Using GPS satellites to detect tsunamis via ionospheric ionization waves | Hacker News Reader

Using GPS satellites to detect tsunamis via ionospheric ionization waves

earthdata.nasa.gov

126 pointsGedxx44 comments

> The GNSS-based Upper Atmospheric Realtime Disaster Information and Alert Network (GUARDIAN) is an ionospheric monitoring yadayadayadascience

That's a pretty impressive number of scrabble points for a project acronym, and I guess bonus points for building that acronym on top of another acronym (GNSS = Global Navigation Satellite System, generic term for America's GPS).

I know government projects have a long, storied history of such wordplay. Anyone have any fun stories on coming up with a really elaborate one? I wonder if chatGPT will unleash a new era of creativity with these...

I still think the USA PATRIOT Act is the most diabolical.

“Uniting and Strengthening America by Providing Appropriate Tools Required to Intercept and Obstruct Terrorism”

Also… goodbye privacy but, you know… Patriotism!

My favorite backfired backronym is definitely the CAN-SPAM act [1].

I get that it's a play on "canning something", but as a strong believer in nominative determinism, it comes as no surprise to me that companies in fact still can spam me.

[1] https://en.wikipedia.org/wiki/CAN-SPAM_Act_of_2003

I love the creativity that goes into naming these projects in the geosciences! I've been a part of several of these projects myself, and have used data and collaborated with teams from many more.

One point of clarification: GNSS is a term that has broader application than you describe, as it encompasses constellations from other countries and political associations as well. For example:

* Galileo - European Union's GNSS system, named after the astronomer * BeiDou - China's GNSS system * GLONASS - Russia's GNSS system * JAXA - Japan's GNSS system

One backronym that I liked from my time doing my PhD was RELAMPAGO, which is a Spanish word for "lightning," but which some group of scientists gave this definition: "Remote sensing of Electrification, Lightning, And Mesoscale/microscale Processes with Adaptive Ground Observations". It was a very cool campaign that produced a ton of amazing data, and catalyzed many dissertations (including one of my close friend's).

GPS was the more generic term until US Navstar-GPS became such the default that it aquired the generic term. And eventually dropped the Navstar, officially.

Sort of a reverse Xerox/Google/Velcro situation.

Maritime Augmented Guidance with Integrated Controls for Carrier Approach and Recovery Precision Enabling Technologies: MAGIC CARPET.

GPS was originally NAVSTAR or Navstar-GPS (names helped, going back to when LORAN was state of the art and other competing dod navigation systems were being developed, like the Navy's TRANSIT sats).

But lots of old school pros like surveyors will still refer to it as 'Navstar', which has resurged with the introduction of competing GNSS systems from other countries. Especially if you want to avoid the GPS/GNSS confusion.

There's debate about whether NAVSTAR itself was ever an acronym/backroom, or just a name.

"NAVigation System using Timing And Ranging"

> I wonder if chatGPT will unleash a new era of creativity with these...

Can confirm I've already submitted at least one bid with a chatGPT-derived acronym, complete with an X for scrabble points.

I'll chalk that one up as an argument in favour of "LLMs will take over the world", as coming up with cool acronyms involving sciency-sounding words genuinely might be one of the most important jobs in the space industry

When I worked at defence it was an important part of getting funding that your project had a catchy acronym. Bonus points for combining multiple TLAs

Gnss includes other systems like glonass and beidou, not just GPS.

Frankly GPS is so outmoded as to be a questionable source of meaningful data for things like ionospheric metrics. Beidou is light years ahead in both speed and fidelity.

I thought GPS was generic and NAVSTAR was the “brand”.

GNSSbUARDIAN

Tangentially related ... I've heard that earthquakes can be detected, perhaps even prior to the event, by changes in the ionosphere.

But last I checked, the serious geologists I worked with had an almost religious aversion to "precursor signals". Has the state of the art changed there?

There are probably some interesting things going on with earthquakes that we don’t understand yet.

On the other hand, earthquakes are very notable events and probably cause all kinds of observational biases that make people interpret other phenomena as related. “Earthquake lights” might just be transformers shorting out, for example.

