Larry Young

Direct Signal Enhanced Semicodeless Processing of GNSS Surface-Reflected Signals

This paper presents an aircraft demonstration of direct-signal enhanced semicodeless processing of global navigation satellite systems (GNSS) signals reflected from the Earths surface. Comparisons are made between this new method and an interferometric approach to GNSS reflectometry. Results show that this technique produces waveforms with greater signal-to-noise compared with the interferometric approach for all GNSS signals currently in use or planned for the near future. Alternatively, the semicodeless technique can have similar performance with smaller antennas for lower hardware costs. The semicodeless approach also has the advantage that different signals along with their different surface spatial resolutions are processed separately, each signals coherent integration time can be optimized, and ground/aircraft experiments and tests are free of spurious signals. The signal processing demands of the semicodeless approach are shown to be proportional to the number of signal components processed when integrated with a GNSS precise orbit determination (POD) receiver.

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.

A novel GPS-based sensor for ocean altimetry

A technique for a novel application of Global Positioning System (GPS) signals to ocean altimetry is described. The entire Earth surface is divided into a triangular grid of points nearly-uniformly spaced. The sea surface heights at the grid points are determined using the GPS signals reflected from the surrounding area. Practical grid size and data sampling rate are determined. An efficient estimation scheme is devised to solve for the thousands of parameters. A simulation analysis shows that global ocean altimetric information can be recovered to better than 10 cm with only 1 day of reflected GPS signals.

TOGA, a prototype for an optimal orbiting GNSS-R instrument

Remotely sensing the Earths surface using GNSS (Global Navigation Satellite System) signals as bi-static radar sources is one of the most challenging applications for radiometric instrument design. As part of NASAs Instrument Incubator Program, our group at JPL is building a prototype instrument, TOGA (Time-shifted, Orthometric, GNSS Array), to address a variety of GNSS science needs. Observing GNSS reflections is major focus of the design/development effort. The TOGA design features an electronically steered antenna (ESA) array which forms simultaneous high-gain beams in multiple directions. Multiple FPGAs provide flexible digital signal processing logic to process both GPS and Galileo reflections. A Linux operating system based science processor serves as experiment scheduler and data post-processor. This paper outlines the TOGA design approach as it applies specifically to observing science quality GNSS-R signals from low Earth orbit.

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...

Orbiting GPS Receiver Modified to Track New L2C Signal

The L2C signal is a great step forward for civil applications of GPS, enabling high-accuracy dual-frequency measurements. Engineers from the Jet Propulsion Laboratory and ITT teamed to reprogram FPGA firmware and add tracking software on an orbiting receiver to track the new GPS L2C signal from SAC-C. SAC-C is an Argentinean science satellite and was launched in November 2000 with a BlackJack GPS receiver. This is a dual-frequency digital receiver with 48 tracking channels and four antennas. On SAC-C, it provides precise orbits, atmospheric occultation data, tests of GPS surface reflections, and serves as an orbiting test bed for new GPS development such as the L2C tracking reported here.