3. The S3-SNOW Products

3.1. Overview

This section will give an overview of all input, intermediate, optional and final products used and generated by the SNAP S3-SNOW processors introduced in the previous chapter.

3.2. Input Products

3.2.1. OLCI L1b TOA Radiance Product

Both reduced and full resolution OLCI L1b TOA radiance products can be used as input data. Tables 3.1 to 3.4 give an overview of OLCI L1b bands, tie point grids and L1b flag coding, respectively. A more detailed description of the OLCI standard L1b product is given in [3]. (Note that in SNAP terminology, the term ‘bands’ does not only refer to ‘spectral bands’ (i.e., 1-21 for OLCI), but for all raster data quantities read or written by SNAP.)


Table 3.1 OLCI bands in L1b product
Name in product Unit Type Description
Oa<nn>_radiance; n=01,..,21 mW/(m^2*sr*nm) float32 TOA radiance in band <nn>
lambda0_band_<nn>; n=01,..,21 nm float32 Central wavelength in band <nn>
FWHM_band_<nn>; n=01,..,21 nm float32 Bandwidth in band <nn>
solar_flux_band_<nn>; n=01,..,21 mW/(m^2*nm) float32 Solar flux in band <nn>
quality_flags dl (flag band) int32 L1b quality flags
altitude m float32 Altitude
latitude degrees float32 Latitude
longitude degrees float32 Longitude
frame_offset dl float32 Resampling frame offset
detector_index dl float32 Detector index


Table 3.2 OLCI instrument channels.
Channel Wavelength (nm)
1 400.0
2 412.5
3 442.5
4 490
5 510
6 560
7 620
8 665
9 673.75
10 681.25
11 708.75
12 753.75
13 761.25
14 764.375
15 767.5
16 778.75
17 865
18 885
19 900
20 940
21 1020


Table 3.3 OLCI tie point grids in L1b product.
Name in product Unit Type Description
TP_latitude deg float32 Latitude of the tie points
TP_longitude deg float32 Longitude of the tie points
SZA deg float32 Sun zenith angle
SAA deg float32 Sun azimuth angle
OZA deg float32 View zenith angle
OAA deg float32 View azimuth angle
horizontal_wind_vector_1 m/s float32 Zonal wind component
horizontal_wind_vector_2 m/s float32 Meridional wind component
total_columnar_water_vapour kg/m^2 float32 Total column of water vapour
sea_level_pressure hPa float32 Mean sea level pressure
ozone kg/m^2 float32 Total ozone
humidity % float32 Relative humidity


Table 3.4 OLCI L1b flag coding.
Bit Flag Description
0-20 saturated_Oa<21-nn> Band <nn> is saturated
21 dubious Pixel is cosmetic
22 sun_glint_risk Pixel has been duplicated
23 duplicated Pixel has glint risk
24 cosmetic Pixel is suspect
25 invalid Pixel is over land,
26 straylight_risk Pixel is bright
27 bright Pixel is part of a coastline
28 tidal_region Pixel is suspect
29 fresh_inland_water Pixel is over land
30 coastline Pixel is bright
31 land Pixel is part of a coastline


3.2.2. OLCI L1b Bottom-of-Rayleigh Reflectances (BRR) Product

As an alternative to the OCLI L1b radiance products, the Snow Properties Processor also accepts as input BRR [1] products which have been generated in a pre-processing step using the SNAP Rayleigh Correction Processor. This processor can be accessed from the SNAP Desktop application as shown in Figure 3.1.

Footnotes

[1]Physically, the term ‘BRR’ used in the Rayleigh Correction Processor should better be called ‘BOAR’ (Bottom-of-atmosphere reflectance). The BOAR is derived from Top-of-atmosphere reflectance by correction for molecular scattering and absorption.
_images/rc_in_snap.png

Figure 3.1 Access to Rayleigh Correction Processor in SNAP Desktop.

The Rayleigh Correction Processor is a SNAP build-in component. For a detailed description see the corresponding SNAP help documentation. For BRR products to be used with the Snow Properties Processor, BRR for spectral bands 1, 6, 17 and 21 (400nm, 560, 865 and 1020nm) must be generated, and in addition the BRRs for all other spectral bands of interest. In Figure 3.2, the parameter settings to generate BRR in bands 1, 6, 17 and 21, and in addition bands 9 and 10, are shown.

_images/rc_params.png

Figure 3.2 Rayleigh Correction: parameters for the generation of BRR in spectral bands 1, 5, 17 and 21, and in addition in spectral bands 9 and 10.

The bands in the resulting BRR product are listed in Table 3.5.

Table 3.5 Bands in BRR product
Name in product Unit Type Description
rBRR_01 dl float32 Bottom-of-Rayleigh reflectance for band 1 (400nm)
rBRR_05 dl float32 Bottom-of-Rayleigh reflectance for band 5 (510nm)
rBRR_17 dl float32 Bottom-of-Rayleigh reflectance for band 17 (865nm)
rBRR_21 dl float32 Bottom-of-Rayleigh reflectance for band 21 (1020nm)
rBRR_<nn> dl float32 Bottom-of-Rayleigh reflectance in band <nn> (optional)
quality_flags dl int32 L1b quality flags

3.3. Intermediate and Optional Products

3.3.1. SNAP IdePix Classification Product

The IdePix classification product is the result of the pixel classification performed on the OLCI L1b product. In return, the IdePix product can be used as optional input for Snow Properties Processor. The IdePix OLCI version for S3-SNOW provides an ‘extended’ classification product containing the bands listed in Table 3.6.

