference in existing data and instruments, and in working to ameliorate its impact. This paper provides a summary of this framework for radio scientists and engineers.

This paper serves as an introduction to the contributions in this Special Issue on ldquoAdvances in Radio Telescopes.rdquo After a very short historical view of the emergence of Radio Astronomy, we refer to earlier IEEE special issues on this subject and mention recent instruments in the domain of millimeter wavelength radio telescopes, developments in very long baseline interferometry and the planned Square Kilometre Array (SKA). After a short discussion of site selection aspects for the new telescopes we conclude with a summary of the major astronomical and astrophysical problems which will be studied by the new instruments described in the following papers.

The Atacama Large Millimeter/Submillimeter Array (ALMA) is an international radio telescope under construction in the Atacama Desert of northern Chile. ALMA is situated on a dry site at 5000 m elevation, allowing excellent atmospheric transmission over the instrument wavelength range of 0.3-10 mm. ALMA will consist of two arrays of high-precision antennas. One, of up to 64 12-m-diameter antennas, is reconfigurable in multiple patterns ranging in size from 150 m up to ~ 15 km. A second array is composed of a set of four 12-m and 12 7-m antennas operating in one of two closely packed configurations ~ 50 m in diameter. The instrument will provide both interferometric and total-power astronomical information on atomic, molecular, and ionized gas and dust in the solar system, our galaxy, and the nearby to high-redshift universe. In this paper, we outline the scientific drivers, technical challenges, and planned progress of ALMA.

The Very Long Baseline Array (VLBA) consists of ten high performance antennas and a 20-station correlator. The 25-m-diameter reflector antennas are located geographically to optimize spatial frequency coverage for high-duality imaging using VLBI techniques. The antennas are equipped with receivers for nine frequency bands over the range 300 MHz to 45 GHz. A wideband, high-density, recording system allows unattended operation at a sustained data rate of 125 Mb/s for a day, and peak rates of up to 512 Mb/s. The antennas operate unattended most of the time, and are controlled by a single operator in the Array Operations Center (AOC) at Socorro, NM. The correlator, located in the AOC, supports processing of all observations with the array.>