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The development of the Galileo System continues under the partnership of the European Commission and the European Space Agency. ESA is charged with the development of the elements of the "Global Component," which will be comprised of the complete constellation of satellites together with the ground segment. The ESA development will culminate in the establishment of an In Orbit Validation (IOV) milestone, which will contain a minimum number of each element of the Full Operational Configuration (FOC). An overview of the Galileo services and requirements is provided. We briefly outline the architecture of the Galileo Global Component. We also introduce the overall development approach and risk mitigation activities such as GSTB-V1/V2 and GIOVE-A/B/M. The interoperability aspects (i.e. GGTO) are highlighted as well. The present master working schedule is presented., Presented at the Annual Precise Time and Time Interval (PTTI) Meeting (39th) held in Long Beach, CA on 26-29 Nov 2007. Published in the Proceedings of the Annual Precise Time and Time Interval (PTTI) Meeting (39th), p5-18, 2007. The original document contains color images.

While the United States has enjoyed an historic advantage in space and the U.S. Air Force (USAF) has enjoyed the luxury of operating relatively unimpeded in this medium, the nation has become vulnerable to threats that could damage or disable its vital satellite constellations. This paper examines the threat to satellites posed by ground-based directed energy weapons and the state of satellite-related nanotechnology research and development (R&D) to demonstrate the applicability for mitigating this type of threat. The paper argues that USAF leaders need to make continued R&D into nanotechnology for satellite applications an investment priority. This will require a long and expensive commitment, only some of which will pay off, but it is necessary if the USAF and the nation are to maintain space supremacy. The paper begins by describing the threat to U.S. satellites posed by directed energy weapons. The paper specifically examines both lasers and high-powered microwaves, describing what they are and how they may threaten satellites. It then discusses what countries currently possess these capabilities and what capability adversaries might possess in the 2025 timeframe. From these discussions, the paper draws upon common satellite orbits to show the role orbits play in satellite vulnerability. It also looks at common satellite subsystems and explores how ground-based directed energy weapons could damage these systems. Several definitions for nanotechnology are explored. The paper explains how nanomaterials are different from materials at the micro- or macro-scale, and details the different properties nanomaterials possess. The paper then explains how the work from nanotechnology R&D projects can be applied to satellite subsystems, for example, surface coatings that could be used to improve satellite thermal control and electrical conductivity, and nanomaterials to improve the radiation hardness of satellite microprocessors and other structures., Blue Horizons Paper.

Satellite constellations around Earth can be used for observing and/or communicating with targets on the surface. This research mainly addressed maneuvering existing satellite constellations in order to improve coverage of multiple targets over a timespan of 30 to 120 days. However, designing new satellite constellations can also be addressed by using a portion of this research regarding coverage estimation. This research identified a direct relationship between a satellite's orbital geometry and the coverage provided by that satellite. This is accomplished by (1) identifying the view of the satellite orbit from an inertial sphere centered on the Earth, and (2) utilizing information from all the orbital views across the target's inertial latitude on order to arrive at lower and upper bounds on coverage. Altering a satellite orbit also alters the coverage that it provides. Gauss' variational equations were used to find maneuvering strategies that effect maximal changes in orbital geometry. These distinct maneuvering strategies were then complied into a list that will be used in the subsequent optimization.

A 1996 paper estimated "that the total number of space-borne atomic frequency standards (AFS) is no more than several hundred" [1, p. 24]. The number of atomic frequency standards in space was dominated by the GPS, GLONASS, and Milstar satellite constellations. In this paper, we present an update to the historical review of the AFSs used in space systems. We will briefly review the systems existing in 1996, and add the newer systems (Galileo, GPS IIR and IIF, GLONASS-M, Cassini-Huygens, AEHF, and QZSS) and potential space systems (PARCS, PHARAO, Beidou) using atomic frequency standards. We have estimated that there have been a total of 452 atomic frequency standards launched into orbit for use on communications and scientific payloads. We conclude the paper with a review of the 1996 predictions for future AFSs and discuss the future as seen in 2006., See also ADM002029. Presented at the Annual Precise Time and Time Interval Systems and Applications Meeting (38th) held in Reston, VA on 5-7 Dec 2006. Published in the Proceedings of the Annual Precise Time and Time Interval Systems and Applications Meeting (38th), p69-80, 2006. The original document contains color images.

The Transformational Communications Office's (TCO) 17 December 2003 report states, "The current SATCOM and data relay systems are unable to meet future bandwidth demands. They lack capacity, in both aggregate data rate and the number of users they can support.... Furthermore, the life expectancies of the existing space segments and much of their associated terminal and management segments do not extend beyond the 2010-2015 time frame." These shortfalls and the military's insatiable demand for bandwidth led to the creation of the Department of Defense's (DOD) TSAT. TSAT's five-satellite constellation will be capable of delivering advanced capabilities to the war fighter via 8,000 radio frequency (RF) links and between 20 and 50 laser communication (lasercom) links. These advanced capabilities will deliver significant communications bandwidth by incorporating advanced laser and RF technologies, software-configurable terminals, packet switching, network management, and interface standards. All of these technologies will rely on Internet protocol (IP) interoperability as the enabling technology for connecting the war fighter. The thesis of this paper is that the advanced capabilities provided by TSAT are limited and will not be sufficient to serve the ground-based portion of the communications network supporting network-centric warfare (NCW). To validate this proposition, this study will start by identifying space-based systems that will enable NCW, discuss the requirements for ground-based NCW, and finally determine the combination of spaced-based systems sufficient to deliver advanced capabilities to the war fighter., Wright Flyer Paper no. 27.

