The question “are we alone in the Universe?” has more of a philosophical flavor than pure science. First, its underlying concept may be too complex to approach scientifically. In this article the author concern himself with the scientific aspect of the question and examine recent science-based findings.

To improve the uplink capability of the Deep Space Network atX-band frequencies near 7.2 GHz and to potentially prepare for retirement of the aging 70-m antennas, the National Aeronautics and Space Administration (NASA) has sponsored two experimental campaigns at the Jet Propulsion Laboratory (JPL, Pasadena, CA) and one at Harris Corporation (Palm Bay, FL) to show the feasibility of uplink arraying for deep space communication. These three efforts made significant progress in demonstrating the uplink arraying concept and in advancing our understanding of the associated error budget. These efforts focused primarily on demonstrating the feasibility of uplink arraying for communications applications. Uplink arraying is also useful for applications other than routine communications, mainly solar system radar and radio science. Among topics investigated are features and characteristics such as the array bandwidth, atmospheric calibration, array phase and amplitude stability, array blind pointing, and array delay calibration. No insurmountable obstacles are identified for the application of uplink arraying to noncommunications services. However, additional studies are recommended to minimize risk to radar and radio science services.

Deep-space communication and navigation is faced with two challenges in the future: (1) the potential retirement of the largest antennas of NASA's Deep Space Network and (2) an anticipated need for increasing ground system capacity so as to support higher data rates to and from missions operating at remote locations in the solar system, as well as in anticipation of a larger number of simultaneously flying missions. In the transmitting, or uplink, direction, one approach to increasing the effective transmitted power is to array multiple antennas. This is attractive mainly because it promises a lower construction cost than equivalent (large) single antenna systems. In addition, it has the potential for increasing the reliability of the uplink and reducing maintenance costs. This paper introduces the concept of uplink arraying by examining technological challenges and possible solutions to them. Arraying principles are presented and error sources described. The main challenge is to maintain carrier phase alignment among the antennas, and this must be done by periodic calibration. Presently, two calibration methods are being developed at the Jet Propulsion Laboratory as part of an uplink arraying demonstration effort. These methods are briefly discussed.

The seventeen papers in this special issue are devoted to technical advances in deep-space communications and tracking.

The twelve articles in this special issue are devoted to the topic of deep space communication.

During the last 40 years, deep space radio communication systems have experienced a move toward shorter wavelengths. In the 1960s, a transition from L- to S-band occurred, which was followed by a transition from S- to X-band in the 1970s. Both these transitions provided deep space links with wider bandwidths and improved radio metrics capability. Now, in the 2000s, a new change is taking place: namely, a move to the Ka-band region of the radio frequency spectrum. Ka-band will soon replace X-band as the frequency of choice for deep space communications, providing ample spectrum for the high data rate requirements of future missions. The low-noise receivers of deep space networks have a great need for link management techniques that can mitigate weather effects. In this paper, three approaches for managing Ka-band Earth-space links are investigated. The first approach uses aggregate annual statistics, the second one uses monthly statistics, and the third is based on the short-term forecasting of the local weather. An example of weather forecasting for Ka-band link performance prediction is presented. Furthermore, spacecraft commanding schemes suitable for Ka-band link management are investigated. Theses schemes will be demonstrated using NASA's Mars Reconnaissance Orbiter spacecraft in the 2007-2008 period, and the demonstration findings will be reported in a future publication.

Accurate estimates of the propagation impairments that affect link quality and availability and determine signal interference fields are essential for the reliable design of telecommunication systems and the efficient use of the electromagnetic spectrum. Recent announcements by commercial entities of their intent to use Ka-band spectrum to supply satellite services have heightened interest in propagation data and models for these frequencies. This paper provides a brief overview of Ka-band Earth-satellite systems and requirements in relation to the need for specific types of propagation data.

In designing a mobile satellite network, engineers and technologists are faced with wide-ranging issues for which there is no prior database. System engineers must address such issues as adequate margin to combat multipath fading, the level of adjacent channel protection required to allow transmission in narrow-band channels, the cochannel protection required to allow for frequency reuse in a multiplebeam system, and the level of intermodulation distortion tolerable for single-channel-per-carrier operation. Technologists, on the other hand, must determine the performance of various system components. For example, in the ground segment, modem and speech codec performance must be evaluated in the presence of thermal noise, fading, and other impairments. To enable system designers and technologists to optimize their design and pursue their research and development, the Jet Propulsion Laboratory has developed a mobile satellite channel simulator in hardware. This simulator has sufficient flexibility to facilitate many forms of tests and experiments.

This paper treats the problem of phase recovery loop performance loss due to a nonideal RF channel. The linear channel under consideration is assumed to suffer from a variety of impairments such as bandwidth limitation, hardware imperfections, and asymmetry of its transfer function. The phase error probability density function of a first-order Costas loop is presented. It has been determined that the phase error is biased in general, and the condition for eliminating this bias is pointed out. To avoid computational complexity and elude computer programming, a simple method is presented for hand computation of the ISI intensity in the loop. Channel bandwidth reduction also introduces phase jitter which can be controlled by proper signal spectrum shaping. For example, it is possible to communicate with a spectral efficiency of 1 bit/s/Hz without degrading the loop performance. Examples have been presented to examine a few practical problems.