Laboratory Demonstration of the Local Oscillator Concept for the Event Horizon Imager

Abstract

Black hole imaging challenges the third-generation space VLBI, the Very Long Baseline Interferometry, to operate on a 500[Formula: see text]GHz band. The coherent integration time needed here is 450[Formula: see text]s though the available space oscillators cannot offer more than 10[Formula: see text]s. Self-calibration methods might solve this issue in an interferometer formed by three antenna/satellite systems, but the need for the third satellite increases the mission costs. A frequency transfer is of special interest to alleviate both performance and cost issues. A concept of two-way optical frequency transfer is examined to investigate its suitability to enable space-to-space interferometry, in particular, to image the “shadows” of black holes from space. The concept, promising on paper, has been demonstrated by tests. The laboratory test set-up is presented and the verification of the temporal stability using standard analysis tool as TimePod has been passed. The resulting Allan Deviation is dominated by the 1/[Formula: see text] phase noise trend since the frequency transfer timescale of interest is shorter than 0.2[Formula: see text]s. This trend continues into longer integration times, as proven by the longest tests spanning over a few hours. The Allan Deviation between derived 103.2[Formula: see text]GHz oscillators is [Formula: see text]/[Formula: see text] within 10[Formula: see text][Formula: see text][Formula: see text]s that degrades twice towards the longest delay of 0.2[Formula: see text]s. The worst case satisfies the requirement with a margin of 11 times. The obtained coherence in the range of 0.997[Formula: see text]0.9998 is beneficial for space VLBI at 557[Formula: see text]GHz. The result is of special interest to future science missions for black hole imaging from space.

TriHex: Combining Formation Flying, General Circular Orbits, and Alias-Free Imaging, for High-Resolution L-Band Aperture Synthesis

“…TriHex relies on the Upper Side Band Syntonization (USBS) concept described for two spacecraft in [37] but extended to N=3 spacecraft [38]. The USBS concept was successfully demonstrated in the ESTEC RF Laboratory [39]. For synchronization TriHex relies on spatial symmetry, in practice realized by (a) manufacturing identical circuits with phase similarity to within 117° at 250 MHz, (b) cutting optical fibres with 3-millimetre accuracy and (c) having the spacecraft in relative navigation with an accuracy of 1 cm.…”

Imaging the event horizon of M87* from space on different timescales

“…The accurate positioning of the satellites and interchange of local oscillator signals permit the phase calibration of EHI observations and, thus, the use of complex visibilities (Kudriashov et al 2021b). Both the proposed on-the-fly positioning and the on-board data processing require a working inter-satellite link (ISL) to perform highly accurate ranging measurements, as well as to exchange observed signals and local oscillator components for coherent operation (Martin-Neira et al 2019;Kudriashov et al 2021a). This implies a direct line of sight between the satellites.…”