LEO PNT – reality or hype?

These services are now provided by four global Medium Earth Orbit (MEO) satellite constellations, with each constellation containing around 30 satellites, orbiting approximately 20,000km above the surface of the earth. GNSS are also supported by several regional augmentation systems, which provide additional services over limited regions, generally using geostationary satellites and satellites in geo-synchronous orbits above the region they serve. Whilst GNSS is now almost ubiquitous it is not the solution to every PNT problem. The main drawbacks are:

• Signals received on the ground are weak and therefore easily jammed, intentionally or unintentionally
• The weak signals limit the ability to penetrate buildings and areas without sky visibility
• Deep urban and cluttered areas suffer from multipath which can degrade performance significantly

In recent years there has been an explosion in the availability of satellite communication networks using much smaller lower cost satellites in Low Earth Orbit (LEO) constellations, typically 400-1500km above the surface of the earth. There are many such constellations emerging, such as Starlink, OneWeb, Kuiper, Iridium etc., which are the most well-known. These LEO satellite communication systems are often referred to as non-terrestrial networks (NTNs) and are increasingly being seen as an extension to ground based cellular networks, some with PNT capabilities. In addition to these LEO communication networks several organisations are exploring and propose the use of LEO constellations having PNT as their primary role with communications a secondary function.

This raises the question of whether LEO satellite constellations offer advantages for PNT, either in addition to conventional MEO GNSS's, or perhaps even in place of them. Establishing the advantages and disadvantages that new LEO systems might offer is an area in which a considerable amount of research and investigation has already been started. In this article we will consider some of the challenges and opportunities facing satellite receiver manufacturers.

Main differences between LEO, MEO, and other orbits

One fundamental difference between LEO and MEO is the choice of orbit height. For LEO this is in the range 400-1500km, whereas for MEO it is around 20,000km – as used in the four global GNSS constellations: GPS, Galileo, BeiDou and GLONASS. However, there are many implications of this parameter, which lead to design challenges and opportunities, such as:

• Number of satellites
• Orbital periodicity and transit times
• Stability of orbit and orbit prediction
• Orbit management and distribution of ephemerides (ground support infrastructure)
• Signal power (transmitted and received) and propagation path loss
• Choice of frequency bands
• Choice of signal type and structure

Figure 1. LEO Earth’s surface coverage

Disentangling the myths of LEO PNT

We hear and read about the many advantages that LEO PNT will offer from researchers and proponents and whilst much of this is true it is not often balanced against the challenges and complexities introduced. Let us go through the main benefits being promoted and look at them from the GNSS receiver manufacturer perspective balancing pros and cons.

Higher signal power leads to better indoor coverage

For signals in the conventional RNSS bands (L1/E1, L5/E5, L2, E6) this is not necessarily true because power for some radio navigation spectrum is regulated according to the received signal strength on the ground. However, the satellites transmit at lower power levels which contributes to smaller less expensive satellites rather than higher received signal levels on the ground.

Another consideration is that the relative signal path length ratio between horizon and zenith is greater for LEO, so signal strength variation during satellite transit is greater than for MEO.

However, signals may be transmitted in new bands not previously used for satellite PNT, e.g. S, C, K or other bands. This could give significant benefits in terms of received signal power and management of ionospheric effects, but it will increase the complexity of the receiver. This additional complexity will likely impact cost and power requirements for the receiver, as well as making antennas more expensive.

It is not possible to say that LEO automatically means significantly higher signal powers at the receiver, but with the added complexity associated with using new radio bands, or changes to the way current RNSS bands are regulated, this could become a future reality.

LEO PNT will lead to better accuracy

Having more usable and visible satellites could lead to improved accuracy, however, with four MEO constellations and as many as 40 visible satellites already in orbit, adding more satellites does not necessarily lead to significant additional benefit.

