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  • Other Sources  (485)
  • 2000-2004  (230)
  • 1995-1999  (255)
  • 1935-1939
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  • 1
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    The 1994 international transatlantic two-way satellite time and frequency transfer experiment: Preliminary results (1995)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: The international transatlantic time and frequency transfer experiment was designed by participating laboratories and has been implemented during 1994 to test the international communications path involving a large number of transmitting stations. This paper will present empirically determined clock and time scale differences, time and frequency domain instabilities, and a representative power spectral density analysis. The experiments by the method of co-location which will allow absolute calibration of the participating laboratories have been performed. Absolute time differences and accuracy levels of this experiment will be assessed in the near future.
    Keywords: COMMUNICATIONS AND RADAR
    Type: NASA. Goddard Space Flight Center, The 26th Annual Precise Time and Time Interval (PTTI) Applications and Planning Meeting; p 39-49
    Format: application/pdf
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  • 2
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    Results of the Calibration of the Delays of Earth Stations for TWSTFT Using the VSL Satellite Simulator Method (1996)
    In:  CASI
    Details
    Publication Date: 2016-06-07
    Description: Two-way satellite time and frequency transfer (TWSTFT) is the most accurate and precise method of comparing two remote clocks or time scales. The accuracy obtained is dependent on the accuracy of the determination of the non-reciprocal delays of the transmit and the receive paths. When the same transponders in the satellite at the same frequencies are used, then the non-reciprocity in the Earth stations is the limiting factor for absolute time transfer.
    Keywords: Physics (General)
    Type: 27th Annual Precise Time and Time Interval (PTTI) Applications and Planning Meeting; 359-372; NASA-CP-3334
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  • 3
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    Preliminary Comparison of Two-Way Satellite Time and Frequency Transfer and GPS Common-View Time Transfer During the INTELSAT Field Trial (1996)
    In:  CASI
    Details
    Publication Date: 2016-06-07
    Description: For a decade and a half Global Positioning System (GPS) common-view time transfer has greatly served the needs of primary timing laboratories for regular intercomparisons of remote atomic clocks. However, GPS as a one-way technique has natural limits and may not meet all challenges of the comparison of the coming new generation of atomic clocks. Two-way satellite time and frequency transfer (TWSTFT) is a promising technique which may successfully complement GPS. For two years, regular TWSTFT's have been performed between eight laboratories situated in both Europe and North America, using INTELSAT satellites. This has enabled an extensive direct comparison to be made between these two high performance time transfer methods. The performance of the TWSTFT and GPS common view methods are compared over a number of time-transfer links. These links use a variety of time-transfer hardware and atomic clocks and have baselines of substantially different lengths. The relative merits of the two time-transfer systems are discussed.
    Keywords: Physics (General)
    Type: 27th Annual Precise Time and Time Interval (PTTI) Applications and Planning Meeting; 347-358; NASA-CP-3334
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  • 4
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    Vorticity Transport and Wave Emission In A Protoplanetary Disk (2002)
    In:  Other Sources
    Details
    Publication Date: 2019-07-18
    Description: Higher order numerical algorithms (4th order in time, 3rd order in space) are applied to the Euler equations and are used to examine vorticity transport and wave motion in a non-self gravitating, initially isentropic Keplerian disk. In this talk we will examine the response of the disk to an isolated vortex with a circulation about equal to the rotation rate of Jupiter. The vortex is located on the 4 AU circle and the nebula is simulated from 1 to 24 AU. We show that the vortex emits pressure-supported density and Rossby-type wave packets before it decays within a few orbits. The acoustic density waves evolve into weak (non entropy preserving) shock waves that propagate over the entire disk. The Rossby waves remain in the vicinity of the initial vortex disturbance, but are rapidly damped. Temporal frequencies and spatial wavenumbers are derived from the nonlinear simulation data and correlated with analytical dispersion relations from the linearized Euler and energy equations.
