Michael W. Fitzmaurice

Measurements of Turbulence Profiles in the Troposphere

Temperature structure coefficients were measured with balloon-borne temperature sensors. Data converted to refractive-index-structure coefficients are reported. These extend knowledge of this coefficient to the upper troposphere. The results are discussed with reference to possible meteorological origins for turbulence.

Wavelength Dependence of Laser-Beam Scintillation*

An experiment has been performed to confirm the proportionality between log-amplitude variance and the 7/6 power of wavenumber predicted by Tatarski for horizontal propagation from a spherical-wave transmitter to a point detector. The validity of this proportionality was tested for two wavelengths: 0.632 and 10.6 μ. Beams from a helium-neon and a CO2 laser were simultaneously transmitted over a folded 1.2-km horizontal path and were detected with a photomultiplier and a gold-doped germanium detector. The primary scintillation statistic, log-amplitude variance, was evaluated for each wavelength with a digital computer and the ratio of variances at 0.632 and 10.6 μ was found to be in close agreement with predictions. Power spectral density, autocorrelation, and cumulative probability density were also evaluated for each wavelength. Scintillation statistics at 10.6 μ were found to be log normal, as in the visible.

Optical design and performance of the NGST wavefront control testbed

The NGST Wavefront Control Testbed (WCT) is a joint technology program managed by the Goddard Space Flight Center (GSFC) and the Jet Propulsion Laboratory (JPL) for the purpose of developing technologies relevant to the NGST optical system. The WCT provides a flexible testing environment that supports the development of wavefront sensing and control algorithms that may be used to align and control a segmented optical system. WCT is a modular system consisting of a Source Module (SM), Telescope Simulator Module (TSM) and an Aft-Optics (AO) bench. The SM incorporates multiple sources, neutral density filters and bandpass filters to provide a customized point source for the TSM. The telescope simulator module contains a flip-in mirror that selects between a small deformable mirror and three actively controlled spherical mirror segments. The TSM is capable of delivering a wide range of aberrated, unaberrated, continuous and segmented wavefronts to the AO optical bench for analysis. The AO bench consists of a series of reflective and transmissive optics that images the exit pupil of the TSM onto a 349 actuator deformable mirror that is used for wavefront correction. A Fast Steering Mirror (FSM) may be inserted into the system (AO bench) to investigate image stability and to compensate for systematic jitter when operated in a closed loop mode. We will describe the optical design and performance of the WCT hardware and discuss the impact of environmental factors on system performance.

Measurement of log-amplitude variance.

Log amplitude variance calculation methods compared in statistics of optical scintillation by application to measurements with laser

Space station laser communication transceiver

NASA/Goddard Space Flight Center (GSFC) has initiated the development of experimental optical communication system which will be installed on Space Station Freedom. This system is part of the Space Station Attached a Payloads Program and is currently scheduled for a 1997 launch. The system is being designed to carry out comprehensive set of tests to evaluate and demonstrate the capabilities of this relatively new technology. Communication tests at rates up to 1200 mbps will be conducted over the space-to-ground link using an existing tracking facility at the GSFC. GaAlAs semiconductor lasers will be intensity modulated using 4 slot pulse position modulation format. Direct detection receivers using silicon avalanche photodiodes will be utilized, and 1 microradian accuracy pointing will be achieved with 2 cascaded pointing stages. Successful completion of this in-orbit test program should demonstrate both the technical maturity and readiness of this technology for follow-up operational missions.

Laser communication transceiver for space station Freedom

This paper describes the design of the Laser Communication Transceiver (LCT) system which was planned to be flight tested as an attached payload on Space Station Freedom. The objective in building and flight-testing the LCT is to perform a broad class of tests addressing the critical aspects of space-based optical communications systems, providing a base of experience for applying laser communications technology toward future communications needs. The LCTs functional and performance requirements and capabilities with respect to acquisition, spatial tracking and pointing, communications, and attitude determination are discussed.

Tracking and Delay Relay Satellite System (TDRSS)-LIGHT: a space communications architecture

There is a growing interest in applying the resources of the Tracking and Data Relay Satellite System (TDRSS) as the primary support capability for future small satellite users. This interest is based on a variety of benefits offered by the TDRSS, and not available with globally-distributed space-ground links. An architecture based on an optical augmentation to the current TDRSS space network is discussed, including a candidate design for the user and relay terminals.

Laser terminal attitude determination via autonomous star tracking

Freespace laser communication systems require an initial spatial acquisition process to coalign the cooperating terminal line of sites (typically to sub-microrad accuracy). The complexity and time required for this process can grow dramatically with the size of the host platform(s) initial uncertainty field-of-view. The feasibility of significantly refining the laser terminal attitude knowledge prior to the initiation of the spatial acquisition process via a preliminary star tracking operation is addressed. It is shown that currently planned laser terminal designs are capable of supporting this star tracking function with minimal hardware augmentation. An analysis of stellar spectral irradiance is presented which enables the computation of the tracking noise equivalent angle versus stellar class and visual magnitude.

A Shuttle Based Laser System For Space Communication

The Goddard Space Flight Center (GSFC) is planning a series of space communication experiments to validate laser technology for future NASA missions. Requirements include sevelal hundred MBPS data relay in the near earth environment and approximately one MBPS over the deep space to earth link. A key element in this program is a Shuttle-based laser system called the Laser Technology Experiments Facility (LTEF). This Facility will be designed to communicate with a cooperative laser system under development for the Advanced Communication Technology Satellite (ACTS) and will conduct a comprehensive set of acquisition, tracking and communication experiments. This report presents the results of the initial study of this Facility with particular emphasis on the challenges associated with LTEF acquisition of the ACTS downlink beacon laser.

NASA Ground-Based And Space-Based Laser Ranging Systems

The development of laser ranging systems within NASA started in the early 1960s, soon after the invention of the laser. This program has grown substantially in the intervening years, and has produced important results in the areas of precision orbit determination and gravity field determination. In addition, laser ranging is expected in the near future to produce some unique results on earth crustal motions. These results may be very important for understanding the mechanisms which cause earthquakes. In this document we present an overview of the NASA laser ranging program. The dis-cussion is organized as outlined in Figure 1. The principal applications these systems address are outlined, and the characteristics of typical systems built during the 1960s and early 1970s are described. The current state-of-the-art as exemplified by some of the more recent ranging systems are also discussed with respect to performance levels and the error sources which limit this performance. The final section discusses the plans within NASA for using laser ranging systems aboard the Space Shuttle.