Measured spectral response of the L2-channel flight mixer: The red squares are the uncorrected T_rec (DSB) vs. local oscillator frequency measured with the GREAT system at the ground-support facility in Dryden. The blue diamonds are the normalized direct-detection response measured in a test cryostat with the FTS setup at the Cologne lab. The absorption feature in the FTS response at 1868 GHz is caused by water in the beam path. The black solid line is the calculated atmospheric transmission for the FTS setup in Cologne.