GRITSS

GRITSS [ISISpace]

GRITSS (Geodetic Reference Instrument Transponder for Small Satellites) is a 12U XL CubeSat technology demonstration developed by NASA to improve the accuracy of the International Terrestrial Reference Frame (ITRF). The spacecraft will provide a common space-based reference point for Global Navigation Satellite System (GNSS), Very Long Baseline Interferometry (VLBI) and Satellite Laser Ranging (SLR) measurements, reducing errors caused by differences between ground stations.

The mission addresses a fundamental problem in space geodesy. Measurements from GNSS, VLBI and SLR are normally combined using precisely surveyed relationships between their ground stations, known as site ties. Small errors in these ties propagate into the ITRF, limiting its accuracy. GRITSS is designed to correlate observations from the three techniques directly and support a target accuracy of 1 mm for the terrestrial reference frame.

The spacecraft carries a modified GPS receiver, a 10 MHz ultra-stable oscillator, S-band and X-band transmitters, and a laser retroreflector. The GPS receiver digitizes signals from GNSS satellites, while onboard ASIC and FPGA electronics process and synchronize the data. The resulting signals are frequency-translated to S-band and X-band and transmitted to VLBI stations. Simultaneous observations of the spacecraft with the radio systems and SLR allow the distances between ground stations to be determined with millimeter-level precision.

The payload includes the NavCube 3 Mini (NC3m) GPS receiver, S-Band Transmitter, X-TTSN X-band Transmitter, 10 MHz Reference Module and Laser Retroreflector. The geodetic-grade GPS receiver is designed to provide the positioning accuracy and robustness required for precise orbit determination in a CubeSat-class spacecraft. Miniature S- and X-band antennas provide stable and accurately characterized phase patterns for the geodetic measurements.

ISISPACE is responsible for construction of the 12U XL CubeSat and will provide mission design, payload integration, launch services, spacecraft registration, commissioning and one year of nominal operations in collaboration with the GRITSS team. The resulting technology is intended to support future high-precision Earth observation and geodesy missions, including measurements of sea-level change, ice sheets, land deformation and freshwater distribution.