Due to their lower orbital height and proximity to the mass of the earth LEO satellite orbits are likely to be less stable than MEOs. After all the rocky body of the earth is not a homogenous mass. So, unless their orbits can be accurately modelled using more advanced orbital ephemerides the orbital errors introduced by LEO satellites might offset other advantages. One way of doing this could be, for example, by making use of GNSS receivers on the LEO satellites tracking MEO signals.

Another challenge for high accuracy positioning is modelling of satellite antenna effects, which are exacerbated when compared to MEO satellites, due to the larger off-nadir angles introduced by the lower orbits.

With the lower orbital heights of LEO, the transit time of the satellite from horizon to horizon is much shorter than for MEOs. This means that the receiver must be able to acquire new signals quickly and cope with higher Doppler shifts. On the one hand this adds complexity, but on the other it opens up opportunities to exploit the new signal characteristics.

Figure 2. MEO vs. LEO Eath’s surface coverage

Despite these hurdles, LEO does provide the significant advantage of much faster change of relative geometry between the satellite and user.  This offers a significant advantage for high precision positioning methods such as RTK and PPP where convergence time is reduced by the more rapid motion of the satellite across the sky. The use of LEO constellations for PNT could lead to convergence times for PPP of less than one minute, whereas for traditional MEO GNSS solutions convergence times are typically a minimum of several minutes.

LEO PNT could lead to improved accuracy but there are many challenges to overcome before this is achieved.

LEO PNT leads to better multipath resilience

The much shorter transit time for LEOs may provide benefits to the receiver if it is able to take advantage of these characteristics. It causes a more rapidly changing radio path as the satellite moves across the sky. This changing radio path leads to faster changes in the channel and thus a multipath environment which changes more quickly. Using modern signal processing techniques and longer coherent integration of the signal the receiver could potentially exploit the more rapidly changing multipath conditions to provide improved multipath mitigation.

LEO PNT should lead to more robust multipath mitigation and resilience, but this will depend on constellation design and the signal processing used in the receiver. Further study will be required to determine the scale of the benefit that can be achieved.

LEO PNT will provide improved jamming and spoofing resilience

Depending on how the LEO PNT signals are designed and implemented there could be significant improvements in resilience against jamming and spoofing. The use of new frequency bands, e.g. S and C, would lead to greater signal diversity giving better resilience against jamming. Some bands might allow for higher signal powers as well.

New signal designs could incorporate enhanced security and anti-spoof measures supporting both data and signal authenticity checks. Therefore, improved anti-jam and anti-spoof capabilities could be one of the big benefits of LEO PNT if the systems are designed for this from day one.

LEO PNT will lead to higher integrity GNSS positioning

GNSS positioning integrity relies on being able to accurately model errors experienced at the receiver and to detect and mitigate rare errors caused by ‘feared events’ such as an undetected satellite failure or geomagnetic storm. LEO does not automatically mean that higher positioning integrity will be achieved, but if appropriate design decisions are taken the use of new signals and bands could lead to improved integrity solutions going together with improved security.

Shorter life of satellites leads to faster technology evolution

Satellites are smaller and cheaper with shorter operational lifetimes. This could lead to quicker evolution of the technology deployed with new features being introduced more quickly than for traditional MEO constellations.

Summary

In short LEO PNT could make a significant contribution to overcoming some of today’s limitations of GNSS. If the right design decisions are made it could be a game changer for some applications. In reality, a mix of MEO, GEO and LEO satellites is likely to be needed to cover the greatest range of applications possible.

There is still considerable work needed to investigate the best LEO solutions: choice of signal bands, signal coding, security, and the proper assessment of the performance achievable.

LEO presents exciting opportunities for the business of satellite PNT and could herald the next innovative phase in the evolution of the technology. It will require both constellation owners and receiver manufacturers to work closely together to achieve the right balance of innovation that preserves the cost effectiveness of Satellite PNT and enhanced performance.

By David Bartlett, Head of GNSS Positioning Technology, Olivier Julien, Senior Principal Engineer, Chris Hide Senior Principal Engineer, and Jos Prakash, Senior Research Engineer at u-blox