Supporting the Flock: Building a Ground Station Network for Autonomy and Reliability

Planet Labs is on a mission to image the whole earth every day. This requires a large orbital constellation and a distributed, autonomous ground station network. The network currently includes 11 geographically diverse sites, many with multiple antenna systems. Antennas systems are deployed with uniform equipment to simplify interfaces and ease operations. Additionally, tools have been developed for automated monitoring and remote troubleshooting of RF chains. Predictive modeling tools help to plan for future need as the orbital constellation grows. Lessons learned and ideas for automation and monitoring are also discussed. A NEW IMAGE OF THE EARTH EVERY DAY Planet Labs is on a mission to image the whole world every day and make global change visible, accessible, and actionable. We strive to improve the world by providing a medium-resolution rapidly-updating data set. Recently our data has been used to raise awareness about illegal mining in a natural reserve[1]. In response to the recent fires in Alberta [2] and earthquake in Ecuador [3], we released our imagery of the areas to aid government and humanitarian relief efforts. As of May 2016, we have successfully launched 133 Dove satellites from 11 different launch vehicles and the International Space Station, and we are rapidly scaling operations to meet our mission. Our current constellation consists of fifty-five 3U CubeSats. Planet has built the largest constellation of commercial earth imaging satellites, and our Doves image anytime they are over land. This constellation generates massive amounts of image data that we need to bring to the ground in a quick and efficient manner. In addition, we collect normal housekeeping telemetry, update software, and refresh the on-board schedules. MEETING THE NEEDS OF AGILE AEROSPACE Agile aerospace is a new set of principles based on the agile software development concepts including rapid design iteration, a continuous improvement process, and easy scalability. As a result of Planet’s agile approach, recent ISS deployments have dramatically increased our constellation of satellites (or “Flock of Doves”) to fifty-five on-orbit CubeSats. Our Doves take approximately 650 passes per day on 33 antenna systems spread across our 11 active ground station sites. The average amount of picture data downlinked per day is approximately 550 GB, with a maximum of 777 GB [4]. Once these latest satellites finish commissioning and begin operations, downlink numbers will rise further. Our distributed ground station network provides several benefits; it reduces the impact of any one failure, serves multiple orbital planes, reduces the time between image capture and downlink, and spreads the load of satellite downlinks more evenly across the globe. At the same time, the multiple sites increase complexity of the network and result in a global, “always-on” ground station network that is maintained, monitored, developed, and expanded by a relatively small team of five people. Autonomy and reliability are necessary. Passes and tasks are scheduled by a computer model, faults are detected and cleared as much as possible by the individual stations, and those not automatically resolved are escalated to the on-call engineer. We rely on software tools to aid in managing a network capable of supporting the current and future constellation. Building Uniform Ground Stations The first Dove CubeSats were launched in April 2013, and there have been 14 satellite hardware revisions Colton 1 30 Annual AIAA/USU Conference on Small Satellites since then that have included new processors, better persistent data storage, newer cameras, and more advanced optics. Even with these hardware changes the link interface has been consistent throughout this time period. Aside from better antennas and a recent baud rate change (from 24 Mbaud to 60 Mbaud) that required a hardware upgrade, all of the changes have been software. On the ground station side of the link, there have been several hardware upgrades and countless software updates during the past three years. Hardware changes include new internet firewall appliances and ethernet switches for increased network security, new transceivers with reduced EMI and spurious signals, GPS time synchronization, and new computer servers with more processing power and remote management features. Figure 1: Planet’s Ground Station Network. Our ground station network includes 11 active global sites. While Figure 1 shows that most of our ground station sites are located in the United States, we do have sites scattered around the world. As discussed previously [5, 6], most CubeSats are using amateur radio frequencies, which are licensed for noncommercial use worldwide. Since we are a commercial company based in the United States, Planet acquired commercial licenses for our satellites [7]. Working with the different frequency licensing organizations around the world can be challenging and time consuming. Uniformity, however, aids in licensing. Licenses often require knowledge of the hardware, and by keeping it similar from site to site, we can begin the licensing exploration and application process in advance. Tracking, Telemetry, and Command The Dove satellites use UHF for TT&C and orbit determination [9], so the constellation benefits greatly from having frequent and global UHF link opportunities. The Planet Labs UHF ground station uses simple, inexpensive COTS components to support the need for diverse coverage at a reasonable cost and with easy maintenance. They use standard Yagi antennas positioned by a Yaesu rotor for both uplink and downlink. Figure 2 shows the antennas on a typical UHF ground station. Figure 2: Example UHF Station. The only non-COTS component in our ground stations is the custom-built SpaceTalker transceiver. It is based on a Texas Instruments CC1110 wireless MCU, which has both a UHF transceiver and microcontroller unit in a single chip. The server communicates with the transceiver over USB [8]. Both the ground SpaceTalker radio and space hardware use extremely similar architecture and interfaces, increasing the system’s predictability and reliability. Figure 3 shows a block diagram of our UHF systems. Figure 3: UHF TT&C Block Diagram. Colton 2 30 Annual AIAA/USU Conference on Small Satellites