Introduction
How can we trust that a satellite orbiting hundreds of kilometers above Earth is measuring the planet with precision? Surprisingly, part of the answer lies on the ground. In this experiment, researchers use an Integrated Geodetic Reference Station, IGRS for short, to calibrate radar satellites. This process makes Earth observation more accurate—crucial for tracking climate change, monitoring infrastructure, and assessing environmental risks.
Radar satellites routinely capture ground images, almost on a daily basis. Using Interferometric Synthetic Aperture Radar (InSAR) techniques, researchers can extract information on ground stability and detect subtle changes over time from such radar images. Because radar satellites are designed to emit microwave signals (3.1, 5.6, and 24 cm for X-band, C-band, and L-band, respectively), the theoretical precision of InSAR standard products, deformation maps, and deformation time series of the radar scatterers, is at the level of millimeters. However, satellite positioning errors limit the geo-location accuracy of radar scatterers to the decimeter or meter scale. This is insufficient for many Earth monitoring applications. Moreover, without a ground reference, like GPS and corner reflector, radar measurements remain relative in time and space, making it impossible to align them with other geodetic measurements.
What are we investigating?
The goal of this experiment is to calibrate radar satellite measurements using a fixed, highly reliable reference point on Earth – IGRS. The station is part of a wider network stations in the Netherlands and Belgium (click for link). With this, the researchers can measure e.g. the effects of salt mining in Hengelo, or the effects of underground gas storage in Gronau.
How does it work?
At the core of the setup is the Integrated Geodetic Reference Station (IGRS), which combines multiple instruments to ensure accurate calibration:
(1) Double BackFlip (DBF) corner reflectors – precisely engineered to bounce radar signals back to the satellite, enabling correction of measurement deviations. The two corner reflectors are designed to facilitate radar satellites in both ascending and descending orbits. Key parameters:
- Inner leg of each corner reflector: 90 cm
- Nominal Radar Cross Section (RCS) for Sentinel-1 C-band radar satellites: 28.5 dBm2
- Standard deviation of the double-difference in Sentinel-1 satellite line-of-sight direction: 0.28 mm
(2) GNSS receiver and antenna – tracks the exact position and ground movements, with millimeter resolution. It is composed of a GNSS antenna and a receiver unit. Main composition:
- PolaRx5 UNAVCO receiver
- Multi-frequency chokering antenna
- Cable and adaptor
(3) Laser reflector platform – serves as calibration target for airborne laser scanning campaigns, which produces the Actueel Hoogtebestand Nederland (AHN) digital elevation model of the Netherlands.
(4) Leveling bolts – used to link the IGRS steady to the Dutch NAP elevation system.
(5) 360° camera – monitors local surroundings and weather conditions, allowing researchers to obtain and analyze the factors in vegetation and atmospheric effects on radar reflections.
Together, these instruments make it possible to tie the satellite data to a global reference frame, such as the International Terrestrial Reference Frame (ITRF), ensuring consistency across the world.
Why is this important?
Reliable satellite measurements are essential for applications such as monitoring groundwater table changes, detecting subsidence in cities, and tracking infrastructure instability. By calibrating from the ground, we ensure that this radar data is not only accurate but also globally comparable.
Contact
This experiment is done in collaboration with the radar group at Delft University of Technology.