William W. Edmonson

Survey of Inter-Satellite Communication for Small Satellite Systems: Physical Layer to Network Layer View

Small satellite systems enable a whole new class of missions for navigation, communications, remote sensing, and scientific research for both civilian and military purposes. As individual spacecraft are limited by the size, mass, and power constraints, mass-produced small satellites in large constellations or clusters could be useful in many science missions such as gravity mapping, tracking of forest fires, finding water resources, etc. The proliferation of small satellites will enable a better understanding of the near-Earth environment and provide an efficient and economical means to access the space through the use of multi-satellite solution. Constellation of satellites provide improved spatial and temporal resolution of the target. Small satellite constellations contribute innovative applications by replacing a single asset with several very capable spacecraft, which opens the door to new applications. Future space missions are envisioned to become more complex and operate farther from Earth, and will need to support autonomous operations with minimal human intervention. With increasing levels of autonomy, there will be a need for remote communication networks to enable communication between spacecraft. These space-based networks will need to configure and maintain dynamic routes, manage intermediate nodes, and reconfigure themselves to achieve mission objectives. Hence, inter-satellite communication is a key aspect when satellites fly in formation. In this survey, we present the various research being conducted in the small satellite community for implementing inter-satellite communications based on the open system interconnection (OSI) model. This survey also reviews the various design parameters applicable to the first three layers of the OSI model, i.e., physical, data link, and network layer. Based on the survey, we also present a comprehensive list of design parameters useful for achieving inter-satellite communications for multiple small satellite missions. Specific topics include proposed solutions for some of the challenges faced by small satellite systems, enabling operations using a network of small satellites, and some examples of small satellite missions involving formation flying aspects.

Small satellite systems design methodology: A formal and agile design process

We propose to develop a model-based systems engineering process that results in high-confidence designs for small satellite systems in the pico-/nano-class, i.e. <; 50kg. This objective will be achieved through the integration of formal methods and model based systems engineering to develop an agile framework for high-confidence designs for these small systems. We propose, Reliable and Formal Design (RFD) process whose results are correct by construction, formally verified, and responsive to system requirement changes. This paper develops an intelligent framework that ties requirements, models, and simulations in a cogent manner. Furthermore, this papers provides a formulation for consistency and traceability, where the latter enforces a condition on the relationship between abstraction layers, that is, the function that refines any layer of abstraction into a successive layer must have a dual. An example of this refinement is illustrated using PVS to express the logical requirement formulation and for providing type checking proof.

Systems engineering of inter-satellite communications for distributed systems of small satellites

Present design processes for satellite networks mainly involve interconnected system models and their parameter-based simulations without emphasizing high level requirements in the process. What is missing is a systems engineering approach and a system of verification and validation based on formal representation and analysis. We propose a systems design methodology for inter-satellite communication based on the Responsive and Formal Design (RFD) [1]-[3], which addresses the need for a model-based system engineering approach coupled with a system of validation and verification based on formal representation and analysis. The goal is to apply the RFD process to provide a solution for the Inter-Satellite Communications (ISC) problem within the Open Systems Interconnection (OSI) framework. The OSI represents a standard framework for communication between devices. It divides the communication process into seven layers, with each one providing well-defined functions and services. We will address the integration of the RFD process with the OSI framework for ISC. The RFD process involves levels of design abstraction and refinement which correspond to layers or groups of layers of the OSI model. The process illustrates how the complete framework for ISC unfolds in an iterative manner.

Software Defined Radio implementation of DS-CDMA in inter-satellite communications for small satellites

In this paper, we study the problem of multi-user inter-satellite communications in a network of small satellites and design and implement of an optimum CDMA-based multiple access communication using Software Defined Radios. Inter-satellite links (ISL) enable small satellites to exchange information and share resources while reducing the traffic load to the ground. The ISL assist in performing advanced functions including distributed processing and autonomous applications. By utilizing the ISL in maintaining the relative distance between the satellites, navigation, and positioning accuracy is improved greatly. The ideology proposed here is to implement remote modifications in inter-satellite communication after launch using Software Defined Radios (SDRs), while accommodating an adaptive autonomous small satellite network. Remote upgrades from the ground as well as the potential to accommodate new applications and future services without hardware changes is very promising. Software defined radio-based implementation of an ISL can assist in enabling an adaptive and reconfigurable communication system, which can achieve higher data rates and modification of frequencies. In this paper, we designed and implemented a multi-user inter-satellite communication network using SDRs, where Code Division Multiple Access (CDMA) technique is utilized to manage the multiple access to shared communication channel among the satellites. This research is the first work to study and implement the utilization of SDRs for inter-satellite communications in small satellite systems.

Utility of Light Emitting Diodes for inter-satellite communication in multi-satellite networks

Future space missions will be designed to take advantage of multi-satellite network deployment because of their potential for providing improved spatial and temporal resolutions of a target and/or can provide a more comprehensive observation of the Earth. To enable interoperability within this satellite network particularly for autanomous operations and provide on-orbit data handling through distributed processing, a reliable inter-satellite communication (ISC) is required. In this paper, we examine the suitability of Light-Emitting Diodes (LEDs) for ISC in multi-satellite networks, and further propose an approach of the physical layer design that meets the requirements of the platform in terms of the size, weight and power (SWaP) of the satellite. An analytical model of the ISL is developed to test feasibility of using LEDs for reliable data communication in the presence of steady solar background illumination. Finally, the performance of the ISL is evaluated in terms of the Bit Error Rate (BER) and achievable data rates for four different intensity modulation and direct detection (IM/DD) schemes.

