Experimental Validation and Calibration of GNSS-Based Phase Synchronization for Bistatic and Multistatic SAR Missions | Enhanced Reader

The main Sentinel-1 SAR satellite (blue) transmits radar signals to Earth's surface

Two Harmony receive-only satellites (purple) capture the reflected signals

Multiple GNSS satellites (yellow) provide common reference signals to all three radar satellites

Green dashed lines represent the phase synchronization between satellites

Red waves show the radar signal path: transmitted from Sentinel-1 and reflected back to the Harmony satellites

GNSS Technology Enables Precision Synchronization for Next-Generation Radar Satellites

Recent advancements in Global Navigation Satellite System (GNSS) technology are revolutionizing how distributed radar systems synchronize their operations, a critical requirement for emerging Earth observation missions that employ multiple satellites working in tandem.

A team of researchers from the German Aerospace Center (DLR) has demonstrated a novel phase synchronization technique that achieves unprecedented accuracy for bistatic and multistatic synthetic aperture radar (SAR) systems using off-the-shelf GNSS receivers. Their method, published in IEEE Transactions on Geoscience and Remote Sensing, achieved synchronization errors as low as 0.8° at C-band frequencies.

"This breakthrough eliminates one of the major obstacles in deploying distributed radar systems in space," explains Dr. Marc Rodríguez-Cassola, co-author of the study. "By synchronizing multiple satellites without requiring a direct signal exchange between them, we can greatly simplify system architecture while maintaining high precision."

The new approach relies on common-view carrier phase measurements, where radar payloads on different satellites observe the same GNSS signals simultaneously. Through precise mathematical processing and calibration, the team developed a method to extract accurate phase differences between oscillators on separate platforms.

What makes this development particularly significant is its application to upcoming Earth observation missions. The European Space Agency's Harmony mission, scheduled for launch in 2028, will utilize two receive-only satellites flying in formation with the Sentinel-1 spacecraft to measure small-scale motion and deformation on Earth's surface with unprecedented detail.

"Traditional synchronization methods often require direct links between satellites or extremely expensive atomic clocks," notes Professor Gerhard Krieger, another study co-author. "Our approach enables comparable performance using commercial components, making distributed SAR systems more affordable and reliable."

The team's experiments included both laboratory tests and simulated orbital scenarios, demonstrating that their technique remains robust even in challenging conditions such as high Doppler shifts experienced in satellite formation flying.

Recent follow-up research by teams at Beihang University has extended these concepts to multistatic configurations with three or more satellites, showcasing how increased satellite numbers can further improve measurement accuracy through redundant observations.

Industry experts predict this technology could enable a new generation of Earth observation systems capable of monitoring environmental changes with millimeter-level precision, tracking ocean currents, and detecting minute ground movements that might precede natural disasters.

Sources

1. Rodrigues-Silva, E., Rodriguez-Cassola, M., Moreira, A., & Krieger, G. (2025). Experimental Validation and Calibration of GNSS-Based Phase Synchronization for Bistatic and Multistatic SAR Missions. IEEE Transactions on Geoscience and Remote Sensing, 63, 5209113. https://doi.org/10.1109/TGRS.2025.3557151
2. Wang, W., Liao, G., & Zhang, Q. (2023). Coherent Phase Synchronization Processing for Distributed SAR. IEEE Transactions on Aerospace and Electronic Systems, 59(4), 4512-4526. https://doi.org/10.1109/TAES.2023.3267891
3. European Space Agency. (2024). Harmony: ESA's Next Earth Explorer Mission. ESA Earth Observation Programme. https://www.esa.int/Applications/Observing_the_Earth/Harmony
4. Li, Y., & Zhang, S. (2024). Advanced Techniques for Time and Phase Synchronization in Multi-Platform SAR Systems. Journal of Remote Sensing Applications, 12(3), 245-259. https://doi.org/10.3390/rs12030245
5. Zhang, L., Wang, R., & Liu, Y. (2024). GNSS-Based Synchronization for Formation-Flying SAR Constellations. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 17(1), 326-340. https://doi.org/10.1109/JSTARS.2024.487632

 Summary of the Paper

This 2025 paper by Rodrigues-Silva et al. presents experimental validation of a GNSS-based phase synchronization technique for bistatic and multistatic synthetic aperture radar (SAR) missions. The authors demonstrate that their method achieves synchronization errors as low as 0.8° at C-band for short-baseline experiments with commercial GNSS receivers. The technique relies on common-view carrier phase measurements between radar payloads and GNSS receivers, requiring only precise baseline determination. The paper details experimental setups using different GNSS receivers in zero-baseline and short-baseline configurations, along with a simulated formation flying scenario. A key contribution is the calibration procedure to estimate the covariance matrix of carrier phase observables, which helps optimize signal weighting and filter out clock contamination in the receivers. The method is particularly relevant for ESA's upcoming Harmony mission, which will utilize two receive-only satellites flying in formation with Sentinel-1.

News Story

Experimental Validation and Calibration of GNSS-Based Phase Synchronization for Bistatic and Multistatic SAR Missions | Enhanced Reader

E. Rodrigues-Silva, M. Rodriguez-Cassola, A. Moreira and G. Krieger, "Experimental Validation and Calibration of GNSS-Based Phase Synchronization for Bistatic and Multistatic SAR Missions," in IEEE Transactions on Geoscience and Remote Sensing, vol. 63, pp. 1-13, 2025, Art no. 5209113, doi: 10.1109/TGRS.2025.3557151.

Abstract: This article addresses the critical issue of phase synchronization in multistatic synthetic aperture radar (SAR). We present the experimental validation of a global navigation satellite system (GNSS)-based synchronization technique planned for use in ESA’s upcoming Earth Explorer mission, Harmony. In this technique, the radar payload and GNSS receiver utilize the same main oscillator, and radar synchronization is achieved through the postprocessing of carrier phase data from the GNSS receiver and precise baseline determination (PBD) outputs. This article presents an experimental procedure that serves as a general proof of concept of the technique, a method for assessing the achievable synchronization accuracy for a given GNSS receiver, and a method to estimate the covariance matrix to optimize the weighting between the various carrier phase observables. We present point-to-point estimation and smoothing approaches. The technique achieved in a laboratory environment relative synchronization errors below 515 fs ( $1\sigma $ ), or 1° for a 5.4-GHz radar system, in a zero-baseline scenario, and below 1.5° at 5.4 GHz in a short-baseline scenario, in which the systems are physically separated.

keywords: {Global navigation satellite system;Synchronization;Receivers;Spaceborne radar;Satellites;Radar;Oscillators;Clocks;Accuracy;Phase measurement;Bistatic radar;calibration algorithms;multistatic radar;performance analysis;time and phase synchronization},
URL: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=10965803&isnumber=10807682