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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 505 records · Page 28

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NASA NTRS1970

Multipath error in range rate measurement by PLL-transponder/GRARR/TDRS

Range rate errors due to specular and diffuse multipath are calculated for a tracking and data relay satellite (TDRS) using an S band Goddard range and range rate (GRARR) system modified with a phase-locked loop transponder. Carrier signal processing in the coherent turn-around transponder and the GRARR reciever is taken into account. The root-mean-square (rms) range rate error was computed for the GRARR Doppler extractor and N-cycle count range rate measurement. Curves of worst-case range rate error are presented as a function of grazing angle at the reflection point. At very low grazing angles specular scattering predominates over diffuse scattering as expected, whereas for grazing angles greater than approximately 15 deg, the diffuse multipath predominates. The range rate errors at different low orbit altutudes peaked between 5 and 10 deg grazing angles.

Sohn, S. J.↗

NASA NTRS1970

Multipath signal model development

The development and use of mathematical models of signals received through the multipath environmental of a TDRS-to-user spacecraft link and vice versa are discussed. The TDRS (tracking and data relay satellite) will be in synchronous orbit. The user spacecraft will be in a low altitude orbit between 200 and 4000 km.

Ghais, A. F.↗

NASA NTRS1970

Modifications of the wideband FM TDRS system

The wideband FM scheme for the Tracking and Data Relay Satellite (TDRS) system is discussed. The technique achieves a spread-spectrum advantage against multipath and interference through the inclusion of a large deviation FM subcarrier. The signal also permits co-channel spread-spectrum multipath access (SSMA) on the downlink from mission spacecraft to TDRS. Each user is assigned a unique combination of carrier and subcarrier which allows the ground receiver to separate user signals with overlapping spectra. The system was modified by the addition of a low-deviation fine range tone to both the uplink and the downlink, separate from the wide-deviation subcarrier. The magnitude of the interference components due to other SSMA signals are analyzed. A carrier and subcarrier frequency plan was worked out based on the modified signal format. A configuration to accommodate 42 and 60 users is described. The inclusion of ambiguity resolving ranging sidetones is also discussed.

Wachsman, R. H.↗

NASA NTRS1970

Multipath errors in range rate measurement by a TDRS/VHF - GRARR

Range rate errors due to multipath reflection are calculated for a tracking and data relay satellite system using the VHF Goddard range and range rate (GRARR) system. At VHF the reflection is primarily specular, and the strength of the multipath relative to the direct path can be modeled in terms of the geometry and the surface characteristics, specifically the root-mean-square (rms) ocean wave height. The uplink and downlink multipath introduces phase jitter on the GRARR carrier and subcarrier. The derivation of these effects is reviewed leading to an expression for the rms range rate error. The derivation assumed the worst-case orbital configurations in which there was very little relative specular Doppler. This means that the specular multipath interference was not attenuated by the carrier and subcarrier PLL transfer functions. Curves of range rate error are presented as a function of grazing angle with wave height 0.3 to 0.7 meters and spacecraft altitude 100 to 700 miles as parameters.

Sohn, S. J.↗

NASA NTRS1972

Earth orbit navigation study. Volume 2: System evaluation

An overall systems evaluation was made of five candidate navigation systems in support of earth orbit missions. The five systems were horizon sensor system, unkown landmark tracking system, ground transponder system, manned space flight network, and tracking and data relay satellite system. Two reference missions were chosen: a low earth orbit mission and a transfer trajectory mission from low earth orbit to geosynchronous orbit. The specific areas addressed in the evaluation were performance, multifunction utilization, system mechanization, and cost.

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NASA NTRS1971

A channel simulator design study

A propagation path simulator was designed for the channel between a Tracking and Data Relay Satellite in geostationary orbit and a user spacecraft orbiting the earth at an altitude between 200 and 4000 kilometers. The simulator is required to duplicate the time varying parameters of the propagation channel.

Devito, D. M.↗

NASA NTRS1973

Earth resources ground data handling systems for the 1980's

The system requirements of an operational data handling system for earth resources in the decade of the 1980's are investigated. Attention is drawn to problems encountered in meeting the stringent agricultural user requirements of that time frame. Such an understanding of requirements is essential not only in designing the ground system that will ultimately handle the data, but also in design studies of the earth resources platform, sensors, and data relay satellites which may be needed.

