European Space Agency to measure Earth at millimeter scale | Hacker News Reader

European Space Agency to measure Earth at millimeter scale

theregister.com

79 pointspseudolus43 comments

This does not really seem to be about measuring the earth; it's about more accurate GNSS. Then if you have very accurate GNSS you can also get more accuracy in the ITRF.

Also to add context: there are many hurdles to hitting a 1mm accuracy for a satellite based positioning service.

One can already achieve accuracy of a few cm, but it's a painful process. You basically place a receiver somewhere, keep it there for a few hours, then download the pseudoranges, and post-process on a computer. In post-processing you correct for ionosphere error and satellite ephemerides (the satellites broadcast their rough location and clock readings, but you can download much more accurate data for these later to get a much better idea of where the satellites were). You need to do post-processing after some delay, because ionosphere readings and satellite ephemerides are not available immediately (the accuracy increases as you wait longer!).

You can also extend this with what's called "Precise Point Positioning" (PPP) using one ground-station that broadcasts an extra signal to moving receivers nearby, so there are ways to get higher accuracy on moving receivers, but it's still painful.

So probably 1mm will come soon to fixed ITRF monitoring stations (basically the ITRF tries to track tectonic movements, etc as those are at the mm level), but it's very far from coming to drones or cars.

PPP doesn’t utilise ground stations, and to achieve cm accuracy, you need to utilise phase information and solve the ambiguity problem

1mm accuracy almost certainly relies on ionosphere and troposphere observations at high temporal and spatial frequency. The process of post processing is essentially correct, but there is a point at which the ionosphere solution won’t get better without an increase in density of observations

Ah thanks for the corrections, I guess RTK is what uses the ground station/mobile receiver concept.

Could you track satellites by imaging them from earth? Some are visible to the naked eye at night. It seems like that would be instant and very accurate data.

I believe that's how satellites are tracked at all? Rather than use visible light, though, tracking stations most certainly use radar, and probably highly directional antennas.

I think they are actually measured with lasers, IIRC. Did a lot of research about GNSS when setting up an RTK base station, and used PPP to get the initial location.

Will this be available in consumer products like phones?

What could one do with such accuracy, and how to interpret this when sensor is larger than precision?

I wonder if you could make a completely offline location service by correlating the results of a local 3d scan of the ground to the global model?

Cruise missiles already do something similar. How big is a mm scale model of the globe?

It would almost completely negate the need for easily jammed and expensive to maintain gps satellites.

Apparently the US Navy has this kind of system for positioning of submarines to correct inertial navigation system errors. Allegedly they also have very good maps of large swaths of the ocean floor to make it useful.

Very accurately measuring gravitational anomalies is important for this too.

Plate tectonics would've been accepted much earlier if the seafloor magnetism maps hadn't been classified.

I imagine the earth isn't particularly static on the mm level.

True - buildings and exposed rock probably stay still, leaf litter and trees definitely don’t. You’d need to find enough of a stable reference to run your correlation - probably more useful in an urban area.

On a millimeter scale I expect weather/seasons will cause movement of the ground as the soil swells with water from rain, or water freezing.

Taller buildings can flex up to a few feet in each direction actually!

I believe tectonic plates generally move about 15mm a year - which is pretty fast when you think about it!

https://en.wikipedia.org/wiki/Plate_tectonics#Key_principles

> Plate motions range from 10 to 40 mm/year at the Mid-Atlantic Ridge (about as fast as fingernails grow), to about 160 mm/year for the Nazca Plate (about as fast as hair grows)

> I wonder if you could make a completely offline location service by correlating the results of a local 3d scan of the ground to the global model?

It's possible, see https://news.ycombinator.com/item?id=39507267

Baffling. The earth isn’t a perfectly smooth solid sphere that never deforms. It is a dynamic object being shaped by all sorts of forces. How do they guarantee 0.1mm error over a year. It must be limited to a small number of high value targets given the techniques used on the article, but still unclear. Just seems wildly complicated to get right.

