Performance and Science Opportunities with the upGREAT Spectrometer onboard of SOFIA

The high-resolution far-infrared spectrometer GREAT is now routinely operated onboard the NASA/DLR airborne observatory SOFIA. The instrument runs simultaneously two receivers in science selected frequency windows between 1.25 and 4.7 THz. Because the instrument has seen major upgrades during the last years we will briefly review the present instrument configurations and performances. The extension of GREAT to mid-size heterodyne arrays is under way. The commissioning of our low-frequency array, operating 2 x 7 HEB mixers at 1.9 THz in two polarizations in a hexagonal configuration, was successfully concluded in 2015, and will be available for community projects during SOFIA's cycle 4. A second (high-frequency) array with 7 pixels aiming at the frequency of the [01] fine-structure line at 4.7 THz is under construction, with commissioning planned for late 2016. In more than 60 successful science flights, most of them performing community projects, a wide variety of astrophysical questions has been addressed, with particular interest in velocity-resolved spectroscopy of the most important ISM cooling lines accessible to GREAT, namely the [CII] and [OI] atomic fine-structure and the mid-J CO rotational transitions. On the example of a few selected science cases the performance of the instrument will be illustrated, and the science opportunities with the instrument in its current and near-future configurations will be addressed.