Cassini Radio Science

Cassini radio science investigations will be conducted both during the cruise (gravitational wave and conjunction experiments) and the Saturnian tour of the mission (atmospheric and ionospheric occultations, ring occultations, determinations of masses and gravity fields). New technologies in the construction of the instrument, which consists of a portion on-board the spacecraft and another portion on the ground, including the use of the Ka-band signal in addition to that of the S- and X-bands, open opportunities for important discoveries in each of the above scientific areas, due to increased accuracy, resolution, sensitivity, and dynamic range.This revised version was published online in July 2005 with a corrected cover date.     

Saturn's Rings: pre-Cassini Status and Mission Goals

Theoretical and observational progress in studies of Saturn's ring system since the mid-1980s is reviewed, focussing on advances in configuration and dynamics, composition and size distribution, dust and meteoroids, interactions of the rings with the planet and the magnetosphere, and relationships between the rings and various satellites. The Cassini instrument suite of greatest relevance to ring studies is also summarized, emphasizing how the individual instruments might work together to solve outstanding problems. The Cassini tour is described from the standpoint of ring studies, and major ring science goals are summarized. This revised version was published online in August 2006 with corrections to the Cover Date.     

Software approach to access UWB interference on GPS receivers

Ever since the FCC approved the use of UWB devices in commercial and federal bands, various agencies whose operations and/or products rely on the integrity of signals within certain "restricted" radio frequency bands have voiced concerns over the potential impact of the UWB interference. GPS signals are among these "restricted" bands. Several groups in the GPS community have conducted experimental studies concerning the impact of UWB interference on the performance of various grades of commercial and aviation GPS receivers. In this paper, we present a software approach to simulate and evaluate UWB interference on GPS receivers. The software approach provides greater flexibility in the design of testing scenarios, such as the inclusion of a large number of aggregated UWB devices, the generation of new UWB signals and modulation schemes, and the possibility of extending the study to new GPS signals. The paper discusses a general framework for developing algorithms to evaluate UWB and GPS interference under a wide variety of hardware and software conditions. This framework consists of three classes of components: input, processing, and analysis. The input components are responsible for the generation of UWB signal waveforms and modulation schemes, and GPS signals. The processing components include a simulated model of GPS RF front end and software implementation of GPS processing blocks, such as acquisition, tracking, and post-processing. The analysis components focus on the study of specific receiver processing component outputs. Both real and simulated UWB signals can be used in the study. The real UWB signals are primarily used to validate the simulation procedure, whereas the simulated UWB signals are used to allow the immediate incorporation of new UWB waveforms and modulations in the evaluations. This paper presents details of the software components developed and the preliminary results achieved     

Europe's major challenge for the 21st century: Access to space

Like the other great space powers of the 21st century, Europe has its own means of accessing space: Ariane, which has guaranteed its independence in the launching of civil and military satellites for almost 30 years and has won a significant part of the highly competitive commercial market. This market provides the Ariane system with the production volume indispensable for its reliability, which also benefits institutional launches. Europe's commercial market share will be even larger if the launch system is flexible and adaptable to the diversity of demand. Probable future technological changes make flexibility more necessary still. Two technical characteristics will be key: a large enough payload capacity and the injection of satellites into energetic orbits, including final geostationary orbit. But carrying out such missions will only be possible if a new generation upper stage is used. The November 2008 ESA ministerial meeting opted to wait until 2011 to decide whether this is necessary, making it doubtful whether Arianespace will be able to maintain leadership in the commercial market. The authors urge a rethink of this position.     

Parametric study of the diversion of a near Earth object on an Earth intersecting trajectory

To date, NASA's “Near Earth Object Program” has discovered over 5500 comets and asteroids on trajectories that bring them within “the neighborhood” of Earth's orbit. Nearly 1000 of these objects are classified as “potentially hazardous,” passing within 0.05 astronomical units of Earth's orbit. Discovery rates of such threatening bodies increase each year. Given this multitude of threats, in addition to evidence that the planet has absorbed many impacts over its history, it is reasonable to assume that another object will strike the Earth at some point in the future. Consequently, researchers have studied and proposed several mitigation techniques for such an occurrence. This study seeks to determine how effectively the attachment of a tether and ballast mass would divert the trajectory of such threatening objects. Specifically, the study analyzes the effects over time of such a system on objects of varying orbital semimajor axis and eccentricity, using various tether lengths and ballast masses. It was determined that the technique is most effective for NEOs with high eccentricity and small semimajor axis, and that system performance increases as tether length and ballast mass increase.     

Automobile Navigation: Where is it Going?

Automobile navigation systems based on dead reckoning, map matching, satellite positioning and other navigation technologies are under active development. ``Map intelligent' systems achieve high relative accuracy by matching dead-reckoned paths with road geometry encoded in a digital map data base that may also serve other functions such as vehicle routing and geocoding. Satellite-based navigation systems achieve high absolute accuracy but require dead reckoning augmentation because of signal aberrations in the automotive environment. Future systems will probably include multiple navigation technologies. Issues influencing the design of future systems include safety concerns regarding the driver interface, and the future availability of comprehensive map data bases, real-time traffic data, and mobile data communication links necessary for on-board generation of optimum routes.