GRITSS: Geodetic Reference Instrument Transponder for Small Satellites

Overview

NASA’s Geodetic Reference Instrument Transponder for Small Satellites (GRITSS) is a CubeSat technology demonstration designed to eliminate “site tie” errors between GNSS, VLBI, and Satellite Laser Ranging (SLR) ground stations. Currently, combining data from these independent systems introduces millimeter-level errors into the International Terrestrial Reference Frame (ITRF). GRITSS solves this by acting as a unified space-based reference point observable by all three techniques. Carrying a modified GPS receiver to convert signals for VLBI alongside a traditional laser retroreflector, GRITSS bridges these systems to help achieve the Earth Science Decadal Survey’s goal of 1-millimeter frame accuracy, ultimately improving global Earth science and precision navigation.

Science Area

Space geodesy and modern positioning, navigation, and timing rely on the global ITRF. A highly accurate ITRF is essential for tracking millimeter-scale changes in water resources, sea levels, vegetation, ice sheets, and land. By improving ITRF accuracy, GRITSS data will enhance geodetic missions studying complex phenomena like global sea-level change and ocean-climate cycles such as El Niño. Furthermore, it provides a highly stable baseline for next-generation topography missions and enables future low-Earth orbit constellations to more accurately track the planet’s shape, continental movements, and freshwater distribution.

Technology

To bridge different ground-based tracking systems, GRITSS upconverts GPS signals into S-band and X-band frequencies that VLBI stations can detect. First, a modified GPS receiver digitizes incoming signals. Onboard processors (an ASIC and FPGA) package this data, synchronized precisely by a 10 MHz ultra-stable oscillator linked to GPS time. The data is frequency-translated by custom S-band and X-band transmitters, then broadcast to Earth via miniature, highly stable antennas. Along with a concurrently mounted laser retroreflector, this perfectly synchronized data allows scientists to determine the exact distances between ground stations with unprecedented millimeter-level accuracy.

Advancements

  • Enhanced Reference Frames — GRITSS provides a space-based geodetic reference point to directly connect independent ground-based measurement, continuing to improving the accuracy and stability of the International Terrestrial Reference Frame (ITRF).
  • Geodetic quality GPS receiver for CubeSats – The GPS receiver implemented on GRITSS goes beyond traditional GPS receivers available for CubeSats by providing geodetic quality positioning accuracy and robustness needed for precision orbit determination in a small package.
  • Precision miniature X and S band antennas – The miniature GRITSS X and S band antennas have extremely accurate and stable phase patterns not previously available in a size suitable for CubeSats.

Principal Investigator

Dr. Christopher Beaudoin is a principle radio frequency engineer at Draper Laboratory in Cambridge, Massachusetts. He has more than 20 years of experience developing advanced microwave, radar, and radio-frequency systems for scientific, aerospace, and defense 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 (EHT) 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 for his supporting roles in the EHT and the NASA Space Geodesy Project.