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 and SICE 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 outlined in the previous chapter, the S3-SNOW Snow Properties Processor also accepts BRR [1] products as an alternative to the OCLI L1b radiance products, whereas the SICE processor needs them as mandatory input in addition to the L1b products. These BRR [1] products must 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]	(1, 2) 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.

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. For the SICE processor, bands 10 and 11 (681 and 708nm) are also mandatory in addition to bands 1, 6, 17 and 21. In the example shown 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.

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_08	dl	float32	Bottom-of-Rayleigh reflectance for band 8 (665nm)
rBRR_09	dl	float32	Bottom-of-Rayleigh reflectance for band 9 (673nm)
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 classification product generated with default user options containing the flag bands listed in Table 3.6. Optionally, more bands can be written to the classification product, as described in more detail in the IdePix processor help documentation. However, the S3-SNOW and SICE just need the pixel classification band.

Table 3.6 Bands in IdePix pixel classification product

Name in product	Unit	Type	Description
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 Harmonisation Product

The O2 harmonisation 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 harmonisation is also available as a standalone SNAP plugin with a separate target product, its default contents are listed in Table 3.8 for completeness.

Table 3.8 Bands in O2 Harmonisation 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 and SICE final products contain various snow properties, depending on the processing parameters specified by the user. The maximum number of bands which can be generated with the S3-SNOW processor 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 BB visible range
albedo_bb_spherical_nir	dl	float32	Spherical albedo in BB near infrared range
albedo_bb_spherical_sw	dl	float32	Spherical albedo in Bb shortwave range
albedo_bb_planar_vis	dl	float32	Planar albedo in BB visible range
albedo_bb_planar_nir	dl	float32	Planar albedo in BB near infrared range
albedo_bb_planar_sw	dl	float32	Planar albedo in BB shortwave range
albedo_spectral_spherical_<nn>	dl	float32	Spectral spherical albedo in band <nn>
albedo_spectral_planar_<nn>	dl	float32	Spectral planar albedo in band <nn>
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	Snow impurity absorption coefficient
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)

The maximum number of bands which can be generated with the SICE processor is given in Table 3.11. Again, for more detailed explanations of the physical meaning of the parameters, see [2].

Table 3.11 Bands in final SICE snow properties product

Name in product	Unit	Type	Description
albedo_bb_spherical_vis	dl	float32	Spherical albedo in BB visible range
albedo_bb_spherical_nir	dl	float32	Spherical albedo in BB near infrared range
albedo_bb_spherical_sw	dl	float32	Spherical albedo in BB shortwave range
albedo_bb_planar_vis	dl	float32	Planar albedo in BB visible range
albedo_bb_planar_nir	dl	float32	Planar albedo in BB near infrared range
albedo_bb_planar_sw	dl	float32	Planar albedo in BB shortwave range
albedo_spectral_spherical_<nn>	dl	float32	Spectral spherical albedo in band <nn>
albedo_spectral_planar_<nn>	dl	float32	Spectral planar albedo in band <nn>
grain_diameter	mm	float32	Snow grain diameter
snow_specific_area	m^2/kg	float32	Snow specific surface area
scattering_angle	dl	float32	Scattering angle.
concentration_of_pollutants	dl	float32	Concentration of pollutants
ndbi	dl	float32	NDBI ratio.
ndsi	dl	float32	NDSI ratio.
sice_pollution_type_flags	dl	int16	SICE pollution type flags.
sice_ground_type_flags	dl	int16	SICE ground type flags.
quality_flags	dl	int16	L1b quality flags
pixel_classif_flags	dl	int16	Pixel classification flags