# SCIAMACHY_L2_NADIR_IR1_CH4

## Variables

The table below lists the variables that are present in the HARP product that results from an ingestion of `SCIAMACHY_L2_NADIR_IR1_CH4` data.

field name | type | dimensions | unit | description
--- | --- | --- | --- | ---
datetime_start | double | {time} | [seconds since 2000-01-01] | measurement start time
datetime_length | double | {time} | [s] | measurement integration time
orbit_index | int32 |  |  | absolute orbit number
latitude | double | {time} | [degree_north] | center latitude for each nadir pixel
longitude | double | {time} | [degree_east] | center longitude for each nadir pixel
latitude_bounds | double | {time, 4} | [degree_north] | corner latitudes for each nadir pixel
longitude_bounds | double | {time, 4} | [degree_east] | corner longitudes for each nadir pixel
solar_zenith_angle | double | {time} | [degree] | solar zenith angle at top of atmosphere
viewing_zenith_angle | double | {time} | [degree] | line of sight zenith angle at top of atmosphere
relative_azimuth_angle | double | {time} | [degree] | relative azimuth angle at top of atmosphere
scan_direction_type | int8 | {time} |  | scan direction for each measurement; enumeration values: forward (0), backward (1), mixed (2)
CH4_column_number_density | double | {time} | [molec/cm^2] | CH4 vertical column density
CH4_column_number_density_uncertainty | double | {time} | [molec/cm^2] | error on the CH4 vertical column density
CH4_column_number_density_validity | int32 | {time} |  | flag describing the CH4 vertical column density
cloud_fraction | double | {time} | [] | average cloud fraction of footprint
index | int32 | {time} |  | zero-based index of the sample within the source product

## Ingestion options

The table below lists the available ingestion options for `SCIAMACHY_L2` products.

option name | legal values | description
--- | --- | ---

dataset

nad_uv0_o3, nad_uv1_no2, nad_uv3_bro, nad_uv4_h2co, nad_uv5_so2, nad_uv6_oclo, nad_uv7_so2, nad_uv8_h2o, nad_uv9_chocho, nad_ir0_h2o, nad_ir1_ch4, nad_ir2_n2o, nad_ir3_co, nad_ir4_co2, lim_uv0_o3, lim_uv1_no2, lim_uv3_bro, clouds_aerosol

the dataset of the L2 product to ingest; each dataset is a combination of nadir/limb choice, retrieval window, and main quantity; option values are ‘nad_uv0_o3’ (default), ‘nad_uv1_no2’, ‘nad_uv3_bro’, ‘nad_uv4_h2co’, ‘nad_uv5_so2’, ‘nad_uv6_oclo’, ‘nad_uv7_so2’, ‘nad_uv8_h2o’, ‘nad_uv9_chocho’, ‘nad_ir0_h2o’, ‘nad_ir1_ch4’, ‘nad_ir2_n2o’, ‘nad_ir3_co’, ‘nad_ir4_co2’, ‘lim_uv0_o3’, ‘lim_uv1_no2’, ‘lim_uv3_bro’, ‘clouds_aerosol’

This definition is only applicable when: dataset=nad_ir1_ch4

## Mapping description

The table below details where and how each variable was retrieved from the input product.

