Global ena Image Simulations

The energetic neutral atom (ENA) images obtained by the ISEE and POLAR satellites pointed the way toward global imaging of the magnetospheric plasmas. The Imager for Magnetopause to Aurora Global Exploration (IMAGE) is the first mission to dedicate multiple neutral atom imagers: HENA, MENA and LENA, to monitor the ion distributions in high-, medium- and low-energy ranges, respectively. Since the start of science operation, HENA, MENA and LENA have been continuously sending down images of the ring current, ionospheric outflow, and magnetosheath enhancements from high pressure solar wind. To unfold multiple-dimensional (equal or greater than 3) plasma distributions from 2-dimensional images is not a trivial task. Comparison with simulated ENA images from a modeled ion distribution provides an important basis for interpretation of features in the observed images. Another approach is to develop image inversion methods to extract ion information from ENA images. Simulation studies have successfully reproduced and explained energetic ion drift dynamics, the transition from open to closed drift paths, and the magnetosheath response to extreme solar wind conditions. On the other hand, HENA has observed storm-time ion enhancement on the nightside toward dawn that differs from simple concepts but can be explained using more sophisticated models. LENA images from perigee passes reveal unexpected characteristics that now can be interpreted as evidence for a transient superthermal exospheric component that is gravitationally-influenced if not bound. In this paper, we will report ENA simulations performed during several IMAGE observed events. These simulations provide insight and explanations to the ENA features that were not readily understandable previously.     

GPS code and carrier multipath mitigation using a multiantennasystem

Multipath is a major source of error in high precision Global Positioning System (GPS) static and kinematic differential positioning. Multipath accounts for most of the total error budget in carrier phase measurements in a spacecraft attitude determination system. It is a major concern in reference stations, such as in Local Area Augmentation Systems (LAAS), whereby corrections generated by a reference station, which are based on multipath corrupted measurements, can significantly influence the position accuracy of differential users. Code range, carrier phase, and signal-to-noise (SNR) measurements are all affected by multipath, and the effect is spatially correlated within a small area. In order to estimate and remove code and carrier phase multipath, a system comprising a cluster of five GPS receivers and antennas is used at a reference station location. The spatial correlation of the receiver data, and the known geometry among the antennas, are exploited to estimate multipath for each satellite in each antenna in the system. Generic receiver code and carrier tracking loop discriminator functions are analyzed, and relationships between receiver data, such as code range, carrier phase, and SNR measurements, are formulated and related to various multipath parameters. A Kalman filter is described which uses a combination of the available information from the antennas (receivers) in the multiantenna cluster to estimate various multipath parameters. From the multipath parameters, the code range and carrier phase multipath is estimated and compensated. The technique is first tested on simulated data in a controlled multipath environment. Results are then presented using field data and show a significant reduction in multipath error     

The Magnetic Field Pile-up and Density Depletion in the Martian Magnetosheath: A Comparison with the Plasma Depletion Layer Upstream of the Earth's Magnetopause

Using magnetometer and electron observations from the Mars Global Surveyor (MGS) and the Wind spacecraft we show that the region of magnetic field pile-up and density decrease located between the Martian ionosphere and bow shock exhibit strong similarities with the plasma depletion layer (PDL) observed upstream of the Earth's magnetopause in the absence of magnetic reconnection when the magnetopause is a solid obstacle in the solar wind. A PDL is formed upstream of the terrestrial magnetopause when the magnetic field piles up against the obstacle and particles in the pile-up region are squeezed away from the high magnetic pressure region along the field lines as the flux tubes convect toward the magnetopause. We here discuss the possibility that at least part of the region of magnetic field pile-up and density depletion upstream of Mars may be formed by the same physical processes which generate the PDL upstream of the Earth's magnetopause. More complete ion, electron, and neutral measurements are needed to conclusively determine the relative importance of the plasma depletion process versus exospheric processes.     

Prefiltering: cascaded stages of decimation-by-two

A very efficient method of reducing the bandwidth of a sampled signal is presented. The method is implemented as a sequence of filtering and decimation stages, where the data rate is halved in each stage. Part of the efficiency is gained through the use of symmetrical linear-phase finite-duration impulse response filters, which have the added benefit that the operation is free of any phase distortion     

Radar receiver optimization for a continuously scanning antenna

The optimum coherent radar receiver configuration is derived for a continuously scanning antenna on the basis of maximizing the available energy for a given processor complexity. Included in the analysis are the length of the coherent dwell, the size of the discrete Fourier transform and the degree of weighting used for Doppler filtering, and the use of overlapped processing windows. Gaussian shapes for the processing window and antenna mainbeam are assumed in order to make the analysis tractable     

Earth observation and UK science policy

This paper examines the way in which Earth observation is linked to UK science policy through the 1993 UK science White Paper and subsequent policy statements, and how the central theme of partnership has been developed in practice. An example of crop yield prediction is given to illustrate the strengths and the weaknesses of the approach. The UK science and space strategies are set in the context of the European Strategy for Space and links are made to the US global change research strategy.     

Modelling the neutral gas environment of comets with special application to P/Halley

A technique has been developed which allows relatively accurate modelling of cometary gas production from nothing more than a visible light curve. Application to P/Halley suggests the production rate of parent molecules will be about 2.6 × 10²⁹ per second on March 10, 1986, for example. The uncertainties and intrinsic limitations in this approach are outlined. The theory is then extended to predictions of abundance of other gaseous species, and a photometric model of these gases provided. Combined with the dust model of N. Divine, preliminary predictions of the luminance of P/Halley as seen in any direction from inside the coma or outside can be provided for λλ3000–7000.     

Phase Detector Response to Noisy and Noisy Fading Signals

Transfer characteristics of sinusoidal, triangular, and sawtooth-type phase detectors in response to noisy and noisy fading signals have been studied in detail. A new analytical model of the swatooth-type phase detector has been suggested. Detailed experimental results have been supplemented in support of the theoretical findings. Both the theoretical findings and the experimental results clearly indicate that the superiority of a sawtooth-type phase detector over the other two varieties is completely lost in noisy and noisy fading environments.     

Microelectronics effects as seen on CRRES.

A MicroElectronics Test Package (MEP) measured total dose degradation and single event upsets (SEUs) on 60 device types on the Combined Release and Radiation Effects Satellite (CRRES) in an 18 degrees inclination orbit between 350 km and 36000 km from July 1990 to October 1991. Simultaneous measurements of the high energy particle environment were used to make a direct cause and effect comparison of the energetic particle backgrounds and microelectronic performance characteristics. The galactic cosmic ray background for the period of the CRRES mission was at a minimum. The SEUs experienced from the cosmic ray background were correspondingly few in number, but surprisingly produced an equal probability of upset over an L-shell range of 8.5 Earth radii (RE) down to less than 3.0 RE. Cosmic ray induced upset frequencies in proton sensitive chips were over 2 orders of magnitude lower than those produced by protons in the heart of the inner proton radiation belts. Multiple upsets, those produced when a single particle upsets more than one memory location, were just as common from protons as from cosmic rays.