Iñigo del Portillo

Nanosatellite optical downlink experiment: design, simulation, and prototyping

Abstract. The nanosatellite optical downlink experiment (NODE) implements a free-space optical communications (lasercom) capability on a CubeSat platform that can support low earth orbit (LEO) to ground downlink rates>10  Mbps. A primary goal of NODE is to leverage commercially available technologies to provide a scalable and cost-effective alternative to radio-frequency-based communications. The NODE transmitter uses a 200-mW 1550-nm master-oscillator power-amplifier design using power-efficient M-ary pulse position modulation. To facilitate pointing the 0.12-deg downlink beam, NODE augments spacecraft body pointing with a microelectromechanical fast steering mirror (FSM) and uses an 850-nm uplink beacon to an onboard CCD camera. The 30-cm aperture ground telescope uses an infrared camera and FSM for tracking to an avalanche photodiode detector-based receiver. Here, we describe our approach to transition prototype transmitter and receiver designs to a full end-to-end CubeSat-scale system. This includes link budget refinement, drive electronics miniaturization, packaging reduction, improvements to pointing and attitude estimation, implementation of modulation, coding, and interleaving, and ground station receiver design. We capture trades and technology development needs and outline plans for integrated system ground testing.

Approximation methods for estimating the availability of optical ground networks

Optical communications are a key technology enabler to return increasing amounts of data from space exploration platforms such as robotic spacecraft in Earth orbit or across the solar system. However, several challenges have hindered the deployment and utilization of this technology in an operational context, most notably its sensitivity to atmospheric impairments such as cloud coverage. To mitigate this problem, building a network of interconnected and geographically disperse ground stations has been proposed as a possible solution to ensure that, at any point in time, at least one space-to-ground optical link is available to contact the space-based platforms. In this paper, we present a new approach for quantifying the availability of an optical ground network that is both computationally inexpensive and suitable for high-level architectural concept studies. Based on the cloud fraction data set, several approximation methods are used to estimate the probability of having a certain number of space-to-ground links fail due to cloud coverage. They are developed in order to capture increasingly complex atmospheric factors, from sites with independent weather conditions, to stations that are both temporally and spatially correlated. Then, the proposed approximation methods are benchmarked and recommendations on how to utilize and implement them are finally summarized.

Integrated Tradespace Analysis of Space Network Architectures

Methods to design space communication networks at the link level are well understood and abound in the literature. Nevertheless, models that analyze the performance and cost of the entire network are scarce, and they typically rely on computationally expensive simulations that can only be applied to specific network designs. This paper presents an architectural model to quantitatively optimize space communication networks given future customer demands, communication technology, and contract modalities to deploy the network. The model is implemented and validated against NASA’s Tracking and Data Relay Satellite System. It is then used to evaluate new architectures for the fourth-generation Tracking and Data Relay Satellite System given the capabilities of new optical and Ka-band technologies, as well as the possibility to deploy network assets as hosted payloads. Results indicate that optical technology can provide a significant improvement in the network capabilities and lifecycle cost, especially when pl...

Architecting the ground segment of an optical space communication network

Optical communications are envisioned as a key technology for space communication in the near future. This transition to optical terminals is being pushed by the higher data volume demand of certain missions (i.e.: missions DESDyNI (now cancelled) and NISAR had together a requirement of 60 Tb/day, whereas the data-volume transmitted daily by the Space Network nowadays is roughly 40 Tb) and by the spectrum encroachment in current RF bands. In addition, recent missions like LLCD and OPALS have demonstrated that optical systems present multiple advantages with respect to RF terminals, such as their lower mass, size and power and the higher data-rate they offer (up to 10 Gbps). However, one of the main issues of using optical systems is the space-to-ground link, due to the difficulty of penetrating through atmospheric clouds. Geographic diversity of ground stations has been proposed as an alternative to mitigate these effects. The goal of this paper is to analyze different architectures for the ground segment of a fully optical space relay-communications network to serve LEO missions. In particular, we analyze the tradespace characterized by the decisions 1) number and location of optical ground stations, 2) use of GEO relay satellites vs. direct to Earth (DTE) approach and 3) presence of crosslinks among relay satellites. To that end, we use historical NOAAs weather data and the cloud fraction dataset from Aquas and Terras MODIS instruments to characterize weather conditions across the globe. We later use these models to determine the best locations to place ground stations that support optical terminals. Next, we present ONGSA, a network simulator that incorporates the cloud models in order to simulate end-to-end operations of the optical network. Finally we exercise ONGSA to explore the aforementioned tradespace and analyze both cost and performance (in terms of availability) for each architecture.

On autonomous software architectures for distributed spacecraft: A Local-Global Policy

New trends such as satellite swarms or fractionated spacecraft have experienced a very significant growth in the last decade. Migration from monolithic satellite architectures to new mission architectures involving large constellations of collaborative spacecraft is enabled by several hardware technologies and the application of modularity-driven designs, and presents numerous benefits such as low development costs and times and high flexibility. This has forced the exploration of new techniques and designs which have been often tackled from the hardware perspective but scarcely approached from the software architecture standpoint. This paper presents an autonomous software architecture and a management policy targeted for the broad range of distributed architecture missions. The paper presents the Local-Global approach, an adaptive management policy based on the collaboration between two levels of control which is aimed at enabling distributed mission control in dynamic and changing environments with limited computational capabilities. The Local- Global policy establishes the behaviouralmodel of a systemcomposed of a master scheduler and an arbitrary number of local schedulers, and describes the parameters that can be adjusted to reduce the amount of information processed by the master node which makes it suitable for different distributed spacecraft architectures.