Vanvleck, E. M.↗

NASA NTRS1973

Adaptive ground implemented phase array

The simulation of an adaptive ground implemented phased array of five antenna elements is reported for a very high frequency system design that is tolerant to the radio frequency interference environment encountered by a tracking data relay satellite. Signals originating from satellites are received by the VHF ring array and both horizontal and vertical polarizations from each of the five elements are multiplexed and transmitted down to ground station. A panel on the transmitting end of the simulation chamber contains up to 10 S-band RFI sources along with the desired signal to simulate the dynamic relationship between user and TDRS. The 10 input channels are summed, and desired and interference signals are separated and corrected until the resultant sum signal-to-interference ratio is maximized. Testing performed with this simulation equipment demonstrates good correlation between predicted and actual results.

Spearing, R. E.↗

NASA NTRS1970

Laser communication experiment. Volume 1: Design study report: Spacecraft transceiver. Part 1: Transceiver design

The ATS-F Laser Communications Experiment (LCE) is the first significant step in the application of laser systems to space communications. The space-qualified laser communications system being developed in this experiment, and the data resulting from its successful deployment in space, will be applicable to the use of laser communications systems in a wide variety of manned as well as unmanned space missions, both near earth and in deep space. Particular future NASA missions which can benefit from this effort are the Tracking and Data Relay Satellite System and the Earth Resources Satellites. The LCE makes use of carbon dioxide lasers to establish simultaneous, two-way communication between the ATS-F synchronous satellite and a ground station. In addition, the LCE is designed to permit communication with a similar spacecraft transceiver proposed to be flown on ATS-G, nominally one year after the launch of ATS-F. This would be the first attempt to employ lasers for satellite-to-satellite communications.

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NASA NTRS1973

A digital communications system for manned spaceflight applications.

A highly efficient, all-digital communications signal design employing convolutional coding and PN spectrum spreading is described for two-way transmission of voice and data between a manned spacecraft and ground. Variable-slope delta modulation is selected for analog/digital conversion of the voice signal, and a convolutional decoder utilizing the Viterbi decoding algorithm is selected for use at each receiving terminal. A PN spread spectrum technique is implemented to protect against multipath effects and to reduce the energy density (per unit bandwidth) impinging on the earth's surface to a value within the guidelines adopted by international agreement. Performance predictions are presented for transmission via a TDRS (tracking and data relay satellite) system and for direct transmission between the spacecraft and earth. Hardware estimates are provided for a flight-qualified communications system employing the coded digital signal design.

Batson, B. H.↗

NASA NTRS1973

Adaptive ground implemented phased array

Tests were conducted to determine the feasibility of using an adaptive ground implemented phased array (AGIPA) to overcome the limitations of the radio frequency interference limited low data Tracking and Data Relay Satellite VHF return link. A feasibility demonstration model of a single user channel AFIPA system was designed, developed, fabricated, and evaluated. By scaling the frequency and aperture geometry from VHF to S-band, the system performance was more easily demonstrated in the controlled environment of an anechoic chamber. The testing procedure employs an AGIPA in which received signals from each element of the array are processed on the ground to form an adaptive, independent, computer controlled beam for each user.

Smith, J. M.↗

NASA NTRS1973

A one way Doppler tracking system for satellite to satellite tracking

An S-band tracking system employing one way Doppler applicable to the tracking and data relay system (TDRS) was evaluated. This evaluation is limited to the measurement system parameters. The analysis assumes state-of-the-art components such as spacecraft oscillator long term stability of one part in 10 to the 8th power and ground station oscillator stability of one part in 10 to the 11th power. Signal characteristics comparable with the present ranging systems are utilized in the analysis. Predicated upon measurement system parameters, position uncertainties for the low orbiting spacecraft vary from 5 meters to 2 kilometers along track, 4 meters to 1 kilometer cross track, and 1 meter to 180 meters radially depending upon the tracking geometry and the high satellite position and velocity error assumptions.

Bryan, J. W.↗

NASA NTRS1974

Integrated source and channel encoded digital communications system design study

Studies on the digital communication system for the direct communication links from ground to space shuttle and the links involving the Tracking and Data Relay Satellite (TDRS). Three main tasks were performed:(1) Channel encoding/decoding parameter optimization for forward and reverse TDRS links,(2)integration of command encoding/decoding and channel encoding/decoding; and (3) modulation coding interface study. The general communication environment is presented to provide the necessary background for the tasks and to provide an understanding of the implications of the results of the studies.

