Jeffrey Y. Tien

An overview of formation flying technology development for the Terrestrial Planet Finder mission

The objective of the Terrestrial Planet Finder (TPF) mission is to find and characterize earth-like planets orbiting other stars. Three architectural options are under consideration for this mission: a formation-flying interferometer (FFI), a structurally-connected interferometer, and a coronagraph. One of these options can be selected as the TPF baseline design in 2006. This paper describes the technology tasks underway to establish the viability of precision formation flying for the FFI option. In particular, interferometric science observations require autonomous precise control and maneuvering of five spacecraft to an accuracy of 2 cm in range and 1 arc-minute in bearing. This precision must be maintained over interspacecraft ranges varying from a few meters to hundreds of meters. Autonomous operations, ranging from formation acquisition and formation maneuvering to high precision formation flying during science observations, are required. Challenges lie in meeting the demanding performance requirements as well as in demonstrating the long-term robustness of the autonomous formation flying system. These challenges are unprecedented for deep space missions. To address them, research is under way in the areas of formation control algorithms, relative sensor technologies, system design, end-to-end real-time system simulation, and ground-based and micro-g end-to-end system demonstrations. Four interrelated testbeds are under development concurrently with the FFI system design. The testbeds include the formation algorithms & simulation testbed (FAST), the formation sensor testbed (FST), the formation control testbed (FCT) and the synchronized position hold engage re-orient experimental satellites (SPHERES) experiment. Formation flying technologies developed under the StarLight project and the NASA Distributed Spacecraft Technology (DST) program are being leveraged and expanded to meet the TPF requirements. This paper provides an overview of the ongoing precision formation flying technology development activities.

Technology validation of the autonomous formation flying sensor for precision formation flying

A Radio Frequency (RF) based sensor, called the Autonomous Formation Flying (AFF) sensor, has been developed to enable deep space precision formation flying by measuring the relative range and bearing angles between multiple spacecraft. The AFF sensor operates at Ka-band and uses signal-processing schemes inherited from the Global Positioning System (GPS). The key features of the AFF sensor are: (a) it operates autonomously without the aid of spacecraft or ground control, (b) it simultaneously provides a wide field of view and accurate angle and bearing angle measurements and it provides accuracy better than 2 cm and 1 arcmin (1-/spl sigma/) near the bore-sight of the antenna, and (c) it provides telemetry among the constellation elements. In this paper we describe the key technology challenges, the approach to resolving them through analysis and testbed activities, and the results of the testbed activities.

Formation acquisition sensor for the Terrestrial Planet Finder (TPF) mission

The Terrestrial Planet Finder (TPF) pre-project, an element of NASAs Origins program, is currently investigating multiple implementation architectures for finding Earth-like planets around other stars. One of the technologies being developed is the Formation Flying Interferometer (FFI). The FFI is envisioned to consist of up to seven spacecraft, each with an infrared telescope, flying in precise formation within /spl plusmn/1 cm of pre-determined trajectories for synchronized observations. The spacecraft-to-spacecraft separations are variable between 16 m and 100 m during observations to support various interferometer configurations in the planet-finding mode. The challenges involved with TPF autonomous operations, ranging from formation acquisition and formation maneuvering, to high precision formation flying during science observations are unprecedented for deep space missions. To meet these challenges, the Formation Sensor Testbed (FST) under the TPF technology program develops and demonstrates the key technology of the formation acquisition sensor. Key performance targets for the acquisition sensor are an instantaneous 4/spl pi/-steradian field of view and simultaneous range and bearing-angle measurements for multiple spacecraft with accuracy better than 50 cm and 1 degree, respectively. This paper describes the TPF FFI mission concept, the requirements for the acquisition sensor, design trades, the resulting sensor, and the technology to be demonstrated by the testbeds.