The Global Navigation Satellite Systems (GNSS) allow it to study many exciting new problems in geodesy and geodynamics. Together with the other space-geodetic techniques, in particular VLBI and SLR, they are also the state-of-the-art technique today to monitor Earth rotation, in particular polar motion (PM) and Length of Day
(LoD).

The key characteristics of PM and and LoD are reviewed and illustrated using a simple tool to analyze GNSS-derived and other Earth rotation series from more than 150 years of Earth rotation monitoring. Time resolution and accuracy of the Earth rotation parameters improved substantially with the evolving methods, but also the problem itself slightly altered as a consequence of this process.

Space use of global navigation satellite systems (GNSS) has grown rapidly since the first operational example on Landsat-4 in 1982. It is now used routinely by spacecraft in low Earth orbit for spacecraft navigation and timing, and for science uses as well, including radio occultation, reflectometry, and geodesy. The International Space Station features several GPS receivers for attitude determination, and projects like the NASA Autonomous Flight Termination System (AFTS) are expanding its usage to launch vehicle range operations.
Starting in the late 1990s, the first experiments in so-called high-altitude GNSS reception were launched, by the US Air Force Academy, the European Space Agency, and AMSAT, respectively. These experiments demonstrated that reception of GPS in particular was possible at altitudes above the GPS constellation itself, via signals that spill past the limb of the Earth, or are broadcast as part of the transmit antenna sidelobes. This utilization was formalized as the GPS Space Service Volume (SSV), which describes signal reception characteristics between the altitudes of 8,000 km and 36,000 km. The first US operational civil missions in the SSV were the NASA Magnetospheric Multiscale (MMS) mission, which launched in 2015, and the NASA/NOAA Geostationary Operational Environmental Satellite (GOES) R-series, the first of which launched in 2016. GOES-R and its companion spacecraft are currently operating in geostationary orbit using highly-strenuous GPS-based navigation techniques, and MMS is receiving GPS at a record-breaking altitude of 29 Earth radii, approximately 50% of the distance to the moon.
Recent studies based on these two missions show that with capable receivers like those already flying, and with appropriate high-gain antennas, traditional GPS and GNSS reception is possible even in lunar orbit. This new frontier in GNSS-based navigation is being explored as part of the NASA Moon to Mars campaign, which seeks to land humans on the Moon by 2024 as a stepping stone to human exploration of Mars. GNSS-based navigation has many benefits for this application, including maintaining high-accuracy spacecraft clocks, providing precise and responsive navigation to vehicles and science instruments, providing the basis for augmentation signals in lunar orbit and on its surface, and supporting infrastructure programs like communications relays.
GNSS-based navigation in lunar orbit and for journeys beyond will be an international initiative, and it supported through the activities of the United Nations (UN) International Committee on GNSS (ICG) through its Interoperable GNSS Space Service Volume project. The ICG defined the SSV concept for the multi-GNSS environment in collaboration with all six GNSS providers in 2018, and is embarking on a project to expand upon that work to support the international push to the Moon and beyond. The International Space Exploration Coordination Group has identified 10–20 individual missions to the Moon in the next 10 years as part of its Global Exploration Roadmap, and identified numerous technology needs that are addressed via use of GNSS.
This talk will explore the history, state of the art, and the significant new frontiers in GNSS for navigation in cis-lunar space and beyond.