Jay L. Gao

Performance evaluation of the CCSDS file delivery protocol - latency and storage requirement

To support robotic and human explorations of Mars in the coming decade, the 2005 Mars Reconnaissance Orbiter (MRO) and the 2009 Mars Telecommunications Orbiter (MTO), as part of the Mars Network, will provide file transfer services to other Marscrafts using the CCSDS File Delivery Protocol (CFDP). CFDP was designed to provide file-based data and storage management, store-and-forward relay, and reliable data transfer over space links characterized by large propagation delay and intermittent availability. This paper will describe how MTO can use CFDP to relay operational data and bulk science data for surface missions such as the Mars Science Laboratory (MSL). Performance metrics for latency and storage requirement are derived from mathematical analysis as well as simulation of anticipated MTO-MSL mission scenarios

A network architecture for precision formation flying using the IEEE 802.11 MAC protocol

Precision formation flying (PFF) missions involve the tracking and maintenance of spacecraft in a desired geometric configuration. Autonomous control of the distributed spacecraft requires inter-spacecraft communications with guaranteed performance. We present a network architecture that supports PFF control across the various phases of mission operations, ranging from initial random deployment to precision formation. The architecture incorporates the IEEE 802.11 MAC protocol and utilizes both its distributed control function (DCF) and point coordination function (PCF) modes as appropriate to the PFF operational phase. A proactive routing protocol provides timely topology status. A new application layer protocol is incorporated which provides a simple interface between the PFF control application and the underlying communications network

Communications architecture for space-based sensor networks

Numerous planned and proposed future space exploration missions employ multiple spacecraft that perform multipoint sensing. Distributed space-based sensing missions can significantly benefit from incorporation of cross-link communications capabilities, thereby forming space-based networks, by enabling continuous access to any/all spacecraft via a single ground contact, real-time coordinated observations, and autonomous in situ processing within a spatial neighborhood of spacecraft. We present a communications architecture for space-based sensor networks. Because of the large inter-spacecraft distances, directional antennas are used, with a single half-duplex transceiver per spacecraft to achieve low cost. Orbital motion induces a dynamic albeit predictable geometry (and topology) among the spacecraft. Primary offered traffic is sensor telemetry destined to the Earth ground station, although other traffic patterns are also treated. We present a technique that derives the link activation schedule (transmit/receive mode and communications neighbor selection) and routes used for efficient traffic relay through the network, leveraging the Florens and McEliece algorithm for tree networks. An illustrative example is presented, and throughput and latency performance are evaluated. An extension to the networking method is described that is traffic adaptive.

Optical retroreflector-based sensor networks for in-situ science applications

In this paper we discuss the communication services requirements and constraints for future in-situ science network applications and present a peer-to-peer communication architecture using space-division optical retro-reflectors. We described how to operate the optical retro-reflectors links in both full and half-duplex modes, and analyze the energy efficiency and the trade-offs between sensor traffic, network size, link budget, duplex mode and the optical characteristics of the system. Our analysis shows full-duplex operation has the potential to double the energy efficiency achieved by traditional halfduplex operation when in-situ peer-to-peer communications becomes a dominant component in the overall network traffic.

Space QoS framework over a delay/disruption tolerant network

In space as well as terrestrial MANET networks the cost of additional capacity is high. In order to support increasing demand for network resources it becomes necessary to intelligently reallocate resources among applications. It is envisioned that future space applications will involve a dynamic mixture of real-time and delay tolerant applications such as two-way voice, streaming video, telemetry, command, and delay tolerant file transfers with some deadline constraint. It is also envisioned that the future network in space will be DTN-capable, allowing for store-and-forward operation in a frequently disconnected environment. In this paper we present a Quality-of-Service framework for future space applications and a test implementation of that framework over DTN. 1 2

On the performance of adaptive data rate over deep space Ka-band link: Case study using Kepler data

