The interstellar medium of galaxies including our Milky Way is threaded by magnetic fields. The core aim of GMIMS is to measure the three-dimensional structure of the magnetic field. We use a technique called Faraday synthesis. The idea of Faraday synthesis was written down in 1966 but only became feasible with the advent of modern wideband radio telescopes. To achieve the resolution required for Faraday synthesis of typical Milky Way magnetic field structures, we require long wavelengths — corresponding to a frequency of ≈300 MHz. To be sensitive to the large magnetic structures we see in the Milky Way, we require short wavelengths, corresponding to ≈1500 MHz. This sets the required observing bandwidth of the GMIMS project.
We cannot observe this entire range (more than two octaves) with one observation. Therefore, we employ multiple surveys to cover the full wavelength range. We published the first low-frequency component in 2019 and the first high-frequency component in 2021. As of 2022, additional surveys that cover the wavelength gap are under way.
See the introduction of Wolleben et al (2021) for a more detailed description of the GMIMS survey.