This thesis produces models of satellite constellations using finite state automata (FSA) or finite automata (FA) and optimizes the sequence of targets for two missions. Two simplified FSA models of satellite constellations with one ground control station (GCS) are developed. The first model is of a single spacecraft and the second includes two spacecraft. Based upon the language states and state transitions of each model the author transforms the FA into a network and enumerates the shortest paths for indicative lists of meta-tasks from each model. The first model is provisionally implemented in MATLAB. The author finds two separate optimal target selection sequences for randomly generated sample target sets using commercial off-the-shelf optimization software. Although stochastically fabricated the sample target sets reflect valid scenarios for a satellite imagery mission. The first sequence a traveling salesman problem minimizes the time required for processing all targets given a multiple orbit mission. For a representative sample target set this is 2.34 orbits. The second sequence a prize collecting traveling salesman problem maximizes the number of targets processed given a dual orbit mission. For the same sample target set two orbits permit the processing of seven targets., The original document contains color images.

The use of satellites as a tactical asset to support theater operations is a desired capability for future space operations. Unlike traditional satellite systems designed to provide coverage over the entire globe or large regions, tactical satellites would provide coverage over a small region which can be modeled as a single ground point defined by a latitude and longitude. In order to provide sufficient utility as a theater asset, a satellite should be placed in an orbit that provides a maximum amount of coverage of the target ground point. This study examined the optimization of orbit parameters to maximize the number of passes made over a target. An optimization algorithm was developed to maximize the number of passes made while also minimizing the distance from the satellite to the target. Single satellite coverage properties as well as two and three satellite constellations were analyzed., The original document contains color images.

This thesis develops optimization models for prioritizing payloads for inclusion on satellite buses with volume, power, weight and budget constraints. The first model considers a single satellite launch for which the budget is uncertain and constellation requirements are not considered. Subsequently, we include constellation requirements and provide a more enhanced model. Both single-launch models provide a prioritized list of payloads to include on the launch before the budget is realized. The single-launch models are subsequently extended to a sequence of multiple launches in two cases, both of which incorporate an explicit dependence on the constellation composition at each launch epoch. The first case ignores future launches and solves a series of independent single-launch problems. The second case considers all launches simultaneously. The optimization models for single- and multiple-launch cases are evaluated through a computational study. It was found that, when the budget distribution is skewed, the prioritization model outperforms a greedy payload selection heuristic in the single-launch model. For the multiple-launch models, it was found that the consideration of future launches can significantly improve the objective function values., The original document contains color images.

We propose a definition for responsive space that encompasses the speed with which a space system - broadly defined - can be made to react to various forms of uncertainty, ranging from geopolitical operational requirements to technical failures to fluctuations in the acquisition funding stream. We note that the ever-larger monolithic spacecraft of today are notoriously unresponsive. We then suggest a novel architectural paradigm, which we call fractionated spacecraft, whereby a satellite is decomposed into a set of similar or dissimilar component modules which interact wirelessly while in cluster orbits. A perfunctory survey of enabling technologies and an overview of the objectives of the forthcoming System F6 initiative at DARPA is provided. Given our more formal definition of responsiveness and this new proposed architecture, we conclude that spacecraft fractionation results in responsiveness across all possible scales of spacecraft size, including very large spacecraft systems that are only made possible by fractionation., Prepared in collaboration with Booze Allen Hamilton. Presented at the Responsive Space Conference (4th), Los Angles, CA on 24-27 APR 2006 and published as AIAA-RS4-2006-1002.

This paper intends to provide a description of the LAREDO tool including architecture, design choices and mathematical formulations. The goal of LAREDO, which is a six DOF simulator developed under Matlab/Simulink-based environment, is to reproduce the Rendezvous and Docking (RvD) sequences applicable to different exploration mission-scenarios and to provide detailed analysis of formation flyings (FF) satellite constellations using highly coupled GNC algorithms, which integrate attitude, navigation and maneuvers. Among those, it is possible to select: Earth, Moon, Mars, Venus, Deimos, Triton and Europa as final destination. Within the LAREDO simulator a launching optimization module is implemented to optimize the launching phase once it has lifted off from the Earth, Moon or Mars surfaces. This optimization is performed such that maximum payload is delivered into orbit. The tool simulates the rocket stages and also evaluates the launch opportunities for optimum launch. The orbit control can be performed by impulsive or continuous maneuvers both on circular and elliptical orbits. The type of maneuvers, their order and schedule is completely configurable by the users. The maneuvers are very accurate even under perturbations. In case the identified safety limits are trespassed, a collision avoidance maneuver (CAM) will be automatically activated, moving the spacecraft to a safety region. The simulator propagates considering various orbit perturbations (aerodynamic, non-sphericity, solar radiation and third body perturbations), communications (both with ground stations and between satellites), and provides full analysis of the trajectory. The GNC modules incorporate sensor models for LIDAR, camera, IMU and RF devices, and both orbit and attitude guidance and control. Furthermore, LAREDO offers powerful 3-D visualization of trajectories and attitude, as well as other important features such as consumed V, ground station contacts, eclipse times, and others., Presented at the Meeting on Integration of Space-Based Assets within Full Spectrum Operations (Integration des moyens spatiaux dans toute la gamme des operations), held in Colorado Springs, CO, on 10-12 Oct 2005. Published in the Proceedings of the Meeting, p11-1 through 11-16, 2005. Prepared in cooperation with GMV Space Systems, Rockville, MD. Document includes briefing charts in addition to text. See also ADM202138, RTO-MP-SCI-150. The original document contains color images.

