Session P3b: Methods and Algorithms for Timing Applications and Timescales

Frequency Standard Contributions to Limitations on the Signal-to-Noise Ratio of Very Long Baseline Interferometry Observations
E.A. Burt, T.A. Ely, M. Anderson, J. Lazio, JPL;, G.C. Bower Academia Sinica Institute of Astronomy and Astrophysics; S. Hernandez, Blue Origin; and E.A. Doughty, Continuum Space Systems
Location: Seaview A/B
Date/Time: Wednesday, Jan. 29, 4:00 p.m.

Since its observation in 2019, the first image of a super-massive black hole using Very Long Baseline Interferometry (VLBI) between a network of Earth-based radio telescopes has generated much scientific and public interest, including the possible extension to include one or more space-based radio antennas in order to obtain higher image resolution. The VLBI technique assumes that each radio telescope has its own frequency standard or “clock,” and a key requirement for that frequency standard is that it have sufficient stability and precision to enable the phase-coherent combination of signals from the independent radio telescopes. For ground-based radio telescopes, the state-of-practice is to have a hydrogen maser co-located. The number of frequency standards that are both space-qualified and perform well enough for this type of VLBI is a small subset of those available on the ground. Among these, it will be necessary to determine what type of frequency standard performance is really required. In addition, given the high cost of launching instruments into space, it may be necessary to make difficult trade-offs between instrument size, and performance. To facilitate this trade space, we derive a metric that directly links frequency standard noise characteristics to the signal-to-noise (S/N) ratio as a function of averaging time for the central VLBI observable, known as the “visibility.” Using this metric, we find that among the existing space clock candidates, only an Ultra-Stable Oscillator (USO) and space hydrogen maser are potentially viable. Furthermore, a USO will only be effective for relatively short integration times for the high observational frequencies required, which are typically of order 100 GHz or higher. While a hydrogen maser can extend the averaging time, it is likely to have prohibitive size and cost for a space VLBI node. We also investigate emerging frequency standards, including optical clocks, and find that the best option may be the optical local oscillator used in an optical clock by itself without the atomic reference. This approach can greatly reduce size and complexity of the frequency standard by eliminating the atomic reference while providing a limit on VLBI visibility S/N significantly higher than what is possible with a hydrogen maser.