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We study the use of distributed computation in a representative multi-robot planetary exploration mission. We model a network of small rovers with access to computing resources from a static base station based on current design efforts and extrapolation from the Mars 2020 rover autonomy. The key algorithmic problem is simultaneous scheduling of computation, communication, and caching of data, as informed by an autonomous mission planner. We consider scheduling of a dependency chain of required and optional (but rewarding) tasks and present a consensus-backed scheduler for sharedworld, distributed scheduling based on an Integer Linear Program. We validate the pipeline with simulation and field results. Our results are intended to provide a baseline comparison and motivating application domain for future research into network-aware decentralized scheduling and resource allocation.

Delay tolerant networks (DTNs) offer a set of standardized protocols to enable Internet-like connectivity across the solar system. Unlike other protocols such as the Transmission Control Protocol (...

This paper explores the ability to ferry data between Mars and Earth, and Jupiter and Earth, using a network of small spacecraft (termed “data mules”) placed in phase-shifted cycler orbits between both planets. These cycler orbits enable the data mules to periodically visit the destination planet without requiring large amounts of fuel. However, their long periodicity also limits the cadence of visits since several years may elapse between consecutive data mule flybys. To increase total data return to Earth, we compare two alternative concepts of operations. First, we assume that each data mule carries an optical terminal capable of establishing a very high-rate inter-satellite link with a spacecraft orbiting Mars and Jupiter (where all data to be returned is stored). This inter-satellite link operates for a short period of time, nominally during 1 day around closest approach of the data mule to the planetary body of interest. This is compared to normal Deep Space Network (DSN) operations, in which we optimistically assume a continuous direct-to-Earth link with a ground antenna. Through our analysis, we show that the concept of data mules can substantially increase the total amount of data returnable from planetary bodies when used as a complement to current DSN support. We also show how the system performance, measured in terms of total returnable data volume or amortized yearly data rate, depends on technological constraints to implement the deep space and proximity links, as well as geometrical constraints imposed by cycler orbits and the solar system.

This paper presents an adaptive-sweep algorithm that can be used in conjunction with a Phased-Lock Loop to acquire and track the carrier of a Binary Phase-shift Keying signal with residual carrier. This algorithm is useful in proximity links where no Doppler pre-compensation is available at the transmitter, and allows the receiver to acquire the incoming carrier even if the experienced Doppler shift is several orders of magnitude larger than the PLL bandwidth. Other applications for this algorithm include Multiple Uplink per Antenna at the Deep Space Network, and generation of observables (Doppler shift, Doppler rate, range rate) for in-situ navigation purposes. This paper enhances a previously presented adaptive-sweep algorithm and its performance analysis in two fronts. First, the algorithm now uses the direction function to automatically estimate the required frequency jump to achieve PLL lock. This enables a fully adaptive-sweep scheme in which system implementer does not need to manually select thresholds. On the other hand, we study the system performance in the presence of Additive White Gaussian Noise and derive necessary conditions for the system to operate at low signal-to-noise conditions. This allows us to create design rules that specify all parameters of the algorithm given a set of system-level requirements. To test the performance of the proposed algorithm, we implement it in GNU Radio, an open-source Software-Defined Radio that interfaces with several commercial radio peripherals. We use this implementation to demonstrate carrier lock and quantify system performance under different noise conditions. We also test the algorithm using open-loop recordings of two downlinks between the Lunar Reconnaissance Orbiter and the Deep Space Network. We show that even after large frequency jumps (tens of kHz), the proposed system is able to rapidly reacquire the carrier and continue operation.