This thesis proposes and analyzes a new measure of functional capability for satellite constellations that incorporates the instantaneous availability and mission effectiveness of individual satellites. The capability measure yields a continuous score between zero and one accounting for the degree to which the constellation meets operational requirements. The measure is computed from an average of satellite capabilities, composed of the product of the satellite's instantaneous availability and value score. Instantaneous availability is acquired by modeling the satellite degradation status as either a time-homogenous, continuous-time Markov chain (CTMC) if it possesses functions with exponential lifetime distributions, or as a time-homogenous, semi-Markov process (SMP) if the function lifetime distributions are not exponential. The satellite value score represents the individual satellite's contribution to the overall constellation mission and is obtained using multi-attribute value theory. For illustrative purposes, analytical results were compared with those obtained via the Monte Carlo method and were found to be indistinguishable with substantially less computational effort.

This thesis develops a methodology to determine an optimal policy for maintaining a satellite constellation that degrades over time. Previous work has developed a methodology to compute an optimal replacement policy for a satellite constellation in which satellites were viewed as binary entities, either operational or failed. This research extends the previous models by developing an optimal maintenance policy for satellite constellations in which each satellite may operate in a finite number of degraded states. The constellation is assumed to consist of a finite number of satellites, each with a finite number of functions with distinct failure mechanisms. Assuming each function lifetime is exponentially distributed, the stochastic degradation process is modelled as a discrete-time Markov chain. The degradation process is subsequently used to formulate an optimization problem as a finite planning horizon Markov decision process in which the total expected loss of utility is minimized. The maintenance actions considered include on-orbit repairs and satellite replacements, each of which have an associated level of risk. Numerical examples are presented to illustrate the model, and a parametric sensitivity analysis is performed using notional data to determine conditions for which the resulting policy consists of only replacements, only on-orbit repairs, or mixtures of replacements and on-orbit repairs.

The use of Satellite Communications (SATCOM) has become essential to operations in both Afghanistan and Iraq. In particular, the Iridium satellite constellation has demonstrated its usefulness and flexibility. It has had significant impact on how operations are conducted. Iridium provides users both voice and data services. There are two approaches to sending data over the Iridium network: a circuit-switched data service and message-switched data service. Use of the circuit-switched data service requires the overhead of a setup in the same manner as a voice call. There are two message-switched data services: Short Burst Data (SBD) and Short Message Service (SMS). Our testing involves the SBD message-switched data service, which is optimized for high capacity and efficiency when sending small amounts of data through the Iridium network. While there are many users for these services, little information can be found about their actual performance in fielded systems. The best sources of information are specifications or modeling of the performance of these services. We perceived that understanding the underlying network performance was needed, and we established experiments to capture that performance. We present results from both circuit-switched data service and message-switched SBD data service. We also address insights into the use of these services., Presented at the International Command and Control Research and Technology Symposium (ICCRTS) (10th), held in McLean, VA, on 13-16 Jun 2005. Published in the Proceedings of ICCRTS, Paper #233, 2005. Prepared in collaboration with ISDI Technologies, Inc., Honolulu, HI. Includes briefing charts in addition to text. The original document contains color images.

Despite its vital importance to the welfare and security of the nation, the Global Positioning System (GPS) constellation is populated with numerous satellites operating well beyond their design life. These aging satellites are more likely to malfunction, which can reduce service coverage, degrade accuracy, and in some cases, transmit dangerously inaccurate data. The Air Force currently subscribes to a launch to sustain (LTS) GPS satellite replenishment strategy. Under this strategy, new satellites are launched only after a satellite failure or just prior to a failure. The purpose of this monograph is to investigate whether the Air Force should forgo its LTS replenishment strategy in favor of a more aggressive launch to augment (LTA) strategy in order to proactively replace high-risk satellites and to accelerate modernization timelines. It will be shown that the explosive growth of GPS over the past fifteen years has outpaced the Air Force's strategy on satellite replenishment. The growing importance of GPS must be matched with a progressive replenishment strategy that sustains its reliability and improves its utility for military, civil, and commercial users. The LTS strategy has placed a disproportionate emphasis on operational efficiency at the expense of operational effectiveness and has impaired the timely insertion of critical new capabilities into the constellation. A LTA strategy would help to reverse this trend., The original document contains color images.

