Executive Summary

This report documents the system characterization of the Indian Space Research Organisation Resourcesat-2A (Indian Space Research Organisation, 2023) Advanced Wide Field Sensor (AWiFS) and is part of a series of system characterization reports produced by the U.S. Geological Survey Earth Resources Observation and Science Cal/Val Center of Excellence (U.S. Geological Survey, 2021). These reports describe the methodology and procedures used for characterization, present technical and operational information about the specific sensing system being evaluated, and provide a summary of test measurements, data retention practices, data analysis results, and conclusions.

Resourcesat-2A was launched in 2016 on the Polar Satellite Launch Vehicle-C36; it is identical to Resourcesat-2, and together, they decrease imaging revisit time from 5 days to 2–3 days, providing data continuity and improved temporal resolution. Resourcesat-2 and -2A carry the AWiFS, Linear Imaging Self Scanning-3, and Linear Imaging Self Scanning-4 medium-resolution imaging sensors, continuing the legacy of the Indian Space Research Organisation’s Indian Remote Sensing-1C/1D/P3 satellite programs. More information about Indian Space Research Organisation satellites and sensors is available through the Joint Agency Commercial Imagery Evaluation Earth Observing Satellites Online Compendium (Clauson and others, 2024) and from the Indian Space Research Organisation at https://www.isro.gov.in/.

The Earth Resources Observation and Science Cal/Val Center of Excellence system characterization team assessed the geometric, radiometric, and spatial performance of the Resourcesat-2A AWiFS sensor. Geometric performance is divided into the interior geometric performance of band-to-band registration and the exterior geometric performance of geolocation accuracy. The interior geometric performance had offsets in the range of −1.10 meters (m; −0.020 pixel) to 3.67 m (0.066 pixel) in easting and −5.68 m (−0.101 pixel) to 10.38 m (0.185 pixel) in northing with root mean square error values from 5.60 m (0.100 pixel) to 11.31 m (0.202 pixel) in easting and from 3.00 m (0.054 pixel) to 13.52 m (0.241 pixel) in northing.

The exterior geometric performance had mean offsets of −25.29 m in easting and 16.22 m northing with root mean square error values of 26.07 m in easting and 17.60 m in northing compared to the Landsat 8 Operational Land Imager sensor (Earth Resources Observation and Science Center, 2020). The radiometric performance had offsets from −0.002 to 0.029 and slopes from 0.733 to 1.012. Spatial performance was in the range of 1.354 to 1.639 pixels for full width at half maximum with a modulation transfer function at a Nyquist frequency in the range of 0.108 to 0.174.

Introduction

This report documents the system characterization of the Indian Space Research Organisation Resourcesat-2A (Indian Space Research Organisation, 2023) Advanced Wide Field Sensor (AWiFS) and is part of a series of system characterization reports produced by the U.S. Geological Survey (USGS) Earth Resources Observation and Science (EROS) Cal/Val Center of Excellence (ECCOE; U.S. Geological Survey, 2021a). These reports describe the methodology and procedures used for characterization, present technical and operational information about the specific sensing system being evaluated, and provide a summary of test measurements, data retention practices, data analysis results, and conclusions.

The Resourcesat-2A AWiFS is a wide-angle medium-resolution camera consisting of four bands: green, red, near infrared, and shortwave infrared (Indian Space Research Organisation, 2023). The camera has a swath width of 740 kilometers, enabling AWiFS to provide a 5-day repeat capability. The primary objectives for data acquired by AWiFS include vegetation and crop monitoring, forest mapping, land cover/land use mapping, change detection, and regional resource assessment.

The data analysis results provided in this report have been derived from Joint Agency Commercial Imagery Evaluation (JACIE) processes and procedures. JACIE was formed to leverage resources from several Federal agencies for the characterization of remote sensing data and to share those results across the remote sensing community (U.S. Geological Survey, 2021b).

Purpose and Scope

The purpose of this report is to describe the specific sensor or sensing system, test its performance in three categories, complete related data analyses to quantify these performances, and report the results in a standardized document. In this chapter, the AWiFS sensor is described. The performance assessment of the system is limited to geometric, radiometric, and spatial analyses. The scope of the geometric assessment is limited to testing the interior alignments of spectral bands against each other and testing the exterior alignment in reference to the Landsat 8 Operational Land Imager (OLI; Earth Resources Observation and Science Center, 2020; U.S. Geological Survey, 2021c).

The system characterization process used by the ECCOE team (U.S. Geological Survey, 2021a) follows the USGS Fundamental Science Practices, which include maintaining data, information, and documentation needed to reproduce and validate the scientific analysis documented in this report. Additional information and guidance about Fundamental Science Practices are available at https://www.usgs.gov/office-of-science-quality-and-integrity/fundamental-science-practices. For additional information related to the report, please contact ECCOE at eccoe@usgs.gov.