Future deep space links are migrating towards higher frequency bands such as Ka-band and optical. These links are susceptible to non-linear effects (e.g. cloud coverage, atmospheric turbulence, scintillation, antenna mis-pointing, jitter, relative motion, etc.) that cause various degrees of fading loss over various time scales. We consider a channel experiencing block fading where the channel coherence time is a lot longer than the length of a codeword. As long as the fading duration is not excessive, an interleaver can be used in a coded-communication system that randomizes the fades among symbols of multiple codewords. This achieves an “averaging” effect that spreads the signal energy more evenly among the codewords, and improves the likelihood that each of them can be decoded correctly. For simplicity, we consider a block interleaver where adjacent symbols of a codeword have a fixed delay (or separation) with each other, and investigate the error-rate performance of a coded-interleaving system (with fixed and finite interleaving depth) in the presence of block fading. Unlike prior research which assumes theoretical channel coherence models, we aim to build upon our analysis using empirical spacecraft signal measurements obtained with the Deep Space Network, and to use the fundamental theories of probability and statistics (e.g. variations of Law of Large Numbers) to quantify the error-rate performance of the coded-interleaving system. We propose to utilize these results in the design of a two-dimensional coded-interleaving system for fading channels.

In this paper, we use an adaptive sweep carrier acquisition and tracking algorithm to acquire and track high dynamic Doppler frequency profiles with high Doppler shift, Doppler rate, and Doppler acceleration. The acquisition and tracking system is a Phase-Locked Loop combined with a bidirectional smart sweeping algorithm. This system is a modified version of previously proposed method that enhances the performance in high dynamic Doppler scenarios. Therefore, our goal is to acquire the received carrier in the presence of high dynamics, over a short acquisition time, while operating at the lowest possible received carrier to noise ratios experienced in realistic flight scenarios. Additionally, in the paper we discuss the system parameters that are key to have a robust acquisition and tracking system for such scenarios. For simulations, we considered Doppler profiles of Goldstone to Mars Science Laboratory (MSL)- Curiosity Rover, Entry, Descent, and Landing (EDL), and Lunar South Pole (LSP) to Lunar Relay Satellite (LRS). Both S-Band, and X-Band were considered for Doppler profiles. For simulations and results, we used MATLAB Simulink models to prototype the system. The true and estimated Doppler frequency, threshold, and direction functions, as well as Doppler estimation errors during tracking mode were obtained from simulations. Similarly, the standard deviation of Doppler estimation error was measured at various received carrier to noise ratios and compared with an approximated analytical result both for a 2nd and a 3rd order loop.

We present the Pluggable Distributed Resource Allocator (PDRA), a middleware for distributed computing in heterogeneous mobile robotic networks. PDRA enables autonomous robotic agents to share computational resources for computationally expensive tasks such as localization and path planning. It sits between an existing single-agent planner/executor and existing computational resources (e.g. ROS packages), intercepts the executor's requests and, if needed, transparently routes them to other robots for execution. PDRA is pluggable: it can be integrated in an existing single-robot autonomy stack with minimal modifications. Task allocation decisions are performed by a mixed-integer programming algorithm, solved in a shared-world fashion, that models CPU resources, latency requirements, and multi-hop, periodic, bandwidth-limited network communications; the algorithm can minimize overall energy usage or maximize the reward for completing optional tasks. Simulation results show that PDRA can reduce energy and CPU usage by over 50% in representative multi-robot scenarios compared to a naive scheduler; runs on embedded platforms; and performs well in delay- and disruption-tolerant networks (DTNs). PDRA is available to the community under an open-source license., Extended version of manuscript presented at IROS 2020. In v2, numerical results are updated and parts of the paper are rewritten and expanded for clarity. In v3, a minor author metadata error is fixed. All code is available under Apache license at https://github.com/nasa/mosaic

NASA's human exploration program is currently working towards landing astronauts on the surface of the Moon by 2024, close the lunar South Pole. To guarantee astronaut safety and maximize science data return, NASA is in the process of defining the communication architecture that will support all astronaut activities from launch to surface operations. Of particular interest to this paper are links from the lunar surface back to Earth without any intermediate relays. We show that the system geometry is such that antennas on the landing system will need to be pointed at low elevation angles, thus potentially causing multi-path fading effects not typically encountered in space communications. This paper is organized in three parts. First, we characterize the multi-path fading effects expected in links between the lunar South Pole and Earth and show that for moderate data rates (less than 1 Mbps) the links suffer from slow fading. We then show that for this operations regime the performance of forward error correction schemes is significantly worse for traditional Additive White Gaussian Noise channels. Finally, we investigate multi-copy mechanisms to mitigate the effects of fading, most notably repetition schemes and Automatic Repeat Request.