Sparse military satellite constellations were designed using two methods: a traditional approach and a genetic algorithm. One of the traditional constellation designs was the Discoverer II space based radar. Discoverer II was an 8 plane, 24 satellite, Low Earth Orbit (LEO), Walker constellation designed to provide high-range resolution ground moving target indication (HRR-GMTI), synthetic aperture radar (SAR) imaging and high resolution digital terrain mapping. The traditional method designed 9-ball, 12-ball, 18-ball, and 24-ball Walker constellations. The genetic algorithm created constellations by deriving a phenotype from a triploid genotype encoding of orbital elements. The performance of both design methods were compared using a computer simulation. The fitness of each constellation was calculated using maximum gap time, maximum revisit time, and percent coverage. The goal was to determine if one design method would consistently outperform the other. The genetic algorithm offered a fitness improvement over traditional constellation design methods in all cases except the 24-ball constellation where it demonstrated comparable results. The genetic algorithm improvement over the traditional constellations increased as the number of satellites per constellation decreased. A derived equation related revisit time to the number of ship tracks maintained., The original document contains color images.

A new class of lowcost midwave infrared (MWIR) Earth observation (EO) data will become available with the flight of miniature MWIR EO instruments in satellite constellations. Due to the frequent ground repeat times inherent in constellations, this data set would provide a unique alternative for those wishing to analyse trends or rapidly detect anomalous changes in the MWIR characteristics of the earth's surface (e.g fire detection) or atmosphere. To date, the MWIR imagers have been based on highly responsive cooled detector technology, which traditionally has been the only real option for collecting useful data in this waveband from space. However, state-of-the art microbolometers, adapted from their original design for operation in the LWIR, are thought to be a potential alternative for low-cost MWIR constellations. Following the laboratory evaluation of a modified microbolometer arrays in the MWIR, a low-mass instrument concept was designed and evaluated for a variety of candidate MWlR mission areas. If implemented, the imager concept would complement a larger imaging suite (visible, near IR, and long-wave IR) on a sub-100kg Surrey Space Technology Ltd. (SSTL) micro-satellite and open up several new potential mission areas for the SSTL-engineered Disaster Monitoring Constellation., Prepared in cooperation with ULIS Uncooled Infrared Detectors, Voroize, France

To protect Australia's economic concerns and its coastline from attack there is a need for surveillance of a large area of Australia's Sea Air Gap (SAG). Satellites have the advantage of viewing large areas of the earth regularly. But currently there is no indigenous facility to launch satellites dedicated to the surveillance of Australia. With the advent of new MEMS systems, small nano/pico satellites are being built and launched at a fraction of the cost of conventional satellites. This has allowed for the invention and investigation of new concepts for satellite missions. For example, the idea of collaborating clusters of micro-satellites has been used to control arrays of satellites where the functionality is distributed across a group of satellites. Missions such as MIT SPHERES formation flying testbed and the Stanford ORION program are showing the benefits of distributed satellite systems. These benefits include increased survivability, reduced development costs, easier maintenance, and improved revisit times and resolution. Auspace (Tactical Satellites Study: Interim Report RTP-TACSAT-0001-AUS 17-April 2003) is investigating the feasibility of the design of small satellites for short-term missions to be launched into low orbit on demand. This report investigates the use of linear programming to optimize the performance of small satellite constellations where the constellation design for a particular mission is known. The purpose of the proposed constellations is to observe the SAG, and the performance of the constellation is measured in terms of percentage coverage of the SAG. Work done by Auspace Ltd. that demonstrated the use of three constellations to cover the SAC was reproduced using Satellite Tool Kit (STK). The results demonstrate how careful scheduling of assets can lower the size of a constellation designed for surveillance and hence the cost of building and launching such a constellation. (7 tables, 9 figures, 9 refs.)

The key to integrating surveillance operations and architectures is the allocation and tasking of multiple surveillance assets for the purpose of collectively satisfying specified surveillance information requirements subject to time, space, and resource constraints. A generic framework (GAMBIT) for addressing this issue was developed by P.E. Berry & D.A.B. Fogg (DSTO Research Report in draft 2003). The framework assumes that the supporting technologies in terms of communications bandwidth and processing power are in place. The issues associated with the maximal exploitation of space-based surveillance assets are unique due to the nature of the platforms, their sensors, and the communications links to the ground station for target information processing and sensor tasking. This report provides optimal solutions to the problem of dynamically determining the allocation and control of space-based surveillance resources for the purpose of detecting, locating, and tracking surface maritime targets using the GAMBIT formalism. The sensors' performances were modelled in terms of their probabilities of detection and false alarm. Three types of target motion model were considered: deterministic, Gauss-Markov, and conservative. Multiple targets were allowed. The solution progressed by cyclical target motion prediction and Bayesian update. Significant computational efficiency was achieved by use of a localized formulation of the problem. To illustrate the implementation of this process, the use of SAR satellites with several modes allowing different swaths and resolutions was considered. Optimal selection of swaths for target tracking, by choosing the swath for each access that minimizes the expected entropy (MOE) across the region of interest, was demonstrated. Three other MOE were used to select the swath: random, maximum probability, and maximum sum of probability. All were shown to be better than fixed swath or random swath selection. (3 tables, 12 figures, 10 refs.)

This Military Standard Order (MSO) prescribes the minimum performance standards that airborne area navigation sensors using the Global Positioning System (GPS) / Precise Positioning Service (PPS) must meet to be identified with the applicable MSO marking. The similarity of this MSO with Technical Standard Order (TSO) C-145, Airborne Navigation Sensors using the Global Positioning System (GPS) augmented by the Wide Area Augmentation System (WAAS), is intentional.

