## Why a new implementation of the UV Tables in GILDAS?

The UV Tables in the original GDFV1 data format, created in 1989, handled visibilities and their required associated data parameters in a very similar way as the UVFITS data format used to export data from the VLA at that epoch. A visibility consisted in

- 7 associated data parameters (daps), in fixed order: u, v, w, date, time, first antenna, second antenna;
- a so-called complex visibility element for each channel, i.e., a real part, an imaginary part, and a weight.

The daps precede the complex visibilities elements.

The 7 (daps) were real numbers. As Plateau de Bure is a 2-D array with small field of view, w was most often replaced by the scan number scan for debugging purpose. The number of channels was simply derived from the visibility size with

(1)
In the early 2000s, an extension was made to this constraining layout by optionally adding two (real) numbers after the channel values. This was made necessary by the development of the ALMA simulator to study the impact of the Atacama Compact Array (ACA).

With further developments coming on, such as polarization, on-the-fly mosaicking, multi-source UVFITS files from CASA,... this scheme was becoming far too constraining. In addition, UV data sets could readily exceed the 2 GByte size limit of the GDFV1 data format. We thus decided to take advantage of the changes required to handle large data sets to implement a more flexible UV data format.