Yep. The ground itself moves with the tides.. so it's never static. https://en.wikipedia.org/wiki/Earth_tide

With the gravity of the moon, and tectonics. It will be interesting to see what insights this will reveal. That all gets down to how frequently they sample areas. If anything, the gravitational impact on the planet beyond waves would be useful for augmenting seismic measurements/events.

I wonder how this experiment will take into account the expansion and contraction of the Earth - certainly at millimeter scales, this will be a factor? Is it not also true that some forms of weather systems can impact topography at those scales?

IIRC... the European geographic grid is tied to the continental plate, and there are sampling points around the perimeter of the continent whose positions are updated with cm-scale accuracy at least yearly.

My guess is that we will get final orbits and clocks to do PPP more quickly.

Today it's about a week.

I'm guessing we will get this at the speed we currently get rapid.

All guesswork... As a gnss guy who doesn't have knowledge of this project

there was a geologist saying more or less: "volcaninc eruptions in the winter are inconvenient, the snow on the ground doesn't allow fine measurement of the surface deformation, wind and waves swamp the seismometers, and clouds tend to obscure satellite photos". Long story short, it will be a very nice fair weather map.

Aren't there privacy implications for this? Can I opt to have my property blurred like I can for Google Street View?

I don't think they are capturing imagery across the globe.

This effort is more about devices (sensors on the ground) having more accurate positioning information.

Doesn’t your local government have precise records of property boundaries and building dimensions and locations already?

Is the LEO-PNT part of this funding really going to make indoor localization a reality? That seems like a big deal.

Why?

A personal reason: I'm building a global simulator for shadows [1]. I started with SRTM based elevation data which allowed me to cast mountain shadows. I'm now offering some LiDAR data which also includes buildings, structures and trees (~50cm resolution). People use my website for sun mapping their gardens, real estate, farms, events, photography, academic research, solar systems. It would be nice to amp up that resolution by another factor.

[1] https://shademap.app

Edit: after reading the article it seems less about surveying the Earth's surface and more about accurate triangulation like GPS :(

Making better maps seems like an obvious usecase, and by "maps" I mostly mean technical documentation. I assume most EU countries have a requirement that new building permits need to come with precisely measured outlines, this could help increase their precision and/or make the measurements cheaper.

In case we need to shrink the Earth for Intergalactic travel....Somebody knows something...

Sometimes we need to give awesome tools to creative people and see what they come up with, even when we don't understand the implications ourselves.

I think millimeter accurate GPS is one of those tools. It has the power to enable so many things. Things we cannot imagine without using the tool itself.

40 cm vs 1 mm is the difference between landing a quadcopter smoothly or crashing it into the ground.

20 cm vs 1 mm is the difference between a robot navigating through a door or crashing into the wall.

20 cm vs 1 mm is the difference between mowing the lawn or cutting through your flower bed.

Unfortunately it doesn't look like we'll be getting millimeter accurate GPS anytime soon. The Genesis satellite might be a prerequisite though.

The government artificially limits civilian gps resolution. Millimeter gps would be a boon for hellfire R9X type explosive free ballistics.

This hasn’t been true for 24 years. Selective Availability was discontinued in May of 2000.

https://en.m.wikipedia.org/wiki/Global_Positioning_System

Why reveal your entire hand?

- The satellite will accomplish this [precision] by having the usual main Earth-measuring techniques co-located on board [satelite navigation, interferometry, laser ..] When used together, the ESA expects to be able to correct for biases inherent in each technique.

- An updated International Terrestrial Reference Frame (ITRF) will have immediate benefits on satellite-based systems, impacting Galileo-enabled applications in fields like aviation, traffic management, autonomous vehicles, positioning and navigation

- The space agency added that meteorology, natural hazard prediction, monitoring climate change effects, land management and surveying – as well as the study of gravitational and non-gravitational forces as fields – would also see benefits.

- ...

> Why?

So somebody can tell if the neighbour’s fence infringes on their property line.

Sea level rise.

To make another one to verify if 42 is indeed the answer to everything. ;)

They will discover that the earth isn't constant in size at that scale.

As ever, there's an XKCD for that:

https://xkcd.com/2170/

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