Architecting space communication networks under mission demand uncertainty

NASAs Space Network has been a successful program that has provided reliable communication and navigation services for three decades. As the third generation of satellites is being launched, alternatives to the current architecture of the system are being studied in order to improve the performance of the system, reduce its costs and facilitate its integration with the Near Earth Network and the Deep Space Network. Within this context, past research has proven the feasibility of efficiently exploring a large space of alternative network architectures using a tradespace search framework. Architecting a space communication network is a complex task that requires consideration of uncertainty, namely (1) factoring in customer demand variability, (2) predicting technology improvements and (3) considering possible budgetary constraints. This paper focuses on adding uncertainty associated with (1) to the existing communications network architecture tool by describing a heuristic-based model to derive mission concept of operations (conops) as a function of communication requirements. The accuracy of the model is assessed by comparing real conops from current TDRSS-supported missions with the predicted concept of operations. The model is used to analyze how customer forecast uncertainty affects the choice of the future network architecture. In particular, four customer scenarios are generated and compared with the current TDRSS capabilities.

On scalability of Fractionated Satellite Network architectures

Fractionated Satellite Networks are a popular concept in space systems. On these networks, several satellites cooperate and collaborate by exchanging resources wirelessly in order to obtain an aggregated network capability higher than the sum of the individual capabilities of the different satellites that compose it. Fractionated Satellite Networks are a generalization of Fractionated Satellites. Scalability is defined as the ability of a system to maintain its performance and function, and retain all its desired properties when its scale is increased greatly without having a corresponding increase in the systems complexity. The whole concept of fractionation (both at spacecraft level and network level) is based on the use of multiple satellites that jointly perform a function that can be further expanded by adding new satellites to the system. Because of this expandable nature of Fractionated Satellite Networks, the concept of scalability is critical on these architectures, as systems that do not scale well present a very poor performance when adding new agents, increasing costs and harming quality of service and stakeholder satisfaction. This paper presents a model and a framework for analyzing scalability of fractionated networks. Our model includes descriptions of the system at the resource, satellite, network and mission level. Connections and resource transfer among nodes are modelled using graphs whereas the study is approached from a resource allocation problem perspective. Finally, the utility and applications of the developed methodology is demonstrated through the analysis of a case study of a potential fractionated network.

Optimal location of optical ground stations to serve LEO spacecraft

Free space optical communications (FSO) are envisioned as a disruptive technology for space communications. Among its advantages, FSO will allow higher throughputs (in the order of tenths of Gbps, which represents an improvement of 10 to 100 times with respect to current RF technology), together with significant reductions in size, weight, and power. However, the main drawback of FSO when compared to RF is the reduced link availability due to outages caused by cloud coverage over the receiving ground stations. Site diversity has already been proven to be an effective mitigation technique against cloud outage for geostationary satellites, but its usefulness in the context of low Earth orbit satellites can be challenged by correlated cloud coverage among all visible ground stations. This consideration, along with trade-offs between minimal cloud probability, minimal latency and proximity to supporting infrastructure should all be taken into account when selecting locations for networks of ground stations. This paper presents a model to optimally determine the location of optical ground stations to serve LEO missions, considering the aforementioned trade-offs. First we describe the atmospheric, latency, and infrastructure models used to evaluate the goodness of a network. Second, we statistically characterize the orbits of the customer missions that the ground network will serve. Finally, we present two case studies: The first one selects the best stations among a group of existing assets (stations in the Near Earth Network, other governmental agencies, and commercial facilities from ground segment operators). The second one determines the optimal locations for the ground stations considering an unconstrained scenario in which facilities can be placed at any point on the Earths surface. For each of these scenarios, we report the availability, latency and cost of ground stations of the Pareto-optimal networks.

Performance characterization of a multiplexed space-to-ground optical network

Advances in phased array systems for multi-beam free space optical communications are a key enabler for a new space-to-ground network architecture, namely a multiplexed optical architecture. The fundamental idea of a multiplexed space-to-ground optical network is the utilization of a multi-beam optical payload that allows each spacecraft to establish links with multiple ground stations within its line of sight. Information is then downlinked in parallel, from the satellite to the ground, through the subset of links not disrupted by clouds. In this paper we evaluate the performance of a multiplexed optical space-to-ground architecture from a systems perspective, with particular emphasis on the effect of cloud correlation in the network throughput. In particular, we first derive the expected data volume returned in a multiplexed architecture as a function of the optical network availability and the system total capacity. Then, we compare the performance of the proposed multiplexed architecture against a traditional single-beam downlink system that utilizes site diversity to mitigate cloud coverage effects. This comparison is based on two canonical scenarios, a global highly uncorrelated network representative of a geosynchronous satellite; and local, highly correlated, network representative of a low Earth orbit spacecraft. Through this analysis, we demonstrate that multiplexed architectures can improve the throughput of a space-to-ground optical network as compared to that of a single ground telescope without requiring a beam switching mechanism.

Uncertainty quantification of network availability for networks of optical ground stations

This paper analyzes differences in the availability of networks of optical ground stations computed using different methods and datasets, and quantifies the uncertainty of the results. For that purpose, we first review existing methods proposed in the literature, and then existing cloud coverage datasets, and we compare the results obtained using different methods and datasets for several scenarios. Finally, we propose a new probabilistic global cloud coverage model that aggregates values from existing datasets and quantifies the uncertainty in measuring cloud probability, and present a method to compute the availability of a network of multiple optical ground stations, along with the corresponding uncertainty.