2021PASP..133d4503G

The digital data produced by each telescope within the observation process of Very Long Baseline Interferometry (VLBI) can be referred to as raw data. The raw telescope data represents the initial data stage (Level 0 data) of the VLBI processing chain, consisting of correlation, fringe-fitting, and geodetic/astrometric parameter estimation. Any systematic effects which are compensated and used for parameter estimation within this chain, are present in the raw telescope data streams. The group delay, although the primary geodetic VLBI observable, can be considered to be the most prominent effect. In this publication, we present a new software package implemented in MATLAB that can simulate raw telescope data for VLBI. The software is called VieRDS (https://github.com/TUW-VieVS/VieRDS) and features tools to simulate a delay, a delay rate, a phase offset, and a frequency response of arbitrary magnitude for each channel of the telescope data stream. The signal model consists of a radio source component of selectable flux density, a system noise component, and a phase calibration signal. Furthermore, a tool for one and two bit quantization and a module to store the simulated data in VDIF data format is implemented. For the first time, the simulation of a dispersive group delay for a VGOS broadband frequency setup, and the simulation of the characteristic station frequency response of two VGOS telescopes are shown for demonstration purposes. The simulated data can be correlated and fringe-fitted by commonly used VLBI software correlators and fringe-fitting programs. VieRDS is thus an ideal tool for testing hypotheses of adverse observational effects in a controlled environment by deliberately causing these effects and studying the responses in the correlator output. In this article, we focus on the description of the key characteristic of the simulation concept, the digital signal creation, the digital signal processing algorithms, and the software architecture.