The Autonomous Sciencecraft Experiment flight demonstration (ASE) will fly onboard the US Air Force's TechSat-21 constellation, an unclassified mission scheduled for launch in 2004. ASE will use onboard science analysis, replanning, robust execution, and formation flying to radically increase science return by enabling intelligent downlink selection and autonomous retargeting. Demonstration of these capabilities in a flight environment will open up tremendous new opportunities in planetary science, space physics, and earth science that would be unreachable without this technology., See also ADM001519. Presented at the RTO AVT Course on "Intelligent Systems for Aeronautics" held in Rhode-Saint-Genese, Belgium on 13-17 May 2002. Published in RTO-EN-022, p7-1-7-10, 2002. Sponsored in part by NASA. The original document contains color images.

Earth monitoring systems of the future may include large numbers of inexpensive small satellites, tasked in a coordinated fashion to observe both long term and transient targets. For best performance, a tool which helps operators optimally assign targets to satellites will be required. We present the design of algorithms developed for real-time optimized autonomous planning of large numbers of small single-sensor Earth observation satellites. The algorithms will reduce requirements on the human operators of such a system of satellites, ensure good utilization of system resources, and provide the capability to dynamically respond to temporal terrestrial phenomena. Our initial real-time system model consists of approximately 100 satellites and large number of points of interest on Earth (e.g., hurricanes, volcanoes, and forest fires) with the objective to maximize the total science value of observations over time. Several options for calculating the science value of observations include the following: (1) total observation time, (2) number of observations, and the (3) quality (a function of e.g., sensor type, range, slant angle) of the observations. An integrated approach using integer programming, optimization and astrodynamics is used to calculate optimized observation and sensor tasking plans., See also ADM001519. Presented at the RTO AVT Course on "Intelligent Systems for Aeronautics" held in Rhode-Saint-Genese, Belgium on 13-17 May 2002. Published in RTO-EN-022, Paper 3, p3-1-3-12, 2003. The original document contains color images.

In 1973, the U.S. Department of Defense invested $12 billion to develop a space-based navigation system known as the Global Positioning System (GPS). GPS provides free navigation and timing signals to worldwide users. Although GPS was originally developed as a military system, during the last decade the GPS industry has exploded with new applications in transportation, mapping, and time synchronization, thus becoming the newest "global utility." GPS is the dominant player in the Global Navigation Satellite System market, which is expected to reach $50 billion by 2010. Driven by the success of GPS and the huge satellite navigation market potential, the European Union is planning on developing its own satellite system, called Galileo. Galileo will be under civil control and its performance will be similar to a modernized GPS, however, Galileo users will have to pay a fee for its premium service. The deployment of Galileo raises several military and commercial concerns that could affect U.S. national interests. Before Galileo becomes operational, United States representatives and their European counterparts must resolve many issues such as signal allocation, protection from unauthorized use of the signal, regulatory restrictions, and interoperability of the two systems., The original document contains color images.

Three Corner Sat (3CS) is a part of the University Nanosatellite Program sponsored by AFOSR, DARPA and NASA GSFC. A joint collaboration among Arizona State University (ASU), the University of Colorado at Boulder (CU), and New Mexico State University (NMSU), the project demonstrates the feasibility of using nanosatellite technology for useful scientific endeavors while minimizing manufacturing and launch costs. 3CS includes a stack of three nearly identical satellites that will be deployed from the Multi-Satellite Deployment System (MSDS), designed by AFRL, after ejection from the Shuttle Hitchhiker Experiment Launch System ?SHELS) in the NASA Space Shuttle. While the project has not been manifested on a particular shuttle launch, late 2003 is the anticipated launch date. Primary mission objectives include virtual formation flying, imaging, and end-to-end command and data handling. Secondary mission objectives include demonstrating MEMS micropropulsiontechnology, modular spacecraft bus design, and student education. This final report is concerned with ASU's responsibilities and designated tasks required to successfully complete the team mission: Program Management of the project at the university level; Configuration Management and Safety; Structures, Mechanisms, Thermal and Radiation; Electrical Power System; Ground Support Equipment; MEMS Micropropulsion Experiment; Integration and Test; and SisterSat.

Analysis of the performance of all on-orbit Navstar space vehicle clocks and Global Positioning System (GPS) monitor station reference clocks is performed by the Naval Research Laboratory (NRL), in cooperation with the GPS Master Control Station, under the sponsorship of the GPS Joint Program Office (JPO). The measurements used in the analysis are collected by multi-channel GPS receivers located at the Air Force and National Imagery and Mapping Agency (NIMA) monitor stations. The offset of each Navstar clock is computed with respect to the Department of Defense Master Clock. The resultant Navstar clock offsets are then used to compute frequency offset, drift offset, frequency stability profiles, and frequency stability histories. The performance of selected cesium and rubidium atomic clocks will be presented. Frequency stability results are calculated using sample times that vary from 15 minutes to several days. The operational Navstar timing signals will be ranked (according to frequency stability) for sample times of 6 hours and 1 day. In addition, the Block IIR clocks will be compared to results from an ongoing NRL laboratory life-test on two flight-qualified Block IIR rubidium atomic frequency standards. Observed clock behavior and anomalies will be discussed., See also ADM001482. Presented at the Annual Precise Time and Time Interval (PTTI) Systems and Applications Meeting (33rd) held in Long Beach, CA on 27-29 Nov 2001. Published in the Proceedings of the Annual Precise Time and Time Interval (PTTI) Systems and Applications Meeting (33rd), p469-480, 2001. The original document contains color images.

