ADS-B is increasingly used for air traffic control in areas covered by terrestrial receivers; however, its limited range makes it unsuitable for other areas such as the oceans. To overcome this limitation, it has been proposed to receive ADS-B signals from low earth orbit nano-satellites and relay them to the terrestrial receivers. This paper gives an overview of the GATOSS mission and of its highly-sensitive ADS-B software-defined radio receiver payload. Details of the design and implementation of the receiver's decoder are introduced. The first real-life, space-based results show that ADS-B signals are indeed successfully received in space and retransmitted to a terrestrial station by the GATOSS nano-satellite orbiting at 700+ km altitudes, thus showing that GATOSS is capable of tracking flights, including transoceanic ones, from space. I. MISSION OVERVIEWAir traffic management (ATM) increasingly relies on automatic dependent surveillance-broadcast (ADS-B) [1] technology. ADS-B is currently being rolled out around the world through the NEXTGEN [2], SESAR [3] and AIRE/ENGAGE [4] programs. The advantages of ADS-B over traditional radar and radio systems include lower infrastructure costs and improved situational awareness, both for ATMs that have access to more accurate data about the position, status and routes of all aircraft within range and for ADS-B equipped aircraft that receive information about other aircraft in the nearby airspace.However, as over-the-horizon communication is not possible, ADS-B cannot accurately track flights passing over areas without ground stations. As a result, a large part of the airspace still remains unsupervised e.g., oceanic or arctic regions [5] and areas scarcely fitted with ADS-B ground stations. To enable global ADS-B coverage, we propose a space-based ADS-B surveillance infrastructure that i) collects flight information from a constellation of low Earth orbit (LEO) nano-satellites equipped with sensitive ADS-B receivers, and ii) relays the collected flight information to ATM operators through ground stations. We refer to this as global air traffic awareness and optimization through spaceborne surveillance (GATOSS).The authors thank the Danish National Advanced Technology Foundation for partial funding. Yannick Le Moullec was affiliated with Aalborg University when the work began.Two scenarios for providing space-based ADS-B service have been envisaged:• Off-line data: a few (three to six) nano-satellites sample the airspace and provide information for off-line data processing; in this case the data is downlinked with delay when the nano-satellites pass over one or more ground stations.• On-line data: a near real-time picture of the airspace can be achieved by means of a larger fleet (40 to 70 nano-satellites) that communicate to the ATM infrastructure via geostationary data relay satellites. Fig. 1. GATOSS demonstration mission: ADS-B out signals transmitted from the aircraft are received and decoded by the proposed payload on-board the Cubesat nano-satellite and re-transm...

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