What are the Next-Generation GNSS Constellations?


What are the Next-Generation GNSS Constellations?

Global Navigation Satellite Systems (GNSS) have unleashed innovative commercial, scientific, and military applications since the US Global Positioning System (GPS) entered service in the 1980s. Today, GNSS constellations from the US, Europe, China, and Russia circle the planet to provide civilian and military positioning services. This article will explain how global GNSS constellations will evolve in the coming decades.

America’s Global Positioning System

GPS Modernization is the next chapter in the United States’ pioneering satellite positioning system. Getting to these new capabilities requires more than just new satellites. The US Space Force is also upgrading its control segment and shepherding the development of next-generation GPS receivers.

Among the new capabilities GPS Modernization will deliver are four new signals to make GPS more accurate and resilient. These upgrades will improve the military’s war-fighting efforts while enabling new industries such as autonomous vehicles and urban air mobility.

L2C for dual-frequency accuracy

The original GPS satellites broadcast signals on two frequencies, L1 and L2, but the L2 signal’s navigation message was restricted to US military users. This limited GPS civilian applications to the lower-accuracy L1 message. High-precision civilian users were able to use the L2 frequency without the military’s navigation message.

With GPS Modernization, civilians get a dedicated message on the L2 frequency. The L2C signal will let civilian receivers correct for ionospheric effects and acquire signals faster. These dual-frequency receivers will operate in a wider range of environments with improved reception within buildings and under trees.

Currently transmitting from a number of GPS satellites, L2C is expected to reach final operating capability in the US government’s 2024 fiscal year.

L5 for aviation and other transportation modes

From automotive to aviation, GPS has transformed transportation. However, the accuracy of the civilian L1 signal has become a bottleneck for future progress. GPS Modernization will introduce a third civilian signal, L5, specifically to provide the high-accuracy positioning moving vehicles need.

The new L5 signal will enhance FAA’s Wide Area Augmentation System, allowing more efficient routing of civilian aircraft and supporting instrument-only guidance to within 200 feet of the runway.

This signal has been pre-operational since 2014. L5 will reach initial operational capability in FY2024 and final operating capability in FY2028.

L1C for GNSS interoperability

Another signal on the civilian L1 band will make GPS interoperable with Galileo and other GNSS constellations. Interoperability allows receivers designed to recognize multiple constellations to deliver more accurate positioning under a wider range of conditions.

A receiver compatible with every constellation could detect signals from dozens of satellites under ideal conditions. In cities, mountain valleys, or forests, a multi-constellation receiver will detect enough signals to acquire a reasonable accuracy.

L1C signals will not be fully available until the late 2020s when Block III designs comprise the entire 24-satellite constellation.

M-Code for restricted users

A new encrypted signal for the American military and other authorized users will eliminate the need to bootstrap from the civilian C/A code. In addition, M-Code signals will be more secure and resistant to adversarial jamming.

M-Code positioning and initial operating capabilities are scheduled for FY2025.

Europe’s Galileo

The European Union’s Galileo constellation is nearly complete, with twenty-eight of the planned thirty medium Earth orbit (MEO) satellites operational at the end of 2022. This was enough capacity to make three of Europe’s GNSS services operational.

  • Open Access Service delivers meter-scale accuracy to the public.
  • Public Regulated Service delivers centimeter-scale accuracy to authorized users.
  • Search-and-Rescue (SAR) relays distress messages to European ground stations.

In early 2023, Galileo’s High Accuracy Service became operational. With sub-meter accuracy in Europe, the new service will support drone navigation, precision agriculture, and other applications. Once the full service goes into effect in 2024, receivers in Europe will get better performance and faster convergence times.

Europe’s software-based design approach allows the Galileo constellation to evolve without replacing satellites. For example, an upcoming software update will improve convergence times and compatibility with augmentation services.