Galileo Avionica S.p.A. (GA), Italy, is industrializing under the European Space Agency supervision a passive hydrogen maser (PHM), to be used in the GalileoSat constellation as master onboard clock. The Engineering Model (EM) phase is being completed under the lead of the Observatory of Neuchatel (ON, Switzerland) with the cooperation of GA and Temex Neuchatel Time (TNT, Switzerland). Current activity regards integration at ON of the EM electronics with the EM physics package. Tuning is in progress and preliminary results are good. Test completion is planned by the end of this year. The parallel industrialization activity, with GA leading the industrial consortium of the above-mentioned companies, is aimed at PHM design consolidation for future flight production. While GA produces the electronics and integrates the PHM, TNT with ON support produces the physics package. In this frame the EM design is being thoroughly reviewed in the scope of performance optimization and cost reduction, while keeping the critical performances unchanged., See also ADM001507. Presented at the Annual Precise Time and Time Interval (PTTI) Planning Meeting (34th), held in Reston, VA, on 3-5 December 2002. Published in the proceedings of the meeting, p161-170, 2002. Prepared in cooperation with Observatory of Neuchatel, Switzerland, and Temex Neuchatel Time, Switzerland. The original document contains color images.

The MITIAFRL Distributed Satellite Systems program examines the motivation, analysis and development of technology associated with the distribution of assets and functionality over a number of cooperating satellites. A complete framework (GINA) for the analysis and comparison of widely varying architectures has been developed and applied to various test cases both existing and in development. Many technologies have been studied, including space based interferometric radar and imaging, formation flying dynamics and control of clusters, micropropulsion, launch vehicle selection, software development and mechanisms for end of life disposal. Two test beds have been developed for the purpose of validating the theories and exercising autonomy and control. This paper summarizes the findings of this research., The original document contains color images. All DTIC reproductions will be in black and white.

IDCSP Constellation Provides Unique Challenge: a Family of Known US Spacecraft; Distinctive Orbital Characteristics; at the Limit of Current Capabilities. Recovery Will Provide Measure of New Sensor Performance., See Also ADM001334, Presented at the 2001 Space Control Conference (19th Annual) held in Lincoln Laboratory, Hanscom AFB, MA on 3-5 April 2001. Prepared in collaboration with Boeing Co., Seattle Washington, Lockheed Martin and Ariel Research. The original document contains color images. Viewgraphs only.

Many organizations, including the National Aeronautics and Space Administration and the U.S. Department of Defense, want to use constellations or fleets of autonomous spacecraft working together to accomplish complex mission objectives. The Air Force Research Laboratory's Space Vehicles Directorate is now developing architectures for commanding and controlling a cluster of cooperating satellites through autonomous software development for the TechSat 21 program. Many space missions require large, monolithic satellites. This often results in costly, complex, failure-prone vehicles whose physical size constraints limit their performance characteristics. Recently, various organizations have begun to explore how distributed clusters of cooperating satellites can replace their larger monolithic counterparts to reduce overall costs, enhance mission performance, and increase system fault tolerance. Large clusters of satellites flying in formation must have some level of onboard autonomy to fly within specified tolerance levels; avoid collisions; address fault detection, isolation, and resolution (FDIR); share knowledge; and plan and schedule activities. Commanding and controlling a large cluster of satellites can be very burdensome for ground operators as well. This article describes the authors' efforts to address these issues through the technology development for TechSat 21 program., Published in IEEE Intelligent Systems, p8-13, November-December 2000. Prepared in collaboration with Princeton Satellite Systems, Princeton, NJ. The original document contains color images.

Invisible lines of satellite information are rapidly supplementing the sea lanes, roads, and cables of today and yesterday. Television, voice, weather, images, location, and other data stream down to Earth from satellites orbiting above -- all of which are operated by military, civil, or commercial entities. These satellites perform functions similar to those of terrestrial public utilities, providing needed goods and services. Unlike their earthbound counterparts, which service only a neighbor hood or city, these utilities are used on almost every continent by billions of people and may thus be appropriately labeled "global utilities." They are critically important to the national security, economies, and safety of the user nations. Over the past years, the reliance on satellites for all types of global utilities has increased, and future loss of any of these satellites, whether through operator error or subversion, would have drastic implications. Satellite services are invaluable to the United States and its allies. The use of space is one of this country's greatest strengths, but extensive reliance on global utilities also represents a substantial liability. Currently no physical system exists for protecting these global utilities. We can bring to bear economic, political, and other multilateral pressures on an offending nation or group, and we are party to treaties and agreements that prohibit certain activities -- these have worked well in the past. But what if the threat comes from nongovernment organizations, terrorist groups, or an adversarial nation? Or what if we are unable to identify the sources of the offense? Treaties and sanctions may not prove so effective. The argument presented here is that protecting global utilities is a natural extension of the Air Force mission to protect other high-value airborne assets; it is also an extension of the Department of Defense's protection of all friendly assets., Published in the Aerospace Power Journal, v14 n2 p37-41, Summer 2000.

