[Paper Review] Progress on VLBI Ecliptic Plane Survey

Fengchun Shu, Leonid Petrov+6 more|arXiv (Cornell University)|May 23, 2016

GNSS positioning and interference3 citations

TL;DR

The VLBI Ecliptic Plane Survey (VEPS) aims to identify and precisely position compact radio sources within 7.5° of the ecliptic for use as phase calibrators in deep-space tracking. By observing 2216 bright sources (S > 100 mJy at 5 GHz) and 4802 fainter ones (50–100 mJy), the program detected 435 sources in three or more sessions and improved position accuracy to below 1.8 nrad for 71 calibrators via high-sensitivity VLBA observations, significantly enhancing the calibrator network for space missions.

ABSTRACT

We launched the VLBI Ecliptic Plane Survey program in 2015. The goal of this program is to find all compact sources within 7.5 degrees of the ecliptic plane which are suitable as phase calibrators for anticipated phase referencing observations of spacecrafts. We planned to observe a complete sample of the sources brighter than 50 mJy at 5 GHz listed in the PMN and GB6 catalogues that have not yet been observed with VLBI. By April 2016, eight 24-hour sessions have been performed and processed. Among 2227 observed sources, 435 sources were detected in three or more observations. We have also run three 8-hour segments with VLBA for improving positions of 71 ecliptic sources.

Motivation & Objective

• To discover new compact radio sources within 7.5° of the ecliptic plane that can serve as phase calibrators for deep-space missions.
• To improve the astrometric accuracy of existing calibrators with position uncertainties >1.5 nrad.
• To observe sources brighter than 50 mJy at 5 GHz from PMN and GB6 catalogues not previously detected via VLBI.
• To establish a high-sensitivity VLBI network using Asian and Oceanian antennas for future calibrator refinement.
• To reduce position uncertainties below 1.5 nrad for calibrators used in phase-referencing observations of spacecraft.

Proposed method

• Conducted 8 multi-day 24-hour VLBI sessions between February 2015 and March 2016 using a core network of Chinese stations (Seshan25, Kunming, Urumqi) and international stations (Kashima34, Sejong, Hobart26).
• Used X-band (800 MHz bandwidth, 16 IF channels, 2048 Mbps data rate) with 2-bit sampling, adjusting for different station capabilities (e.g., 1-bit sampling at Sejong).
• Correlated data using the DiFX correlator, implementing custom processing for mixed-mode data (e.g., 1-bit vs. 2-bit, 16 MHz vs. 32 MHz bandwidth) via frequency-domain transformation.
• Performed three 8-hour VLBA sessions at 2.3 and 8.6 GHz (2 Gbps) to improve positions of 73 weak calibrators with correlated fluxes between 30–50 mJy at 5000 km baselines.
• Applied dual-band S/X observations to achieve sub-nanoradian position accuracy, with a priori positions derived from single-band VLBI data.
• Used statistical analysis of position uncertainties before and after observations to quantify improvements, with error distributions tracked over time.

Experimental results

Research questions

• RQ1 How many compact radio sources within 7.5° of the ecliptic plane can be detected via VLBI among sources brighter than 50 mJy at 5 GHz?
• RQ2 What is the achievable position accuracy for weak calibrators after high-sensitivity S/X-band VLBI observations?
• RQ3 How do baseline-specific system equivalent flux densities (SEFD) affect detection limits and data quality in the VEPS program?
• RQ4 To what extent can the inclusion of Asian and Oceanian antennas improve the sensitivity and dynamic range of future calibrator surveys?
• RQ5 What is the impact of solar contamination on source visibility in the PMN survey, and how can it be mitigated in future VEPS sessions?

Key findings

• Among 2227 observed sources, 435 were detected in three or more observations, indicating a high detection rate for the survey.
• Position uncertainties for 71 calibrators were reduced from a median of 6.2 nrad to 1.8 nrad after high-sensitivity VLBA observations at 2.3 and 8.6 GHz.
• The number of calibrators with position errors <1.5 nrad increased from 191 (February 2016) to 293 (May 2016), reflecting significant progress in astrometric refinement.
• Detection limits for most baselines were better than 30 mJy, except for Sejong, which showed SEFD variations from 3000 to 5000, indicating potential issues with pointing or system efficiency.
• The survey identified two gaps in source coverage near 200° ecliptic longitude due to solar contamination in the PMN survey, which will be addressed in future sessions using alternative catalogues.
• The total number of known calibrators within ±7.5° of the ecliptic plane reached 1167 by May 2016, with 23% having position accuracy better than 1.5 nrad.