Constellations of small satellites are useful for a number of earth observation and space exploration missions. The Heterogeneous Spacecraft Network project is defining operations concepts and promising technology that can provide greater capability at lower cost. Typically, such spacecraft can communicate with each other in orbit and with ground stations for spacecraft operation and downlink of science data. However, small spacecraft often cannot utilize the capability delivered by networks such as the Universal Space Network, even if the mission could afford the cost. Small spacecraft have significant constraints in terms of power availability, attitude stability and overall mass and volume, requiring innovative technology for implementing highly functional satellites. A major challenge for such missions is selecting communications technology able to function in the space environment, able to meet the requirements for both inter-satellite and space-toground data links and fit within the resources available on small satellites. Moreover, the cost of the technology needs to be as low as possible to facilitate participation by a broad range of organizations. Finally, the communications networks should conform to standards allowing broad adoption and the use of common infrastructure for multiple missions. Communications technology based on the IEEE 802 family of local area and metropolitan area network standards can be adapted to meet the needs of such missions. This paper will identify possible development paths for improved communication between small satellites and to the ground by reviewing and evaluating standards-based technology for use by small satellite missions. Methods for greatly extending both range and data rate will be proposed and analyzed. It will review and evaluate the IEEE 802.11 wireless network standards, the ITU WCDMA 3G cell phone standard and the IEEE 802.15.4 Personal Area Network standard. A simple set of communication requirements will define the trade offs between standards and identify the technical capability needed for such missions. Specifically, the improvements needed to the Physical Layer to extend range to 1200 Km and the ability to comply with spectrum management constraints will be investigated. Authentication and encryption will be addressed along 1 with adjustments to the Media Access Control layer that can optimize data transfer rates over a broad range of distances and conditions. The paper concludes with recommendations for standards-based communication technology development for small satellites supported by the results of this trade study. The primary objective is to greatly reduce the cost of data communication for small satellites by establishing a common infrastructure able to meet the needs of most missions.

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Heterogeneous Spacecraft Networks (HSNs) are network environments in which spacecraft from different missions and institutions can communicate with each other at low cost and with low impact on overall system resources. The Mission Design Center (MDC) at NASA Ames Research Center has been studying solutions for low cost multi spacecraft systems for a number of years. One may now build on the idea to interconnect clusters of spacecraft with each other to have them act as mobile nodes belonging to the same collaborative mission. Recent progress in small satellite technology is significant, and one of the advantages of small satellites lies precisely in the large quantity of spacecraft that can be produced at accessible costs. It follows naturally that small satellites are an interesting candidate platform for development and demonstration of the HSN concept. This paper is the second in a series of three companion papers. The general concept of operations for HSNs in LEO and a number of future applications are proposed in the first paper [6], while enabling technology such as devices and lower layer protocols are discussed in paper three [7]. In this paper, we pick up the scenario of a low-cost and multi-institutional network of Earth Observation (EO) missions in LEO and conduct network performance analysis using the AGI System Tool Kit (STK) and the open-source Network Simulator (NS-3). A multi spacecraft network consolidates the individual capabilities of each spacecraft from different institutions by combining benefits of both frequent revisit and concentrated observation. Complementary and correlated data could be collected simultaneously from a large set of distributed spacecraft utilizing HSN capability. In this specific configuration, communication distance between spacecraft, related delays and error rate are the major factors in network performance.Also, average duration of communication opportunities between spacecraft is usually very limited. Thus, it is important to simulate orbital dynamics, link margins, and protocols simultaneously to analyze network performances. In this paper, we compare some existing protocols to obtain a measure for the practical performance of the candidate network. We focus on best-effort data delivery, an approach necessitated by the severe constraints on communications resulting from low-cost and low system resource small spacecraft. In the application layer, we show that packet size and data rate of a source node also affect overall performance of the network. We present the resulting figures of merit from our simulations. The paper concludes with a summary of the simulation results.

This work provides an efficiency analysis of the LightForce space debris collision avoidance scheme in the current debris environment and describes a simulation approach to assess its impact on the long-term evolution of the space debris environment. LightForce aims to provide just-in-time collision avoidance by utilizing photon pressure from ground-based industrial lasers. These ground stations impart minimal accelerations to increase the miss distance for a predicted conjunction between two objects. In the first part of this paper we will present research that investigates the short-term effect of a few systems consisting of 20 kW class lasers directed by 1.5 m diameter telescopes using adaptive optics. The results found such a network of ground stations to mitigate more than 85 percent of conjunctions and could lower the expected number of collisions in Low Earth Orbit (LEO) by an order of magnitude. While these are impressive numbers that indicate LightForce's utility in the short-term, the remaining 15 % of possible collisions contain (among others) conjunctions between two massive objects that would add large amount of debris if they collide. Still, conjunctions between massive objects and smaller objects can be mitigated. Hence, we choose to expand the capabilities of the simulation software to investigate the overall effect of a network of LightForce stations on the long-term debris evolution. In the second part of this paper, we will present the planned simulation approach for that effort. For the efficiency analysis of collision avoidance in the current debris environment, we utilize a simulation approach that uses the entire Two Line Element (TLE) catalog in LEO for a given day as initial input. These objects are propagated for one year and an all-on-all conjunction analysis is performed. For conjunctions that fall below a range threshold, we calculate the probability of collision and record those values. To assess efficiency, we compare a baseline (without collision avoidance) conjunction analysis with an analysis where LightForce is active. Using that approach, we take into account that collision avoidance maneuvers could have effects on third objects. Performing all-on-all conjunction analyses for extended period of time requires significant computer resources; hence we implemented this simulation utilizing a highly parallel approach on the NASA Pleiades supercomputer.

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