would have on TSIS pointing. In particular, very little information was available about ISS jitter and most of  it focused on ISS vibration as it related to structural safety at non-ELC locations and not to pointing  performance for external payloads on ELCs. The few payload studies that did exist attempted to apply  translational acceleration measurements across the ISS structure in order to estimate angular jitter on the  ELCs, and these estimates were near or close to the TSIS maximum allowable jitter. Additionally, due to  the large physical scale and dynamic configuration of payloads, modules, and visiting vehicles, past  predictions and measurements may differ or change spatially depending on mounting location and  temporally as the station configuration evolves. Finally, it became clear that no procedural mechanism  existed (or will exist) to manage ISS jitter, so instrument payloads (e.g., TSIS) must design their instruments  to be robust to ISS jitter based on their own performance requirements and risk posture.   As an aside and a sign of progress, in the past year several ELC-mounted payloads, including OPALS [3],  SAGE III, and now TSIS, have been able to successfully measure the jitter environment at their specific  ELC mounting locations. It is anticipated that these new measurements will reduce the uncertainty in the  ISS jitter environment for future payloads.  Actuator Selection  Faced with the uncertain jitter environment and other design accommodations specific to ISS, the LASP  engineering team performed a detailed comparison between stepper-based actuators used on similar LASP  pointing systems, including the TIM Pointing System for the Glory mission and a previous iteration of TSIS  for the NPOESS mission, and direct-drive BLDC actuators. The next section will describe many of the  differences in these actuators along with some of the key advantages BLDC motors can offer for precision  pointing applications.  For context, it is worth mentioning that stepper-based actuators are usually intended for coarse pointing  applications such as solar array drive mechanisms and antenna pointing mechanisms with typical pointing  accuracies around 1 degree. By contrast, TSIS requires precision solar pointing with less than 60  arcseconds 1σ of jitter that is only marginally compatible with a common cardinal step size of ~30  arcseconds.  BLDC Advantages Compared to Stepper-Based Actuators  This section provides a qualitative description of many of the advantages that direct-drive BLDC actuators  provide compared to stepper-based actuators in the hopes that it may be helpful to others trying to select  an appropriate actuator type for precision pointing applications. For this comparison of space-borne  actuators, it is assumed that the stepper motors are mated to harmonic drive based gear train to increase  torque output and reduce step size. It is also assumed that the reader is familiar with both technologies, but  expertise at the first-principals mathematical level is not required.  Continuous Torque Command Authority  A distinct advantage to stepper motors is their functional simplicity. The controller requests that the motor  moves a number of steps (angle) and the motor responds by moving that amount. For simple applications  where the step size is small relative to required angular precision, this open-loop approach is ideal for its  set-it-and-forget-it simplicity.  However, for more precise applications, these benefits can become a liability. At an abstracted level, the  stepper motor operates in a discrete manner as governed by the cardinal step size of the motor in  combination with any additional gearing in the actuator. This is accomplished by applying a step change in  power to the motor windings that results in a corresponding change in torque to the rotor. This torque moves  the rotor along with the load to the next angular position with associated dynamic overshoot. If the desired  angle is smaller than the step size, the controller is unable to reduce the angular error. This effect can be  reduced somewhat through microstepping, but it comes at a cost of increased complexity and reduced  holding torque while still subject to most of the limitations of stepper motors.   NASA/CP—2018-219887 272