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GLA02 Records: Release 33

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Level-1A atmospheric data (GLA02) include the normalized relative backscatter for the 532 nm and 1064 nm channels, and low-level instrument corrections such as laser energy normalization for both channels, background subtraction, range-square correction, dead-time correction for 532 nm, photon coincidence for 532 nm, and detector-gain correction for 1064 nm. Vertical profiles for both channels are shown in the following diagrams:

532 nm profile



1064 nm profile

The vertical resolution, calculated from each return bin's width, is 76.8 m. GLA02 consists of 1 sec records with the time and position information corresponding to the first shot (of 40) of that second. Linear interpolation between seconds should be used to obtain location and time information for each of the 40 Hz profiles within a second.

Note that the height of the top bin within each layer is not constant. The height above the ellipsoid at which GLAS acquires lidar data depends on the height of the local topography below the nadir point of the satellite. GLAS is designed to acquire lidar data such that the end of the profile, or the last bin of the lowest layer, is always roughly 1.5 km below the geoid. Keeping this profile over changing topography causes the height of the top bin, and all subsequent bins of the profile, to change too. This "vertical shifting" only occurs on second boundaries. Within a second, all 40 shots are acquired starting at the same height. To determine the height of the top bin, examine the "i_Rng2PCProf" variable (byte offset 28476) for the 532 nm channel and "i_rng2CDProf" variable (byte offset 29500) for the 1064 nm channel. Subtracting these values from the height of the spacecraft, "i_Hsat" (byte offset 56636) yields the height above the geoid for the top bin.


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.


