GRITSS: Geodetic Reference Instrument Transponder for Small Satellites

Overview

The NASA GRITSS (Geodetic Reference Instrument Transponder for Small Satellites) CubeSat is a technology demonstration mission designed to eliminate “site tie” bias errors that frequently misalign ground station networks. Many space missions — from ICESat-2 and NISAR to SWOT and CYGNSS — need a flawlessly precise 3D coordinate system for Earth. This grid, the Terrestrial Reference Frame (TRF), relies on co-located GPS and Very Long Baseline Interferometry (VLBI) ground stations. However, because these stations use different radio frequencies, measuring the exact distance between them on the ground creates tiny, millimeter-level errors. GRITSS solves this problem by acting as a space-based relay transponder, using a modified GPS receiver to convert GPS signals into a frequency VLBI dishes can read. The GRITSS technology could help cross-correlate geodetic datasets, yielding improvements for a vast range of Earth science measurements.

Science Area

Space geodesy relies on the global TRF system. A highly accurate TRF is essential for tracking millimeter-scale changes like tectonic shifts and sea-level rise. By making the TRF more precise, GRITSS data could help improve the accuracy of many geodetic missions focused on phenomena like regional and global sea-level change and complex ocean-climate cycles, such as El Niño. It could also provide a highly stable baseline for next-generation topography missions, enabling high-precision ocean, ice, and land measurements. In addition, GRITSS could also allow future low-Earth orbit (LEO) constellations to track changes in the planet’s shape, continental movements, and fresh water distribution more accurately

Technology

To function as a “tie” that bridges different types of radio tracking networks, GRITSS upconverts GPS signals into S-band and X-band signals VLBI stations can detect. First, a modified GPS receiver digitizes incoming GPS L1 and L2C signals. Next, an Application-Specific Integrated Circuit (ASIC) captures these signals, while a Field-Programmable Gate Array (FPGA) packages the digitized data with precise synchronization frames. To transmit this data to VLBI ground stations, GRITSS employs a commercial X-band digital data transmitter using OQPSK modulation. Finally, a custom frequency reference block—utilizing phase-locked microwave synthesizers and mixers—locks the transmitter’s carrier frequency to an ultra-stable 10 MHz onboard clock, ensuring flawless phase coherence and allowing VLBI stations to process the translated signals with unprecedented millimeter-level accuracy.

Advancements

  • Enhanced Reference Frames — allows GRITSS to directly connect independent ground-based tracking systems in space, significantly improving the accuracy and stability of the International Terrestrial Reference Frame (ITRF).
  • Custom frequency block — strips the transmitter’s natural carrier phase and uses microwave synthesizers to lock the transmission directly to an onboard master clock, ensuring perfectly synchronized, drift-free data transmission.
  • Space-based geodetic site ties — pioneers a novel configuration for calibrating navigation satellites, measuring the location of true electronic signals instead of physical hardware for unprecedented accuracy.

Principal Investigator

Christopher Beaudoin is a radio frequency engineer at Draper Laboratory in Cambridge, Massachusetts. He has more than 18 years of experience developing advanced microwave, radar, and radio-frequency systems for scientific and aerospace applications. He previously worked as a Senior Radar engineer at the University of Massachusetts, Lowell, as well as a Research Engineer at MIT’s Haystack Observatory, where he contributed to the Event Horizon Telescope project that produced the first image of a black hole. He’s received the National Science Foundation’s Antarctic Service Medal and NASA’s Robert H. Goddard Exceptional Achievement Award. The GRITSS project benefits from significant partnerships with NASA Goddard Space Flight Center and ISISpace Group.