field name | mapping description
--- | ---
datetime_start | path | /nad_ir1_ch4[]/dsr_time
datetime_length | path | /nad_ir1_ch4[]/integr_time
orbit_index | path | /mph/abs_orbit
latitude | condition | No co-adding needed
path | /geolocation_nadir[]/cen_coor_nad/latitude
description | center latitude for each nadir pixel
condition | Co-adding needed and all N geolocations are within a single scan (N is not divisible by 5)
path | /geolocation_nadir[]/cor_coor_nad[]
description | the latitude of the geographic average of cor_coor_nad[2] and cor_coor_nad[3] of the N/2-th pixel
condition | Co-adding needed of both forward and backward scan pixels (N is divisible by 5)
path | /geolocation_nadir[]/cen_coor_nad, /geolocation_nadir[]/cor_coor_nad[]
description | the latitude of the geographic average of 1: the geographic average of cor_coor_nad[2] and cor_coor_nad[3] of the second pixel and 2: cen_coor_nad of the N-th pixel
longitude | condition | No co-adding needed
path | /geolocation_nadir[]/cen_coor_nad/longitude
description | center longitude for each nadir pixel
condition | Co-adding needed and all N geolocations are within a single scan (N is not divisible by 5)
path | /geolocation_nadir[]/cor_coor_nad[]
description | the longitude of the geographic average of cor_coor_nad[2] and cor_coor_nad[3] of the N/2-th pixel
condition | Co-adding needed of both forward and backward scan pixels (N is divisible by 5)
path | /geolocation_nadir[]/cen_coor_nad, /geolocation_nadir[]/cor_coor_nad[]
description | the longitude of the geographic average of 1: the geographic average of cor_coor_nad[2] and cor_coor_nad[3] of the second pixel and 2: cen_coor_nad of the N-th pixel
latitude_bounds | condition | No co-adding needed
path | /geolocation_nadir[]/cor_coor_nad[]/latitude
description | corners are rearranged in the following way: 0,2,3,1
condition | Co-adding needed and all N geolocations are within a single scan (N is not divisible by 5)
path | /geolocation_nadir[]/cor_coor_nad[]
description | cor_coor_nad[0] and cor_coor_nad[1] are taken from the first pixel and cor_coor_nad[2] and cor_coor_nad[3] are taken from the N-th pixel; corners are rearranged in the following way: 0,2,3,1
condition | Co-adding needed of both forward and backward scan pixels (N is divisible by 5)
path | /geolocation_nadir[]/cor_coor_nad[]
description | cor_coor_nad[0] is taken from the first pixel, cor_coor_nad[2] is taken from the fourth pixel, cor_coor_nad[1] is taken from cor_coor_nad[3] of the N-th pixel, and cor_coor_nad[3] is taken from cor_coor_nad[1] from the N-th pixel; corners are rearranged in the following way: 0,2,3,1
longitude_bounds | condition | No co-adding needed
path | /geolocation_nadir[]/cor_coor_nad[]/longitude
description | corners are rearranged in the following way: 0,2,3,1
condition | Co-adding needed and all N geolocations are within a single scan (N is not divisible by 5)
path | /geolocation_nadir[]/cor_coor_nad[]
description | cor_coor_nad[0] and cor_coor_nad[1] are taken from the first pixel and cor_coor_nad[2] and cor_coor_nad[3] are taken from the N-th pixel; corners are rearranged in the following way: 0,2,3,1
condition | Co-adding needed of both forward and backward scan pixels (N is divisible by 5)
path | /geolocation_nadir[]/cor_coor_nad[]
description | cor_coor_nad[0] is taken from the first pixel, cor_coor_nad[2] is taken from the fourth pixel, cor_coor_nad[1] is taken from cor_coor_nad[3] of the N-th pixel, and cor_coor_nad[3] is taken from cor_coor_nad[1] from the N-th pixel; corners are rearranged in the following way: 0,2,3,1
solar_zenith_angle | condition | No co-adding needed
path | /geolocation_nadir[]/sol_zen_angle_toa[1]
condition | Co-adding needed and all N geolocations are within a single scan (N is not divisible by 5)
path | /geolocation_nadir[]/sol_zen_angle_toa[2]
description | the value at end of integration time of the N/2-th geolocation
condition | Co-adding needed of both forward and backward scan pixels (N is divisible by 5)
path | /geolocation_nadir[]/sol_zen_angle_toa[1], /geolocation_nadir[]/sol_zen_angle_toa[2]
description | the average of the value at end of integration time of the second record and the value at middle of integration time of the N-th record
viewing_zenith_angle | condition | No co-adding needed
path | /geolocation_nadir[]/los_zen_angle_toa[1]
condition | Co-adding needed and all N geolocations are within a single scan (N is not divisible by 5)
path | /geolocation_nadir[]/los_zen_angle_toa[2]
description | the value at end of integration time of the N/2-th geolocation
condition | Co-adding needed of both forward and backward scan pixels (N is divisible by 5)
path | /geolocation_nadir[]/los_zen_angle_toa[1], /geolocation_nadir[]/los_zen_angle_toa[2]
description | the average of the value at end of integration time of the second record and the value at middle of integration time of the N-th record
relative_azimuth_angle | condition | No co-adding needed
path | /geolocation_nadir[]/rel_azi_angle_toa[1]
condition | Co-adding needed and all N geolocations are within a single scan (N is not divisible by 5)
path | /geolocation_nadir[]/rel_azi_angle_toa[2]
description | the value at end of integration time of the N/2-th geolocation
condition | Co-adding needed of both forward and backward scan pixels (N is divisible by 5)
path | /geolocation_nadir[]/rel_azi_angle_toa[1], /geolocation_nadir[]/rel_azi_angle_toa[2]
description | the average of the value at end of integration time of the second record and the value at middle of integration time of the N-th record
scan_direction_type | path | /geolocation_nadir[]/corner_coord[], /geolocation_nadir[]/dsr_time

description

When the integration time is higher than 1s we are dealing with a mixed (2) pixel, otherwise the scan direction is based on the corner coordinates of the first ground pixel of the measurement. The first geolocation pixel is a backscan (1) pixel if the inproduct of the unit vector of the third corner with the outproduct of the unit vector of the first corner and the unit vector of the second corner is negative (otherwise it is part of a forward (0) scan)

CH4_column_number_density | path | /nad_ir1_ch4[]/vcd[0]
CH4_column_number_density_uncertainty | path | /nad_ir1_ch4[]/vcd_err[0], /nad_ir1_ch4[]/vcd[0]
description | relative error is converted to absolute error by multiplying with measured value
CH4_column_number_density_validity | path | /nad_ir1_ch4[]/flag_vcd_flags
cloud_fraction | path | /clouds_aerosol[]/cl_frac
description | in case of co-adding, the average of all values within the integration time is taken