Autonomous access links using laser communications

ABSTRACT We have validated an autonomous acquisition scheme that is critical for achieving data transfer over proximity links with ranges up to a few thousand kilome ters. The sun-illuminated Internationa l Space Station (ISS) against a dark sky background during terminator passes over Southern California was used to validate the autonomous acquisition and tracking scheme. A root mean square (rms) accuracy of 83 P rad was achieved. Keywords: Laser Communication, Autonomous Acquisition, Tracking 1. INTRODUCTION As free-space laser communication technology evolves an incr easing number of niche appli cations offering significant performance advantages are expected. The performance advantages will stem from increased bandwidth and information throughput with reduced size, weight and power. In this paper we discuss one such application, namely proximity or access links that operate to fa cilitate high-rate data transfer between assets separated by distances of a few thousand kilometers. The optical access link was originally conceived for transferring information from surface assets on Mars to science orbiters around Mars, for subsequent rela y back to Earth. Today such access links operate at UHF frequencies and have returned a bulk of the information recei ved [1] from the Mars Exploration Rove rs (Spirit and Opportunity) [2] and the recent Phoenix Mission [3]. The upcoming Mars Science Laboratory and other futu re lander/rover missions will rely on similar access links as a primary means for returning data to Ea rth. Typical data volumes expected in the MSL-MRO era are 1-4 gigabits per Martian day (Gb/sol). Recently we reported on analysis, transceiver prototyping and link emulation for Mars optical access links, that can improve data return to at least 90 Gb/sol, nearly two orders of magnitude greater than state- of-the art UHF systems [4]. Optical access links can easily boost the data return relative to currently used UHF systems by 1 to 2 orders of magnitude. Moreover, this performance improvement can be achieved with mass, power and operational complexity comparable to those of UHF systems. Typically the links will be asymmetric requiring high volume data transfer from the surface to the orbiter and low-rate command capability from the orbiter to the surface. Instantaneous rates of 50-200 megabits per second (Mb/s) for the return or uplink and 0.05  1 Mb/s for the forward or downlink are currently targeted. The optical access link systems will potentially be deployed on science orbiters around Mars (300-400 km altitude circular orbits, or 150 x 6200 km elliptical orbits) and surface landers or rovers. The communication ranges will be limited to a few thousand kilometers. Optical access link systems can also potentially be deployed at the Moon for relaying information from surface assets to Earth via lunar relay satellites. An important difference between the lunar and Mars scenarios is the atmospheric attenuation at Mars wh ich due to dust loading of the atmosphere can be of the order of 10 dB at zenith angles of 75-80 degrees. In this paper we first describe, in Section 2, the concept of operations for an optical access link. Section 3 describes experiments performed to validate autonomous acquisition, a critical step in the overall concept of operations. The experiments involve the use of prototype transceivers to passively acquire and track the sunlit International Space Station (ISS) during terminator passes over Southern California i.e. ISS passes that occur just after sunset or just before sunrise so that the spacecraft appears brig ht against a dark sky background.