Commercial low Earth orbit (LEO) satellite constellations have the potential to introduce new positioning, navigation and timing (PNT) signals to complement existing global navigation satellite systems (GNSS), providing resilience, security, and high precision to navigation users. More than 20x closer to Earth compared to medium Earth orbit (MEO), providing stronger signals from LEO is attractive due to reduced path loss and smaller satellite footprint. Faster satellite motion brings rapid convergence of Precise Point Positioning (PPP) along with shorter decorrelation time to aid with effects such as multipath. Novel signal designs bring authentication and encryption for cybersecurity. These are just some of the features attractive to industries from automotive autonomy to critical infrastructure and mass mobile.
launched its first demonstration commercial LEO navigation satellite, Huginn, in May of 2022 to showcase these benefits. The subsequent phase of Xona deployment targets the launch of approximately 30 satellites to provide a 1-in-view LEO GNSS enhancement service over mid-latitude population centers. Ultimately, Xona targets the deployment of a constellation of nearly 260 satellites to deliver GPS-level satellite visibility and geometry for full positioning services that provide resilient accuracy with security. These satellites are designed for on-orbit flexibility and faster refresh cycles (5 years) to keep pace with ever-increasing PNT demands.
Testing LEO PNT
With only one Xona demonstration satellite currently on orbit, live sky testing is limited to only a few minutes at a time with a handful of opportunities per day. Although this showcases the core satellite technology, the final constellation will take time to deploy which is where radio frequency (RF) signal simulation capability becomes critical - for performance evaluation of the expected 10+ continuous Xona LEO satellites in view, in final deployment, for PNT receiver and equipment development.
This is whyis so important – it brings commercial PNT further into the hands of developers and all of us one step closer to hardened LEO PNT user equipment deployment. GNSS signal simulators are an invaluable tool when used in the development and integration of systems that use GNSS. Most current implementations simulate signals from existing GNSS in MEO and Geosynchronous (GSO) orbits. Xona recently worked closely alongside Spirent to certify the first GNSS simulator capable of producing signals representative of Xona’s demo satellite - but at a scale that builds the entirety of the fully populated constellation of near 260 satellites.
Results have been obtained from both hardware and software receivers which gives flexibility for receiver and algorithm development. This already showcases several important factors: (1) LEO satellite dynamics shown from the rapid geometry change and Doppler profiles; (2) higher receive signal power compared to GPS (~20 dB or ~100x); (3) code and phase noise indicating the ability to perform high accuracy positioning; and (4) the ability for third party simulator and receiver partners to create successful implementations for Xona’s PNT signals.
Scenarios and mechanics
While the Spirent GSS7000 simulation platform was used in our initial Xona performance characterization, the capabilities also apply to Spirent’s GSS9000. In creating scenarios, LEO satellite orbits can be defined via Keplerian elements at a given epoch or via *.sp3 file. PRNs can be assigned individually to each satellite where navigation data is provided via a file. One fun note is that Xona’s constellation has more than 99 satellite identifiers, which breaks the current sp3 standard. This is one of many hurdles that had to be overcome for a successful implementation.
Having defined the scenario, the high-performance computing platform (C50r) generates baseband In-phase and Quadrature (I/Q) components, which are uploaded to the Spirent GSS7000 or GSS9000 simulator platform to generate a corresponding Xona RF signal – coherently with GNSS. This gives flexibility to work with either software receivers at the I/Q level, or with hardware receivers that leverage full RF.
The power of this is immense. First, it allows us to dig in with end users early to understand and quantify the performance boost offered by Xona’s LEO PNT service while working with specific receiver hardware. We can leverage Spirent’s complex scenario obscuration, multipath, and interference sources which can then be combined with SimXona to showcase the advantages that Xona’s service offers in specific scenarios.
At Xona, both internal and external facing testing benefits significantly from Spirent’s latest SimXona development. Externally, this allows us to demonstrate the full Xona LEO PNT system potential to customers and partners before the full constellation is fielded. We have seen a clear pull for this type of hardware simulation capability from industry to aid with both user equipment development and to showcase the boost in performance expected in specific use cases. Interest in such capability has been expressed by Xona partners across all sectors from automotive to heavy machinery to critical infrastructure.
Internally, the SimXona module allows us at Xona to experiment with new system elements as the system design first matures for production, and later as it continues to evolve. Changes in orbit, data messages, and other parameters can be evaluated on the ground and their effect on receiver hardware in a controlled environment in the lab. For example, Xona’s demo mission, Huginn, was a rideshare mission onboard the SpaceX Tranporter-5 and is currently at an altitude of 520 km, though production satellites target altitudes above 1000 km. Furthermore, SimXona currently simulates representative signals from Huginn, which will see changes in production based on lessons learned from this first satellite. As designs mature and experimental features are released, the Spirent SimXona testbed conceivably could be used to check for any significant ramifications to users before pushing changes to the constellation itself.
Perhaps most important is how features like SimXona act as a first step in receiver certification, not just by Xona but by receiver developers themselves. This testbed gives the tools to create a standardized approach and test vectors to not only aid in receiver development but to empower receiver manufacturers for self-certification. With the first Xona production-class satellite launch planned for 2024, this provides a path for readying third-party user equipment prior to launch.
To find out more about SimXona and how it could benefit your future PNT provision, see the