GNSS, or Global Navigation Satellite System, are the satellite constellations that provides positioning, navigation, and timing (PNT) services on a global or a regional scale. GPS is just one of the 6 operational constellations, with more being developed.
First creted in the 1960s with the US military system called Transit, GNSS has gone through many changes and advancements. What are some of the upcoming innovations and changes for GNSS?
Currently, GNSS technology uses microwave clocks – an atomic clock whose timekeeping mechanism operates in the microwave region. They have been used for GPS because of their precision. A new generation of atomic clocks is on its way to replace microwave atomic clocks – optical atomic clocks.
What does this mean for GNSS? Over the last decade, optical clocks are being used more on Earth in laboratories and other places where high precision is vital. Because optical clocks surpass the technology of microwave clocks, they could be used to replace them or provide a backup.
These new clocks can improve position determination thanks to the lower frequency instabilities they bring to the table. Researchers with the National Institute of Standards and Technology (NIST) have even redefined the length of a second using the precision of these new clocks. The researchers at NIST estimate that optical clocks are capable of being 10 to 100 times more accurate than measured in the study.
GNSS architecture describes the configuration of the system. For GPS it consists of three segments: space, ground, and user. Researchers in a study published in April of 2021 in GPS Solutions, propose this new technology can and is enabling new GNSS architectures.
The example given is the Kepler constellation proposed by the German aerospace Center DLR. This new constellation is special because of the addition of ‘innovative key features of optical inter-satellite links (ISL) delivering highly precise range measurements and of optical frequency references enabling a perfect time synchronization within the complete constellation.’ This new constellation shows significant improvements on the geocenter estimates and on reliability.
GPS is going to the Moon
GPS currently is only used on Earth, but NASA aims to change that. NASA’s Space Communications and Navigations program is developing GPS capabilities for NASA missions, starting with the Artemis missions to the Moon. The aim is to take advantage of GPS navigations and critical timing applications in space.
Key to the success of this project is getting multiple GNSS constellation to work together – known as the interoperability of the GNSS constellations. By using multiple constellations, you get more available signals. The improved accuracy from multiple signals could be helpful at higher altitudes where GNSS signals are sparser. The unique design of each constellation is a large part of the challenge of this endeavor. It’s a bit like trying to build with different types of building blocks – you need an adaptor for the connections.
The satellite Bobcat-1 is the next step in studying GNSS signals 250 miles above the Earth. Over a period of 9 months, Bobcat-1 will measure signals from each constellation. This will give NASA engineers a better handle on the current performance of GPS at higher altitudes.
A mission to the Moon might seem like a long way away from our activities on Earth. But, just like the Apollo missions, the innovations during the Artemis mission are likely to make a noticeable difference for technology on Earth.