ISRO Launches Innovative GNSS-Reflectometry Instrument on EOS-08 Satellite for Advanced Remote Sensing
On August 18, 2024, ISRO marked a significant milestone in its remote sensing capabilities with the successful initiation of the GNSS-Reflectometry (GNSS-R) instrument onboard the EOS-08 satellite. This advanced instrument represents a new frontier in how we observe and interpret Earth’s surface, offering a resource-efficient method to gather critical environmental data.
GNSS-Reflectometry is a novel remote sensing technique that leverages signals from Global and Regional Navigation Satellite Systems (GNSS/RNSS), such as GPS and India's NavIC. Instead of relying on traditional, dedicated transmitters, GNSS-R captures signals that are naturally reflected off various Earth surfaces—ranging from oceans to agricultural lands and rivers. These reflected signals are then collected by a precision receiver housed within the satellite, which orbits the Earth at an altitude of 475 kilometers.
This mode of sensing is particularly noteworthy for its efficiency. The GNSS-R instrument, developed by the Space Applications Centre (SAC-ISRO) in Ahmedabad, is India’s first space-borne precision receiver of its kind. It is lightweight, requires minimal power, and its compact size makes it ideal for deployment in large constellations, potentially allowing for faster and more comprehensive global coverage. The system operates by collecting ground-reflected GNSS signals, analyzing their power, and interpreting other signal characteristics. This data is then used to derive crucial scientific information, such as soil moisture levels, surface inundation, and ocean surface wind and wave measurements.
The collected raw data is processed at the National Remote Sensing Centre (NRSC-ISRO) in Shadnagar, Hyderabad. Here, sophisticated algorithms and data processing software, developed by SAC-ISRO, are employed to generate multiple levels of data products. The primary outputs of this processing are Delay-Doppler Maps (DDMs), which provide a visual representation of the reflected signals. These DDMs are then used to calculate parameters like reflectivity and Normalized Bistatic Radar Cross-Section (NBRCS), which are essential for retrieving various scientific parameters.
The instrument’s resolution capabilities are impressive. Over oceans, it can provide data with a resolution of 15 km x 15 km, while over land, it offers an even finer resolution of better than 1 km x 1 km. This makes it particularly valuable for detailed environmental monitoring. The first land data collected by the GNSS-R instrument was over the Sahara Desert, using a high-resolution mode that outperforms similar sensors like the contemporary CYGNSS. The data, processed to assess soil moisture levels, yielded results well within the expected range. A subsequent dataset from the Amazon Rainforest, acquired on August 21, was used to generate surface inundation maps, demonstrating the instrument’s sensitivity to features as narrow as sub-kilometer river widths.
The instrument also began gathering ocean data on August 19, starting with a section of the Pacific Ocean. This data was crucial for assessing ocean surface conditions, such as wind speed and significant wave height. Once again, the results were consistent with expectations, affirming the instrument’s accuracy and reliability.
While the GNSS-R instrument is still in its calibration and validation phase, these early results highlight its immense potential. Its ability to provide high-resolution, reliable data with minimal resource consumption positions it as a powerful tool for a wide range of scientific studies and practical applications, from agricultural monitoring to climate research and disaster management. As ISRO continues to refine and expand its capabilities, the GNSS-Reflectometry instrument on EOS-08 is set to become a cornerstone of India’s space-based remote sensing efforts.
