Missions to Mercury

Mercury is a very difficult planet to observe from the Earth, and space missions that target Mercury are essential for a comprehensive understanding of the planet. At the same time, it is also difficult to orbit because it is deep inside the Sun’s gravitational well. Only one mission has visited Mercury; that was Mariner 10 in the 1970s. This paper provides a brief history of Mariner 10 and the numerous imaginative but unsuccessful mission proposals since the 1970s for another Mercury mission. In the late 1990s, two missions—MESSENGER and BepiColombo—received the go-ahead; MESSENGER is on its way to its first encounter with Mercury in January 2008. The history, scientific objectives, mission designs, and payloads of both these missions are described in detail.     

Notes on SCPC-FM System Design and Performance

In many domestic satellite communication systems single-channel per-carrier FM (SCPC-FM) has become the accepted transmission method for thin route communication. The SCPC-FM modem operates over a wide range of carrier frequencies and is designed to use satellite power and bandwidth efficiently. It incorporates elements which make the signal transfer complex, so the system behavior is not immediately obvious. This article investigates the signal processes and analyzes the performance of a companded and preemphasized SCPC-FM system and it furnishes insight into the choice of crucial system parameters.     

Conopulse Radar Angle Tracking Accuracy

An analysis is given of the tracking accuracy of a conopulse radar that angle-tracks on target echo pulses. The analysis includes effects of noises in both sum and difference channels, the effect of the tracking range gate's duration, and applies to any shape of radio frequency (RF) pulse envelope. For large single-pulse signalto-noise ratio and any shape of RF pulse envelope, accuracy approaches the theoretical accuracy of a conopulse system when a short-duration range gate is used. The inportant case of rectangular RF pulses is solved in detail.     

Recursive temporal-spatial information fusion with applications totarget identification

Centralized/distributed recursive algorithms for temporal-spatial information integration using the Dempster-Shafer technique are developed. Compared with the Bayesian approach, the Dempster-Shafer technique has the strong capability of handling information uncertainties, which are particularly desirable in many applications. In the centralized integration algorithm, all information is pooled into the central processor and integrated. In contrast, the distributed integration algorithm shares the computational burden among the local processors, which increases the computational efficiency. The developed algorithms are effectively applied to a target identification problem with three sensors: identification of friend-foe-neutral (IFFN), electronic support measurement (ESM), and infrared search and track (IRST)     

Solar physics assessment of future high-resolution observations in the Grazing-Incidence domain

The project for a Grazing Incidence Solar Telescope (GRIST) offers, for the first time, the combinations of high spatial (1) and spectral resolution in the extreme-ultraviolet wavelength range. The 3-dimensional electron density and temperature structure of the transition region and corona will be determined. The dynamics of the structures which make up the corona will be studied. GRIST can be expected to provide definitive improvement in the understanding of the coronal heating problem.Proceedings of the Conference Solar Physics from Space, held at the Swiss Federal Institute of Technology Zurich (ETHZ), 11–14 November 1980.     

Type Pi 1–2 magnetic field pulsations

The relationships of type Pi (broadband) pulsations to various other substorm-related phenomena are reviewed. Several of the more popular mechanisms for the origin of Pi activity are discussed in the light of the observations. There is only one mechanism in sight that tentatively accounts for observed characteristics of Pi 1–2 activity at auroral oval and polar cap latitudes and that is the three-dimensional current loop mechanism. If two or more mechanisms are involved in the generation of Pi noise, then it is possible that the garden-hose overstability and/or a drift Alfvén wave mechanism operating in the plasma sheet contribute to the observed pulsations.The common feature of all Pi 1–2 events is not the presence of temporal precipitation pulsations but the presence of an E-region, suggesting that enhanced conductivity and E-region currents are required. Pi activity appears to be closely related to unsteady convection in progress. Pi data promise to provide useful information on convection and field-aligned and ionospheric currents.     

Optical identification of X-ray sources in the Einstein observatory medium and deep surveys

Methods are discussed for establishing the optical identification of X ray sources in the medium and deep X-ray surveys of the Einstein Observatory. Of the 63 X-ray sources with a statistical significance of 5 in the medium survey (Maccacaro et al. 1981), optical identification work is summarized for 51, of which identifications have been made with 30 active galactic nuclei. The optical properties of some of these X-ray selected objects are briefly discussed.The Einstein deep survey of Pavo (Griffiths et al. 1981) is used to illustrate the problems and methods used for securing optical identifications for X-ray sources in the deep survey fields. Identifications have been made with 4 QSOs at the bright end of the optical candidate distribution (together with 3 G stars) and it is shown that a further 7 fainter objects are also likely to be QSOs.     

High-Order Intermodulation Effects in Digital Satellite Channels

The phenomenon of intermodulation generation due to passive and normally linear components is sometimes called passive intermodulation. A unique characteristic of passive intermodulation interference in communications satellites is that the products causing trouble are of high orders. Due to the immense combinatorial complexity of the problem, both exact analysis and direct simulation are almost impossible. In this paper the problem is simplified by considering a composite interference model consisting of 1) the lowest order intermodulation product (of those high-order ones) at a frequency of interest, 2) a constant-envelope carrier with random phase to account for all other intermodulation interferences by direct overlap or spectrum spreading in the vicinity, and 3) white Gaussian noise (receiver noise). An approximation of the tail statistics of this composite interference is derived, and numerical results (checked by simulation) show that the tail distribution may be very different from Gaussian; careful consideration must be rendered before resorting to the a priori Gaussian assumption.     

The Resolution Performance of Range Estimators

The theoretical basis and methods of implementation of a moment algorithm for the range separation estimation of two closely spaced point targets are presented. Moment estimation and noise filtering techniques introduced here result in a considerable improvement over Baum's algorithm. The error bounds are established and it is shown that the spectral moment estimator exhibits optimum (zero bias, minimum variance) performance when the target separation normalized to the standard deviation of the Gaussian pulse is 2?1.5. Monte Carlo simulation is performed to verify the approximations made and to demonstrate the feasibility of the working models.