[mgsouth]

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

[Qqqwxs]

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.

[mgsouth]

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