Engineering topics
Murphy, David W.
Publications and source records attributed to Murphy, David W..
Frequency comparisons via GPS carrier-phase: jump processing, temperature compensation and zero/short-baseline noise-floors
GPS carrier-phase receivers have been used to produce high quality time/frequency comparisons between clocks and frequency standards. Current ways of processing the carrier-phase data, however, result in phase jumps and data gaps that make the data difficult to use effectively and repeatedly for long datasets. In addition, temperature fluctuations generally perturb receiver output detrimentally. We describe the steps of a simple but effective automated algorithm for comparing frequencies that can compensate for data gaps, day boundary jumps, and other phase jumps as well as for temperature disturbances. We describe process features and focus on zero-baseline noise-floors, measuring Allan deviations of the Deep Space Atomic Clock mission’s receiver down to 1x10-17 at 5x105 seconds, with an upper confidence level of 3x10-17. The zero-baseline noise floors we present give confidence in our algorithm and temperature compensation’s robustness.
Systems Engineering and Application of System Performance Modeling in SIM Lite Mission
The SIM Lite Astrometric Observatory will be the first space-based Michelson interferometer operating in the visible wavelength, with the ability to perform ultra-high precision astrometric measurements on distant celestial objects. SIM Lite data will address in a fundamental way questions such as characterization of Earth-mass planets around nearby stars. To accomplish these goals it is necessary to rely on a model-based systems engineering approach - much more so than most other space missions. This paper will describe in further detail the components of this end-to-end performance model, called "SIM-sim", and show how it has helped the systems engineering process.
SIMsim: An End-to-end Simulator for the SIM Mission
In this paper we discuss the use of an innovative SIM simulator, called SIMsim, to perform end-to end simulations of the SIM mission. The inputs to the simulator are a physically-based parameterization of the major SIM error sources and the output is the mission astrometric accuracy for various observing scenarios such as narrow-angle (NA) and wide-angle (WA) observations. The primary role of SIMsim is to validate the SIM astrometric error budget (AEB), bit it is also being used to study a variety of mission performance issues as well as being a test-bed for prototype data reduction algorithms. SIMsim is giving us confidence that the SIM AEB is a valid estimate of mission performance. It also is illustrating where analytical formulas for estimating certain effects breakdown and a numerical approach has to be adopted.
The VSOP-2 Space VLBI Mission
Following the success of the VLBI Space Observatory Program (VSOP), a next generation space VLBI mission, VSOP-2, is currently being planned. Higher observing frequencies, cooled receivers, increased bandwidths and larger telescope diameters will result in gains in resolution and interferometer sensitivity by factors of 10 over the VSOP mission. The use of phase-referencing by fast switching between a calibrator source and the target source is now being studied as this technique allows sources 50-150 times weaker to be observed depending on the frequency band. Such a capability would greatly enhance the VSOP-2 mission. Several other enhancements to the VSOP-2 mission are also presently under investigation including the VSOP-2 spacecraft operating at the same time as a US spacecraft to form what has come to be known as the iARISE (international ARISE) mission.
Discovery of a Sub-Parsec Radio counterjet in the Nucleus of Centarus A
A sub-parsec scale radio counterjet has been detected in the nucleus of the closest radio galaxy, Centarus A (NGC 5128), with VLBI imaging at 2.3 and 8.4 GHz.
2.3 GHz VLBI Images of Southern Hemisphere Radio Galaxies and Quasars
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Preliminary Space VLBI Requirements for Observing Time on Ground Radio Telescopes
An initial estimate has been made of the observing time required on ground radio telescopes by the space VLBI missions Radioastron and VSOP. Typical science programs have been adopted for both missions.