Four signi?cant factors have led us to update this text. The ?rst is the breathtaking progress in technology, especially in receiver and digital techniques. The second is the advance of radio astronomy to shorter wavelengths, and the increased availab- ity of astronomical satellites. The third is a need to reorganize some of the chapters in order to separate the basic theory, that seldom changes, from practical aspects that change often. Finally, it is our desire to enhance the text by including problem sets for each chapter. In view of this ambitious plan, we have expanded the number of authors. In the reorganization of this edition, we have divided Chap. 4 of the 4th edition into two Chaps. 4 and 5. The ?rst remains Chap. 4, with a slightly different - tle, Signal Processing and Receivers: Theory. This was expanded to include digital processing and components including samplers and digitizers. In Chap. 5, Prac- cal Receiver Systems. we have relegated the presentations of maser and parametric ampli?er front ends, which are no longer commonly used as microwave receivers in radio astronomy, to a short section on “historical developments” and We have retained and improved the presentations of current state-of-the-art devices, cooled transistor and superconducting front ends. We have also included descriptions of local oscillators and phase lock loops. Chapters 5 and 6 in the 4th edition has now become Chap. 6, Fundamentals of Antenna Theory and Chap.