System Description

This section describes the satellite and operational details for Resourcesat-2A and provides information about the AWiFS sensor. Resourcesat-2A was launched in 2016 on the Polar Satellite Launch Vehicle-C36; it is identical to Resourcesat-2, and together, they decrease imaging revisit time from 5 days to 2–3 days, providing data continuity and improved temporal resolution. Resourcesat-2 and -2A carry the AWiFS, Linear Imaging Self Scanning-3, and Linear Imaging Self Scanning-4 medium-resolution imaging sensors, continuing the legacy of the Indian Space Research Organisation’s Indian Remote Sensing-1C/1D/P3 satellite programs. More information about Indian Space Research Organisation satellites and sensors is available through the Joint Agency Commercial Imagery Evaluation Earth Observing Satellites Online Compendium (Clauson and others, 2024) and from the Indian Space Research Organisation at https://www.isro.gov.in/.

Sensor Information

The spectral characteristics and the relative spectral response of the AWiFS are listed in table 2 and shown in figure 1, respectively.

Table 2.    

Imaging sensor details for Resourcesat-2A Advanced Wide Field Sensor (Indian Space Research Organisation, 2023).

[The Resourcesat-2A Advanced Wide Field Sensor (AWiFS) has a swath width of 740 kilometers; μm, micrometer; m, meter; NIR, near infrared; SWIR, shortwave infrared]

Table 2.    Imaging sensor details for Resourcesat-2A Advanced Wide Field Sensor (Indian Space Research Organisation, 2023).

Spectral band(s) details	Resourcesat-2A AWiFS
	Lower band (µm)	Upper band (µm)	Radiometric resolution (bits)	Ground sample distance (m)
Band 2—green	0.52	0.59	10	56
Band 3—red	0.62	0.68	10	56
Band 4—NIR	0.77	0.86	10	56
Band 5—SWIR	1.55	1.70	10	56

Procedures

ECCOE has established standard processes to identify Earth observing systems of interest and to assess the geometric, radiometric, and spatial qualities of data products from these systems.

The assessment steps are as follows:

Measurements

The observed USGS measurements are listed in table 3. The mean error and root mean square error (RMSE) values for interior (band-to-band) and exterior (image-to-image) geometric performance are listed in meters (pixels). These values are derived from tables 4, 5, 6, and 7 and are summarized here. The values for interior and exterior geometry and for radiometry are the averages of three datasets used for the analysis. The spatial performance is assessed on a single scene and is reported without averages. Details about the methodologies used are outlined in the “Analysis” section.

Analysis

This section describes the geometric, radiometric, and spatial performance of AWiFS.

Geometric Performance

The geometric performance for AWiFS is characterized in terms of the interior (band-to-band alignment) and exterior (geometric location accuracy) geometric analysis results.

Interior (Band to Band)

The band-to-band alignment analysis was completed using the EROS System Characterization (EROSSC) software (Cantrell and Christopherson, 2024) on three separate images. Band combinations were registered against each other to determine the mean error and RMSE values as listed in table 4 with results represented in pixels resampled to a 60-meter (m) GSD from the original 56-m GSD. Example error scatterplots and histograms for scene identifier 239329911_R2A_AW__11-NOV-2023_100_063_GEOREF (Indian Space Research Organisation, 2023) are shown in figures 2, 3, and 4.

Exterior (Geometric Location Accuracy)

For this analysis, band 2 (green) of the AWiFS data was compared against the corresponding band from the Landsat 8 OLI image using the EROSSC software. Conjugate points in the reference and search images were identified automatically and refined using similarity measures such as normalized cross-correlation metrics. The mean error and RMSE results for three image pairs are listed in table 5 with results represented in meters at a 60-m GSD. Please note that the OLI (30-m GSD) and AWiFS (56-m GSD) images were resampled to 60 m. A geometric error map showing the directional shift and relative magnitude of the shift between AWiFS and Landsat 8 OLI is shown in figure 5. A corresponding error scatterplot and histograms for scene identifier 239329911_R2A_AW__11-NOV-2023_100_063_GEOREF are provided in figure 6.

Radiometric Performance

For this analysis, cloud-free regions of interest were analyzed within three AWiFS and Landsat 8 OLI scene pairs using the EROSSC software. Raw digital number-to-radiance conversion coefficients were obtained from the Indian Space Research Organisation. The scatterplots in figure 7 show the reference sensor on the x-axis and the comparison sensor on the y-axis. The linear regression represents Top of Atmosphere (TOA) reflectance relative to that of the reference sensor. Ideally, the slope should be near unity, and the offset should be near zero. For instance, if the slope is greater than unity, the comparison sensor is overestimating the TOA reflectance compared to the reference sensor.

TOA reflectance comparison results of the three scene pairs used for the analyses are listed in table 6. A band-by-band graphical comparison between AWiFS scene identifier 239329911_R2A_AW__11-NOV-2023_100_063_GEOREF and the corresponding Landsat 8 OLI band is shown in figure 7.

AWiFS radiometric quality is also assessed by comparing it with Radiometric Calibration Network (RadCalNet; Bouvet and others, 2019) coincident measurements. RadCalNet provides automated TOA reflectance measurements that are used to calibrate and validate optical satellite sensors. AWiFS was compared to measurements from the RadCalNet instrumentation and a coincident Landsat 9 OLI image over the Railroad Valley playa, Nevada, site. The AWiFS footprint over Railroad Valley playa is shown in figure 8, and the red box represents the 700-m x 700-m region of interest used to extract AWiFS and Landsat 9 OLI TOA reflectance.

