The TPF-Darwin planet-finding concept is a space-based mid-IR nulling interferometer consisting of four formation flying reflecting mirrors focusing light into a separate beam combiner spacecraft. As a flagship mission it is a highly capable instrument with the principal science goal of detecting Earth-like planets in the habitable zone of nearby stars (up to 15 parsec) from their thermal emission, and characterizing their physical properties. Measurements can be made of the size, temperature, orbit, and of the presence of an atmosphere with moderate resolution (R = 50) spectra to reveal bio-markers such as ozone, carbon dioxide, methane, and water vapor. The sensitivity is such that a whole planetary system clown to half Earth-size planets can be detected in a single day and in a five-year mission life, revisits and repeat detections would reveal the system dynamics. For the most interesting objects, longer measurement series would reveal the detailed thermal spectrum; these measurements would take place in the later part of the mission. The system also has a general astrophysics capability for investigations of distant compact objects and the mission timeline provides ample opportunities for such observations. The mission would use a single heavy launch vehicle to place the system at L2 Sun-Earth halo orbit. This poster shows the main mission elements, the spacecraft design and formation flying, and xenon-ion thruster technologies. Many of the ideas have been developed with European colleagues and the mission has potential to be a collaborative effort between the US and European space agencies.

A document describes the proposed addition of a radar function to the Autonomous Formation Flying Sensor, making possible coarse relative-position control to prevent collisions in the event of failure of one of the spacecraft. According to the proposal, in addition to tracking GPS-like one-way ranging signals transmitted by the other normally functioning spacecraft, each spacecraft could simultaneously track the reflection of its own ranging signal from a disabled, non-transmitting spacecraft. From the round-trip travel time, the approximate distance to the disabled spacecraft could be estimated. To prevent jamming of the receiver by the transmitter on the same spacecraft, the receiver would be switched off during transmission.

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.

The Terrestrial Planet Finder (TPF) pre-project, an element of NASA s Origins program, is currently developing two architectures for a mission to search for earth-like planets around nearby stars. One of the architectures being developed is the Formation Flying Interferometer (FFI). The FFI is envisioned to consist of up to seven spacecraft (as many as six collectors with IR telescopes, and a combiner) flying in precise formation within +/-1 cm of pre-determined trajectories for synchronized observations. The spacecraft-to-spacecraft separations are variable between 20 m and 100 m or more during observations to support various configurations of the interferometer in the planet-finding mode. The challenges involved with TPF autonomous operations, ranging from formation acquisition and formation maneuvering to high precision formation control during science observations, are unprecedented. In this paper we discuss the development of the formation acquisition sensor, which uses novel modulation and duplexing schemes to enable fast signal acquisition, multiple-spacecraft operation, and mitigation of inherent jamming conditions, while providing precise formation sensing and integrated radar capability. This approach performs delay synthesis and carrier cycle ambiguity resolution to improve range measurement, and uses differential carrier cycle ambiguity resolution to make precise bearing angle measurements without calibration maneuvers.

The Terrestrial Planet Finder (TPF) pre-project, an element of NASA's 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.

Ka-band integrated range and bearing-angle formation sensor called the Autonomous Formation Flying (AFF) Sensor has been developed to enable deep-space formation flying of multiple spacecraft. The AFF Sensor concept is similar to that of the Global Positioning System (GPS), but the AFF Sensor would not use the GPS. The AFF Sensor would reside in radio transceivers and signal-processing subsystems aboard the formation-flying spacecraft. A version of the AFF Sensor has been developed for initial application to the two-spacecraft StarLight optical-interferometry mission, and several design investigations have been performed. From the prototype development, it has been concluded that the AFF Sensor can be expected to measure distances and directions with standard deviations of 2 cm and 1 arc minute, respectively, for spacecraft separations ranging up to about 1 km. It has also been concluded that it is necessary to optimize performance of the overall mission through design trade-offs among the performance of the AFF Sensor, the field of view of the AFF Sensor, the designs of the spacecraft and the scientific instruments that they will carry, the spacecraft maneuvers required for formation flying, and the design of a formation-control system.

A formation-flying sensor that can determine spacecraft separation with a max- imum uncertainty of 2 cm, measure the bearing angles of the remote spacecraft with a maximum uncertainty of a minute of arc, and operate with a wide field of view autonomously in deep space has been designed and prototyped. It is the au- tonomous formation-flying (AFF) sensor that operates at 32 GHz (Ka-band) using technology similar to that of the Global Positioning System (GPS). A significant challenge lies in the simultaneous requirements for precision and a wide field of view, mandating a substantial technology development effort and design of a sensor with some novel features. Through development of a prototype, the AFF sensor has been extensively characterized and the key technology risks have been retired. It has been concluded that the AFF sensor can meet the (2-cm, 1-arcmin) require- ment within the StarLight two-spacecraft stellar optical interferometer mission. An overview of the sensor design, results of the technology development, conclusions of the technology investigations, and highlights of the related inter-spacecraft issues are presented in this article.

A Radio Frequency (RF) based sensor, called the Autonomous Formation Flying (AFF) sensor(2), 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 range and bearing angle measurements and it provides accuracy better than 2 cm and 1 arcmin (1-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.

The StarLight Mission, an element of NASA's Origins Program, was designed for first-time demonstration of two technologies: formation flying optical interferometry between spacecraft and autonomous precise formation flying of an array of spacecraft to support optical interferometry. The design overview and results of the technology effort are presented in this paper.