In order to effectively communicate with Earth from deep space there is a need for network automation similar to that of the Internet. The existing automated network protocols, such as TCP and IP, cannot work in deep space due to the assumptions under which they were designed. Specifically, protocols assume the existence of an end-to-end path between the source and destination for the entirety of a communication session and the path being traversable in a negligible amount of time. In contrast, a Delay Tolerant Network is a set of protocols that allows networking in environments where links suffer from high-delay or disruptions (e.g. Deep Space). These protocols rely on different assumptions such as time synchronization and suitable memory allocation. In this paper, we consider the problem of autonomously avoiding memory overflows in a Delay Tolerant Node. To that end, we propose using Reinforcement Learning to automate buffer management given that we can easily measure the relative rates of data coming in and out of the DTN node. In the case of detecting overflow, we let the autonomous agent choose between three actions: slowing down the client, requesting more resources from the Deep Space Network, or selectively dropping packets once the buffer nears capacity. Furthermore, we show that all of these actions can be realistically implemented in real-life operations given current and planned capabilities of Delay Tolerant Networking and the Deep Space Network. Similarly, we also show that using Reinforcement Learning for this problem is well suited to this application due to the number of possible states and variables, as well as the fact that large distances between deep space spacecraft and Earth prevent human-in-the-loop intervention.

In this paper, we propose an adaptive sweep signal acquisition and tracking algorithm that (1) takes into account high-order uplink signal dynamics (Doppler shift, Doppler rate, Doppler acceleration), (2) dynamically adapts the carrier search direction (positive or negative in frequency error) and (3) dynamically adapts the carrier search step. The combination of (2) and (3) are the main innovation of this paper, since radios do not need to restart at its initial search state when the carrier is missed. This, in turn, minimizes the time required for carrier lock and thus makes the system more robust to the loss of lock failures. Note that the proposed system improves upon the frequency sweeping algorithm used in current flight radios (e.g., JPL's Electra and IRIS) which use constant search direction and search step. There are three motivating applications for our proposed system: Proximity links at Mars or the Moon, support of Multiple Access per Antenna at the Deep Space Network, and generation of Doppler measurements for navigation purposes. As indicated later in the paper, all these scenarios require spacecraft to establish highly dynamic links for which no Doppler compensation can be performed on the ground. Therefore, the receiver's ability to properly acquire and track the received carrier is paramount for transfer of information, relative positioning and navigation.

In this paper we design and analyze an end-to-end communication system between a lander/rover on the surface of the lunar South Pole and an Earth station. The acquisition and tracking system is discussed in detail. The communication system on the lander or rover could be used for the Earth-to-Moon communication. To communicate to and from the lander/rover on the lunar South Pole, low and/or medium directional antennas onboard the lander/rover will have to be pointed at low elevation angles between 2 to 10 degrees, thus causing multipath fading effects due to reflection of a portion of the transmitted electromagnetic waves from the surface of the Moon that are not commonly encountered in traditional deep space communications between a spacecraft and a ground station. To design and analyze such a communication system, and in particular the acquisition and tracking system, in the presence of multipath fading, first we model the fading channel based on existing and simulated data. In addition to multipath fading, the channel also introduces Doppler frequency up to 11.5 KHz, and Doppler rate up to 0.735 Hz/sec. For coherent reception the Doppler frequency, which is time varying, should be acquired and the incoming carrier phase, which includes the fading phase, should be tracked in the presence of multipath fading. For this communication system in addition to estimating the received carrier phase, the amplitude of the fading signal should also be estimated, in particular to be used in the decoder. In addition to acquisition and tracking, we consider simple modulation and coding schemes. Space diversity using two antennas on Earth to mitigate the effects of fading could also be used. We design phase-locked loops and frequency sweeping schemes robust to the attenuations due to fading. After designing various components of the communication system, we use Simulink models to obtain the end-to-end performance of the communication link under investigation. Based on the available data, the fading channel can be accurately modeled as a Rician fading channel with Rician parameter of 10 dB, and Doppler spread that depends on the Doppler frequency and the transmit/receive antenna patterns. The challenge is how to make such a communication system robust in the presence of the multipath fading where the Doppler spread changes in time and thus produces fading with time-varying durations (short and very long fades). In summary, this paper covers communication system design, performance analysis, and simulations for performance of Doppler frequency acquisition, tracking, uncoded system, and coded system under ideal interleaving assumption with hard decision over communication link between a lander/rover at the Lunar south pole and a DSN Earth station in presence of Rician fading.