Future missions envisioned for both human and robotic exploration demand increasing communication capacity through the use of Ka-band communications. The Ka-band channel, being more sensitive to weather impairments, presents a unique trade-offs between data storage, latency, data volume and reliability. While there are many possible techniques for optimizing Ka-band operations such as adaptive modulation and coding and site-diversity, this study focus exclusively on the use of adaptive data rate (ADR) to achieve significant improvement in the data volume-availability trade-off over a wide range of link distances for near Earth and Mars exploration. Four years of Kepler Ka-band downlink symbol signal-to-noise (SNR) data reported by the Deep Space Network were utilized to characterize the Ka-band channel statistics at each site and conduct various what-if performance analysis for different link distances. We model a notional closed-loop adaptive data rate system in which an algorithm predicts the channel condition two-way light time (TWLT) into the future using symbol SNR reported in near-real time by the ground receiver and determines the best data rate to use. Fixed and adaptive margins were used to mitigate errors in channel prediction. The performance of this closed-loop adaptive data rate approach is quantified in terms of data volume and availability and compared to the actual mission configuration and a hypothetical, optimized single rate configuration assuming full a priori channel knowledge.

Space network time distribution and synchronization protocol development for Mars proximity link

Time distribution and synchronization in deep space network are challenging due to long propagation delays, spacecraft movements, and relativistic effects. Further, the Network Time Protocol (NTP) designed for terrestrial networks may not work properly in space. In this work, we consider the time distribution protocol based on time message exchanges similar to Network Time Protocol (NTP). We present the Proximity-1 Space Link Interleaved Time Synchronization (PITS) algorithm that can work with the CCSDS Proximity-1 Space Data Link Protocol. The PITS algorithm provides faster time synchronization via two-way time transfer over proximity links, improves scalability as the number of spacecraft increase, lowers storage space requirement for collecting time samples, and is robust against packet loss and duplication which underlying protocol mechanisms provide.

A study of adaptive coding and modulation over free space optical link using OCTL data

In this paper we utilized the downlink channel data captured at the Optical Communication Test Laboratory (OCTL) in support of the Lunar Laser Communications Demonstration (LLCD) to evaluate the potential performance gain of using adaptive coding and modulation (ACM) scheme to increase data volume while mitigating weather impairment. The study assumed that the estimated signal and noise photon counts statistics generated by concurrent processing of the downlink signal is available in real time to drive a closed-loop control of the code rate and the PPM order of the downlink. Downlink data from two representative passes were used. Pass no. 12, which occurred on October 29, 2013 and lasted 24 minutes and 15 second, recorded several link outages due to thin cloud and cloud interruption and is used to calibrate the symbol error constraint for the adaptive modulation algorithm. Pass no. 20, which was 19 minutes and 16 seconds long and occurred on November 18, 2013, operated over very clear sky and large sun-earth-probe (SEP) angle (approx. 170 degrees) and provided a representative sample of a clean optical channel. This study ignores constraints such as peak and average power limitations and processing overhead. Impacts of delay in the control loop are not addressed in this study, therefore any performance enhancement indicated by this study is only applicable to a short propagation delay environment. For deep space environment, further analysis will be required to determine the benefit of ACM.

The Benefits of Packet Service in Evolving Space Communications Provider Networks

We advocate a general packet interface for space missions. This service interface is essential to support the delivery of bursty and variable-size data units across the ProviderUser boundary for all missions using CCSDS Space Packet, Internet Protocol (IP), Delay Tolerant Networking (DTN), CFDP, or user-defined packets. This service bridges the ground and space domains, allowing optimization within either domain without impacting the other. The proposed Packet Service enables the service provider systems to directly engage in the execution of space link optimization and enhancement by applying methods including adaptive coding, modulation, and data rate as well as automatic retransmission. Packet Service reduces the user ground system complexity and provides a more natural interface to user applications. In addition, Packet Service offers a crucial step in the transition toward the future Solar System Internetwork.

Space-based networking technology developments in the Interplanetary Network Directorate Information Technology Program

We present advanced technology developments in space networking achieved under the Interplanetary Network Directorate Information Technology Program. Progress has been made in three primary areas: (1) distributed on-board scheduling as it applies to communications; (2) middleware services; and (3) communications protocols. Demonstrations of these capabilities have been provided using a simulation environment capable of providing a realistic representation of the underlying space communications network environment