Name Short Description Byte Offset Data Type Total Bytes
i_rec_ndx GLAS record index 0 i4b 4
i_UTCTime Transmit time of first shot in frame in J2000 (referenced from noon on 01 January 2000) 4 i4b(2) 8
i1_pred_lat Predicted geodetic latitude of the laser footprint 12 i4b 4
i1_pred_lon Predicted geodetic longitude of the laser footprint 16 i4b 4
i_DEMmin DEM minimum 20 i2b 2
i_DEMmax DEM maximum 22 i2b 2
i_g_lid_qf 532 nm lidar data quality flag 24 i1b(12), unsigned 12
i40_g_lid 532 nm lidar data for 10.5 to -1.5 km segment 36 i4b(148,40) 23680
i5_g_lid 532 nm lidar data for 20.5 to 10.5 km segment 23716 i4b(132,5) 2640
i1_g_lid 532 nm lidar data for 40 to 20.5 km segment 26356 i4b(268) 1072
i40_g_sat_f 532 nm saturation flag for 10.5 to -1.5 km segment 27428 i1b(740), unsigned 740
i5_g_sat_f 532 nm saturation flag for 20.5 to 10.5 km segment 28168 i1b(84), unsigned 84
i1_g_sat_f 532 nm saturation flag for 40 to 20.5 km segment 28252 i1b(36), unsigned 36
i40_g_TxNrg_EU 532 nm laser transmit energy at 40 Hz 28288 i4b(40) 160
i5_g_TxNrg_EU 532 laser transmit energy at 5 Hz 28448 i4b(5) 20
i1_g_TxNrg_EU 532 laser transmit energy at 1 Hz 28468 i4b(4) 16
i_g_IntRet 532 nm integrated return, 40 km to 20.5 km 28472 i4b 4
i_Rng2PCProf Start range of 532 nm backscatter profile 28476 i4b 4
i_Rng_PkRt Range from spacecraft to peak of return 28480 i4b 4
i40_g_bg 532 nm background at 40 Hz 28484 i4b(4,40) 640
i5_g_bg 532 nm background at 5 Hz 29124 i4b(4,5) 80
i1_g_bg 532 nm background at 1 Hz 29204 i4b(4) 16
i_gPredCldTop 532 nm predicted cloud top height at 5 Hz 29220 i2b(5) 10
i_g_shot_ctr 532 nm lidar data shot counter 29230 i2b 2
i_SpcmBg2Del SPCM background 2 delay 29232 i2b, unsigned 2
i_SpcmRngDel SPCM range delay 29234 i2b, unsigned 2
i_SpcmGateDel SPCM gate delay 29236 i2b, unsigned 2
i_SpcmBg1Del SPCM background 1 delay 29238 i2b, unsigned 2
i_spcm_stat SPCM status 29240 i2b, unsigned 2
i_g_TxNrg_Cts 532 nm laser transmit energy, counts 29242 i1b(40), unsigned 40
i_g_TxNrg_qf 532 nm laser transmit energy quality flag 29282 i1b(10), unsigned 10
i_g_IntRet_qf Integrated return quality flag 29292 i1b, unsigned 1
i_spare2 Spares 29293 i1b, unsigned 1
i_ir_lid_qf 1064 nm lidar data quality flag 29294 i1b(12), unsigned 12
i_ir_shot_ctr 1064 nm cloud digitizer shot counter 29306 i2b 2
i_spcm_cts SPCM raw counts 29308 i1b(8), unsigned 8
i_pc_rbias Photon counter range bias 29316 i4b 4
i40_ir_TxNrgEU 1064 nm laser transmit energy at 40 Hz 29320 i4b(40) 160
i5_ir_TxNrgEU 1064 nm laser transmit energy at 5 Hz 29480 i4b(5) 20
i_rng2CDProf Start range of the 1064 nm backscatter profile 29500 i4b 4
i40_ir_bg 1064 nm background at 40 Hz 29504 i4b(4,40) 640
i5_ir_bg 1064 nm background at 5 Hz 30144 i4b(4,5) 80
i40_ir_lid 1064 nm lidar data from 10.5 km to -1.5 km 30224 i4b(148,40) 23680
i5_ir_lid 1064 nm LIDAR data from 20.5 km to 10.5 km 53904 i4b(132,5) 2640
i_CdBg2_Del Cloud digitizer background 2 delay 56544 i2b, unsigned 2
i_RngGate_Del Cloud digitizer range gate delay 56546 i2b, unsigned 2
i_cd_bg1_del Cloud digitizer background 1 delay 56548 i2b, unsigned 2
i_cd_det_stat Cloud digitizer detector status 56550 i2b, unsigned 2
i_cd_rbias Cloud digitizer range bias 56552 i4b 4
i_cd_ad_out A/D output 56556 i1b, unsigned 1
i_cd_att_set Attenuation setting 56557 i1b, unsigned 1
i_CldPkSig Cloud return peak signal 56558 i1b(5) 5
i_gndret_pksig Ground return peak signal 56563 i1b(5) 5
i_gnd_ret_loc Ground return location 56568 i1b(5) 5
i_et_cal_mode Etalon calibration - current mode 56573 i1b 1
i_ir_TxNrg_qf 1064 nm laser transmit energy quality flag 56574 i1b(10) 10
i_EtHtrC37j_c Etalon heater current, Ch 37j 56584 i2b 2
i_EtC37d_t Etalon temperature, Ch 37d 56586 i2b 2
i_ETsettleTime Etalon temperature settle time 56588 i2b, unsigned 2
i_et_Flags Etalon flags 56590 i1b, unsigned 1
i_et_update_ctr Etalon averaging update counter 56591 i1b 1
i_et_StartTemp Start temperature 56592 i1b 1
i_et_StopTemp Stop temperature 56593 i1b 1
i_et_TempStep Temperature step 56594 i1b 1
i_et_spare Spares 56595 i1b(3), unsigned 3
i_et_acqavg_tm Etalon averaging time for acquire command 56598 i1b 1
i_spare6 Spares 56599 i1b, unsigned 1
i_et_temperr Etalon temperature error 56600 i4b 4
i_ET_state Etalon state 56604 i1b 1
i_spare3 Spares 56605 i1b, unsigned 1
i_et_acqset_tm Etalon temperature settle time for acquire command 56606 12b, unsigned 2
i_et_onax_xmit Etalon averaged on-axis transmission 56608 i4b 4
i_et_offax_xmit Etalon averaged off-axis transmission 56612 i4b 4
i_et_trkfltout Etalon tracking loop filter output 56616 i4b 4
i_et_trkfltavg Etalon tracking failure average 56620 i4b 4
i_APID_AvFlg APID Data Availability Flag 56624 i1b 8
i_OrbFlg Orbit flag 56632 i2b, unsigned 2
i_HoffMin Offset to minimum DEM height 56634 i2b 2
i_Hsat Geodetic altitude of satellite above Earth 56636 i4b 4
i_4nsBgMean 4 ns background mean value 56640 i4b(40) 160
i_4nsBgSDev 4 ns background standard deviation 56800 i4b(40) 160
i_DualPinA Dual Pin A data 56960 i1b(40), unsigned 40
i_DualPinB Dual Pin B data 57000 i4b(40), unsigned 40
i_spare4 Spares 57040 i1b, unsigned 1
i_DitheringEnabledFlag Dithering enabled flag 57041 i1b 1
i_timecorflg Time-correction flag 57042 i2b 2
spare5 Spares 57044 i1b(12) 12

Page last updated: 01/11/13