An iterative LiDAR DEM-aided algorithm for GNSS positioning in obstructed/rapidly undulating environments

This paper describes a new algorithm to aid stand-alone GNSS positioning in areas of bad signal reception using a Digital Elevation Model (DEM). Traditional Height-Aiding (HA) algorithms assume either a preset (fixed) value for the receiver elevation or rely on the elevation value that corresponds to the nearest available position fix. This may lead in erroneous receiver elevation estimates that, under circumstances, are inefficient to aid effectively GNSS positioning. In this study, the receiver elevation is updated at every iteration step of the navigation solution through dynamic interpolation of the elevation model. The algorithm, because of its ability to extract and fully exploit the elevation information derived from a digital model, it can prove particularly useful in forested areas with steep-sloped terrain. Extended test runs were undertaken to validate the correctness of the mathematical model and the feasibility of the algorithm and associated software. Particularly, analysis of a dataset acquired in a forested, rapidly undulating environment reveals significant average improvement in all performance metrics of positioning, namely the GNSS position availability (50%), accuracy (56%) and external reliability (86%) compared to the Standard Point Positioning (SPP) solution. Moreover, it was found that the method can cope successfully in marginal operating conditions with situations of bad satellite geometry and satellite signals affected by interference due to tree canopy.     

Lunar ground penetrating radar: Minimizing potential data artifacts caused by signal interaction with a rover body

Ground-penetrating radar (GPR) is the leading geophysical candidate technology for future lunar missions aimed at mapping shallow stratigraphy (<5 m). The instrument’s exploration depth and resolution capabilities in lunar materials, as well as its small size and lightweight components, make it a very attractive option from both a scientific and engineering perspective. However, the interaction between a GPR signal and the rover body is poorly understood and must be investigated prior to a space mission. In doing so, engineering and survey design strategies should be developed to enhance GPR performance in the context of the scientific question being asked. This paper explores the effects of a rover (simulated with a vertical metal plate) on GPR results for a range of heights above the surface and antenna configurations at two sites: (i) a standard GPR testing site with targets of known position, size, and material properties, and; (ii) a frozen lake for surface reflectivity experiments. Our results demonstrate that the GPR antenna configuration is a key variable dictating instrument design, with the XX polarization considered optimal for minimizing data artifact generation. These findings could thus be used to help guide design requirements for an eventual flight instrument.     

First Results of the THEMIS Search Coil Magnetometers

We present the first data from the THEMIS Search Coil Magnetometers (SCM), taken between March and June 2007 while the THEMIS constellation apogee moved from the duskside toward the dawnside. Data reduction, especially the SCM calibration method and spurious noise reduction process, is described. The signatures of magnetic fluctuations in key magnetospheric regions such as the bow shock, the magnetopause and the magnetotail during a substorm, are described. We also discuss the role that magnetic fluctuations could play in plasma transport, acceleration and heating.     

The THEMIS ESA Plasma Instrument and In-flight Calibration

The THEMIS plasma instrument is designed to measure the ion and electron distribution functions over the energy range from a few eV up to 30 keV for electrons and 25 keV for ions. The instrument consists of a pair of “top hat” electrostatic analyzers with common 180°×6° fields-of-view that sweep out 4π steradians each 3 s spin period. Particles are detected by microchannel plate detectors and binned into six distributions whose energy, angle, and time resolution depend upon instrument mode. On-board moments are calculated, and processing includes corrections for spacecraft potential. This paper focuses on the ground and in-flight calibrations of the 10 sensors on five spacecraft. Cross-calibrations were facilitated by having all the plasma measurements available with the same resolution and format, along with spacecraft potential and magnetic field measurements in the same data set. Lessons learned from this effort should be useful for future multi-satellite missions.     

