13 SYSTEMS: INTERNATIONAL SPACE STATION PLANNING—A ROADMAP TO GETTING IT ALL DONE CHAPTER 1 and coordinate replanning. This process continues around the clock, 7 days a week tweaks to the schedule, whether small or large, occur on a daily basis. The real-time OPS PLAN team is also responsible for reviewing and approving all messages to be uplinked daily for the crew’s Daily Execute Package. A significant part of the package is a Daily Summary. As the name implies, this part provides a high-level summary of notable activities and constraints (e.g., “The thruster will be fired at a specific time today, so ensure the window shutters are closed to prevent contamination.”) as well as follow-up questions to the crew (e.g., “Last week, a piece of hardware was reported broken. Can you please provide the serial number of the item?”) or answers to questions the crew had asked (e.g., “Can I move my exercise on Wednesday to later in the day?”). These messages also include procedure updates, activity overviews (e.g., a big-picture plan for an upcoming spacewalk), or system data. This is the final product generated by the planning team in preparation for plan execution. Changes to the plan that occur during the day of plan execution, due to anomalies encountered or for a variety of other reasons, are fed back into the planning process by the planners. Constraints for completing activities or urgency to implement new activities in response to system failures (e.g., exercise equipment breakage, toilet troubles, laptop failures, computer network problems, etc.) dictate how quickly plan changes need to be implemented. Again, the PPCR system is used to document these plan changes and work them into future plans. Scheduling Challenges During ISS operations, planners routinely grapple with a number of scheduling problems. For example, scheduling activities that need communications satellite coverage, managing resources and temporal relationships (e.g., Activity B must occur no earlier than 30 minutes after the end of Activity A), scheduling activities for globally distributed users, handling uncertainty in task duration, and wrestling with on- board stowage and worksite issues. Mission planners and crews continue to evolve the understanding of types of information the crews need and how to more effectively tie crews into the planning process during increment execution. Communication relay satellite scheduling can be especially challenging, given the complex nature of communications requirements, the competition for services with other users (e.g., the Hubble Space Telescope, the Department of Defense), and the uncertainties in coverage quality with variances in vehicle attitude. Science payloads, major events such as visiting vehicles dockings and undockings, spacewalks, and video events intended for the public generally all require using NASA’s Tracking and Data Relay Satellite (TDRS) (Ku- band for video and high-rate data transmission S-band for voice and health and status telemetry. See also Chapter 13). The ISS planners make requests of TDRS services weeks in advance in competition with other TDRS users. Even uncertainties in vehicle attitude, which affects the ability of the radio antennae on the ISS to have the required direct line of sight to the TDRS, and where the solar arrays might be in their constant motion that sometimes can block the signal, may render a communication pass unusable. These factors make it difficult for planners to commit to specific times more than 1 week in advance for ISS TDRS service needs. Late changes may result in a lack of available TDRS time since service is scheduled on a first come, first served basis. If the ISS Program suddenly needs TDRS coverage—such as for an emergency spacewalk—the NASA flight director can declare the TDRS time as critical, thereby forcing other users off the network. Due to the impacts to other uses, which can include loss of science, this is not done unless absolutely required. Scheduling use of the exercise equipment is one of the bigger challenges in daily planning. Three main exercise devices are located on the USOS part of the ISS. These devices include Treadmill 2, a Cycle- Ergometer with Vibration Isolation System, and the Advanced Resistive Exercise Device. Crew members are required to exercise a minimum of 2.5 hours per workday and follow strict exercise programs created by the medical team. Additionally, crew members often have preferences as to when they would like to exercise. Some crew members prefer to complete their exercise in the morning, others prefer to spread it throughout the day, and some like to perform their exercises in a particular order (e.g., aerobic followed by resistive). All six crew members must use the Advanced Resistive Exercise Device, thus compounding the planning. When a piece of exercise equipment breaks, a great deal of replanning is usually required until the equipment is repaired.