Exploiting GNSS signal structure to enhance observability

There are a number of different error sources, such as multipath and thermal noise, which corrupt satellite navigation waveforms from their theoretical structure. However, even under ideal conditions the broadcast signals have some degree of deformation as a result of the practical individual hardware implementation. For the most demanding users of satellite navigation, such as aircraft navigation and landing systems, it is important to characterize the nominal signal structure in order to detect minimal variations resulting from hardware-based errors. Thus far such precorrelation Global Navigation Satellite System (GNSS) signal quality monitoring has been performed through high gain antennas, which allow for raising the GNSS spectrum above the thermal noise floor and observing the structure of the signal directly at the front end output. This paper describes a new approach to achieve such observability based on signal processing techniques, such as dithering and averaging, which leverage the repetitive nature of the GNSS signal. The paper presents how these techniques can drastically improve the signal-to-noise ratio (SNR) in postprocessing, allowing for the direct analysis of GNSS signals using traditional front end designs and conventional antennas. Results are predicted using the appropriate theory and validated using data collected from the Global Positioning System (GPS).     

Geologic field work on Mars: Distance and time issues during surface exploration

Based on field experience at Analogue stations MDRS and FMARS and additional theoretical computations, motorized field work traverses were planned for various surface feature types on Mars, with emphasis on their horizontal dimension, maximal slope angle and cumulative vertical surface undulation or roughness. The aim was to explore the possibilities and characteristics of field work for different terrain types. Those terrain types are reviewed in the present study that have already been analyzed based on remote sensing data, and that are described in the scientific literature.Dunes, gullies, slope streaks, cross sections of valley networks and of lava channels might be analyzed during only one extra vehicular activity (EVA, e.g., pedestrian and vehicular field work) with the objective of a first in-situ exploration along an optimized traverse, in order to provide the most valuable scientific information on their general characteristics and origin. Smaller tectonic faults, lava flows, lobate debris aprons and outcrops of polar layered deposits can be analyzed only by several EVAs together. Analysis of large landslides, calderas and interior layered deposits produce even more difficulties on Mars, and require specialized technology. In cases where several EVAs are necessary for detailed analysis along the best traverse, mobile pressurized vehicles (with pressurized cabin for astronauts without spacesuits) or other methods would be necessary.Many of the geologic structures that have been analyzed only with remotely sensed data could not be surveyed during one field campaign, and in some cases because of high slope angle and large cumulative topographic undulation, their in-situ exploration could not be accomplished with the technological capabilities available, and in-situ analysis requires more advanced technology than long distance rovers can provide now. Hence, the prior location of important sites and the usage of robotic help will be of high importance.     

Water related environment modelling on Mars

During a human Mars exploration because of the lack of time astronauts need fast methods for the interpretation of unexpected observations which give them flexibility and new, important targets. With in-situ modelling it is possible to get information on various past and present processes at the same location on a far wider spectrum than would be realized even during a long mission. This work summarizes the potential technical requirements and benefits of the modelling. Based on a simple estimation with a 300 kg package, and 1-10% of the working time of 1-2 astronauts at the same location, they can get plenty of new and important information for the whole past and present Mars. With the proposed five test groups astronauts will be able to make better and newer kinds of interpretations of observations, and find better targets and methods during the same mission.     

GPS C/N/sub 0/ estimation in the presence of interference and limited quantization levels

The carrier-to-noise density ratio (C/N₀) is considered an important parameter describing the GPS receiver performance. This paper compares the performance of two popular coarse-acquisition (C/A) C/N₀ algorithms appearing in literature: the variance summing method (VSM) (Psiaki et al., 2003, Psiaki, 2001), and the power ratio method (PRM) (Van Dierendonck, 1996, Sayre, 2003), in terms of their estimates in 1) additive white Gaussian noise (AWGN), 2) narrowband continuous wave interference (CWI), 3) their response to quantization and saturation effects, and their 4) dynamic range. The algorithms were implemented as a part of a software receiver. Two LI GPS data sets are examined; one was obtained from a GPS raw data collection setup, while the other was obtained from a GPS signal simulator. The collected set was stored with almost constant C/N₀ level while the simulated one contained variable C/N₀ levels. The effect of adding AWGN on the C/N₀ estimate was directly proportional with the noise power. The C/N₀ estimates suffered more when the CWI frequency was closer to the IF of the receiver. The PRM suffered from saturation at higher C/N₀ levels. The VSM showed good tracking at high C/N₀ levels and better immunity to limited quantization levels, while its C/N₀ estimate suffered from rapid fluctuations in power levels when sudden power steps occurred     

Review of wet environment types on Mars with focus on duration and volumetric issues

The astrobiological significance of certain environment types on Mars strongly depends on the temperature, duration, and chemistry of liquid water that was present there in the past. Recent works have focused on the identification of signs of ancient water on Mars, as it is more difficult to estimate the above-mentioned parameters. In this paper, two important factors are reviewed, the duration and the volume of water at different environment types on past and present Mars. Using currently available information, we can only roughly estimate these values, but as environment types show characteristic differences in this respect, it is worth comparing them and the result may have importance for research in astrobiology. Impact-induced and geothermal hydrothermal systems, lakes, and valley networks were in existence on Mars over the course of from 10(2) to 10(6) years, although they would have experienced substantially different temperature regimes. Ancient oceans, as well as water in outflow channels and gullies, and at the microscopic scale as interfacial water layers, would have had inherently different times of duration and overall volume: oceans may have endured from 10(4) to 10(6) years, while interfacial water would have had the smallest volume and residence time of liquid phase on Mars. Martian wet environments with longer residence times of liquid water are believed to have existed for that amount of time necessary for life to develop on Earth between the Late Heavy Bombardment and the age of the earliest fossil record. The results of this review show the necessity for more detailed analysis of conditions within geothermal heat-induced systems to reconstruct the conditions during weathering and mineral alteration, as well as to search for signs of reoccurring wet periods in ancient crater lakes.     

Carrier loop architectures for tracking weak GPS signals

The performance of various carrier recovery loop architectures (phase lock loop (PLL), Doppler-aided PLL, frequency lock loop (FLL), and Doppler-aided FLL) in tracking weak GPS signals are analyzed and experimentally validated. The effects of phase or frequency detector design, oscillator quality, coherent averaging time, and external Doppler aiding information on delaying loss of lock are quantified. It is shown that for PLLs the metric of total phase jitter is a reliable metric for assessing low C/N performance of the tracking loop provided the loop bandwidth is not too small (~> 5 Hz). For loop bandwidths that are not too small, total phase jitter accurately predicts carrier-to-noise ratio (C/N) at which loss of lock occurs. This predicted C/N is very close to the C/N predicted by bit error rate (BER). However, unlike BER, total phase jitter can be computed in real-time and an estimator for it is developed and experimentally validated. Total phase jitter is not a replacement for BER, since at low bandwidths it is less accurate than BER in that the receiver loses lock at a higher C/N than predicted by the estimator. Similarly, for FLLs operating at small loop bandwidths, it is found that normalized total frequency jitter is not a reliable metric for assessing loss of lock in weak signal or low C/N conditions. At small loop bandwidths, while total frequency jitter may indicate that a loop is still tracking, the Doppler estimates provided by the FLL will be biased.