THEMIS Ground Based Observatory System Design

The comprehensive THEMIS approach to solving the substorm problem calls for monitoring the nightside auroral oval with low-cost, robust white-light imagers and magnetometers that can deliver high time resolution data (0.33 and 2 Hz, respectively). A network of 20 Ground-Based Observatories (GBOs) are deployed across Canada and Alaska to support the collection of data from these instruments. Here we describe the system design of the observatory, with emphasis on how the design meets the environmental and data-collection requirements. We also review the design of the All Sky Imager (ASI), discuss how it was built to survive Arctic deployments, and summarize the optical characterizations performed to qualify the design to meet THEMIS mission requirements.     

The Upgraded CARISMA Magnetometer Array in the THEMIS Era

This review describes the infrastructure and capabilities of the expanded and upgraded Canadian Array for Realtime InvestigationS of Magnetic Activity (CARISMA) magnetometer array in the era of the THEMIS mission. Formerly operated as the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) magnetometer array until 2003, CARISMA capabilities have been extended with the deployment of additional fluxgate magnetometer stations (to a total of 28), the upgrading of the fluxgate magnetometer cadence to a standard data product of 1 sample/s (raw sampled 8 samples/s data stream available on request), and the deployment of a new network of 8 pairs of induction coils (100 samples per second). CARISMA data, GPS-timed and backed up at remote field stations, is collected using Very Small Aperture Terminal (VSAT) satellite internet in real-time providing a real-time monitor for magnetic activity on a continent-wide scale. Operating under the magnetic footprint of the THEMIS probes, data from 5 CARISMA stations at 29–30 samples/s also forms part of the formal THEMIS ground-based observatory (GBO) data-stream. In addition to technical details, in this review we also outline some of the scientific capabilities of the CARISMA array for addressing all three of the scientific objectives of the THEMIS mission, namely: 1. Onset and evolution of the macroscale substorm instability, 2. Production of storm-time MeV electrons, and 3. Control of the solar wind-magnetosphere coupling by the bow shock, magnetosheath, and magnetopause. We further discuss some of the compelling questions related to these three THEMIS mission science objectives which can be addressed with CARISMA.     

Orbit Design for the THEMIS Mission

THEMIS, NASA’s fifth Medium Class Explorer (MIDEX) mission will monitor the onset and macro-scale evolution of magnetospheric substorms. It is a fleet of 5 small satellites (probes) measuring in situ the magnetospheric particles and fields while a network of 20 ground based observatories (GBOs) monitor auroral brightening over Northern America. Three inner probes (～1 day period, 10 R_E apogee) monitor current disruption and two outer probes (～2 day and ～4 day period, 20 R_E and 30 R_E apogees respectively) monitor lobe flux dissipation. In order to time and localize substorm onsets, THEMIS utilizes Sun–Earth aligned conjunctions between the probes when the ground-based observatories are on the nightside. To maintain high recurrence of conjunctions the outer orbits have to be actively adjusted during each observation season. Orbit maintenance is required to rearrange the inner probes for dayside observations and also inject the probes into their science orbits after near-simultaneous release from a common launch vehicle. We present an overview of the orbit strategy, which is primarily driven by the scientific goals of the mission but also represents a compromise between the probe thermal constraints and fuel capabilities. We outline the process of orbit design, describe the mission profile and explain how mission requirements are targeted and evaluated. Mission-specific tools, based on high-fidelity orbit prediction and common magnetospheric models, are also presented. The planning results have been verified by in-flight data from launch through the end of the first primary science seasons and have been used for mission adjustments subject to the early scientific results from the coast phase and first tail season.     

THEMIS ESA First Science Results and Performance Issues

Early observations by the THEMIS ESA plasma instrument have revealed new details of the dayside magnetosphere. As an introduction to THEMIS plasma data, this paper presents observations of plasmaspheric plumes, ionospheric ion outflows, field line resonances, structure at the low latitude boundary layer, flux transfer events at the magnetopause, and wave and particle interactions at the bow shock. These observations demonstrate the capabilities of the plasma sensors and the synergy of its measurements with the other THEMIS experiments. In addition, the paper includes discussions of various performance issues with the ESA instrument such as sources of sensor background, measurement limitations, and data formatting problems. These initial results demonstrate successful achievement of all measurement objectives for the plasma instrument.