Time-transfer users who make use of GPS for one-way synchronization benefit from being able to operate autonomously, and in anonymity. Though autonomous, anonymous operations can prove advantageous, in particular, for activities associated with the military, their anonymity can result in the undesirable side effect of lost connectivity with the GPS community. Recent GPS satellite maintenance activities have exposed some unusual characteristics of certain, but not all, types of GPS time-transfer receivers. As a result, many Department of Defense (DOD) and other government agency users affected by these satellite activities have emerged to provide the GPS community (2 SOPS in particular) with feedback concerning their experiences with GPS Time Transfer. This paper presents general descriptions of the types of users who have emerged, as well as general observations on the received feedback., See also ADM001481. Presented at the Annual Precise Time and Time Interval (PTTI) Meeting (31st), held in Dana Point, CA on 7-9 December 1999. Published in the proceedings of the meeting, p29-42, 1999.

The Time and Frequency Section of the National Physical Laboratory (NPL) has an active program of work in time and frequency metrology. This paper provides an overview of this section's principal activities. These include: maintenance and development of the national time scale UTC (NPL), development of a new cesium fountain clock, time-transfer activities, development of clock algorithms, technology-transfer activities, and time and frequency dissemination in the UK., See also ADM001481. Presented at the Annual Precise Time and Time Interval (PTTI) Meeting (31st), held in Dana Point, CA on 7-9 December 1999. Published in the proceedings of the meeting, p247-256, 1999.

Though the level of precise timekeeping for military satellite communications (milsatcom) applications may not be as stringent as that required for satellite navigation, milsatcom poses its own unique timekeeping problems. For example, milsatcom timekeeping must be precise without putting an undue burden on a ground station's workload. Further, milsatcom timekeeping must be robust, with the ability to autonomously detect and correct timekeeping problems during protracted periods when the ground control station is either unavailable or burdened with other pressing tasks. Here, we discuss three different space-segment timekeeping systems that could be employed in milsatcom, and our numerical simulations investigating their various attributes. These systems include a Master/Slave system (similar to present day Milstar), an Ensembling system (based on NIST's AT1 algorithm),and a Kalman-Filter system (similar to GPS when it goes to crosslink ranging). The timekeeping performance of the three systems is characterized by the median time interval between ground station updates, given a somewhat arbitrary 2-microsec requirement for space-segment timekeeping. Among other effects, our simulations include: satellite temperature variations, satellite clock random noise, satellite clock frequency aging, time-transfer noise between satellites, as well as time-transfer noise between satellites and the ground station. As we will show, milsatcom timekeeping involves a complicated interplay between satellite timekeeping hardware and the space-segment timekeeping system. Judicious choice of the hardware and space-segment system can allow weeks between ground station updates of the constellation's timekeeping., See also ADM001481. Presented at the Precise Time and Time Interval (PTTI) Planning Meeting (31st Annual) held in Dana Point, CA on 7-9 December 1999. Published in the Proceedings of the Precise Time and Time Interval (PTTI) Planning Meeting (31st Annual), p121-132, 1999.

Interest in low cost space operations and exploration has encouraged the investigation of small satellite constellations which enhance the performance of tasks previously performed by a few much larger spacecraft. Satellite constellations consisting of several spacecraft with masses between 0.01 and 100 kg will be used for various missions 1. The growing trend towards the use of large numbers of small spacecraft operating in local clusters or constellations has created a critical requirement for low power efficient propulsion systems.

The commercialization of space missions has led to the need for cost minimization in order to make these ventures commercially viable. In the past, commercial space systems have generally consisted of a single satellite in a geo-stationary orbit. Command and control of these systems required only a single ground station and cost was not an issue. However, with the crowding of the geo-stationary belt and the desire for truly global wireless communication, companies have now begun developing mission concepts using proliferated low Earth orbit (LEO) satellite systems. These systems cannot be commanded by a single ground station. Therefore, in order to minimize cost for the system, the minimum number of ground stations required for command and control operations must be determined. This paper develops a method for determining the minimal number of ground stations required to command and control Motorola's iridium satellite constellation during the mission phase. The method relies on the use of the iridium satellites' ability to communicate with other satellites in the system, that is the satellites' cross-link abilities. Using a computer model, the method will determine which satellite is currently in view of a given "candidate" ground station position. Then, based on delay time for ground station processing and satellite processing, the method will determine how many satellites the current ground station could command using satellite cross-links. It does this for every latitude and longitude position until it finds the position that can command the most satellites in the system. From there, the method determines the next ground station position in the same manner, until all the satellites have been commanded. Limitations of the given method as well as generalizations that can be made to the method in order to determine the minimal number of ground stations required for normal operations of any proliferated LEO satellite system are discussed.

The on-orbit control of future communications satellite constellations poses a challenge greater than the sum of the challenges of controlling each component satellite. New approaches are required which result in viable control systems that are flexible, reliable, and efficient. These approaches must also have the capability to derive control commands targeted at maintaining constellation-level metrics, and not just individual satellite positions and velocities. In addition, a certain level of autonomy is desired in order to minimize ground station communication and activity, and to allow for quick, responsive solutions to on-orbit failures. This work develops and tests a new methodology for the automated station-keeping of satellite constellations. Three conventional station-keeping strategies are proposed and studied, with the use of a constellation station-keeping simulation, for their ability to address the requirements of satellite constellations. In addition, three Lyapunov non-linear control strategies are developed mathematically. Finally, three sets of simulations are conducted to analyze the control and coverage characteristics of the Ellipso constellation. The Automated Station- Keeping Simulator (ASKS), created for this project, combines an optimal n-impulse rendezvous element to provide automated station-keeping for a constellation of satellites using realistic satellite dynamics calculations on a distributed network of workstations.