The TOA reflectance comparison among AWiFS, RadCalNet, and Landsat 9 (Earth Resources Observation and Science Center, 2020) is shown in figure 9A and B on two dates: July 29, 2023 (239330721_R2A_AW__29-JUL-2023_252_045_GEOREF), and September 15, 2023 (239330711_R2A_AW__15-SEP-2023_252_045_GEOREF), respectively. For the radiometric comparison, RadCalNet hyperspectral TOA reflectance is used to simulate AWiFS TOA reflectance using the AWiFS relative spectral response. In figure 9A and B, blue symbols represent the TOA reflectance ratio between AWiFS and RadCalNet, whereas green symbols represent the TOA reflectance ratio between AWiFS and Landsat 9 OLI observations. For the September 15, 2023, comparison in figure 9B, AWiFS agrees with RadCalNet within 13 percent across all the bands, and the shortwave-infrared band has the best agreement (within 5 percent). AWiFS agrees with Landsat 9 OLI within 11 percent for the near-infrared and shortwave-infrared bands, but it is only within 14 percent and 16 percent for the red and green bands, respectively. For the July 29, 2023, comparison, figure 9A shows a better agreement with AWiFS being within 5 percent for RadCalNet and Landsat 9 OLI.

The AWiFS radiometric assessment was also completed by comparing the sensor with Landsats 8 and 9 OLI using pseudoinvariant calibration sites (PICS). Footprints of AWiFS, Landsat 9, and Hyperion (Folkman and others, 2001) over the Libya 1 PICS site in the Sahara Desert in Libya are shown in figure 10. The red box represents the region of interest used to compare AWiFS and Landsat 9 OLI TOA reflectance. The spectral difference between the two sensors is compensated for by calculating a spectral band adjustment factor (SBAF) for AWiFS using Hyperion hyperspectral data.

The comparisons between AWiFS and Landsat using the Libya 1 PICS site and the Algeria 5 PICS site (in the Grand Erg Occidental in Algeria) are shown in figure 11A and B. The mean TOA reflectance ratio between AWiFS and Landsats 8 and 9 OLI is shown in figure 11A. AWiFS agrees with Landsat within about 7 percent, and the best agreement was observed in the red and near-infrared bands. In figure 11B, the TOA reflectance ratio using the individual PICS sites is shown. The reflectance ratios between AWiFS and Landsats 8 and 9 for the Libya 1 site are more consistent than for the Algeria 5 site.

The results of the SBAF analyses are summarized in table 7. The SBAF should be interpreted such that a factor of 1 indicates a perfect alignment of spectral bands and calibration between Landsat and AWiFS.

Summary and Conclusions

This report summarizes the sensor performance of the Resourcesat-2A Advanced Wide Field Sensor (AWiFS) based on the U.S. Geological Survey Earth Resources Observation and Science Cal/Val Center of Excellence (ECCOE) system characterization process.

In summary, ECCOE has determined that this sensor provides an interior geometric performance with band-to-band mean offsets in the range of −1.10 meters (m; −0.020 pixel) to 3.67 m (0.066 pixel) in easting and −5.68 m (−0.101 pixel) to 10.38 m (0.185 pixel) in northing with root mean square error values in the range of 5.60 m (0.100 pixel) to 11.31 m (0.202 pixel) in easting and 3.00 m (0.054 pixel) to 13.52 m (0.241 pixel) in northing.

We have measured the mean exterior geometric error offset to be −25.29 m in easting and 16.22 m in northing with root mean square error values of 26.07 m in easting and 17.60 m in northing in comparison to the Landsat 8 Operational Land Imager sensor.

The measured radiometric performance was in the range of −0.002 to 0.029 in offset and 0.733 to 1.012 in slope, and the spatial performance was in the range of 1.354 to 1.639 pixels for full width at half maximum with a modulation transfer function at a Nyquist frequency in the range of 0.108 to 0.174.

In conclusion, the ECCOE team completed a standardized system characterization of the Resourcesat-2A AWiFS sensing system. Although the team followed characterization procedures that are standardized across the many sensors and sensing systems under evaluation, these procedures are customized to fit the individual sensor as was done with AWiFS. The team acquired the data, defined proper testing methodologies, carried out comparative tests against specific references, recorded measurements, completed data analyses, and quantified sensor performance accordingly. The team archived all data and measurements and documented the evaluation methods, which ensures that all data and measurements remain accessible so that the performance results can be reproduced if necessary.

The ECCOE project and associated Joint Agency Commercial Imagery Evaluation partners are always interested in reviewing sensor and remote sensing application assessments and would like to review and discuss information on similar data and product assessments and reviews. If you would like to discuss system characterization with either the U.S. Geological Survey ECCOE or Joint Agency Commercial Imagery Evaluation teams, please email us at eccoe@usgs.gov.