Sentinel-6 and Sea Level Rise

By Rud Istvan,

Nearly two years ago (January 2019) over a pleasant lunch, Charles urged me to examine whether the NASA satellite altimetry (satalt) measurements of sea level rise (SLR) were fit for purpose. I eventually provided a longish, somewhat technical guest post concluding they were NOT based on NASA’s newest ‘bird’, Jason-3, while also showing that NASA PR was provably obscuring that. Satalt simply does not correspond with the BEST SLR tide gauge measurements by a factor of about 1.5x. That is not rounding error; it is a big climate data problem.

Jason-3’s replacement, Sentinel-6a, has just completed ground testing and is scheduled to launch November 2020 from Vandenberg AFB. A bit of background. These satalts are necessarily in low earth orbit (LEO). That means they encounter slight atmospheric drag (from Earth’s thermosphere), so their orbit deteriorates, so they do not last long: an average of only 5 years operationally. Jason-3 launched in early 2016. This 4Q2020 launch will allow about 6 months of calibration overlap before Jason-3 must be decommissioned thanks to its orbital decay. Going to be close, because the Sentinel-6a launch was originally scheduled for 1Q2020. Conceptual image of Sentinel-6a below is courtesy of ESA. The ‘roof like’ projections are its solar cells. The downward projections are antennas aimed at Earth. And its odd house like configuration explains why thermospheric drag is such a big LEO orbit problem.

Sentinel-6 is actually two identical satellites, (a) and (b), both to be launched into the same LEO orbit as Jason-3 at 1336km mean original altitude. (b) will sit in inventory and launch in ~2025 to replace (a) for a total mission life to about 2030. Both were built in Europe by ESA, incorporating a couple of JPL.NASA developed instruments. Both will ride NASA launches.

NASA’s big press release yesterday on completion of Sentinel-6 operational ground testing says it will provide ‘centimeter’ precision: “measuring down to the centimeter for 90% of the world’s oceans.” Is that true? Dunno, and there is no way to tell yet because after several hours of researching, neither ESA nor NASA have evidently provided a detailed description of the accuracy and precision of their coming Sentinel-6 data products. But we know some relevant stuff…

It is true that Sentinel-6 contains new plus improved instruments. The five specifically named instruments for SLR by both NASA and ESA are:

  • Poseidon-4, a new higher resolution synthetic aperture radar altimeter,
  • AMR-C, a new ‘climate quality’ multi-frequency radiometer for humidity,
  • GNSS-POD, a GPS guided POD (positional orbit detector),
  • LRA, a laser retroflector array for POD,
  • DORIS, a ‘Doppler orbitotography and Radio positioning integrated by satellite’ for POD—whatever that actually is and supposedly does.

This NASA/ESA technobabble requires both translation and then contextual positioning. Recall the three main Jason-3 accuracy/precision weaknesses from my previous technical Jason-3 post: orbital decay, humidity retarded radar altimetry, and ocean surface wave height.

The higher resolution synthetic aperture radar altimeter, at higher pulse rates, enables a rough estimation of wave heights, at least those above the 2 meter arbitrary average assumed by the Jason-3 signal processing. That helps some.

The multi-frequency radiometer (different frequencies for different altitudes) provides a better estimate of humidity retarding effects to the main altimeter.

The last three instruments collectively provide a more robust triangulation of the inevitable orbital decay over time.

So, it is conceptually possible that Sentinel-6 could achieve a statistically robust 1cm sea height resolution. But nowhere that I can today find is this ‘fact’ explained by published technical specs. There simply is nothing specific on line (yet?) about Sentinel-6 overall ‘data product’ accuracy and precision. Deliberate?

Two final thoughts

First, the best long record calibrated (to vertical land motion) tide gauge estimates of SLR are about 2.2mm/year, with NO acceleration, AND closure. So, even if the new Sentinel-6 1cm claims are true, they are still not fit for purpose by a factor of about 4x SLR mm/year. And this satalt only lasts ~5 years.

Second, if Sentinel-6 really is this good, then it should (inaccurately) find about 2.2mm of SLR per year, proving Jason-3 was a goof as its published tech spec showed. Personally, I think the chances of that data driven scientific outcome is near zero, because the Jason-3/Sentinel-6 calibration overlap period enables any necessary Sentinel-6 data processing algorithm ‘adjustments’. We already have such ‘adjustments’ shown many different ways for NOAA/NASA surface temperature UHI homogenization. (See essay ‘When Data Isn’t’ in ebook Blowing Smoke for multiple compelling examples.)

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September 7, 2020 at 12:30PM

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