The mobile exploration system project (MEX) at NASA Ames Research Center has been conducting studies into hybrid communication networks for future planetary missions. These networks consist of space-based communication assets connected to ground-based Internets and planetary surface-based mobile wireless networks. These hybrid mobile networks have been deployed in rugged field locations in the American desert and the Canadian arctic for support of science and simulation activities on at least six occasions. This work has been conducted over the past five years resulting in evolving architectural complexity, improved component characteristics and better analysis and test methods. A rich set of data and techniques have resulted from the development and field testing of the communication network during field expeditions such as the Haughton Mars project and NASA mobile agents project. This paper defines design, analysis and test methods for hybrid mobile communication networks, identifying the key issues and constraints that affect performance in both the radio frequency (RF) and network engineering disciplines. Previous work by the MEX team has addressed the architecture and detailed analysis of wireless networks including the results of field tests. We continue the analysis using a new 802.11b backbone utilizing two repeaters that significantly increase range and coverage but greatly increase latency, which reduces overall network throughput. The addition of a satellite link can result in significant additional throughput loss due to light-speed delays in the space segment interacting with variable latencies in the multi-hop wireless network. The paper analyzes and presents RF domain field test results combined with network performance metrics which describe a comprehensive approach for designing and optimizing future hybrid mobile networks.
The energy storage and attitude control subsystems of the typical satellite are presently distinct and separate. Energy storage is conventionally provided by batteries, either NiCd or NiH, and active attitude control is accomplished with control moment gyros (CMGs) or reaction wheels. An overall system mass savings can be realized if these two subsystems are combined using multiple flywheels for simultaneous kinetic energy storage and momentum transfer. Several authors have studied the control of the flywheels to accomplish this and have published simulation results showing the feasibility and performance. This paper presents the first experimental results showing combined energy storage and momentum control about a single axis using two flywheels.
Contemporary knowledge of the role of fire in the global environment is limited by inadequate measurements of the extent and impact of individual fires. Observations by operational polar-orbiting and geostationary satellites provide an indication of fire occurrence but are ill-suited for estimating the temperature, area, or radiant emissions of active wildland and agricultural fires. Simulations of synthetic remote sensing pixels comprised of observed high-resolution fire data together with ash or vegetation background demonstrate that fire properties including flame temperature, fractional area, and radiant-energy flux can best be estimated from concurrent radiance measurements at wavelengths near 1.6, 3.9, and 12 /spl mu/m. Successful observations at night may be made at scales to at least 1 km for the cluster of fire data simulated. During the daytime, uncertainty in the composition of the background and its reflection of solar radiation would limit successful observations to a scale of approximately 100 m or less. Measurements at three wavelengths in the long-wave infrared would be unaffected by reflected solar radiation and could be applied to separate flame properties in a binary system of flame and background. However, likely variation in the composition of the background and its temperature limit the approach to measurements that are of high resolution in relation to the scale of the flaming front. Alternative approaches using radiances at wavelengths near 4 and 12 /spl mu/m alone must fail absent a correction for the background, yet the correction is made imprecise by uncertainty in composition of the background where it comprises more than one-third of a pixel.
Communications for Telemedicine requires substantial bandwidths to provide timely transmission of large data sets. To provide rural America with modern Telemedicine requires very high bandwidth resources that may take decades to appear. Satellites provide the natural choice for communication between the rural primary care centers and the tertiary care hospital. However there are problems if TCP, as required for Internet access, is to be the protocol of choice. Analysis of multi-station satellite access clearly argues for distributed non-random methods and hence for appropriate handling of TCP data streams
A new methodology has been proposed to enhance inverse dynamics applications in the process of trajectory planning and optimization in terrain following flights (TFFs). The new approach uses a least square scheme to solve a general two-dimensional (2-D) TFF in a vertical plane. In the mathematical process, Chebyshev polynomials are used to model the geographical data of the terrain in a given route in a manner suitable for the aircraft at hand. The aircraft then follows the modeled terrain with sufficient clearance. In this approach the terrain following (TF) problem is effectively converted to an optimal tracking problem. Results show that this method provides a flexible approach to solve the TFF problem especially in conditions where the existing terrain to be flown over is not mathematically well behaved.