However, fallout from Russia’s invasion of Ukraine and delays in the transition of Europe’s launch capacity away from the Ariane 5 has put Galileo’s deployment on hold. The final ten first-generation Galileo satellites, including replacements for aging satellites, will remain on the ground until the Ariane 6 launch vehicle enters service.

While Europe waits, the Galileo program is not standing still. Second-generation Galileo (G2) satellites will start entering service later this decade. More efficient than their predecessors, the G2 satellites will use new electric propulsion systems to maintain their orbits for extended service life. Inter-satellite communications will allow satellites to check their positioning performance against each other, reducing Galileo’s reliance on ground stations. In addition, new GNSS technologies will enable more accurate positioning and make Galileo more resistant to jamming and spoofing.

Looking further out, the European Space Agency plans in-orbit testing of LEO augmentation Galileo’s MEO constellation.

China’s BeiDou

China’s third-generation positioning system, BeiDou-3, consists of twenty-four MEO satellites providing global coverage plus three satellites each in geostationary orbit (GEO) and inclined geosynchronous orbit (GSO). Civilian signals allow positioning accuracy to within 3 meters globally. Augmentation with signals from GEO and GSO allows 1-meter accuracy in China.

Civilians with BeiDou-enabled devices can send emergency SMS messages similar to the emergency texting service provided by Apple and Globalstar. China’s military and other authorized users have two-way messaging capabilities.

Future BeiDou-3 developments include GNSS augmentation using ground stations and enhanced two-way messaging.

Fourth-generation plans for BeiDou will include more precise atomic clocks and improved communications technology. Official statements indicate the planned completion of this next phase by 2035.

Russia’s GLONASS

Russia’s GLONASS constellation consists of eighteen active MEO satellites in high-inclination orbits to serve Russia’s higher-latitude geography. Over the past ten years, Russia began replacing its second-generation GLONASS-M satellites with the more modern GLONASS-K design. By moving from FDMA signals on L1 and L2 frequencies to CDMA signals on L1, L2, and L3, civilian performance will become equivalent to other GNSS constellations.

GLONASS-K and its successor, GLONASS-K2, also reduce Russia’s dependence on western technologies. However, these efforts have not made GLONASS immune to western sanctions following Russia’s annexation of Crimea and the recent invasion of Ukraine.

By early 2022, nearly two-thirds of operational GLONASS satellites were beyond their planned service life. The first GLONASS-K2 prototype has seen repeated delays from its original 2018 launch date and may not launch until late 2023.

Simulating Multiple GNSS Constellations

GNSS/INS simulations must recreate the many positioning options available in this age of multi-constellation positioning. CAST Navigation GNSS solutions can simultaneously simulate up to three different constellation signal types on each antenna element.

CAST Navigation has been the leading provider of GPS and GNSS simulator capabilities for more than forty years. Over that time, our simulator systems have become more capable to match the evolution of constellations in service. For example, our GNSS simulators have mirrored the evolution of GPS from P(Y) Code through SAASM to M-Code AES and MNSA.

Bringing the World into Your Laboratory

This is just one way CAST brings GNSS/INS testing out of the field and into the lab. From the performance of each visible satellite to interference sources and the effect of vehicle motion, GNSS reception in the real world never matches the ideal. Our powerful simulators account for every variable that impacts GNSS signal reception.

As signals travel from orbiting satellites, they are subject to many sources of natural, industrial, and hostile interference. CAST solutions give you complete control over the atmospheric conditions that degrade positioning accuracy. With our jamming subsystems, you can configure urban and hostile interference sources.

CAST simulators also give your granular control over the effect of vehicle configurations on GNSS reception such as vehicle silhouettes and antenna gain patterns. Scenario profiles can create dynamic, 6-DOF vehicle trajectories while our simulators’ 3D engine accounts for terrain effects along the vehicle’s path.

CAST Navigation simulator solutions bring the world into your lab and generate accurate, precise, and repeatable GNSS radio frequency signals to support your testing and integration projects.

If your applications demand precise and repeatable GNSS RF signals, then contact CAST Navigation today.

June 27, 2023