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Positioning, Navigation and Timing in Low Earth Orbit: Test Challenges and Considerations


Designing PNT systems for LEO constellations and equipment poses many challenges. A webinar from Spirent and Inside GNSS outlines some key test considerations.

A new space race is underway – and it’s happening very close to the Earth. The number of satellites deployed into Low Earth Orbit (LEO) increased from 3,300 in 2019 to 6,300 in 2022, with spending on LEO systems projected to grow at 21.7% CAGR, creating a $16.7bn global market by 2028.

The relatively strong signals and fast data rates achievable with satellites orbiting as low as 500km from the Earth’s surface are opening up many new cases, with much of the impetus coming from global demand for fast mobile broadband. Companies like SpaceX, Iridium and Globalstar are already providing satellite connectivity direct to smartphones, opening up new markets in areas that are not practically reachable with terrestrial or high-altitude balloon-based broadband.

The potential for close-quarters observation of the Earth is another driver, with satellites being deployed to monitor everything from the impact of natural disasters to the effects of climate change. LEO constellations also enable new kinds of positioning, navigation and timing (PNT) signals to be broadcast to ground-based receivers, paving the way for services like high-accuracy indoor positioning and ubiquitous autonomous transportation.

On-demand webinar: PNT test considerations for LEO constellations and equipment

There are many exciting opportunities to be seized, but designing, deploying and managing PNT systems for satellites so close to the Earth’s surface presents many non-trivial challenges.
A January 2023 webinar hosted by Spirent and Inside GNSS takes a detailed look at the PNT test considerations for LEO constellations and LEO equipment, and proposes some practical ways of tackling the many challenges posed by PNT signals to and from LEO constellations.

It’s available to watch on demand for free – and here’s a taster of what you can expect to learn.

1. Test protocols for the use of GNSS signals in Low Earth Orbit

Most LEO satellites rely on PNT signals from GNSS constellations operating in Medium Earth Orbit (MEO), using those signals to perform functions such as precise orbit determination, payload delivery and communications to ground-based receivers. To do that effectively, their PNT systems must be able to overcome significant challenges posed by orbital physics.

Those include the pronounced Doppler shifts caused by the high velocity of LEO satellites in comparison both with GNSS satellites operating in MEO and with ground-based receivers. LEO satellites also differ significantly from MEO and geostationary (GEO) satellites in the way they experience the gravitational pull of the Earth and other celestial bodies, and their lower altitude makes atmospheric drag another key consideration.

Given that field testing of space-borne receivers is not practically or financially viable, these and many other factors must be modeled and simulated accurately in the lab. In his presentation, Spirent’s Dr Günter Heinrichs outlines the simulation requirements, including the need to test with a much lower horizon; the need for accurate modelling of vehicle orbits, vehicle motion and antenna patterns; and the need to test for very high precision.

2. Designing GNSS receivers for use on LEO satellites

Switzerland-based SpacePNT is a pioneer of space-based PNT solutions, developing high-accuracy, high-precision GNSS receivers for use on spacecraft within and beyond Earth’s orbit. In his presentation, Michele Scotti, GNSS Engineer for Space Applications at SpacePNT, outlines some of the real-world challenges he and his team have faced in designing, developing and testing receivers for use in Low Earth Orbit.

Key among these are the considerations relating to testing with hardware in the loop. For reliable test results, this requires signal simulation equipment that is an order of magnitude more precise than the company’s receivers – which, given that some of SpacePNT’s receivers offer 10cm-level accuracy, is not a trivial ask.

Many GNSS signal simulators rely on standard Keplerian algorithms to model orbital trajectory, but high-precision LEO-based receivers require greater computational accuracy owing to the unusual combination of external forces acting on the spacecraft. Michele explains how SpacePNT has tackled this challenge, achieving highly realistic orbit modeling in the lab.

3. Building a lab test environment for LEO constellations

Developing space-borne PNT systems requires simulation at every stage in the product development lifecycle, from initial R&D to real-world testing.

Achieving a lab test environment that can stand in as a realistic proxy for actual conditions in space is a major challenge, but not impossible with the right equipment. In the final presentation, Spirent’s Stuart Smith considers three scenarios in which developers may need to simulate realistic signals, from the existing signals broadcast by GNSS constellations and augmentation systems to new LEO-to-ground PNT signals that will drive new types of positioning and timing services.

Stuart reviews the challenges involved in creating realistic signals and modeling realistic environments, and explains the solutions Spirent has developed to help developers conduct highly realistic LEO testing in the lab.

Watch the full webinar on demand

If your organization is researching or developing LEO-based constellations, equipment or applications, this 1.5 hour webinar provides a detailed introduction to the PNT test considerations involved and some of the ways in which the challenges can be overcome.

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Mia Swain
Mia Swain

Mia Swain is a Product Manager working on simulator products and PNT solutions within the Spirent Positioning Technology business unit. She has an academic background in Physics and worked in R&D, applications engineering and then product management. Mia Swain’s current focus is managing the commercial status and development lifecycle of Spirent’s commercial grade GNSS simulator and its software platform, ensuring that the products maximize their commercial value and next generation products meets the needs of the market.