US Court approves $500m sale of OneWeb to the UK Government

Yeah and if you actually read the project documents what they are refering to is on-chip atomic clocks not to be used for the generation of GPS signals (as it lacks the level of precision required for navigation signals) but to be used for calculating targeting data for the missile tracking function (e.g triangulating heat blooms) and for time stamping satellite to satellite optical communication links so they cant be dummied.

These on chip atomic clocks are already available commercially and only cost about $3000, they are used in military radars and commercial sonars as well as high precision GPS receivers. However they are about 1000th as accurate as a traditional atomic clock and while the the traditional type keep the same precision perpetually the on-chip type lose accuracy and need recalibrating after only a couple of years of use (starting at about 1.7 to the -11 in their first year or two in their third or fourth year they will be losing whole seconds) which makes them unsuitable at present for space applications where they cant be regularly replaced.

There isn’t a need for an absolute reference, accurate over time, for LEO satellites to function as an alternative GPS system. Short term clock accuracy in the LEO satellite can be extended to long term accuracy, provided there is an ability to correct for error/drift. Either the actual clock is corrected/synced/disciplined or the error is tracked and used to offset the clock error.

There are at least two ways to correct error/drift.

1. Reference to existing GPS satellites with their more accurate clocks
2. Reference to fixed ground stations, with accuracy at least as good as GPS satellites

If traditional GPS satellites are disabled then LEO satellites can still reference the ground stations. Arguably routinely accessing both is likely to be more robust.

LEO satellites are likely to have something like 5 year lives. Depending on orbit height they will do a complete orbit of the Earth ~15 times per day (or ~100 minutes per orbit). The orbit then ‘travels’ laterally around the world, so its crossing the equator at different points each orbit. So how often a satellite can be time corrected will depend on the number of ground stations with reference clocks and the rate of orbit travel (in the absence of GPS satellites functioning), but it might be as often as multiple time checks per day. The UK could for example have ground station clocks at Ascension, Diego Garcia, Australia or NZ, Pitcairn and Falklands.

Microsemi is one example of a company producing chip atomic clocks for LEO space applications but SDA is clearly interested in exploring improvements.

https://www.microsemi.com/product-directory/embedded-clocks-frequency-references/5207-space-csac#overview

That’s the Tracking function. The Navigation function is what I am referring to.

If the US loses its GPS satellites and any in-orbit replacements, in part or in whole, then its effectively lost navigation and timing data potentially across its entire military, unless it resorts to third party positioning services at lower accuracy (assuming availability), INS or earth based chip atomic clock equipment. The Navigation function of SDA is intended to at least mitigate such a loss and hopefully dissuade an adversary attempting it in the first place. LEO satellites simply being repeaters for current GPS satellites doesn’t address this threat.

A LEO satellite derived GPS may not be as accurate in initial implementations (see my other reply to you on this topic) but it doesn’t preclude both system and source clock improvements to close any gap.

As I understand it the trickier bit with using LEO satellites for GPS is not the clock accuracies but each satellite knowing its position with sufficient accuracy. With conventional GPS having fewer satellites in higher orbits it is far easier to track their orbits pretty much constantly and update their positional coordinates from ground stations as necessary whereas with a much higher number of satellites orbiting much faster and I think at LEO also the orbital trajectories subject to more minor random perturbations meaning the onboard inertial positioning needs to be re-seeded more often creates a big and possibly insurmountable challenge. It’s very unclear whether LEO GPS satellites could maintain accurate enough positional data to be able to provide a level of accuracy to surface terminals that matches that currently offered by existing GPS constellations.

Similar to time corrections options for positional updates might be with reference to higher orbit GPS nodes that know their position with the required accuracy or in theory from ground stations but the later is far more tricky than with clock adjustment because positional updates are potentially needed multiple times per orbit for each satellite and with the number of satellites and range of orbits that means that a huge number of ground stations would be needed. Another possibility I suppose might be positioning updates by reference to other satellites in the LEO constellation that have had recent ground station updates but the current OneWeb satellites don’t have any inter-satellite links (as opposed to the SpaceX Starlink satellites that do).

Even though the sufficiently accurate LEO GPS bit looks challenging this OneWeb acquisition is still hugely exciting for the UK military simply in terms of the game-changing high bandwidth low latency worldwide connectivity that it will offer if successfully rolled out. Coincidentally Musk tweeted just yesterday in reply to a question about Starlink being “deployable on high-speed moving objects like trains”. His answer was “Yes. Everything is slow to a phased array antenna”. One wonders how far his definition of “everything” extends but maybe a future anti-ship or cruise missile being able to relay targeting/sensor data and take mid-course corrections anywhere in the world from anywhere in the world with <30mS latency? There are lots of possibilities such a worldwide network opens up and with the UK owning a golden share in terms of approving potential OneWeb customers or any changes in equity ownership the UK would I think be unique in the entire world in having such a state controlled capability. The USA would also have such a capability because it has already contracted with SpaceX for various military uses of Starlink but technically since SpaceX is a private company does that count as state controlled? I suppose in reality it does because I’d be quite surprised if the US government didn’t have many levers it could pull to stop SpaceX doing something deemed damaging to US national security (the craziest and obviously totally unrealistic example being to sell itself to the Russian or Chinese government for instance) so maybe in practice it will end up with the UK and the USA being the only countries on the planet to have such a capability. Pretty impressive if the UK can pull that off and see this through to its conclusion (which will take a lot more investment to complete I suspect).

I probably should caveat my comments with qualifications that some of this is by no means easy, or necessarily in the immediate near term.

As you point out the current GPS time and position is routinely validated and corrected if required, Jet Propulsion Labs’ GDGPS provides details. We shouldn’t assume the UK’s future GNSS service will be supported with the current LEO satellites, that seems very unlikely. But since LEO satellites will have a ~5 year life, then future satellites can support inter-satellite links and improving clocks, along with other capabilities.

The GDGPS system indicated above has a significant number of base stations for orbit and time correction. A separate UK system might for example use UK embassy and consulate locations as a first pass for such bases since they probably want the comms capability anyway, but locations don’t have to be limited to these, just as the GDGPS base stations aren’t.

LEO satellite based GNSS is more challenging than a MEO network. Indeed, there seem to be a lot of ‘experts’ that rule it out as possible. They apparently couldn’t wait in order to rush into print to say so. This is reminiscent of other ‘experts’ who said BEVs weren’t commercially possible, landing and re-use of rockets wasn’t possible, and LEO satellite broadband comms isn’t possible, or isn’t possible with low latency. Each time challenges have been overcome and they have been proved wrong, or to have had an agenda. I see LEO GNSS as no different.