Timothy C. Henderson

High-Precision Pointing and Attitude Determination and Control on ExoplanetSat

ExoplanetSat is a 10 10 34-cm, 5-kg space telescope designed to detect exoplanets around the brightest Sun-like stars via the transit method. Achieving this objective while meeting strict mass, volume, and power constraints necessitates an innovative highprecision pointing and attitude determination and control subsystem (ADCS) design. This paper will present the overall ADCS hardware and software design of ExoplanetSat. The software description will focus on the payload operation during slews (ecient window generation for stars entering the eld of view) as well as the guidance, navigation, and control algorithms for high-precision pointing with two-stage actuation. Analyses and results on the achievable pointing precision using a high-delity simulation will be presented. To demonstrate this two-stage control idea in hardware, a testbed on a spherical air bearing was designed, fabricated, and tested at the Massachusetts Institute of Technology (MIT) Space Systems Laboratory (SSL) in collaboration with Draper Laboratory. This hardwarein-the-loop testbed successfully demonstrated precision pointing, raising this subsystem to a technology readiness level (TRL) of 5. Furthermore, results from this testbed were used to validate the simulation results, thereby increasing the condence in results produced by the simulation.

Attitude determination and control system design for the CYGNSS microsatellite

This paper presents the development of the attitude determination and control system design of the Cyclone Global Navigation Satellite System spacecraft. The CYGNSS constellation consists of eight small satellite observatories in 500 km circular orbits at an inclination of 35 deg released from a single launch platform. Each CYGNSS spacecraft will make frequent and accurate measurements of ocean surface winds throughout the life cycle of tropical storms and hurricanes with the objective to fundamentally improve gap-free coverage for hurricane forecast and monitoring. Realising this objective requires the spacecraft to accurately and reliably point its signal collection antennae in desired directions and hold its Earth relative attitude over long time durations to prescribed knowledge and point requirements. Indeed, the microsatellite ADCS regulates all spacecraft estimation and control functionality spanning detumbling; Sun acquisition and hold; pointing control and momentum-management over the micro-satellite lifetime to design requirements. This paper presents the ADCS hardware, software and algorithms used to control the spacecraft in all phases of CYGNSS operations and presents simulation based performance results of the closed-loop estimation and control systems.

Conical scanning approach for Sun pointing on the CYGNSS microsatellite

The small scales of area, volume, and power of small spacecraft, such as NASAs 25-kg Cyclone Global Navigation Satellite System (CYGNSS) satellites, constrain the number of independent subsystems that they can support. Consequently, small satellites often require novel approaches to execute the same mission functions that a larger satellite can easily perform with familiar sensor, actuator and algorithm options. In the case of CYGNSS, the spacecraft must execute a Sun acquisition and pointing phase but the actuator suite does not include 2-axis sun sensors or rate gyros; two measurements that seem like obvious inclusions for the Sun acquisition task. Instead, during Sun acquisition, the CYGNSS attitude control system uses a limited actuator and sensor set consisting of three magnetic torque rods, a three-axis magnetometer, and Sun incidence-angle measurements from three solar panel faces. This paper describes the sensing and control algorithms implemented in CYGNSS flight software to acquire and maintain Sun pointing with the available measurements and actuators. The Sun pointing algorithm uses a conical scanning approach based on traditional RF pointing and target-tracking systems, which consists of two key control loops: (1) a rate loop, which initiates a body spin about the solar-array face axis, and (2) a slower angle controller that tracks the array power gradients measured over the course of the fast spin. A slew toward the peak power eventually drives the solar panel face normal to spin in a cone centered about the Sun vector. The Sun acquisition process has a large convergence basin, is stable in the Lyapunov sense, and demonstrates excellent performance behavior in simulation.

Low-mass high-performance deployable optical baffle for CubeSats

An optical moon baffle for stray light attenuation was developed for use on ExoplanetSat, a 3U CubeSat being developed jointly by MIT and Draper Laboratory which aims to detect transiting exoplanets via precision photometry. This paper will discuss the optical and mechanical design of the baffle, as well as the optical performance as demonstrated through the test of a prototype. The baffle collapses to fit into a small volume around ExoplanetSats lens and deploys on orbit to a full length of 12 cm. The baffle is capable of attenuating moonlight by a factor of 105 at a lunar exclusion angle of 30 degrees.