    Keywords: Astrophysics
    Type: AGU Meeting; May 28, 2002 - May 31, 2002; Washington, DC; United States
    Format: text
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  • 5
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    Recession Curve Generation for the Space Shuttle Solid Rocket Booster Thermal Protection System Coatings (2002)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Ablatable Thermal Protection System (TPS) coatings are used on the Space Shuttle Vehicle Solid Rocket Boosters in order to protect the aluminum structure from experiencing excessive temperatures. The methodology used to characterize the recession of such materials is outlined. Details of the tests, including the facility, test articles and test article processing are also presented. The recession rates are collapsed into an empirical power-law relation. A design curve is defined using a 95-percentile student-t distribution. based on the nominal results. Actual test results are presented for the current acreage TPS material used.
    Keywords: Nonmetallic Materials
    Type: AIAA Paper 2002-3334 , 8th AIAA/ASME Joint Thermophysics and Heat Transfer Conference; Jun 24, 2002 - Jun 27, 2002; Saint Louis, MO; United States
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  • 6
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    Pre-Launch Testing of GPS Receivers for Geodetic Space Missions (2000)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The methodology used and the results obtained in the pre-flight testing of the Blackjack Global Positioning System (GPS) space receiver for the Vegetation Canopy Lidar Mission (VCL) and the Ice, Cloud, and Land Elevation Satellite (ICESat) spacecraft is described. Both real and simulated signals were used to: (1) assess the accuracy and coverage of the navigation solutions, (2) assess the accuracy and stability of the 1-PPS timing signal, (3) assess the precision of the carrier phase observable, and (4) measure the cold-start time to first fix. In addition, an anechoic chamber was used to measure the antenna phase centers with millimeter-level precision. While the test results have generally been excellent and are discussed in this paper, emphasis is placed on describing the test methodology. It is anticipated that future geodetic satellite missions using GPS for navigation, timing, and precise orbit determination (POD) can employ the same tests for pre-launch performance assessment of their particular receiver.
    Keywords: Composite Materials
    Type: Global Positioning Systems; Jan 01, 2000; Salt Lake City, NV; United States
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  • 7
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    Testing of the International Space Station and X-38 Crew Return Vehicle GPS Receiver (1999)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: This paper discusses the process and results of the performance testing of the GPS receiver planned for use on the International Space Station (ISS) and the X-38 Crew Return Vehicle (CRV). The receiver is a Force-19 unit manufactured by Trimble Navigation and modified in software by the NASA Goddard Space Flight Center (GSFC) to perform navigation and attitude determination in space. The receiver is the primary source of navigation and attitude information for ISS and CRV. Engineers at GSFC have developed and tested the new receiver with a Global Simulation Systems Ltd (GSS) GPS Signal Generator (GPSSG). This paper documents the unique aspects of ground testing a GPS receiver that is designed for use in space. A discussion of the design of tests using the GPSSG, documentation, data capture, data analysis, and lessons learned will precede an overview of the performance of the new receiver. A description of the challenges that were overcome during this testing exercise will be presented. Results from testing show that the receiver will be within or near the specifications for ISS attitude and navigation performance. The process for verifying other requirements such as Time to First Fix, Time to First Attitude, selection/deselection of a specific GPS satellite vehicles (SV), minimum signal strength while still obtaining attitude and navigation, navigation and attitude output coverage, GPS week rollover, and Y2K requirements are also given in this paper.
