George Purcell

System and method for measuring ocean surface currents at locations remote from land masses using synthetic aperture radar

This is a system for measuring ocean surface currents from an airborne platform. A radar system having two spaced antennas wherein one antenna is driven and return signals from the ocean surface are detected by both antennas is employed to get raw ocean current data which is saved for later processing. There are a pair of GPS systems including a first antenna carried by the platform at a first location and a second antenna carried by the platform at a second location displaced from the first antenna for determining the position of the antennas from signals from orbiting GPS navigational satellites. This data is also saved for later processing. The saved data is subsequently processed by a ground-based computer system to determine the position, orientation, and velocity of the platform as well as to derive measurements of currents on the ocean surface.

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.

TPF-Emma: concept study of a planet finding space interferometer

A novel space interferometer design originating in Europe has been studied. The interferometer uses the technique of starlight nulling to enable detection of earth-like planets orbiting nearby stars. A set of four telescope spacecraft flying in formation with a fifth, beam-combiner spacecraft forms the interferometer. This particular concept shows potential for reducing the mission cost when compared with previous concepts by greatly reducing the complexity of the telescope spacecraft. These spacecraft have no major deployable systems, have simplified propulsion and a more rugged construction. The formation flying geometry provides for greater average separation between the spacecraft with commensurate risk reduction. Key aspects of the design have been studied at the Jet Propulsion Laboratory with a view to collaborations between NASA and the European Space Agency. An overview of the design study is presented with some comparisons with the TPF-FFI concept.

Design Study for a Planet-Finding Space Interferometer

The characterization of earth like planets that may be orbiting nearby stars will require large observatories capable of detecting a small number of planet photons and separating them from the much larger flux from the parent star. One approach to this is to employ nulling interferometry on a space-based platform utilizing a number of spacecraft. At the Jet Propulsion Laboratory, a linear dual chopped Bracewell array design was studied in depth while in Europe various two-dimensional designs were discussed. This study looks in a little more depth at the design issues for one of the European concepts and concludes that the concept has promise for a significant reduction in spacecraft complexity and mass and therefore would result in a lower cost mission.

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.

Architecture trade study for the Terrestrial Planet Finder Interferometer

The Terrestrial Planet Finder Interferometer (TPF-I) is a space-based NASA mission for the direct detection of Earth-like planets orbiting nearby stars. At the mid-infrared wavelength range of interest, a sun-like star is ~107 times brighter than an earth-like planet, with an angular offset of ~50 mas. A set of formation-flying collector telescopes direct the incoming light to a common location where the beams are combined and detected. The relative locations of the collecting apertures, the way that the beams are routed to the combiner, and the relative amplitudes and phases with which they are combined constitute the architecture of the system. This paper evaluates six of the most promising solutions: the Linear Dual Chopped Bracewell (DCB), X-Array, Diamond DCB, Z-Array, Linear-3 and Triangle architectures. Each architecture is constrained to fit inside the shroud of a Delta IV Heavy launch vehicle using a parametric model for mass and volume. Both single and dual launch options are considered. The maximum separation between spacecraft is limited by stray light considerations. Given these constraints, the performance of each architecture is assessed by modeling the number of stars that can be surveyed and characterized spectroscopically during the mission lifetime, and by modeling the imaging properties of the configuration and the robustness to failures. The cost and risk for each architecture depends on a number of factors, including the number of launches, and mass margin. Quantitative metrics are used where possible. A matrix of the architectures and ~30 weighted discriminators was formed. Each architecture was assigned a score for each discriminator. Then the scores were multiplied by the weights and summed to give a total score for each architecture. The X-Array and Linear DCB were judged to be the strongest candidates. The simplicity of the three-collector architectures was not rated to be sufficient to compensate for their reduced performance and increased risk. The decision process is subjective, but transparent and easily adapted to accommodate new architectures and differing priorities.

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.

GPS measurements of the baseline between Quincy and Platform Harvest

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.

Accurate GPS measurement of the location and orientation of a floating platform

Abstract 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 platforms 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/ac...