Huth, G. K.↗

NASA NTRS1974

Integrated Power/Attitude Control System (IPACS) study. Volume 1: Feasibility studies

An Integrated Power/Attitude Control System (IPACS) concept consisting of an array of spinning flywheels, with or without gimbals, capable of performing the dual function of power storage and generation, as well as attitude control has been investigated. This system provides attitude control through momentum storage, and replaces the storage batteries onboard the spacecraft. The results of the investigation are presented in two volumes. The trade-off studies performed to establish the feasibility, cost effectiveness, required level of development, and boundaries of application of IPACS to a wide variety of spacecraft are discussed. The conceptual designs for a free-flying research application module (RAM), and for a tracking and data relay satellite (TDRS) are presented. Results from dynamic analyses and simulations of the IPACS conceptual designs are included.

Notti, J. E.↗

NASA NTRS1974

Variable-beamwidth monopulse antennas

The merits of nine methods for zooming microwave amplitude-sensing monopulse antenna patterns are discussed. Of these, six are directly related to the TDRSS (Tracking Data Relay Satellite System) and are compatible with a deployable-mesh pseudo-paraboloidal main reflector. The remaining three methods utilize radically different geometrical configurations that depart considerably from the TDRSS parameters existing at this time. Preservation of the monopulse postulates is considered to be of prime importance for any variable-beamwidth candidate, however, it is allowed that approximate satisfaction of the postulates should be accepted for practical reasons. All of the methods discussed admit free choice of the polarization state, and the zooming function is never predicated on polarization. Exploration of the zooming techniques was carried out almost entirely by means of the Kirchhoff-Kottler vector diffraction program. The program generates electric and magnetic field intensity, associated phase, and time-average Poynting vector power flow in the intermediate near-field and far-field zones in both receive and transmit modes of operation. A few of the concepts have been verified experimentally with excellent agreement between theory and practice.

Schmidt, R. F.↗

NASA NTRS1974

Delta-102 Synchronous Meteorological Satellite (SMS-A): Operations summary

The Synchronous Meteorological Satellite (SMS) program is a joint effort of the National Aeronautics and Space Administration and the Department of Commerce. This program is intended to provide systematic worldwide weather coverage on an operational basis. The pilot SMS program is expected to launch three spacecraft: two prototype spacecraft designated SMS-A and SMS-B and one operational spacecraft designated SMS-C. The SMS program will use spacecraft in synchronous orbit to obtain day and night information on the earth's weather by means of earth imaging instruments, retransmission of image data, data collection, data relay, and space environmental monitoring.

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NASA NTRS1974

An evaluation of the ERTS data collection system as a potential operational tool

The Earth Resources Technology Satellite Data Collection System has been shown to be, from the users vantage point, a reliable and simple system for collecting data from U.S. Geological Survey operational field instrumentation. It is technically feasible to expand the ERTS system into an operational polar-orbiting data collection system to gather data from the Geological Survey's Hydrologic Data Network. This could permit more efficient internal management of the Network, and could enable the Geological Survey to make data available to cooperating agencies in near-real time. The Geological Survey is conducting an analysis of the costs and benefits of satellite data-relay systems.

Paulson, R. W.↗

NASA NTRS1973

RFI analysis applied to the TDRSS system

The effect of radio frequency interference (RFI) on the proposed Tracking and Data Relay Satellite System (TDRSS) was assessed. The method of assessing RFI was to create a discrete emitter listing containing all the required parameters of transmitters in the applicable VHF and UHF frequency bands. The transmitter and spacecraft receiver characteristics were used to calculate the RFI contribution due to each emitter. The individual contributions were summed to obtain the total impact in the operational bandwidth. Using an as yet incomplete emitter base, it is concluded that the 136- to 137-MHz band should be used by TDRSS rather than the whole 136- to 138-MHz band because of the higher interference levels in the 137- to 138 MHz band. Even when restricting the link to 136 to 137 MHz, the existing link design is marginal, and it is recommended that interference reduction units, such as the adaptive digital filter, be incorporated in the TDRSS ground station.

Jenny, J. A.↗