[HN Gopher] Using GPS satellites to detect tsunamis via ionosphe...
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Using GPS satellites to detect tsunamis via ionospheric ionization
waves
Author : Gedxx
Score : 99 points
Date : 2024-12-17 14:50 UTC (3 days ago)
(HTM) web link (www.earthdata.nasa.gov)
(TXT) w3m dump (www.earthdata.nasa.gov)
| unsnap_biceps wrote:
| 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.
| hotshot1001 wrote:
| 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.
| magicalhippo wrote:
| 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
| mgsouth wrote:
| 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
| mgsouth wrote:
| 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
| Qqqwxs wrote:
| 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.
| xattt wrote:
| 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?
| magicalhippo wrote:
| Open-access paper here:
| https://link.springer.com/article/10.1007/s10291-022-01365-6
| ygouzerh wrote:
| 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.
| IncreasePosts wrote:
| I presume you mean 2004
| ElectRabbit wrote:
| Having a crazy stable transmitter and clock source allows to
| detect a lot of interesting (side) effects.
| floatrock wrote:
| So is this technically a side-channel attack against nature?
| ElectRabbit wrote:
| I would rather call it misuse of GPS ;-)
| hbrav wrote:
| This sounds very similar to the method discussed in this podcast
| to detect rocket launches:
| https://www.armscontrolwonk.com/archive/1216884/detecting-mi...
| floatrock wrote:
| > 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...
| sailfast wrote:
| 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!
| lxgr wrote:
| 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
| goochphd wrote:
| 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).
| beerandt wrote:
| 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.
| beerandt wrote:
| 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"
| xattt wrote:
| I thought GPS was generic and NAVSTAR was the "brand".
| yieldcrv wrote:
| GNSSbUARDIAN
| sumofproducts wrote:
| Maritime Augmented Guidance with Integrated Controls for
| Carrier Approach and Recovery Precision Enabling Technologies:
| MAGIC CARPET.
| notahacker wrote:
| > 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
| nimbius wrote:
| 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.
| myrmidon wrote:
| 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?
| loxias wrote:
| > 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. :)
| lxgr wrote:
| 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.
| adgjlsfhk1 wrote:
| starlink is in a low enough orbit that it isn't ideal
| (atmospheric drag will make the position harder to accurately
| predict)
| lxgr wrote:
| Yes, this is actively being explored, see for example this for
| Iridium: https://investor.iridium.com/2021-05-24-Iridium-Makes-
| Strate...
| jvanderbot wrote:
| 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?
| aaron695 wrote:
| 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?
| ted_dunning wrote:
| They aren't really predicting events at all.
|
| They are _detecting_ events quickly.
| jinnko wrote:
| 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?
| tonyarkles wrote:
| I believe around 19,500km is the correct number from some past
| work.
| lxgr wrote:
| 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.
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