  In this paper we introduce a software package, the Very Long Baseline
Interferometry Network SIMulator (VNSIM), which provides an integrated platform
assisting radio astronomers to design the Very Long Baseline Interferometry
(VLBI) experiments and evaluate the network performance with a user-friendly
interface. Though VNSIM is motivated to be designed for the East Asia VLBI
Network, it can also be expandable to other VLBI networks and generic
interferometers. The software package not only integrates the functionality of
plotting $(u,v)$ coverage, scheduling the observation, and displaying the dirty
and CLEAN images, but also further extends to add new features such as the
sensitivity calculation of a certain network and multiple-satellite space VLBI
simulations which are useful for future space VLBI mission. In addition, VNSIM
provides flexible interactions on both command line and graphical user
interface and offers friendly support for log report and database management.
VNSIM also supports multiprocessing accelerations, enabling to handle large
survey data. To facilitate the future development and update, each simulation
function is encapsulated in different Python module allowing for independently
invoking and testing. In order to verify the performance of VNSIM, we have
carried out various simulations and compared the results with other simulation
tools. All tests show good consistency.