One of the key performance metrics for satellite constellations is the statistics of the visibility periods between the satellites and points on the ground. Associated with this are other desirable communications statistics such as data through-put, link qualities, etc. Typically, the computation of coverage statistics requires the propagation of the trajectories. For orbits with non-repeating ground tracks, this may require orbit propagation for tens of years per spacecraft. Lo (1994) proposed an approach using ergodic theory which replaced the need to compute the statistics from integrated trajectories by a definite integral over the circular region of the elevation mask of a point on the ground. The effects of J2 due to the non-spherical shape of the Earth are included in the definite integral. The definite integral can be implemented in Excel for quick trade studies. But the simple geometric methods used to derive the integral for circular orbits cannot be readily extended to elliptical orbits. In This work a new algorithm using differential geometry enables us to extend this theory to elliptical orbits.
One of the goals of early stage conceptual design is to execute broad trade studies of possible design concepts, evaluating them for their capability to meet minimum requirements, and choosing the one that best satisfies the goals of the project. To support trade space exploration, we have developed the advanced trade space visualizer (ATSV) that facilitates a design by shopping paradigm, which allows a decision-maker to form a preference a posteriori and use this preference to select a preferred satellite. Design automation has allowed us to implement this paradigm, since a large number of designs can be synthesized in a short period of time. The ATSV uses multidimensional visualization techniques, preference shading, and Pareto frontier display to visualize satellite trade spaces.
Temperature and radiation tolerant electronics, as well as long life survivability are the key capabilities required for future NASA missions. Current approaches to electronics for extreme environments focus on component level robustness and hardening. Compensation techniques such as bias cancellation circuitry have also been employed. However, current technology can only ensure very limited lifetime in extreme environments. This paper presents a novel approach, based on evolvable hardware technology, which allows adaptive in-situ circuit redesign/reconfiguration during operation in extreme environments. This technology complements material/device advancements and increases the mission capability to survive harsh environments. The approach is demonstrated on a mixed-signal programmable chip, which recovers functionality until 280/spl deg/C. We show in this paper the functionality recovery at high temperatures for a variety of circuits, including rectifiers, amplifiers and filters.
The transmission control protocol (TCP) is widely used by Internet connected computers for reliable data transmission, but it's performance over geo-stationary Earth orbit (GEO) satellites is degraded. GEO satellites can provide high-bandwidth data transmission resources to wide geographic areas and are not affected by Earth-based natural disasters that render terrestrial channels unusable. In addition, GEO satellites can provide reliable and fast data transmission when terrestrial communication channel and systems damage is high. Therefore, because of the advantages in using GEO satellites for data transmission, the "TCP over GEO satellite" performance problem must be resolved. This paper describes the multiple segment transmission with majority decoding transport layer protocol, which is a novel transport layer protocol that redundantly transmits segments to provide reliable data transmission. It is shown that variants of this transport layer protocol transmit large data sets over GEO satellites faster than TCP Reno.
TEMPEST is a planner for long-range planetary navigation that bridges the gap between path planning and classical planning and scheduling. In addition to planning routes, our approach yields the timing and placement of actions to conserve and restore expendable resources and that abide by operational constraints. TEMPEST calls upon the incremental search engine (ISE) to enable heuristic path planning and efficient re-planning under global constraints, over a four dimensional state space. We describe our approach, then demonstrate how the planner operates in a simulated Mars science traverse. Following a brief summary of TEMPEST results from a recent rover field experiment, we evaluate our research progress and describe our current and future work.
Actuators are complex electro-hydraulic or mechanical mechanisms utilized in aircraft to drive flight control surfaces, landing gear, cargo doors, and weapon systems. Impact has developed a prognostic and health management (PHM) methodology for these critical systems that includes signal processing and neural network tracking techniques, along with automated reasoning, classification, knowledge fusion, and probabilistic failure mode progression algorithms. The processing utilizes the command/response signal and hydraulic pressure data from the actuators and provides a real-time assessment of the current/future actuator health state. This methodology was applied to F/A-18 stabilator electro-hydraulic servo valves (EHSVs) using test stand data provided by Boeing Phantom works. The automated module demonstrated excellent health state classification results. The prognosis was also successfully performed however no data was available to validate the prediction. These algorithms were developed with consideration to sensor/processing limitations for potential onboard implementation. Many of the PHM elements presented here could also be adapted for other actuator types and applications.
