THOR calibration

The correct interpretation of data from multiple field-of-views requires some calibration procedures not applicable to single-view lidars. THOR's radiometric calibration has three components:

1. Laboratory relative calibration of THOR channels. A calibration sphere is attached to the THOR telescope to illuminate the front lens uniformly and isotropically (see photo in images section). Because the increases in field-of-view size from one channel to the next are known, we can predict how photon counts would increase in an ideal instrument. The deviations between the ideal and the observed increases provide the coefficients for a relative calibration that anchors all channels to Channel 1.
2. In-flight relative calibration of THOR channels. This procedure checks whether THOR's performance changed substantially since the last laboratory calibration. For this, it uses as quasi-uniform light source the moonlight reflected from extensive cloud fields, which is measured between successive laser pulses.
3. Absolute calibration of Channel 1. Rayleigh scattering of laser pulses can provide absolute calibration if THOR flies over a thick layer of cloud-free and aerosol-free air. Because all channels are tied to Channel 1 through relative calibration, this procedure provides absolute calibration for all channels.

Since accurate calibration may not be available on all flights, THOR cloud property retrievals are designed to work even without calibration, albeit with larger uncertainties.

 

            Ratio of photon counts in neighboring THOR channels

The data on the right side was obtained in laboratory experiments that used 10 illumination levels. The data on the left side is from various flight altitudes during the last flight of the 2002 THOR validation campaign.