Table 3.6 Bands in IdePix pixel classification product
Name in product Unit Type Description
Oa21_reflectance dl float32 TOA reflectance for band 21
trans_13 dl float32 Rectified and desmiled transmission for band 13
press_13 hPa float32 Pressure for band 13 neglecting scattering
surface_13 dl float32 Rectified transmission for band 13 neglecting scattering
surface_pressure hPa float32 Pressure at altitude of the surface
altitude m float32 Altitude of the surface (from DEM or L1b product)
cloud_over_snow dl float32 Mask band indicating if there is cloud over snow
pixel_classif_flags dl int16 Standard IdePix classification flag band
quality_flags dl int32 L1b quality flags

The IdePix OLCI classification flag coding is given in Table 3.7.

Table 3.7 Bands in IdePix pixel classification product
Bit Flag Description
0 IDEPIX_INVALID Pixel is invalid
1 IDEPIX_CLOUD Pixel is either ‘cloud sure’ or ‘cloud ambiguous’
2 IDEPIX_CLOUD_AMBIGUOUS Semi-transparent clouds, or detection is uncertain
3 IDEPIX_CLOUD_SURE Fully opaque clouds with full confidence of detection
4 IDEPIX_CLOUD_BUFFER A buffer of N pixels (user option) around a cloud
5 IDEPIX_CLOUD_SHADOW Pixel is affected by a cloud shadow
6 IDEPIX_SNOW_ICE Snow or ice pixel
7 IDEPIX_BRIGHT Pixel is bright
8 IDEPIX_WHITE Pixel is white
9 IDEPIX_COASTLINE Pixel is part of a coastline
10 IDEPIX_LAND Land pixel

3.3.2. O2 Correction Product

The O2 correction step is done as part of the IdePix pixel classification. The results of this step are used by IdePix internally, and no separate products are written in the S3-SNOW processing flow. However, as the O2 correction is also available as a standalone SNAP plugin with a separate target product, its contents are listed in Table 3.8 for completeness.

Table 3.8 Bands in O2 Correction target product
Name in product Unit Type Description
trans_13 dl float32 Rectified and desmiled transmission for band 13
press_13 hPa float32 Pressure for band 13 neglecting scattering
surface_13 dl float32 Rectified transmission for band 13
radiance_13 mW/(m^2*sr*nm) float32 TOA radiance in band 13

3.3.3. Slope Product

A slope product is the output of the SNAP Slope Processor introduced in chapter The SNAP S3-SNOW Processors. As mentioned, this is an optional product which is not directly used in the S3-SNOW processing flow. For completeness, its contents are listed in Table 3.9.

Table 3.9 Bands in Slope Processor target product
Name in product Unit Type Description
slope degrees float32 Terrain slope
aspect degrees float32 Aspect angle in [0, 360] deg
elevation_variance degrees float32 Variance of elevation over 3x3 pixel window
<altitude_band> m float32 Altitude band from input product (optional)


3.4. Final Snow Properties Product

The S3-SNOW final product contains various snow properties, depending on the processing parameters specified by the user. The maximum number of bands which can be generated is given in Table 3.10. For more detailed explanations of the physical meaning of the parameters, see [2].

Table 3.10 Bands in final S3-SNOW snow properties product
Name in product Unit Type Description
albedo_bb_spherical_vis dl float32 Spherical albedo in broadband visible range
albedo_bb_spherical_nir dl float32 Spherical albedo in broadband near infrared range
albedo_bb_spherical_sw dl float32 Spherical albedo in broadband shortwave range
albedo_bb_planar_vis dl float32 Planar albedo in broadband visible range
albedo_bb_planar_nir dl float32 Planar albedo in broadband near infrared range
albedo_bb_planar_sw dl float32 Planar albedo in broadband shortwave range
albedo_spectral_spherical_<nn> dl float32 Spectral spherical albedo in band <nn> (max. 21 bands)
albedo_spectral_planar_<nn> dl float32 Spectral planar albedo in band <nn> (max. 21 bands)
rBRR_<nn> dl float32 BRR in band <nn> (max. 21 bands)
ppa_spectral_<nn> dl float32 PPA in band <nn> (max. 21 bands)
grain_diameter mm float32 Snow grain diameter
snow_specific_area m^2/kg float32 Snow specific surface area
ndbi dl float32 Bare ice indicator
pollution_mask dl float32 Pollution mask
f 1/mm float32 Normalized snow impurity absorption coefficient at 1micron
l mm float32 Effective absorption length
m dl float32 Absorption Angstrom parameter
r_0 dl float32 Reflectance of nonabsorbing snow layer
f_rel_err dl float32 Relative error of parameter f
l_rel_err dl float32 Relative error of parameter l
m_rel_err dl float32 Relative error of parameter m
r_0_rel_err dl float32 Relative error of parameter r_0
ndsi dl float32 NDSI
ndsi_mask dl float32 NDSI mask for snow identification
quality_flags dl int16 L1b quality flags
pixel_classif_flags dl int16 Pixel classification flags (see Table 3.7)