The Pros, Cons, And Predictions For A OneWeb UK GNSS

It was announced last week that a consortium including the UK Government has won the bid to rescue the OneWeb satellite constellation from bankruptcy. This has been much heralded in the press as the opportunity for the UK to secure its own Global Navigation Satellite System (GNSS) after Brexit has reduced our access to the secure components of the Galileo system. Some observers have however declared that the UK has bought the wrong satellites for this. So let’s dive in.

Why does the UK Government need this?

One of the many things that the UK has lost following Brexit, is access to the Public Regulated Service (PRS) signal from the Galileo GNSS. This is the encrypted military positioning service, and the UK was actually key in the design and development of it. The UK will no longer have the rights to use, or further develop, this critical strategic capability. While the UK currently has access to the encrypted M-code GPS signal in a limited fashion, there is no guarantee this will always be the case. A sovereign capability of this type is long overdue for a Nuclear Power. So the UK government has been assessing its own options for a UK GNSS for over a year, but the recent Covid-19 crisis has placed ever-growing strain on the proposed $5Bn budget and so it is no surprise that OneWeb became of interest.

Why OneWeb?

OneWeb is a Low Earth Orbit (LEO) communications system, designed to provide high speed internet globally, and trying to convert it into a Global Navigation Satellite System will certainly require some effort and will reveal some significant challenges. Here are the pros and cons of this option

Pros

• LEO satellites are much closer to Earth than the Medium Earth Orbit (MEO) satellites used by each of the traditional GNSS. This means that receivers on Earth will enjoy much stronger signals than those provided by MEOs, providing better indoor coverage and improved protection from jamming.
• LEOs are easier to launch and replace due to their lower orbits
• LEOs move much more rapidly across the sky than MEOs, which help to reduce the impact of one of the main sources of error on radio positioning systems (multipath interference).
• OneWeb already has access to launch capabilities and a schedule, satellite design and build capabilities, space-qualified hardware, a partially-deployed constellation, and in general represent somewhat of a head start for UK GNSS than other options. This is likely to have been the main driving force in the decision to invest in the system.

Cons

• The OneWeb system does not already include a signal designed for radio positioning, nor will the satellites currently be tracked with an accuracy that is good enough to provide a GNSS. Both aspects can however be rectified in principle
• The platform currently uses carrier frequencies that are much higher than those used by GNSS. While it is possible to build new receivers capable of using very different frequencies, this is not in practice a sensible option, it is a much better idea to broadcast the right frequencies on future OneWeb platforms as part of a dedicated positioning system signal payload.
• Various components on the OneWeb platform and in its ground infrastructure may need to be upgraded to be able to provide high quality positioning signals, and to provide military-grade encryption.
• Due to the much lower orbits, many more LEO satellites are required than MEOs to achieve the same levels of global coverage and a high number of satellites simultaneously in view.
• LEOs need to be replaced much more often than MEOs due to increased atmospheric drag in LEO meaning that their station-keeping manoeuvres burn through fuel at a much higher rate.
• LEO communication systems need to just ensure that every part of the Earth they wish to cover is being served by just one satellite at a time to provide the lowest infrastructure cost, whereas GNSS positioning systems strive to cover every part of the Earth with service from as many satellites as possible to get good positioning accuracy. So there is an immediate issue with a LEO constellation planned for communication not being optimal from a LEO-GNSS viewpoint. This issue of coverage and overlapping satellite footprints will need to be resolved in an updated constellation plan.

Where will it go from here?

This is where I get to have fun with my crystal ball. There are lots of potential directions that this could all take, but here are the key issues and possible solutions.

_Observations

• If the UK GNSS programme is now required by the UK government to use OneWeb, then new requirements for UK GNSS need to be set, and they will need to be realistic given the constraints of small LEO satellites. They need to actually be driven now by the scientists and engineers telling the stakeholders what is physically possible given the system they have to work with, as opposed to an ideal system being designed from scratch to fit the requirements proposed by the stakeholders.
• It will not be possible to source the hundreds of space-qualified atomic clocks required for the system (plus each being replaced every 2-3 years as the satellites run out of fuel). There simply is not a global supply chain for this volume of certified clocks, and the UK does not currently have such manufacturing capability itself. The system will therefore need to not need its own atomic clocks in orbit or the programme will be delayed further while a new supply chain is established.
• Due to the polar orbits required for LEO satellites, the number of satellites visible at the same time varies wildly from the equator (very small number visible) to the poles (very large number visible). Unless all OneWeb satellites contain a positioning payload, it will not be possible to serve the equatorial regions with coverage from enough satellites to provide a standalone GNSS, especially in urban environments. Therefore it would be very sensible to plan for the OneWeb UK GNSS to be an overlay service to the open signals coming from the other GNSS.

_Predictions

Based on these restrictions, in order to provide a useful Positioning, Navigation and Timing (PNT) service from OneWeb I suspect that we will see the following features evolve:

• There will not be a PNT signal from every satellite. They are too small and will have to be replaced too often for a joint communications-and-positioning payload to be viable. The comms signals (Ku band) are too different in frequency from the current GNSS signals (L-band) to be used to provide the PNT service directly. So some subset of the full constellation will be switched over to a dedicated PNT payload. (Note that this will impact the profits of OneWeb, only the communication satellites will be generating revenue). We may even see these dedicated satellites form a new constellation with dedicated orbits separate from the current OneWeb communication satellite orbital planes.
• The system will not be a standalone GNSS. It will actually depend on the open signals of the other GNSS satellites in MEO to provide the LEO satellites with their own orbital and clock data via an onboard GNSS receiver. This will dramatically reduce the overall cost and complexity of the system. This dependancy will not be a problem as the main threat to GNSS is local jamming and spoofing nearby receivers on the Earth, not a catastrophic failure of the space segment. And historically when one system has have a failure (as has happened in the past for GPS, Galileo and Glonass) the others have remained unaffected due to their independence. So designing UK GNSS to be dependant on the signals from any of the existing 4 independent GNSS can actually provide some resilience. It is a major departure from the traditional desire for a fully independent system but one that will be required for the OneWeb jerry-rigging to be practical within the desired budget.
• Further to the above the system may not even be capable of independent position fixes at all, or this may not be the requirement for the system to be providing some of its core benefits. One of the reasons for UK GNSS to exist is to counter concerns over spoofing of the open GNSS signals from other constellations. A receiver only needs to access two secure (encrypted/authenticated) signals from UK GNSS to confirm their agreement or not with the set of open (insecure) signals from GPS, Galileo, etc. If they disagree then a spoofing warning can be raised and the Open GNSS signals ignored from the navigation solution. It will be easier to provide an overlay authentication service to the existing open GNSS signals with OneWeb than to provide an entire standalone UK GNSS.
• The debate will certainly rage on. There have been very mixed opinions about how sensible the OneWeb choice for UK GNSS is, and that is not a good sign. LEOs have always traditionally been discarded as a choice for a GNSS based simply on the economics never making sense. However there are further benefits to the UK maintaining the OneWeb communications system in its own right, even if the constellation is never used for UK GNSS after all.