SAR-Change service specifications

This service provides a Red-Cyan composite based on backscatter values in dB using a pair of Sigma Nought assets at same polarization from SAR Calibrated datasets acquired by the same sensor. It also provides co-located single band assets of the pre- and post-event Sigma0 in dB.

The tutorial of the SAR-Change service is available in this section.

Service Description

The SAR Amplitude Change (SAR-Change) service provides an RGB composite of backscattering from a pair of SAR amplitude images (e.g. pre- and post-event).

This Red-Cyan backscatter composite is useful to visually detect changes in backscatter coefficient (e.g. in a flood scenario). Areas in Red indicate a decrease in backscattering on those areas where new flooded areas occur, dark areas depict regions where the water is standing both dates, while cyan regions are those where water is receding.

Figure 1 - An example of RGB color composite which highlights the change in radar backscattering from a pair of SAR detected images, acquired from Sentinel-1 on 2 and 20 March 2019, during the flood event that occurred in lower Shire River between Malawi and Mozambique (Image credits ESA. This image contains modified Copernicus Sentinel data (2019), processed by ESA)¹.

It also provides backscatter coefficient in dB for the two input images. The SAR Amplitude Change processor inherits its methodology from the SNAC service of the GEP².

Warning

The pre-event and post-event must be SAR calibrated dataset. Such image pairs must come from the same sensor.

Note

For reasons of consistency in the visual inspection of changes, the selected pair of SAR images shall have, when possible, pre- and post-event images acquired under the same radar geometry (incident angle, orbit path).

Workflow

The processing service applies the workflow below:

graph TB c(cos2) --> d1[Dataset pre-event] c --> d2[Dataset post-event] d1 --> b1((SAR calibration)) d2 --> b2((SAR calibration)) b1 --> a1[input pre-event] b2 --> a2[input post-event] subgraph Inputs a1 a2 end subgraph SAR Amplitude Change a1[input pre-event]--> colloc((Collocation stacking)) a2[input post-event]--> colloc((Collocation stacking)) colloc --> colloc1[Collocated pre-event backscatter] colloc --> colloc2[Collocated pre-event backscatter] colloc1 --> c1((Clip range_min,range_max)) colloc2 --> c2((Clip range_min,range_max)) c1 --> r((RGB composite R: pre, G: post, B: post)) c2 --> r((RGB composite R: pre, G: post, B: post)) end subgraph Outputs r --> o1[RGB composite] r --> o2[Red channel] r --> o3[Green channel] r --> o4[Blue channel] colloc1 --> o5[pre-event backscatter] colloc2 --> o6[post-event backscatter] end

This is done for each polarization available in the acquisitions. The range applied above depends on the mission SAR band and polarization. The below Table 1 reports the values used:

Polarization	L	C	X
VV	[-27,0]	[-20,0]	[-22,2]
HH	[-27,0]	[-20,0]	[-22,2]
VH	[-35,-5]	[-26,-5]	[-27,-3]
HV	[-35,-5]	[-26,-5]	[-27,-3]

Table 1 - Valid range in dB adopted for subsetting sigma nought.

Inputs

Input of the SAR Amplitude Change service are at least two SAR Detected amplitude images. In particular, it uses as input calibrated products from the systematic Radar products Calibration. Service inputs are listed and described below.

Parameters

The SAR-change service requires a specified number of mandatory and optional parameters. All service parameters are listed in the below Table 2.

Parameter	Description	Required
Pre-event SAR calibrated dataset	Sigma0 in db from pre-event SAR data	YES
Post-event SAR calibrated dataset	Sigma0 in db from post-event SAR data	YES
Area of Interest	Area of interest expressed in WKT	NO

Table 2 - Service parameters for the SAR-change processor.

Pre- and post-event SAR calibrated datasets

The pre-event and post-event are SAR calibrated datasets, must come from the same sensor and shall have the same radar geometry (incident angle, orbit path).

AOI (optional)

This last parameter (optional) may define the area of interest expressed as a Well-Known Text value. If set, it overrides the automatic determination of the maximium common area between the input-reference products geometry.

Outputs

The SAR-change processor provides as output the following products in GeoTIFF format:

Product A: RGB composite: Red=(Sigma0 post-), Green=(Sigma0 pre-) , Blue=(Sigma0 pre-)
Product B: Pre-event sigma nought product [db]
Product C: Post-event sigma nought product [db].

SAR-Change Product Specifications can be found in the below tables.

SAR-Change overview

Attribute	Value / description
Long Name	SAR Amplitude Change RGB composite
Short Name	overview-PP (where PP is the polarization [HH, HV, VH, VV])
Description	Red-Cyan RGBA Composite (R=Sigma0 from post-, G=Sigma0 from pre-, B=Sigma0 from pre-) including transparency
Processing level	L1
Data Type	UnSigned 8-bit Integer
Band	4
Format	COG
Projection	Native or EPSG:4326 - WGS84
Valid Range	[1 - 255]
Fill Value	0

Pre-event sigma nought

Attribute	Value / description
Long Name	Backscatter coefficient from pre-event SAR calibrated data
Short Name	1.s0_db_B_PP (where PP is the polarization [HH, HV, VH, VV] and B the SAR-band [X,C,L)]
Description	Backscatter gray scale single band image of pre-event product
Processing level	L1
Data Type	Float 32 bit
Band	Single for each polarization
Format	COG
Projection	Native or EPSG:4326 - WGS84
Units	dB

Post-event sigma nought

Attribute	Value / description
Long Name	Backscatter coefficient from post-event SAR calibrated data
Short Name	2.s0_db_B_PP (where PP is the polarization [HH, HV, VH, VV] and B the SAR-band [X,C,L)]
Description	Backscatter gray scale single band image of post-event product
Processing level	L1
Data Type	Float 32 bit
Band	Single for each polarization
Format	COG
Projection	Native or EPSG:4326 - WGS84
Units	dB

ESA Observing the Earth, Flooding following Cyclone Idai, available at ESA Multimedia. ↩
Terradue, SNAC service tutorial, available at https://docs.terradue.com. ↩