Atmospheric Remote Sensing: Measurements


Info (metadata)

The raw lidar data files are in PNG gray-scale image format that is 2000 pixels by 2200 pixels. File naming convention is AK14dddhhmmss.png, where ddd is year day, hh is hour, mm is minute, and ss is second. These describe the time the file was created (UTC). Each column in the image represents a single lidar shot, so each full data file represents about 67 seconds at the lidar repetition rate of 30 Hz. Some partial files contain fewer than 2000 columns. The first 1000 pixels in each column are the 1000 samples of the co-polarized receiver channel. The digitizer has eight bits, so there are 256 levels of gray. The next 1000 pixels are the simultaneous samples from the cross-polarized receiver. The last 200 samples are used to record ancillary information in ASCII format. Data gaps are identified by several columns of zeroes.

The ancillary information for each shot includes (in order): latitude (decimal degrees, bytes 2000-2009), N (for north latitude, byte 2010), longitude (bytes 2011-2020), W (for west longitude, byte 2021), co-polarized receiver gain voltage (bytes 2039-2045, cross-polarized receiver gain voltage (bytes 2047-2053), date (year, bytes 2086-2089; month, bytes 2090-2091; day, bytes 2092-2093) , and GPS time (hour, bytes 2094-2095; minute, bytes 2096-2097; seconds, bytes 2098-2103). Flags set to 1 (binary, not ASCII) in the presence of ice (byte 2150), fog (byte 2151), land (byte 2152), missing data (byte 2155), bad data, including when the aircraft was outside the altitude range (byte 2153), or a plankton layer (byte 2156).

Digitization level is converted into voltage by the following equations:

VCO = 1.41421(0.00173536DL - 0.42225)

VX = 1.41421(0.0017333DL - 0.42262)

where DL is the recorded digitization level between 0 and 255. For either channel, the digitized voltage is converted into photocathode current by:

I = 10-8(V + 0.486) / 50G

where the pmt gain G is found from the recorded receiver gain voltages from:

log(G) = -2.257 + 19.316log(VG) - 8.604[log(VG)]2

The data set includes files that obviously do not contain good data for a number of reasons. These were left in the data set because they do include valid navigation information. Also, the polarizer for the co-polarized channel was actually set in the cross-polarized position for the first four flights. Finally, note that the calibration coefficients required to convert photocathode current into attenuated backscatter have not been finalized.