Journal of Wireless Networking and Communications 2013, 3(3): 19-28 DOI: 10.5923/j.jwnc.20130303.01 User Datagram and Bundle Protocol for Distributed Small Satellite Topologies Paul Muri 1,* , Janise McNair 1 , Joe Antoon 2 , Ann Gordon-Ross 2 , Kathryn Cason 3 , Norman Fitz-Coy 3 1 Wireless and Mobile (WAM) Systems Lab, Department of Electrical and Computer Engineering, Gainesville, FL 32611, USA 2 Center for High Performance Reconfigurable Computing (CHREC), Electrical and Computer Engineering, University of Florida 3 Space Systems Group (SSG) Department of Mechanical and Aerospace Engineering University of Florida Gainesville, FL Abstract Many planned Earth observation satellite missions use distributed satellite systems in constellation or cluster orbits instead of one, large satellite with many instruments. This network of satellites and ground stations allows data to be efficiently downlinked. However, to create a networking protocol for constellations of small Earth observing satellites, several limitations need to be addressed, including distributed topology management, slow downlink data-rates, and single point-to-point communication. Since distributed satellite constellations exacerbate the severity of these limitations, a thorough analysis of a constellation's network performance is required to ensure that task objectives are achievable. In this paper, we compared low Earth orbit(LEO) Earth observing satellite constellations using delay tolerant networking protocols and the user datagram protocol (UDP). The constellations' inter-satellite links and downlinks were evaluated using network metrics such as access window time, drop-ratio, and throughput. We simulated these network metrics using the Network Simulator 3 (ns-3). Previous works have proposed satellite constellations for Earth observation, however, constellations of small satellites have not been analyzed for network metrics. Results show that a sun-synchronous constellation with a repeating ground track outperforms a flower constellation with respect to increased access time, reduced drop-ratio, and higher throughput. The simulations also determined the optimum media access control slot time and packet transmission intervals for long distance satellite links. Additionally, we were able to compare the throughput performance of UDP to abundle protocol by adding nodes with delay tolerant networking protocol implementations. Keywords Communication networks, Delaytolerant networking (DTN), Network topology, Satellite communication, Satellite constellations, Network simulations, User datagram protocol (UDP), Vehicular networks, Wireless mesh networks 1. Introduction Smallsatellites are satellites based on a pico-satellite platform with a mass of less than one kilogram. Even though small satellites have grown in popularity, a size constraint of 10x10x10 cm[1] severely limits these satellites’ power and communication.With these limitations, small satellites have low transmission power due to a limited solar panel capacity of two watts and high latency due to the long propagation distance between satellites and ground stations, which can range up to 750 km for downlinking and 2,000 km for intersatellite lin king. These satellites are exclusively launched in low Earth orbit (LEO)[2], which leaves a limited timewindow to link with ground stations. Depending on the orbital parameters and the location of the ground station, a single LEO satellite may only downlink two to three times per day for ten-minute periods. Thus, a communication protocol is needed to allow bursts * Corresponding author: pmuri@ufl.edu (PaulMuri) Published online at http://journal.sapub.org/jwnc Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved of data to be downlinked efficiently when there is a line of sight connection with the ground station. To mitigate these limitations, researchers are developing constellations with several satellitescoordinating to perform a single task[3]. A constellation’s task dictates the constellation’s specific design. In some cases, multiple candidate constellation types may be appropriate for the same task. Traditionally, to minimize deployment cost, constellation types were selected to minimize the number of satellites given the constellation’s coverage requirements. However, when designing constellations of multiple satellites that communicate over inter-satellite links, the constellation’s network performance (i.e., the quality of the inter-satellite links) becomes a criterion for constellation selection. Delay tolerant networking (DTN) protocols were used in several prior space missions, including the Deep Impact Network Experiment in 2005, the UK-DMC in 2008, the International Space Station in 2009, and the “Internet Router In Space” (IRIS) on the IntelSat-14 in 2011[2]. The Consultative Committee for Space Data Systems (CCSDS) has standardized the bundle protocol for further use on geosynchronous satellite missions, such as the Laser Communications Relay Demonstration.However, small