Time Transfer From GPS for Designing a SmallSat-Based Lunar Navigation Satellite System

With the advent of new frontiers in space exploration, traveling to the Moon serves as a crucial stepping stone for the success of future deep space missions. Many international space agencies, such as the National Aeronautics and Space Administration (NASA), European Space Agency (ESA), China National Space Administration (CNSA), Japan Aerospace Exploration Agency (JAXA), and Roscosmos State Corporation for Space Activities (Roscosmos), have demonstrated a growing interest in building a sustainable human presence on the Moon (Laurini & Gerstenmaier, 2014). More than 40 lunar missions are being planned within the next decade by 10 international space agencies (Tai et al., 2020). After more than 50 years since the Apollo program ended, NASA’s Artemis mission will land humans on the Moon in the mid-2020s, including the first woman and first person of color (Smith et al., 2020). Furthermore, active efforts on traveling to the Moon are also being invested in by private sector companies like SpaceX and Blue Origin. User position, navigation, and timing (PNT) serves as the basis for various lunar applications that include mission activity planning, search-and-rescue operations, and geotagging calibrated scientific payload data samples (Israel et al., 2020). Stanford University Abstract For the future lunar navigation satellite system (LNSS), a GPS-like constellation for the Moon, NASA has expressed interest in using a smallsat platform. However, many design decisions have yet to be finalized, including the orbit, constellation, and onboard clock. Furthermore, designing a dedicated smallsat-based LNSS limits the payload capacity, thus restricting the size, weight, and power (SWaP) of the onboard clock. We propose an LNSS design with time transfer from intermittently available GPS signals wherein we design a Kalman filter to estimate timing corrections for a low-SWaP clock. Additionally, we devise a lunar User Equivalent Range Error (UERE) metric to characterize the ranging accuracy of signals transmitted by an LNSS satellite. We further validate our time-transfer technique with simulations of an LNSS satellite in an elliptical lunar frozen orbit (ELFO) with an onboard chip scale atomic clock (CSAC) while analyzing the visibility effects of GPS, timing errors, and lunar UERE across ELFOs.