This thesis evaluates the survivability of the proposed Iridium Low Earth Orbit (LEO) Satellite Network. In addition to the complete Iridium constellation, three degraded Iridium constellations are analyzed. This analysis occurs via the use of simulation models, which are developed to use three dynamic routing algorithms over three loading levels. The Iridium network models use a common set of operating assumptions and system environments. The constellation survivability was determined by comparing packet rejection rates, hop' counts, and average end to end delay performance between the various network scenarios. It was concluded that, based on the established scenarios, the proposed Iridium constellation was highly survivable. Even with only 45 percent of its satellites functioning (modeled with 36 failed Iridium satellites), the average packet delays were never greater than 178 milliseconds (msec), well within the real time packet delivery constraint of 400 msec. As a result, while additional research is necessary, Iridium has demonstrated the network robustness that is required within the military communications environment.

A low orbit satellite constellation which only provides navigation and positioning services for a certain specially designated region is capable of achieving the functions of global positioning systems. This system may possibly be what most nations hope for. Preliminary results show that, if it is desired to satisfy this requirement, there is a need for a satellite constellation composed of 27 low orbit satellites in five respective orbital planes in order to provide regional navigation and positioning services. From research associated with this article, it is proven that the concept of a regional positioning system formed from a low orbit satellite constellation is feasible. Although the number of individual satellites required is not necessarily a lot fewer compared to original global positioning systems, due to the low orbit satellites, however, delivery energies required by the satellites are, by contrast, much less compared to synchronous orbit satellites. This point should be an advantage which making use of low orbit satellite constellations to provide navigation and positioning functions can achieve., Trans. of Cama, China Astronautics and Missilery Abstracts (China), v2 n4 p1-4, 1995.

For several years, discussions have been ongoing about how to analyze GPS time information in order to get the most accurate and precise time comparison between two locations. Many similar studies were done several years ago before the constellation was dominated by Block If satellites. With the Block II satellites and the use of Selective Availability (S/A), time comparisons have lost some precision and accuracy. This paper presents the results of research that was performed to find not only the best of several time comparison methods, but also to determine to what degree precision and accuracy have declined due to S/A., This article is from 'Proceedings of the Annual Precise Time and Time Interval (PTTI) Applications and Planning Meeting (25th) Held in Marina Del Rey, California on 29 November - 2 December 1993', AD-A280 955, p317-327/328.

The Global Positioning System is an outstanding tool for the dissemination of time. Using mono-channel C/A-code GPS time receivers, the restitution of GPS time through the satellite constellation presents a peak-to-peak discrepancy of several tens of nanoseconds without SA but may be as high as several hundreds of nanoseconds with SA. As a consequence, civil users are more and more interested in implementary hardware and software methods for efficient restitution of GPS time, especially in the framework of the project of a real-time prediction of UTC, UTCp, which could be available in the form of time differences UTCp - GPS time. Previous work, for improving the real-time restitution of GPS time with SA, to the level obtained without SA, focused an the implementation of a Kalman filtering based on past data and updated at each new observation. An alternative solution relies upon the statistical features of the noise brought about by SA: it has already been shown that the SA noise is efficiently reduced by averaging data from numerous satellites observed simultaneously over a sufficiently long time. This method has been successfully applied to data from a GPS time receiver, model AOA TTR4P, connected to the caesium clock kept at the BIPM. This device, a multi-channel, dual-frequency, P-code GPS time receiver, is one of the first TTR-4P units in operation in a civil laboratory. Preliminary comparative studies of this new equipment with conventional GPS time receivers are described in this paper., This article is from 'Proceedings of the Annual Precise Time and Time Interval (PTTI) Applications and Planning Meeting (25th) Held in Marina Del Rey, California on 29 November - 2 December 1993', AD-A280 955, p183-194.

The United States Air Force's Navstar Global Positioning System (GPS) provides high-accuracy space-based navigation and time distribution to suitably- equipped military and civilian users. The system consists of earth-orbiting satellites and a world-wide network of ground stations. A single operational control center, the GPS Master Control Station (MCS) monitors, maintains, and commands the GPS satellite constellation. The on-going deployment of the complete satellite constellation and recent changes in the operational crew structure may invalidate previously used planning and management paradigms. There is currently no analytical method for predicting the impact of these and other environmental changes on system parameters and performance. Extensive testing cannot be performed at the MCS itself due to the criticality of the GPS mission and lack of operational redundancy. This research provides and validates a discrete event simulation model of the MCS operations center task flow, focusing on the creation and testing of a sliding-window MCS activity scheduler. The simulation was validated using MCS historical data. Experiments were conducted by varying the number of ground stations and satellite constellation size available to the simulation. The results, while not quantitatively trustworthy, were used to draw general conclusions about the GPS operational environment.... Navstar Global Positioning System, GPS, Operations control center, Master control station, MCS, activity scheduling, Simulation.