The Merlin instrument is an Integrated Path Differential Absorption (IPDA) Lidar for Methane column density measurements, based on Germany's long heritage in airborne greenhouse gas Lidar systems and on up-to-date innovative concepts of pulsed high power laser systems for space applications.
The instrument measures the light scattered and reflected from the Earth's surface and cloud tops which are illuminated by laser pulses with slightly different wavelengths denoted as on-line (on) and off-line (off).
Measurement concept with Merlin
Thee wavelength on is accurately positioned in the trough of one of the methane absorption line multiplets in the 1.64 µm - 1.67 µm region, in order to dramatically relax the required laser frequency stability/knowledge. The off measurement serves as the reference measurement with negligible methane absorption. The differential optical depth and the column-averaged methane volume mixing ratio are calculated from the ratio of the Pon/Poff Lidar echoes (using mean values).
The satellite has an orbital velocity of 7600 m/s. A laser pulse takes 3.4 ms for a round trip. To raster Earth's surface, the Lidar instrument emits pulse pairs at a frequency of 20 Hz (50 ms) along the ground track. Thus, the measurement points on the surface are spaced around 380 m apart. By integrating the single column contents over a distance of 50 km, an average methane content can be determined. The on and off pulses are 250 µs apart, leading to an along-track offset on the Earth's surface of around 2 m.