    Keywords: Aircraft Communications and Navigation
    Type: GPS Conference; Sep 14, 1999 - Sep 17, 1999; Nashville, TN; United States
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  • 8
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    Testing of the International Space Station and X-38 Crew Return Vehicle GPS Receiver (1999)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: This paper discusses the process and results of the performance testing of the GPS receiver planned for use on the International Space Station (ISS) and the X-38 Crew Return Vehicle (CRV). The receiver is a Force-19 unit manufactured by Trimble Navigation and Modified in software by the NASA Goddard Space Flight Center (GSFC) to perform navigation and attitude determination in space. The receiver is the primary source of navigation and attitude information for ISS and CRV. Engineers at GSFC have developed and tested the new receiver with a Global Simulation Systems Ltd (GSS) GPS Signal Generator (GPSSG). This paper documents the unique aspects of ground testing a GPS receiver that is designed for use in space. A discussion of the design and tests using the GPSSG, documentation, data capture, data analysis, and lessons learned will precede an overview of the performance of the new receiver. A description of the challenges of that were overcome during this testing exercise will be presented. Results from testing show that the receiver will be within or near the specifications for ISS attitude and navigation performance. The process for verifying other requirements such as Time to First Fix, Time to First Attitude, selection/deselection of a specific GPS satellite vehicles (SV), minimum signal strength while still obtaining attitude and navigation, navigation and attitude output coverage, GPS week rollover, and Y2K requirements are also given in this paper.
    Keywords: Space Transportation
    Type: ION GPS Conference; Sep 14, 1999 - Sep 17, 1999; Nashville, TN; United States
    Format: application/pdf
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  • 9
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    Testing of the International Space Station and X-38 Crew Return Vehicle GPS Receiver (1999)
    In:  Other Sources
    Details
    Publication Date: 2019-07-17
    Description: This paper discusses the process and results of the performance testing of the GPS receiver planned for use on the International Space Station (ISS) and the X- 38CrewReturnVehicle(CRV). The receiver is a Force-19 unit manufactured by Trimble Navigation and modified in software by NASA:s Goddard Space Flight Center (GSFC) to perform navigation and attitude determination in space. The receiver is the primary source of navigation and attitude information for ISS and CRV. Engineers at GSFC have developed and tested the new receiver with a Global Simulation Systems Ltd (GSS) GPS Signal Generator (GPSSG). This paper documents the unique aspects of ground testing a GPS receiver that is designed for use in space. A discussion of the design of tests using the GPSSG, documentation, data capture, data analysis, and lessons learned will precede an overview of the performance of the new receiver. A description of the challenges that were overcome during this testing exercise will be presented. Results from testing show that the receiver will be within or near the specifications for ISS attitude and navigation performance. The process for verifying other requirements such as Time to First Fix, Time to First Attitude, selection/deselection of a specific GPS satellite vehicles (SV), minimum signal strength while still obtaining attitude and navigation, navigation and attitude output coverage, GPS week rollover, and Y2K requirements are also given in this paper.
    Keywords: Space Communications, Spacecraft Communications, Command and Tracking
    Type: Institute of Naviation (ION) GPS; Sep 14, 1999 - Sep 17, 1999; Nashville, TN; United States
    Format: text
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  • 10
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    Results from the GPS Flight Experiment on the High Earth Orbit AMSAT OSCAR-40 Spacecraft (2002)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: A GPS receiver flying on the High Earth Orbit (HEO) AMSAT-OSCAR 40 (AO-40) spacecraft has been returning GPS observations from high above the altitude of the GPS constellation. AO-40, an amateur radio satellite launched November 16, 2000, is currently in a low inclination, 1000 by 59000 lan altitude orbit. This low-cost experiment utilizes a mid 1990's era, 6-channel, CIA code receiver configured with high gain receiving antennas for tracking above the GPS constellation. The receiver has performed well, despite operating significantly outside of its original design environment. It has regularly returned GPS observations from points all around the orbit, with over ten weeks of GPS tracking data collected to date. Signal to noise levels as high as 48 B-Hz have been recorded near apogee, when the spacecraft was at an altitude of close to 60000 km. GPS side lobe signals have been tracked on several occasions, primarily from Block IIR GPS satellites. Although the receiver has not computed a solution in real-time, point solutions have been computed on the ground using simultaneous measurements from four satellites. This experiment has provided important experience dealing with the many challenges inherent to GPS tracking at high altitudes, and the measurements returned are providing valuable information about the characteristics of GPS signals available for future HE0 users.
    Keywords: Space Communications, Spacecraft Communications, Command and Tracking
    Type: ION GPS 2002; Sep 24, 2002 - Sep 27, 2002; Portland, OR; United States
    Format: text
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