Routing in networks where nodes move randomly is particularly challenging due their potentially unpredictable, and rapidly changing topology. Several routing algorithms have been presented in the literature to address the needs of such networks, most of them implementing variants of controlled network flooding in the hope of successful data delivery.In this note, we compare the results of previous routing algorithms with Opportunistic Contact Graph Routing (OCGR), an enhanced version of Contact Graph Routing (CGR) that is suitable for networks where contacts cannot always be scheduled ahead of time. To perform the benchmark, we simulate a network of nodes moving in a certain space according to the Random Waypoint Mobility Model, and then take measurements of bundle delivery probabilty and overhead ratio as metrics of performance and cost respectively. Through this exercise, we demonstrate that the performance of OCGR is highly dependent on the type of network under consideration (e.g. very sparse vs. densely connected) and the assumed mobility model.

Experts predict the number of devices connected to Internet of Things will reach 50 billion in 2020. In this paper, we apply the concept of IoT to the airport management industry and investigate the utilization of RFID technology to enhance airport mobility and security. We propose a system to track passengers inside an airport terminal and focus on the optimization of its architecture. To that end, we develop a simulation model incorporating the most important parts of the system, including: An airport model based on Chicago O'Hare International Airport, a passenger trajectory model, a link budget model, a coverage model, a reader placement algorithm, a communication protocol and the limitations of available RFID technology and regulations. Next, we identify a set of architectural decisions, such as the physical limits of the system or the specific technology to be deployed, and develop performance and cost metrics to compare the plausible architectures. Then, based on the developed model and the metrics space, we present the architectural tradespace of the system and analyze the impact each decision has in order to optimize the architecture. Finally, we assess the viability of the chosen architecture in terms of coverage and tracking performance.

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...

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.

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.

In this article we describe the analysis and simulation effort of the end-to-end traffic flow for the Deep Space Network (DSN) in the Human Exploration Era, when DSN will provide communication and navigation services for human missions to distant celestial objects like the Moon, asteroids, and Mars. Using the network traffic derived for the 30-day period within July/August 2039 from the Space Communications Mission Model (SCMM), we simulate the bandwidths of the ground links and the buffer profiles of the network nodes. We also use a 2-state Markov scheme that models the store-and-forward mechanism that regulates the ground network traffic. The network traffic modeling and simulation generates ground bandwidth and buffer statistics, which in turn are used to formulate the future DSN ground network bandwidth and storage requirements.

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.

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., United States. National Aeronautics and Space Administration (NNX11AR70G)

Traditionally, government space agencies have developed aggregated systems that co-host multiple capabilities on shared spacecraft buses. However, in response to cost growth and schedule delays on past programs, leaders in the government space community have expressed an interest in disaggregation, or distributing their capabilities across multiple spacecraft. Since their aggregated National Polar-orbiting Operational Satellite System (NPOESS) program was cancelled in 2010, both the National Oceanic and Atmospheric Administration (NOAA) and the Department of Defense (DoD) have investigated opportunities to reduce program costs through disaggregation. This paper expands their initial investigation and explores the cost impacts of aggregation and disaggregation across a large trade space of candidate architectures for environmental monitoring in low-Earth orbit. We find that on average, aggregated architectures are less costly than fully disaggregated ones but also find opportunities for cost savings by developing semi-aggregated systems, or systems with one or two satellites per orbital plane. Finally, we investigate several trades that are currently under consideration by NOAA and the DoD and make recommendations for future environmental monitoring systems in low-Earth orbit., Massachusetts Institute of Technology (Sandia Corporation Excellence in Engineering Graduate Fellowship), Skolkovo Institute of Science and Technology

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.