Future planetary exploration missions can require rovers to perform difficult tasks in rough terrain, with limited human supervision. Knowledge of terrain physical characteristics would allow a rover to adapt its control and planning strategies to maximize its effectiveness. This paper describes recent and current work at MIT in the area of onboard terrain estimation and sensing utilizing visual, tactile, and vibrational feedback. A vision-based method for measuring wheel sinkage is described. A tactile method for on-line terrain parameter estimation is also presented. Finally, a method for terrain classification based on analysis of vibration in the rover suspension is described. It is shown through simulation and experimental results that these methods can lead to accurate and efficient understanding of a rover's physical surroundings.
Recent developments in mobile router technology include the ability to prioritize selection of the home agent by the mobile unit. This technology was originally developed for route optimization. However, the technology also can be applied to autonomous catastrophic recovery, and robust redundant network control centers. This paper describes a variety of architecture scenarios that can benefit from prioritized home agents including: homeland security, virtual mission operations, mobile command centers and route optimization for aeronautical applications. A demonstration testbed was presented where this technology was proven in the field. In addition, a virtual mission operation center demonstration currently being deployed was described.
Significant technology advances have enabled planetary aircraft to be considered as viable science platforms. Such systems fill a unique planetary science measurement gap, that of regional-scale, near-surface observation, while providing a fresh perspective for potential discovery. Recent efforts have produced mature mission and flight system concepts, ready for flight project implementation. This work summarizes the development of a Mars airplane mission architecture that balances science, implementation risk and cost. Airplane mission performance, flight system design and technology readiness are described.
A mobile ad hoc network is a collection of wireless mobile nodes, dynamically forming a temporary network without the use of any existing network infrastructure or centralized administration. It is an emerging technology for civilian and military applications. However, security in mobile ad hoc networks is hard to achieve due to the vulnerability of the links, the limited physical protection of the nodes, and the absence of a certification authority or centralized management point. Similar to other distributed systems, security in mobile ad hoc networks usually relies on the use of different key management mechanisms. We exploit characteristics of an ad hoc network and present our authentication service to protect network security in the presence of dishonest users. Nodes originally trustable in the network may become malicious due to sudden attacks, so an adequate security support for authentication to deal with dishonest users who issue false public key certificates is crucial. We describe a new authentication service with a well-defined network model and a trust model. These models allow nodes in the network to monitor and rate each other with an authentication metric. We also propose a novel public key certificate operation, incorporating with a trust value update algorithm in public key authentication. The authentication service we propose is able to discover and isolate dishonest users in the network. Finally, we evaluate the proposed solution through simulation to demonstrate the effectiveness of the scheme.
The Autonomous Sciencecraft Experiment (ASE) operates onboard the Earth Orbiter 1 mission in 2004. The ASE software uses onboard continuous planning, robust task and goal-based execution, and onboard machine learning and pattern recognition to radically increase science return by enabling intelligent downlink selection and autonomous retargeting. In This work we discuss how these AI technologies are synergistically integrated in multi-layer control architecture to enable a virtual spacecraft science agent. We also present the preliminary results from flight validation of this experiment. This software demonstrates the potential for space missions to use onboard decision-making to detect, analyze, and respond to science events, and to downlink only the highest value science data. As a result, ground-based mission planning and analysis functions were simplified, thus reducing operations cost.
