FODIS and ILS

The aircraft has a downwelling light sensor, with a cosine collector mounted on the roof bringing light via a fiber optic cable to the instruments. Both CASI and Eagle have systems for measuring this light

• ILS. The NERC ARSF CASI System incorporates an Incident Light Sensor (ILS) measuring downwelling radiation at the aircraft; this is Lumogen coated for enhancement of the blue response below 450 nm.
• FODIS. The Eagle instrument incorporates a downwelling radiation sensor, similar to the CASI ILS, called the Fiber-Optic Downwelling Irradiance Sensor (FODIS).

Physical layout

DCALM's downwelling light collector (prior to cleaning!) mounted on the roof:

Side view of DCALM, showing location of downwelling light collector:

Processing

2011 onwards

The FODIS data processing (2011 onwards) is much the same as for the Eagle data except that each scan line of FODIS data (4 pixels) is averaged together to reduce noise. The resulting processed data will consist of 1 data value per scan line of imagery.

pre-2011

Details on processing the pre-2011 ILS/FODIS data are in the azgcorr manual (to a limited extent - we'll improve this as soon as we get the manual text..). Basically it's an extra data vector in the HDF files, called CAils, with one pixel of upwards-looking data per downwards-looking scanline.

The vector should be present in all CASI and Eagle HDFs, but will be garbage in any flight where the sensor wasn't connected (fibre optic cable from the CCD to the roof-mounted collector). If the data are correct, they should resemble the solar spectrum.

It will also be garbage in any Eagle HDFs preceding mid 2008 as the cable wasn't connected (may be present in some Ethiopia flights) and the processing wasn't working (fixed as of June/08, backprocessing available on request).

Note that due to a bug in ENVI (?), it appears that one gets odd spiky results from directly viewing the HDF vector. You must extract it first with:

azexhdf -Be fodis.bil -d CAils youreagle.hdf