Global networks for crustal strain measurement provide important constraints for studies of tectonic plate motion and deformation. To date, crustal strain measurements have been possible only in terrestrial settings: on continental plates and island sites within oceanic plates. We report the development of technology for horizontal crustal motion determination at seafloor sites, allowing oceanic plates to be monitored where islands are not available. Seafloor crustal monitoring is an important component of global strain measurement because about 70% of the Earth's surface is covered by water, and this region contains most of the tectonic plate boundaries and zones of crustal deformation. Using the Global Positioning System (GPS) satellites and underwater acoustics, we have established a geodetic reference site on the Juan de Fuca plate at 2.6 km depth, approximately 150 km off the northwest coast of North America. We measure the baselines between this site and two terrestrial GPS stations on Vancouver Island, British Columbia. The Juan de Fuca plate site is an appropriate setting to develop seafloor observation methods, since it is a well studied area, easily accessible from west coast Canadian and United States ports. Determination of seafloor motion at this site addresses questions related to convergence between the Juan de Fuca and North American plates across the Cascadia Subduction Zone. At the Juan de Fuca seafloor geodetic reference site, we installed precision acoustic transponders on the seafloor, and measured ranges to them from a sound source at a surface platform (ship or buoy). The platform is equipped with a set of three GPS antennas allowing determination of the sound source position at times of signal transmission and reception. Merging the satellite and acoustic data allows determination of the transponder network location in global reference frame coordinates. Data processing to date suggests repeatabilities of ±0.8 cm north and ±3.9 cm east in the seafloor transponder network position relative to reference points on Vancouver Island.

The DS-3 mission will demonstrate various elements of the technology required for space interferometry, including Autonomous Formation Flying (AFF). It will consist of three spacecraft, each having a degree of autonomy but all comprising a single instrument and constrained to move together at the vertices of an equilateral triangle with sides of 100 to 1000 m. In order to meet the mission's goals, AFF must maintain the distances between spacecraft within 1 cm and the relative orientations of the spacecraft within 1 arcminute per axis.This paper proposes an implementation of AFF that borrows technology from the Global Positioning System (GPS), using measurements of both r-f carrier phase and a ranging code. Each spacecraft wilt have at least one transmitting antenna and three receiving antennas, operating at 30 GHz with a code rate of 100 Mchips/second.To validate the proposed scheme, two sets of covariance analyses have been performed, using batch and sequential processing. Both analyses suppose measurement errors of 1 cm on the ranges and 10 mu m on the phases, but they process the data differently and make somewhat different assumptions about the nature of systematic errors. Nevertheless, the two analyses reach similar results, concluding that with careful calibration the mission requirements can be met by measurements of the specified accuracy.The paper also summarizes various aspects of the proposed implementation that have not been entirely worked out, indicating in general terms how each might be handled and what problems remain to be solved, These problems include initialization, the location and beam shape of the antennas, multipath, systematic measurement errors, and the effect of the local transmitter on received signals.

As part of TOPEX altimeter verification, the global positioning system has been used to measure the baseline between the verification site at oil Platform Harvest and a GPS antenna collocated with the satellite laser ranging site at Quincy, California. Data from Harvest, Quincy, and a global network of stations, collected between September 25, 1992 and December 17, 1993, have been analyzed to obtain 272 single‐day estimates of the baseline. These daily estimates have in turn been fitted with a linear model, yielding a single estimate of the baseline and its rate of change. Changes in the horizontal components of the baseline reflect the relative tectonic motion of the Pacific plate and the Sierra Nevadan microplate, along with local motion at Harvest and Quincy. The vertical component, crucial to verification, is determined with millimeter‐level accuracy and shows no significant variation during the measurement interval.

This paper presents the status of the long-term experiment using a hybrid GPS/acoustic system to determine strain rates across the Cascadia subduction zone.

Signals transmitted by satellites of Global Positioning System (GPS) measure orientation of baseline on ship, aircraft, or other vehicle with accuracy. Two GPS antennas and receivers placed at well separated points on platform. Receivers measure positions of ends of baseline as functions of time. Output processor computes vector difference between two positions and determines orientation of baseline. Combined with conventional GPS data, orientation data allows more precise navigation and mapping and enhances calculations related to performance and control of vehicle.

This article describes the design and initial tests of the GPS portion of a system for making seafloor geodesy measurements. In the planned system, GPS antennas on a floating platform will be used to measure the location of an acoustic transducer, attached below the platform, which interrogates an array of transponders on the seafloor. Since the GPS antennas are necessarily some distance above the transducer, a short‐baseline GPS interferometer consisting of three antennas is used to measure the platform's orientation. A preliminary test of several crucial elements of the system was performed at the Scripps Institution of Oceanography (SIO) in December 1989. The test involved a fixed antenna on the pier and a second antenna floating on a buoy about 80 m away. GPS measurements of the vertical component of this baseline, analyzed independently by two groups using different software, agree with each other and with an independent measurement within a centimeter. The first test of an integrated GPS/acoustic system took place in the Santa Cruz Basin off the coast of southern California in May 1990. In this test a much larger buoy, designed and built at SIO, was equipped with three GPS antennas and an acoustic transducer that interrogated a transponder on the ocean floor. Preliminary analysis indicates that the horizontal position of the transponder can be determined with a precision of about a centimeter. Further analysis will be required to investigate the magnitude of systematic errors.