The ESA Charter Mapper provides access to EO data coming from several missions and sensors and provided using different data formats and metadata.
These acquisitions have heterogeneous formats for the data files and associated metadata.
The list of EO Missions supported by the ESA Charter Mapper can be found here.
To reduce this heterogeneity, the ESA Charter Mapper seeks to pre-process the acquisitions into a common format which, on the one hand, provides a ready-to-inspect and comparable dataset and, on the other, to have downstream processing services that support multi-sensor and multi-mission acquisitions without implementing complex blocks.
Below, an inroduction of the concepts that support this approach.
STAC as the common metadata model
EO data is handled under the ESA Charter Mapper Processing Chains using the SpatioTemporal Asset Catalog (STAC).
STAC consists of a standardized way to catalog and expose multi-source geospatial data.
The basic core of STAC1 is adopted and extended within the ESA Charter Mapper to manage the Charter collection of multi-source satellite imagery using multiple STAC “spatiotemporal” Assets.
Each STAC item (e.g. a single Sentinel-2 MSI L1C product for UTM zone 9 and grid square XK) includes multiple assets (e.g. multiple multispectral bands) and consists of a GeoJSON feature with metadata and thumbnail and can be organized under a STAC Catalog (e.g. multitemporal Sentinel-2 L1C MSI products for UTM zone 9 and grid square XK).
Each STAC Catalog can be then grouped under a STAC Collection (e.g. Sentinel-2 L1C) which can also further complement associated metadata information such as producer, processor, host, license, version, temporal extent, GSD, instrument, off-nadir angle, etc. More information can be found at STAC Common Metadata section of STAC Specification documentation2.
The STAC format is also adopted by USGS for the provision of Landsat-7 and Landsat-8 products, as described in the USGS Data Format Control Book3 which describes the format of the data to be used in Collection 2 processing.
Common Band Names (CBN)
STAC assets of EO data are catalogued in the ESA Charter Mapper processing environment using Common Band Names (CBN). CBN classes refer to common band ranges derived from pre-defined frequency ranges of the Electromagnetic Spectrum made for several popular instruments. Each CBN class is defined by a Band Range in micrometers for optical and in centimeters for SAR data. The CBN classification of the frequency spectrum allows a one-to-one mapping of multi-mission and multi-sensor bands (Optical and SAR). CBN thus ease the handling multi-sensor source EO data.
Optical Common Band Names
To classify a generic band from an optical sensor with the CBN schema (e.g. Worldview-3 Yellow band from 0.584 to 0.632 μm), one way is to derive its centered wavelength in micrometers (0.608 μm) and identify the CBN class having the closest central wavelength (CBN class 05 and centered at 0.6 μm). However, while choosing a CBN class also the bandwidth needs to be considered. Common Band Names "nir" and "lwir" refer, in fact, to wider bands that cover most of the spectral range for NIR (0.75μm to 1.0μm) and TIR (10.5μm to 12.5μm) radiation. On the other hand, narrow bands for "nir" are "nir08" and "nir09", centered at 0.85μm and 0.95μm respectively. This is particularly useful for example to classify sensors having both a wide (e.g. Sentinel-2 MSI Band 8 at 833 nm) and a narrow (Sentinel-2 MSI Band 8a at 865 nm) band over the same portion of the EM spectrum. CBN "nir" will refer then to S2 MSI spectral Band 8 and "nir08" to the S2 MSI spectral band 8a.
Figure 1 - Comparison of Landsat-7 and 8 bands with the ones of Sentinel-2 (Image credit: USGS).
The same applies for "lwir" which is also discretized into two narrow bands "lwir11" and "lwir12", centered at 11μm and 12μm respectively. Thus, the CBN schema is not rigid and allows the definition of new classes as soon as an additional portion of the spectrum is required for a particular scope (e.g. future EO data of the ESA Charter Mapper optical payload having different spectral resolution that cannot be fully classified by using the current CBN schema).
