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GLA07 Records

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GLA07 contains the calibrated, attenuated backscatter for both the 1064 nm and 532 nm channels. Backscatter units are in m^-1 sr^-1, where "sr" is steradian, the International System of Units (SI) unit of solid angular measure.

The 532 nm data are calibrated using the molecular return from about 30 km altitude. The 1064 nm channel is not sensitive enough to measure a molecular return, and is calibrated using the instrument parameters and airborne validation measurements. The molecular backscatter cross section is computed from either standard atmosphere data (temperature and pressure as a function of height) or, when available, the National Center for Environmental Prediction (NCEP) gridded analysis fields of temperature and pressure interpolated to the spacecraft position and time.

The "i_metFlg" flag in GLA07 gives the source of the meteorological data used to compute the molecular backscatter profiles. The complete molecular backscatter cross section profile is stored in GLA07 for both channels. The molecular backscatter profiles are calculated and stored once per second. The 532 nm molecular profile ("i_g_mbscs") spans from 41.1 km to -1.0 km, and the 1064 nm molecular profile ("i_ir_mbscs") extends from 20.5 km. Both have a vertical resolution of 76.8 m.

The calibration constants used to calibrate the data are also stored on the product. Variable names are "i_g_cal_cof" and "i_ir_cal_cof," respectively, for the 532 nm and 1064 nm channels. The 532 nm calibration is a three-word array where the first word is the calibration constant calculated from about 30 km altitude. The second word contains the calibration constant calculated from an altitude of about 10 km. The third word contains the value used to compute the calibrated backscatter. The 1064 nm calibration constant is a two-word array where the first word contains the calculated atmospheric calibration at about the 10 km height. The second word is the value used to compute the calibrated backscatter, which in the case of the 1064 nm channel is likely a constant derived from instrument specifications or validation measurements.

The 532 nm data are stored as 5 Hz profiles from 41.1 km (bin 1) to -1.0 km (bin 548) above the geoid, and 40 Hz profiles from 10.3 km (bin 1) to -1.0 km (bin 148). GLAS' fundamental collection frequency is 40 Hz for data below 10.3 km, 5 Hz for data between 10.3 km and 20.5 km, and 1 Hz for data above 20.5 km. As a result, for the 5 Hz profiles, the data in the layers above 20.5 km are repeated five times. In other words, that portion of the profile does not change within a given second, since the collection frequency of the instrument is 1 Hz above 20.5 km. The vertical bin size, or resolution, is 76.8 m.

The 1064 nm data are stored as 5 Hz profiles from 20.5 km (bin 1) to -1.0 km (bin 280) above the geoid, and 40 Hz profiles from 10.3 km (bin 1) to -1.0 km (bin 148). The fundamental collection frequency of the instrument is 40 Hz for data below 10.3 km and 5 Hz for data between 10.3 km to 20.5 km. No data are collected above 20.5 km for the 1064 nm channel. The vertical bin size, or resolution, is 76.8 m.

GLA07 is designed to vertically align the data to a predetermined altitude range, with respect to the geoid, of -1.0 km to 20.5 km (1064 nm) and -1.0 km to 41.1 km (532 nm). Originally, this product was planned to set the range gate setting bias (the range gate governs the height of the start of data) onboard GLAS to a value resulting in lidar profiles that end 1.5 km below the ground. Following the launch of ICESat, the GLAS science team set the range gate bias somewhat lower (1.75 km) in order to obtain more background information. This changes the altitude range spanned by the raw data. The 1064 nm data actually start at about 19.75 km (40.25 km for 532 nm) above the ground, and extend to about -1.75 km below the ground.

The saturation profiles from the 532 nm channel correspond bin-for-bin to the 40 Hz and 5 Hz atmospheric profiles described above. The saturation profiles are binary flags that indicate the associated 532 nm bin is saturated (1 = "saturation"). The 532 nm channel saturates at a level of about 8 to 10 photons per detector. This occurs mainly from dense water clouds and the ground return. The 532 nm saturation profiles are "packed" on GLA07, meaning each bin of the profiles is represented by a single bit.

Note that the 532 nm lidar profiles in the GLAS Altimetry Data Dictionary are labeled "532 nm merged attenuated backscatter." Whenever the 532 nm channel is saturated, the values can be replaced with a measurement of the cross section from the 1064 nm channel, which is not saturated; hence, the label "merged attenuated backscatter." Any such replacement is controlled by the "i_532AttBS_Flag" (byte offset 70276) in the processing software. View the byte structure of this flag for more information.

Follow the links in the table below to the GLAS Atmosphere Data Dictionary for details of each record, including units and scaling factors. The GLAS science team created this dictionary. Units and scaling factors with a "d" indicate double-precision constants; for example, a value of "1.0d5" is equivalent to 100,000. The variable "pe/bin" represents photo electrons per bin.

The following codes denote data types throughout the remainder of this document.

i1b: 1-byte integer
i2b: 2-byte (short) integer
i4b: 4-byte (long) integer
r4b: 4-byte real
r8b: 8-byte real

Values in parentheses indicate the record size, for example:

i2b (39): 39 records of 2-byte integers
i1b (48,40): 48-record x 40-record array of 1-byte integers

Nearly all integers are signed. Exceptions are noted in the following record table.

Page last updated: 10/01/08