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  • 1
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    Simultaneous dual baseline differential VLBI (1983)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Attention is drawn to the benefits of simultaneous dual baseline radio interferometric measurements for low altitude planetary orbiter navigation. The radio interferometric data types constitute a valuable addition to conventional tracking data in the establishment of orbit-plane orientation, thanks to their measurement of inertial reference cross-track velocity. The present results show that only a few Delta-VLBI measurements, taken at either end of a conventional tracking arc, suffice for the realization of a positional accuracy of better than 1 km for the case of a Venus Orbiting Imaging Radar-type planetary orbiter. This is comparable to Deep Space Net accuracies, despite the use of a shorter baseline.
    Keywords: SPACE COMMUNICATIONS, SPACECRAFT COMMUNICATIONS, COMMAND AND TRACKING
    Type: Journal of the Astronautical Sciences (ISSN 0021-9142); 31; Jan
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    NASA TECHNICAL REPORTS
  • 2
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    Seasat orbit refinement for altimetry application (1980)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: This paper describes the use of stochastic differential correction models in refining the Seasat orbit based on post-flight analysis of tracking data. The objective is to obtain orbital-height precision that is commensurate with the inherent Seasat altimetry data precision level of 10 cms. Local corrections to a mean ballistic arc, perturbed principally by atmospheric drag variations and local gravitational anomalies, are obtained by the introduction of stochastic dynamical models in conjunction with optimal estimation/smoothing techniques. Assessment of the resulting orbit with 'ground truth' provided by Seasat altimetry data shows that the orbital height precision is improved by 32% when compared to a conventional least-squares solution using the same data set. The orbital height precision realized by employing stochastic differential correction models is in the range of 73 cms to 208 cms rms.
    Keywords: ASTRODYNAMICS
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    NASA TECHNICAL REPORTS
  • 3
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    The Venus Radar Mapper (VRM) mission (1984)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The Venus Radar Mapper (VRM) mission is sponsored by NASA to put a single spacecraft in orbit around Venus to map the surface of Venus using a synthetic aperture mapping radar. The spacecraft is scheduled to be launched in April 1988 using a Shuttle-Centaur G combination. The spacecraft arrives at Venus in late July 1988 and begins its mapping mission which lasts for one Venus rotation or 243 days. This paper describes the VRM mission at its present state of design. The science objectives and project constraints are described. Key features of the spacecraft system and radar system are discussed. The interplanetary and mapping orbit design are covered. Navigation strategy is explained, including trajectory maneuvers and mapping phase orbit determination. Finally, the mapping sequences to optimize planet coverage are described.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: AIAA PAPER 84-0212
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    NASA TECHNICAL REPORTS
  • 4
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    Ship navigation using Navstar GPS - An application study (1982)
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    Publication Date: 2019-07-13
    Description: Ocean current measurement applications in physical oceanography require knowledge of inertial ship velocity to a precision of 1-2 cm/sec over a typical five minute averaging interval. The navigation accuracy must be commensurate with data precision obtainable from ship borne acoustic profilers used in sensing ocean currents. The Navstar Global Positioning System is viewed as a step in user technological simplification, extension in coverage availability, and enhancement in performance accuracy as well as reliability over the existing systems, namely, Loran-C, Transit, and Omega. Error analyses have shown the possibility of attaining the 1-2 cm/sec accuracy during active GPS coverage at a data rate of four position fixes per minute under varying sea-states. This paper is intended to present results of data validation exercises leading to design of an experiment at sea for deployment of both a GPS y-set and a direct Doppler measurement system as the autonomous navigation system used in conjunction with an acoustic Doppler as the sensor for ocean current measurement.
    Keywords: COMMUNICATIONS AND RADAR
    Type: AIAA PAPER 82-0314 , Aerospace Sciences Meeting; Jan 11, 1982 - Jan 14, 1982; Orlando, FL
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    NASA TECHNICAL REPORTS
  • 5
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    GPS aiding of ocean current determination (1981)
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    Publication Date: 2019-07-13
    Description: The navigational accuracy of an oceangoing vessel using conventional GPS p-code data is examined. The GPS signal is transmitted over two carrier frequencies in the L-band at 1575.42 and 1227.6 MHz. Achievable navigational uncertainties of differenced positional estimates are presented as a function of the parameters of the problem, with particular attention given to the effect of sea-state, user equivalent range error, uncompensated antenna motion, varying delay intervals, and reduced data rate examined in the unaided mode. The unmodeled errors resulting from satellite ephemeris uncertainties are shown to be negligible for the GPS-NDS (Navigation Development) satellites. Requirements are met in relatively calm seas, but accuracy degradation by a factor of at least 2 must be anticipated in heavier sea states. The aided mode of operation is examined, and it is shown that requirements can be met by using an inertial measurement unit (IMU) to aid the GPS receiver operation. Since the use of an IMU would mean higher costs, direct Doppler from the GPS satellites is presented as a viable alternative.
    Keywords: SPACE COMMUNICATIONS, SPACECRAFT COMMUNICATIONS, COMMAND AND TRACKING
    Type: AAS PAPER 81-208 , Astrodynamics Specialist Conference; Aug 03, 1981 - Aug 05, 1981; Lake Tahoe, NV; US
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    NASA TECHNICAL REPORTS
  • 6
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    Venus Radar Mapper orbit accuracy analysis (1984)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Spacecraft orbital accuracies during the mapping phase of the Venus Radar Mapper Mission are the subject of this paper. The analysis presented describes the navigational strategy for the prime mission in support of mapping the planet Venus using a spacecraft deployed Synthetic Aperture Radar. Achievable orbital accuracies are shown to be limited by accuracy of interferometric data to be used. Implications of tracking system configurations and of expected degradation in data accuracy towards the end of the mission in the proximity of superior solar conjunction are examined.
    Keywords: SPACE COMMUNICATIONS, SPACECRAFT COMMUNICATIONS, COMMAND AND TRACKING
    Type: AIAA PAPER 84-1986
    Format: text
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