A total of 29 CBNs (pan, coastal, blue, yellow, etc.) have been identified for the ESA Charter Mapper which are listed in Table 1. This table also includes a sample application of this CBN schema, made for Landsat-7 ETM+, Landsat-8 OLI/TIRS and Sentinel-2 MSI data.
|CBN code||Common Band Name (CBN)||Band Range (μm)||Landsat 7 ETM+||Landsat 8 OLI / TIRS||Sentinel-2 MSI|
|CBN-01||pan||0.50 - 0.70||8||8|
|CBN-02||coastal||0.40 - 0.45||1||1|
|CBN-03||blue||0.45 - 0.50||1||2||2|
|CBN-04||green||0.50 - 0.60||2||3||3|
|CBN-05||yellow||0.58 - 0.62|
|CBN-06||red||0.60 - 0.70||3||4||4|
|CBN-07||rededge||0.70 - 0.79|
|CBN-08||rededge70||0.69 - 0.71||5|
|CBN-09||rededge74||0.73 - 0.75||6|
|CBN-10||rededge78||0.69 - 0.71||7|
|CBN-11||nir||0.75 - 1.00||4||5||8|
|CBN-12||nir08||0.75 - 0.90||8a|
|CBN-13||nir09||0.85 - 1.05||9|
|CBN-14||swir12||1.19 - 1.21|
|CBN-15||cirrus||1.35 - 1.40||9||10|
|CBN-16||swir16||1.55 - 1.75||5||6||11|
|CBN-17||swir155||1.45 - 1.65|
|CBN-18||swir165||1.65 - 1.75|
|CBN-19||swir173||1.72 - 1.74|
|CBN-20||swir22||2.10 - 2.30||7||7||12|
|CBN-21||swir215||2.13 - 2.17|
|CBN-22||swir220||2.18 - 2.22|
|CBN-23||swir225||2.23 - 2.27|
|CBN-24||swir23||2.28 - 2.32|
|CBN-25||mwir38||3.5 - 4.1|
|CBN-26||lwir||10.5 - 12.5||6|
|CBN-27||lwir09||8.5 - 9.5|
|CBN-28||lwir11||10.5 - 11.5||10|
|CBN-29||lwir12||11.5 - 12.5||11|
Table 1 - The Common Band Name schema of the ESA Charter Mapper and its application for Landsat-7, Landsat-8 and Sentinel-2 missions.
To better explain the advantages in managing multi sensor EO optical data with CBN, an application of CBN schema, made for a selection of the ESA Charter Mapper optical payload is shown in Table 2. This enables a clear mapping of EO data across the EM spectrum and of relative STAC assets.
|CBERS-4||PAN / MUX||P||B||G||R||X|
|CBERS-4A||PAN / MUX||P||B||G||R||X|
|ResourceSat-2 / 2A||LISS-III||G||R||X||X|
|ResourceSat-2 / 2A||LISS-IV||G||R||X|
Table 2 - Common Band Names for a selection of the ESA Charter Mapper Optical payload products. P, C, B, G, Y, and R stands for PAN, Cyan, Blue, Green, Yellow, and Red respectively. With asterisks are marked spectral bands which are not always present in the products delivered for the Charter.
SAR Common Band Names
A similar schema can be applied also for SAR missions. CBN classes for SAR refer to a combination of frequency band and polarization derived from multiple band numbers of several popular SAR instruments for disaster mapping. The CBN classification of the microwave radiation enables a one-to-one mapping of multi-mission and multi-sensor SAR products. A sample application of CBN for PE SAR missions is shown in Table 3.
|Common Name||Wavelength (cm)||ICEYE||Kompsat-5||TerraSAR-X TanDEM-X||GF-3||Radarsat 2||RCM 1/2/3||Sentinel-1 A/B||ALOS-2||SAOCOM-1 A/B|
|X-HH||2.5 - 4||X||X|
|X-HV||2.5 - 4||X||X|
|X-VH||2.5 - 4||X||X|
|X-VV||2.5 - 4||X||X||X|
|C-HH||4 - 8||X||X||X||X|
|C-HV||4 - 8||X||X||X||X|
|C-VH||4 - 8||X||X||X||X|
|C-VV||4 - 8||X||X||X||X|
|L-HH||15 - 30||X||X|
|L-HV||15 - 30||X||X|
|L-VH||15 - 30||X||X|
|L-VV||15 - 30||X||X|
Table 3 - Common Band Names schema for the ESA Charter Mapper SAR payload products.
CBN for SAR are meant to classify SAR Payload using this code “<N>-<XXXX>”, where “N” is the SAR-band and “XXXX” is the polarization (HH, HV, VH or VV). This schema enables the mapping of the SAR products provided as quad (HH+HV+VH+VV), dual (VV+VH or HH+HV), or single polarization (HH or VV). Cross-pol assets are generally provided within dual- or quad-pol products. The ICEYE mission is represented by only the CBN "X-VV" because the GRD products are given only in VV polarization.