The technology of temperature measurement appears to some to be a mature field. However, to many, requirements for improved performance and reliability are a driver for continual scientific and technology advancement. Although Johnson noise has been proposed as a thermometry method for several decades, it is only recently that digital and analog electronics have made it possible to economically fabricate measurement systems based on Johnson noise. Johnson noise, which is a result of fundamental physics, is caused by the random thermal motions of electrons in all conductors. Its fundamental nature allows us to construct temperature measurement systems that do not require periodic calibration. Thus long, unattended operating intervals are feasible. Several unique implementations of Johnson noise thermometry (JNT) are possible. One permits temperature measurement without contacting the measured surface nductive JNT. Another implementation measures the Johnson noise of a resistance element in contact with the measured surface - conductive JNT. The resistive element in conductive JNT can be an RTD. Apparatus have been recently fabricated demonstrating the practicality of both JNT implementations. A demonstration of conductive JNT is planned at a nuclear facility within two years. We present new hardware implementations that allow real-time calibration of the signals that have the potential of allowing a fully-integrated, physically small and robust system to be achieved.
The harsh radiation environment of space, the propensity for SEUs to perturb the operations of silicon-based electronics, the rapid development of microprocessor capabilities and hence software applications, and the high cost (dollars and time) to develop and prove a system, require flexible, reliable, low cost, rapidly developed system solutions. A reconfigurable triple-modular-redundant (TMR) system-on-a-chip (SOC) utilizing field-programmable gate arrays (FPGAs) provides a practical solution for space-based systems. The configurable fault-tolerant processor (CFTP) is such a system, designed specifically for the purpose of testing and evaluating, on orbit, both the reliability of instantiated TMR soft-core microprocessors, the ability to reconfigure the system to support any onboard processor function, and the means for detecting and correcting SEU-induced configuration faults. The CFTP utilizes commercial off-the-shelf (COTS) technology to investigate a low-cost, flexible alternative to processor hardware architecture, with a total-ionizing-dose (TID) tolerant FPGA as the basis for a SOC. The flexibility of a configurable processor, based on FPGA technology, enables on-orbit upgrades, reconfigurations, and modifications to the soft-core architecture in order to support dynamic mission requirements. Single event upsets (SEU) to the data stored in the FPGA-based soft-core processors are detected and corrected by the TMR architecture. SEUs affecting the FPGA configuration itself are corrected by background "scrubbing" of the configuration. The CFTP payload consists of a printed circuit board (PCB) of 5.3 inches/spl times/7.3 inches utilizing a slightly modified PC/104 bus interface. The initial FPGA configuration is an instantiation of a TMR processor, with included error detection and correction (EDAC) and memory controller circuitry. The PCB is designed with requisite supporting circuitry including a configuration controller FPGA, SDRAM, and flash memory in order to allow the greatest variety of possible configurations. The CFTP is currently manifested as a space test program (STP) experimental payload on the Naval Postgraduate School's NPSAT1 and the United States Naval Academy's MidSTAR-1 satellites, which was launched into low earth orbit in March 2003.
Impact technologies have developed a robust modeling paradigm for actuator fault detection and failure prediction. This model-based approach to prognostics and health management (PHM) applies physical modeling and advanced parametric identification techniques, along with fault detection and failure prediction algorithms, in order to predict the time-to-failure for each of the critical, competitive failure modes within the system. Advanced probabilistic fusion strategies are also leveraged to combine both collaborative and competitive sources of evidence, thus producing more reliable health state information. These algorithms operate only on flight control command/response data. This approach for condition-based maintenance provides reliable early detection of developing faults. As an advantage over 'black-box' health-monitoring schemes, faults and failure modes are traced back to physically meaningful system parameters, providing the maintainer with invaluable diagnostic and prognostic information. The developed model-based reasoner was validated and demonstrated on an electromechanical actuator (EMA) provided by Moog, Inc.
Presented herein is the sequentially controlled distributed solar-array power system with maximum power tracking (MPT). The power system controller sequentially activates sufficient solar-array sourced dc-dc converter power-processing channels, of which paralleled-outputs supply power to a common load. A "shared-bus" current-sharing method utilizing distributed MPT control is employed to regulate parallel-connected current-mode converters dedicated to each power channel. Among the activated solar-array power channels, the most recently activated channel provides output bus voltage regulation while the previously activated channels are operated in MPT mode. The remaining inactive power channels consume negligible power resulting in lower component temperatures and improved overall long-term system reliability. Through modeling and simulation of a three-channel MPT power system, the sequentially controlled architecture and control concept are validated.
