Two-image MTC composite workflow

Available with Image Analyst license.

A two-image multitemporal coherence (MTC) composite workflow creates a three-band visualization that highlights surface change and temporal stability using synthetic aperture radar (SAR) data. It combines two types of information:

• Radiometrically terrain-corrected (RTC) backscatter from two separate acquisitions, as the Red and Green bands.
• Coherence between the same two acquisitions, as the Blue band.

This color composite is useful for identifying areas that have changed in either radar brightness (backscatter), phase similarity (coherence), or both. It is especially helpful for detecting land cover changes, monitoring infrastructure, mapping surface moisture variation, and assessing vegetation growth or disturbance.

Data prerequisites

The following data is needed for this workflow:

• This process requires 2 SAR images covering the same area of interest
• Both inputs must have the same sensor mode, track (ascending or descending), and polarization. If the inputs are Sentinel-1 TOPS, they both need to have the same subswath.
• A DEM that overlaps with the SAR images, which will be used to process and create accurate coregistration and terrain correction data.

Processing overview

First, the SAR RTC workflow needs to be run on both SAR scenes to create radiometrically terrain corrected backscatter data, in decibel (dB) units. These two outputs will be used as the Red and Green input bands.

Next, run the Coherence workflow on the SAR image pair. This step estimates the interferometric coherence between the two scenes. If using Sentinel-1, be sure to apply precise orbit correction before computing coherence. Use the Apply Geometric Terrain Correction tool on the coherence raster so that it matches the same map geometry as the RTC backscatter outputs.

Create the MTC composite

The Generate Multitemporal Coherence tool can create the two-image MTC RGB composite by combining the two processed SAR RTC rasters and one coherence raster. All these rasters need to be geometrically aligned before running the tool. Optionally, apply water or layover/shadow masks for the inputs, for easier interpretation.

The two-image MTC RGB composite uses the following input structure.

Band	Acquisition	Processing	Band source	Units
Red band	Scene 1 (Reference)	SAR RTC workflow	Backscatter	dB
Green band	Scene 2 (Secondary)	SAR RTC workflow	Backscatter	dB
Blue band	SAR pair	Coherence workflow	Coherence	No units, with a coherence range between 0 to 1.

Interpretation

Each color in the composite reflects a combination of surface brightness (backscatter) and phase stability (coherence). Interpretation depends on understanding how different land covers respond to radar signals across time.

The table below explains how to interpret the MTC RGB composite output.

Color	Band values	Interpretation
White	Red—high backscatter Green—high backscatter Blue—high backscatter	Bright and stable scatterer with high temporal similarity between dates. This is common for buildings, rocks, and dry bare ground.
Yellow	Red—high backscatter Green—high backscatter Blue—low backscatter	Bright targets that have decorrelation between the two scenes. The radiometry remained high, but the phase stability dropped. This is common for vegetation changes, moving machinery, breaking waves, and construction.
Cyan	Red—medium to high backscatter Green—very high backscatter Blue—medium to high backscatter	The backscatter increased from scene 1 to scene 2, while remaining fairly coherent. This is common for surface wetting or roughening under similar geometries.
Magenta	Red—very high backscatter Green—medium to high backscatter Blue—medium to high backscatter	The backscatter decreased from scene 1 to scene 2, with limited decorrelation. This is common for surface drying or smoothing under similar geometries.
Dark / Black	Red—low backscatter Green—low backscatter Blue—low backscatter	Low backscatter and low coherence exists. This is common for open water, deep shadows, or significant misregistration.

Note:

Interpretation can vary in some cases, and should be approached cautiously and supported by additional information.

Processing considerations

Here are a few key processing considerations to ensure accurate results and proper visualization:

• Use identical parameters in the SAR RTC workflow for both scenes. This includes parameters such as DEM, calibration model, multilooking, and other relevant options.
• Ensure accurate coregistration before a coherence estimation; misregistration reduces coherence and introduces false changes.
• Keep the temporal baseline short for vegetation analysis; this can be a few days to a few weeks. You can use longer baselines for structural change detection.
• Use layover or shadow masks to prevent misinterpretation of radar geometry artifacts as surface changes.
• If coherence is uniformly low, recheck the accuracy of the coregistration, baselines, and acquisition compatibility.
• If the Blue band is noisy, increase the coherence window size.