I think it’s widely accepted that many animals, including dogs, go nuts leading up to an earthquake. They can detect it, so something must be measurable.

This sounds very similar to the method discussed in this podcast to detect rocket launches: https://www.armscontrolwonk.com/archive/1216884/detecting-mi...

This is amazing work. I don't quite understand how they are detecting the tsunamis though. They mention that this works via significant displacements of air. Is the amount of air displacement in the open ocean statistically significant for detection or is it being detected via a slightly different source.

Pretty astounding, isn't it? I don't see a paper, but there was a webinar [1]. There's a technical synopsis at 8:00. The phenomenon they're measuring is actually signficant. It's the total number of (free?) electrons between the satellite and the receiver. Typically its about 10^12 electrons/m^3 (@8:00 in video). The disturbance from the 2011 earthquake and tsunami was, if I'm reading the movie/chart correctly, about +/- 1 TECU, which is 10^16 electrons/m^3 (@10:40). The water elevation may only be a few feet in open ocean, but it's over a vast area. That's a lot of power.

They're measuring it by looking for phase differences in the received L-band (~2GHz) signals, rather than amplitude. That eliminates lots of noise. And they're looking for a particular pattern, which lets you get way below the noise floor. For example, the signal strength of the GNSS (GPS) signal itself might be -125 dBm, while the noise level is -110 dBm [2]. That means the signal is 10^-12 _milliwatts_, and the noise is about 30 times larger. But by looking for a pattern the receiver gets a 43 dB processing boost, putting the effective signal well above the noise.

[1] https://www.youtube.com/watch?v=BEpZmRPPWFo

[2] https://www.nxp.com/docs/en/brochure/75016740.pdf

To expand upon this:

>> They're measuring it by looking for phase differences in the received L-band (~2GHz) signals

The "L-Band signals" are GNSS signals, for example GPS L1 and L2, which use a carrier wavelength of 1575.42 MHz and 1227.6 MHz, respectively. Both L1 and L2 signals are emitted at the same time, but experience differing levels of delay in the ionosphere during their journey to the receiver. The delay is a function of total electron content (TEC) in the ionosphere and the frequency of the carrier wavelength. Since we already know precisely how carrier frequency affects the ionospheric delay, comparing the delay between L1 and L2 signals allows us to calculate the TEC along the signal path.

Another way to think of it is: we have an equation for signal path delay with two unknowns (TEC, freq). Except, it is only one unknown (TEC). Use two signals to solve simultaneously for this unknown. Use additional signals (like L5) to reduce your error and check your variance.

OK, the "typically 10^12 TEC" vs. a +/- 1 TECU (10^16 TEC) disturbance was really bugging me. I think the slide has an error, or there's an apples/oranges issue. The +/- 1 TECU looks to be consistent, but the typical background level is "a few TECU to several hundred" [1]. A Wikipedia page has shows the levels over the US being between 10 - 50 TECU on 2023-11-24, and says that "very small disturbances of 0.1 - 0.5 TEC units" are "primarily generated by gravity waves propagating upward from lower atmosphere." [2].

[1] https://www.swpc.noaa.gov/phenomena/total-electron-content

[2] https://en.wikipedia.org/wiki/Total_electron_content

The way I understood it is that the wave amplitude of a tsunami isn't typically huge, say only a few feet[1], however it's wavelength is. Thus while normal waves can have a much larger amplitude, they're easily filtered by a low-pass filter.

edit: The atmosphere between the surface and the ionosphere forms a natural low-pass filter as well. I imagine typical ocean waves as seen by us are way too high-frequency to make it up to the ionosphere.

There are other, natural disturbances in the ionosphere, such as traveling planetary waves[2], but they have a significantly longer wavelength. As such the paper[3] mentions filtering them out using a high-pass filter.

In the paper they show some preliminary results trying to invert the parameters in order to estimate the height of the tsunami based on the measured ionosphere disturbance based on synthetic data, and the baseline amplitude is 10cm (4 inches), which the model comes quite close to.

[1]: https://earthobservatory.nasa.gov/blogs/fromthefield/2014/04...

[2]: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/201...

