Accession Number : ADP009124

Title :   Providing Hydrogen Maser Timing Stability to Orbiting VLBI Radio Telescope Observations by Post-Measurement Compensation of Linked Frequency Standard Imperfections,

Corporate Author : NEOCOMM SYSTEMS INC LA CANADA-FLINTRIDGE CA

Personal Author(s) : Springett, James C.

Report Date : 02 DEC 1993

Pagination or Media Count : 11

Abstract : Orbiting VLBI (OVLBI) astronomical observations are based upon measurements acquired simultaneously from ground-based and Earth-orbiting radio telescopes. By the mid 1990's, two orbiting VLBI observatories, Russia's Radioastron, and Japan's VSOP, will augment the worldwide VLBI network, providing baselines to Earth radio telescopes as large as 80,000 km. The challenge for OVLBI is to effectuate space to ground radio telescope data cross-correlation (the observation) to a level of integrity currently achieved between ground radio telescopes. But, VLBI radio telescopes require ultra-stable frequency and timing references in order that tong term observations may be made without serious cross-correlation loss due to frequency source drift and phase noise. For this reason, such instruments make use of hydrogen maser frequency standards. Unfortunately, space-qualified hydrogen maser oscillators are currently not available for use on OVLBI satellites. Thus, the necessary long-term stability needed by the orbiting radio telescope may only be obtained by microwave uplinking a ground-based hydrogen maser derived frequency to the satellite. Although the idea of uplinking the frequency standard intrinsically seems simple, there are many contaminations which degrade both the tong and short term stability of the transmitted reference. Factors which corrupt frequency and timing accuracy include additive radio and electronic circuit thermal noise, slow or systematic phase migration due to changes of electronic circuit temporal operating conditions (especially temperature), ionosphere and troposphere induced scintillation's, residual Doppler-incited components, and microwave signal multipath propagation. What is important, though, is to realize that ultimate stability does not have to be achieved in real-time.

Descriptors :   *FREQUENCY STANDARDS, *RADIO INTERFEROMETRY, *EARTH ORBITS, *GROUND STATIONS, *TIME STUDIES, *CORRELATION TECHNIQUES, ARTIFICIAL SATELLITES, COHERENCE, COMPENSATION, CROSS CORRELATION, DEGRADATION, DRIFT, HYDROGEN, IONOSPHERE, MASERS, MEASUREMENT, MICROWAVES, NETWORKS, NOISE, OBSERVATION, OPERATION, OSCILLATORS, PHASE, PROPAGATION, RADIO LINKS, RADIO TELESCOPES, REAL TIME, RESIDUALS, SIGNALS, STABILITY, TIME, TROPOSPHERE, DOPPLER EFFECT, THERMAL PROPERTIES.

Subject Categories : Radiofrequency Wave Propagation
      Unmanned Spacecraft
      Astronomy

Distribution Statement : APPROVED FOR PUBLIC RELEASE