The Effect of Ground Reflections and Scattering on an Interferometer Direction Finder

The azimuth and elevation angles of an airborne beacon transmitter can be determined using a direction finder comprised of two orthogonal interferometers located near the earth's surface. In this paper we consider the reflection of the incident electromagnetic field by the earth's surface and the resultant effect on the direction-finding system. The analysis yields an upper bound or limit to the interferometer phase error and the corresponding error in bearing angle that can result because of specular reflection and diffuse scattering.     

Ionospheric observations made by a time-interleaved Doppler ionosonde

Mid-latitude HF observations of ionospheric Doppler velocity as a function of frequency are reported here as observed over a quiet 24-h period by a KEL IPS71 ionosonde operating at a 5-min sampling rate. The unique time-interleaving technique used in this ionosonde provided a Doppler resolution of 0.04 Hz over a Doppler range of ±2.5 Hz at each sounding frequency via FFT processing and is described here for the first time. The time-interleaving technique can be applied to other types of ionosonde as well as to other applications. The measurements described were made at a middle latitude site (Salisbury, South Australia). Doppler variations (<30 min) were ever present throughout the day and showed short-period TID characteristics. The day-time Doppler shift was found to closely follow the rate-of-change of foF2 as predicted by a simple parabolic layer model. The descending cusp in short-period TIDs is shown to mark an abrupt change with increasing frequency from negative towards positive Doppler shift with the greatest change in Doppler shift being observed below the cusp. The “smilergram” is introduced as observed in both F2 and Sporadic E. The characteristic curve in Doppler versus group height at a single frequency is described and related to changes in reflection symmetry, velocity and depth of moving ionospheric inhomogeneities.     

Digisonde observations of TIDs with frequency and angular sounding technique

A technique for studying ionospheric wavelike phenomena, primarily AGW/TID events, is developed based on the solution of the problem of radio wave propagation in ionospheric plasma disturbed by wavelike processes. A perfectly reflecting surface model is used for representing TIDs propagating at ionospheric heights. This technique is a generalization of the Frequency-and-Angular Sounding (FAS) method developed earlier for oblique TID diagnostics using transmitters of opportunity. Trial measurements were made in November 2003 with two DPS-4 systems at Millstone Hill Observatory, providing experimental validation of the developed method by comparing the results of disturbance diagnostics to those simultaneously obtained with the original (oblique) FAS method. The TID parameters recovered during the November 2003 campaign suggest that the observed disturbances predominately propagated equatorward which likely indicates their sources to be in the auroral region. The equatorward propagating AGW/TIDs are typical for disturbed geomagnetic conditions which were observed during the campaign. Implementation of the generalized FAS technique in the DPS sounder allowed development of a dedicated data acquisition system for ionospheric disturbance diagnostics. Routine measurements with the developed technique using the existing world-wide network of Digisondes (GIRO) will make it possible to conduct large-scale studies of the AGW/TID phenomena.     

Sensor Fault Detection in a System with Random Disturbances

Sensor faults are detected in an operating automatic system by a simplified version of the dedicated observer scheme. Control inputs are augmented by a random disturbance of moderate intensity. The dedicated observer in this case is a Kalman filter, driven by a single sensor. This filter provides estimates of the outputs from the other, nonredundant, sensors. A logical combination of these functionally redundant signals with the actual sensor signals provides prompt detection of incipient faults on all instruments without false alarms. The scheme is applied to a simulation of the lateral axis control system of a hydrofoil boat in which four sensors are to be covered by the fault detection scheme. Tests indicate that the scheme is robust with respect to variations in the intensity of the random disturbance.     

Plasma sheet oscillations and their relation to substorm development: Cluster and double star TC1 case study

We examined two consecutive plasma sheet oscillation and dipolarization events observed by Cluster in the magnetotail, which are associated with a pseudo-breakup and a small substorm monitored by the IMAGE spacecraft. Energy input from the solar wind and an associated enhancement of the cross-tail current lead to current sheet thinning and plasma sheet oscillations of 3–5 min periods, while the pseudo-breakups occur during the loading phase within a spatially limited area, accompanied by a localized dipolarization observed by DSP TC1 or GOES 12. That is, the so-called “growth phase” is a preferable condition for both pseudo-breakup and plasma sheet oscillations in the near-Earth magnetotail. One of the plasma sheet oscillation events occurs before the pseudo-breakup, whereas the other takes place after pseudo-breakup. Thus there is no causal relationship between the plasma sheet oscillation events and pseudo-breakup. As for the contribution to the subsequent small substorm, the onset of the small substorm took place where the preceding plasma sheet oscillations can reach the region.     

The X-ray/Gamma-ray Spectrometer on the Near Earth Asteroid Rendezvous Mission

An X-ray/gamma-ray spectrometer has been developed as part of a rendezvous mission with the near-Earth asteroid, 433 Eros, in an effort to answer fundamental questions about the nature and origin of asteroids and comets. During about 10 months of orbital operations commencing in early 1999, the X-ray/Gamma-ray Spectrometer will develop global maps of the elemental composition of the surface of Eros. The instrument remotely senses characteristic X-ray and gamma-ray emissions to determine composition. Solar excited X-ray fluorescence in the 1 to 10 keV range will be used to measure the surface abundances of Mg, Al, Si, Ca, Ti, and Fe with spatial resolutions down to 2 km. Gamma-ray emissions in the 0.1 to 10 MeV range will be used to measure cosmic-ray excited elements O, Si, Fe, H and naturally radioactive elements K, Th, U to surface depths on the order of 10 cm. The X-ray spectrometer consists of three gas-filled proportional counters with a collimated field of view of 5° and an energy resolution of 850 eV @ 5.9 keV. Two sunward looking X-ray detectors monitor the incident solar flux, one of which is the first flight of a new, miniature solid-state detector which achieves 600 eV resolution @ 5.9 keV. The gamma-ray spectrometer consists of a NaI(Tl) scintillator situated within a Bismuth Germanate (BGO) cup, which provides both active and passive shielding to confine the field of view and eliminate the need for a massive and costly boom. New coincidence techniques enable recovery of single and double escape events in the central detector. The NaI(Tl) and BGO detectors achieve energy resolutions of 8.7% and 14%, respectively @ 0.662 MeV. A data processing unit based on an RTX2010 microprocessor provides the spacecraft interface and produces 256-channel spectra for X-ray detectors and 1024-channel spectra for the raw, coincident, and anti-coincident gamma-ray modes. This paper presents a detailed overview of the X-ray/Gamma-ray Spectrometer and describes the science objectives, measurement objectives, instrument design, and shows some results from early in-flight data.