[3]: https://link.springer.com/article/10.1007/s10291-022-01365-6

If you can imagine this, can you imagine the forms of sensing-by-proxy that’s possible that’s kept under wraps by various Skunkworks orgs?

i believe its the pressure waves propagating through the air itself is pushing the ionosphere upwards, inducing a detectable signal from fluctuations in the electron density.

Open-access paper here: https://link.springer.com/article/10.1007/s10291-022-01365-6

Very interesting article. It also illustrates how dangerous unit conversions are especially combined with thousands separators: 20000km (GNSS satellite altitude) is NOT 12.4 miles.

Does anyone know if it would be feasible nowadays to just use starlink (or other LEO satellites) as a GNSS constellation? Even without precise onboard-clocks, would it not be possible to just bounce clock signals from earth as long as latency is known?

Yes, this is actively being explored, see for example this for Iridium: https://investor.iridium.com/2021-05-24-Iridium-Makes-Strate...

> as long as latency is known?

Ah, but how do you know latency, without accurate clocks? :) Accurate clocks facilitates measuring latency, which is used to calculate distance. :)

True, but the accurate clock doesn’t need to be aboard the satellites as long as they have continuous (low-jitter) connectivity to the timing source, and Iridium and newer Starlink satellites do.

How does NTP work? GPS is stratum 0. Starlink could be effectively stratum 1.

Elon tweeted a while back they could do positioning, but it’s just not a high priority right now

starlink is in a low enough orbit that it isn't ideal (atmospheric drag will make the position harder to accurately predict)

I’d be surprised if that were an issue.

Starlink probably could learn the position of every satellite very accurately based on latency and signal strength and angle data from all the ground stations alone if they wanted to, if they don’t already.

you would need nanosecond accurate clocks in the satellites and groundstations

Wow that would be great! The high number of casualties during the 2002 tsunami mostly was due to a lack of early alerting, as not enough tsunami buoys were deployed. This could have save so many lives.

Like the other commenter, I assume you mean the 2004 Indian Ocean Tsunami. The lack of early warning was not due to the lack of buoys, but due to the lack of public and institutional knowledge about Tsunamis itself. Few people had even heard of the word Tsunami (including much of the governments) and there were certainly none in the living memory. There were a few TV documentaries about Tsunamis in Japan, but few people had seen those either. Internet had just started penetrating these regions at that time and disaster preparedness was poor. Consequently, there were no tsunami buoys around at all.

Many areas that suffered the waves had earlier felt the earthquake that caused it (including here in India) and the sea receded in some places. Almost no one was alarmed by this or understood what was to follow. Even the news media didn't initially understand what they were reporting.

That situation has changed now. Sufficient buoys are deployed along with public warning systems. People are also much more sensitive to the warning signs. I don't think anybody will be caught by surprise like that in the future. That said, any new concepts and advancements are welcome. The more the merrier.

PS: We're coming up on the 20th anniversary of that disaster - the 26th of this month. Please remember the more than 128K people that perished on that day. Some of them were younger than much of the HN audience. May the victims and the lessons of that day never be forgotten.

I presume you mean 2004

I don't get it, are the figures here typos?

> Given that GNSS satellites typically travel in medium Earth orbit, approximately 20,000 km or 12.4 miles above the surface, GNSS systems are well suited for detecting fluctuations in ionospheric density. > > Further, because ground stations can detect GNSS satellites from such a significant distance (up to 1,200 km),...

Should that be 2000 km and 1240 mi?

No, GNSS constellations are indeed usually in MEO, not LEO.

Using MEO means that they'll need fewer satellites for global coverage at acceptable elevation angles than they would in LEO, and since navigation signals are very low data rate, power is usually not a limiting factor either.

I believe around 19,500km is the correct number from some past work.

Having a crazy stable transmitter and clock source allows to detect a lot of interesting (side) effects.

So is this technically a side-channel attack against nature?

I would rather call it misuse of GPS ;-)

Live map here - https://guardian.jpl.nasa.gov/

I don't understand, something happens every day. It's been running for years. Are the predicting it our not?

Have they timestamped a prediction/analysis beating other methods and had it confirmed afterwards?

How often is it wrong, how often is it right when they make calls real time?

They aren't really predicting events at all.

They are detecting events quickly.

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