Systems Engineering Education for East Africa

The goal of this paper is to explore the need for Systems Engineering (SE) in East African countries and how best to educate engineers in the field of SE. It provides a comprehensive overview on the usefulness of SE education for East African nations and proposes SE curriculum. Presently SE has been given little attention in East African countries. However, these countries are in the beginning of industrialization with many new mega projects and infrastructure expansions that demands SE professionals. This motivates the need to introduce SE education in the region. Systems engineering education demands the development of SE curriculum, which will be multidisciplinary and considers social and psychological factors by taking into account the active participation of the community. Towards this end, the paper demonstrates the necessity of SE education via designing and managing ongoing mega projects. It also proposes SE curriculum by incorporating courses covering both foundations of SE and the practice of SE. Furthermore, it suggests that these courses will be delivered in collaboration with industry and government entities.

A non-cooperative game theoretic approach for power allocation in intersatellite communication

Static interference management techniques to enable multiple access intersatellite communication in small satellite networks can be inefficient noting the unpredictable variations in network conditions. Game Theory (GT) is a powerful mathematical tool to model the interactions among the cognitive small satellites to observe the network variations as well as actions of other satellites and make the best decisions to optimize their performance. In this paper, a network of multiple slave satellites that want to communicate with a single master satellite over an interference channel is considered. We propose a distributed power allocation mechanism using a non-cooperative game theoretic model to enable the slave satellites to select the optimal power strategy to reduce the interference level and maximize their utility functions. Using a pricing algorithm to control the aggressive behavior of the players of the game, the optimum pricing factor for the system and the optimal values of the power for slave satellites are defined. To the best of our knowledge, this paper is the first one to propose a decentralized power allocation mechanism in autonomous small satellite networks using a game theoretic model. The simulation results show the performance of this proposed model in enhancing the network sum-throughput.

Link performance improvement via design variables optimization in LED-based VLC system for inter-satellite communication

In our previous paper, we examined the utility of LEDs for inter-satellite communication (ISC) in multiple small satellite networks and proposed an approach of the physical layer design that meets the requirements of the platform in terms of the critical physical layer design variables. These variables (or parameters) include the LED transmit power, photodetector active area, receiver bandwidth and link distance. One of the most important tasks for the visible light communication (VLC) system designer is how to ensure the required balance or trade-off among these variables in order to achieve the desired performance. In this work, we employed multi-objective optimization to determine physical layer design variables at which the signal-to-noise ratio (SNR) at the VLC receiver is maximized. We used the Non-dominated Sorting Genetic Algorithm II (NSGA-II) in MATLAB to determine the Pareto front of two conflicting objective functions, and then extracted the optimal solution using the Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS). Analysis of the optimal solution showed that it yielded the maximum SNR within the set of non-dominated solutions at the Pareto front. We showed that using multi-objective optimization techniques for assignment of parameter values can yield more than 3 dB improvement in the SNR.

Fuzzy classification context for the responsive and formal design process

This paper presents an application of a fuzzy relation in system modeling (from requirements) to be used for a Systems Engineering (SE) methodology. We define fuzzy classifications (models for distributed systems), extract component and system theories (sets of logical expressions), and ensure consistency of requirements for the Responsive and Formal Design (RFD) Process. The RFD process is a SE methodology that relates a set of requirements, associated models, simulations, and the relationship between them, by integrating Model-Based Systems Engineering (MBSE) to manage system modeling complexity with formal methods to ensure that designs are verifiably correct against their requirements. To translate informal requirements to logical expressions in the RFD process, we first model requirements using a 3-tuple structure called a classification formulated from Barwise and Seligmans channel theory. A classification consists of tokens (observed situations) and types (situation features) and a binary relation classifying tokens with types. However, classifying tokens using types as present (represented as ‘1’) or absent (represented as ‘0’) as used in channel theory is not always possible (since it involves vagueness and imprecission) and the representation lacks expressiveness to reason about relations among such types (vague situation features). Hence, a binary classification doesnt capture uncertainity. In this paper, we consider a degree of truth in the relation between tokens and types to define a fuzzy classification. We then develop an algorithm that extracts a theory from a fuzzy classification. This helps in formal proof for checking consistency (no contradiction) and deducing to requirements (verifying properties). We demonstrate our development using three small satellites measurement system whose goal is to image the colorful auroral ovals seen around Earths magnetic poles.

Designing a Remote In Situ Soil Moisture Sensor Network for Small Satellite Data Retrieval

This chapter introduces an application of small satellite for environmental monitoring using real-time data that focuses on measured soil moisture and temperature using in situ sensor network. Soil Moisture Active Passive Mission (SMAP) uses microwave radar and radiometer to sense surface soil moisture condition but gives coarser result than the in situ data. To overcome the limitation of accuracy in SMAP mission, the proposed architecture will provide sensor data via a Ground Monitoring Wireless Sensor Network (GM-WSN) where data is collected by small satellite(s) operating in Lower Earth Orbit (LEO). The satellite will store the data until it passes over a ground station whereby it is communicated back to earth. The motivation for developing satellite accessible in situ measurements is to retrieve information from remote areas like Lake Tana in Ethiopia, where human accessibility is difficult. Hence, those important and unattended places would be covered by the proposed system. Another key attribute of this architecture is that it addresses four (climate, carbon, weather, and water) out of six NASA’s earth science strategic focus areas. The GM-WSN will consist of a sensor network that will measure soil moisture and temperature. The base station function is to fuse the data from the sensors, provide a time stamp, and format the data to be transmitted to satellite. It will also act as transceiver for ground to space communication using the amateur VHF/UHF radio band that has a maximum data rate of 9.6 kbps and will provide health maintenance and power management of network.