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.)
I’m still waiting on OCO-II revelations and I’m cynical about any new satellite measurements because of it. All the FUD bestowed on current satellite temperature measurements, even though they correlate with balloon instruments, proves my point. If it doesn’t match their bias they’ll either ignore it or make one that will.
Rud ==> Even if the new satellite could be accurate to 1 cm (previous versions were 2-3 cm) — that is for a stationary surface. Like a baseball field with a GPS target in the middle.
As you and I know full well, the surface of the seas are not stationary – they are not smooth — they are not at a constant level.
The adjustments made for any one sat measurement of a single point are at least one order of magnitude greater (in some cases multiple orders of magnitude greater) than the ANNUAL change claimed to being measured. Tide levels, waves, chop — not to mention the occasional just plain odd “bump” –are all being “controlled for” in ad hoc manners.
Satellite sea level measurements may help us in some way, but they will not be able to tell us how much the surface of the seas are rising or falling during this historical period when the magnitude is believed to be in the single-digit mm range.
We’ve seen that post glacial rebound is fast enough that water gauges along the east coast disagree markedly on how much sea level rise there is. One tiny technical detail is that we are using GPS to measure the post glacial rebound. example
Given that the Earth’s surface isn’t static, my first question would be, what are these satellites using for a reference? My second reaction would be that there’s too much math involved in getting the data. Math always involves assumptions and the tiniest mistake in any of the assumptions can increase the errors greatly. Anyway, we need a robust explanation for why the satellites disagree with the tide gauges.
I don’t understand the need for an expensive satellite solution when we have a cheap ground based system that works IE tide gauges.
In order to get centimeter level sea level measurements, you are going to have to know the altitude of the satellite to a value significantly less than a centimeter. My guess is you will have to know the altitude to within a tenth of a centimeter.
That simply is not possible. Heck, knowing it to within a centimeter isn’t possible.
So to sum it up. It doesn’t matter how precise the capability of a satellite or instruments are in taking measurements if the people compiling the data and crunching the numbers are not committed to producing accurate results.
So, even if the new Sentinel-6 1cm claims are true
3.4 mm rise per year, with a measurement that is only accurate to within 10mm.
At least it is better than the current model whose measurement is only accurate to within 25.4mm.
Don’t worry about satellite calibration, Rud, I have a fail-safe technique. When I have to wade through seawater to get to my favorite outdoor restaurant, the Avalon, along Ocean Drive in South Miami Beach, I will send you an alert. Until then everything else is background noise. You’re welcome.
121 in LA we are all going to die.
I have a great scientific interest in all things SLR related …
Persistent acceleration in global sea-level rise since the 1960s
https://www.nature.com/articles/s41558-019-0531-8
A Spatially Variable Time Series of Sea Level Change Due to Artificial Water Impoundment
https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2020EF001497
Isn’t this a measurement where we need to know both the accuracy and resolution of the measurements? The measurements might have errors but if these are repeatable (systemic) then we might be able to detect rate of SLR?
And we’re forgetting some of the predictions of SLR over a century exceed 1 metre – 10 cm in a decade. In which case even this junk should be able to give us some evidence either way.
Satellite orbital paths are not on a set of sky-train tracks. Add to that the hubris of believing that they can measure an utterly uneven surface such as the oceans to 1cm accuracy. This is a waste of money to even send up there. Although, living in the Adjustocene era as we do, I’m sure there will be firm data claims from NASA.
Well looks like they’re not in agreement with your starting point Mr Istvan:
From ESA : “Records show that, on average, global sea level rose by 3.2 mm a year between 1993 and 2018, but hidden within this average is the fact that the rate of rise has been accelerating over the last few years. Taking measurements of the height of the sea surface is essential to monitoring this worrying trend – and the Sentinel-6 mission is on the way to being ready to do just this.”
Hidden also in a corner they also say this : “On average, sea level has been rising at a rate of 3.2 mm a year
since 1993, but there are regional differences within this trend.”
With 4.3 billion euros spent in the period 2014 to 2020 I hope it’s all worth it.
The image looks like a pitched roof house.
The incredible AGW industry that just keeps on giving. Thank you Mr and Mrs taxpayer.
The water levels in the open ocean goes up and down with the tides, high and low pressure systems passing overhead, waves that vary from nothing to 10 meters or more, wind blown surges, heating due to subsea volcanoes and a variety of other things. Couple that with a satellite that also wobbles due to gravitational changes in the Earth below it and constant drag.
Add in the fact it only measures a spot on the Earth once a day and we expect it to give us a robust sea level measurement of the entire oceans to one centimeter (or less)? I think not.
“DORIS, a ‘Doppler orbitotography and Radio positioning integrated by satellite’ for POD—whatever that actually is and supposedly does.”
Doppler Orbitography and Radiopositioning Integrated by Satellite or, in French, Determination d’Orbite et Radiopositionnement Integre par Satellite is a French satellite system used for the determination of satellite orbits and for positioning. It uses 50-60 ground based beacons transmitting on 401.25/2036.25 MHz and satellite based receivers with a precision of about 10cm. Basically, it’s sort of an “inverted” GPS with the reference transmitters on the ground and the receiver on the satellite
Even though the precision of any single measurement is about 10cm, they get to average a LOT of measurements. And the satellite movement is heavily constrained by Newton’s laws which should help a lot in data fitting by ruling out impossible/improbable solutions. MAYBE they can manage sub 1cm radial average error.
Jason 1 also had something called Blackjack which was intended to try to determine position via GPS. If that worked out, it’s probably integrated with their DORIS system. GPS should probably be more accurate for satellite users than for ground users because satellite users shouldn’t need to worry about tropospheric or (most?) ionospheric phenomena
Tangentially, when you get down to cm range, the whole “satellite position” thing becomes a little nebulous. You’re probably looking at some “position” related to the radar antenna or receiver hardware, not at the center of mass of the platform. And you probably need to worry about the orientation of the satellite/radar antenna relative to the Earth (Vehicle Attitude)
All in all, I’m glad I never had to worry about this sort of stuff
Some years ago on a flight to Madeira I saw a huge area of smoothed ocean surface, literally tens of thousands of square miles. It has been suggested that this was the result of phytoplankton oil/surfactant release. Wave breaking was suppressed up to what I judged to be about Force 4.
Question. If wave suppression took place under one of the satellites, what would be the result on the output? Humanity is altering enough of the ocean’s biome to alter the surface — nitrogen runoff, dissolved silica, poisonous effluent would all change the balance between calcareous phytos, diatoms, DMS producers. Would alteration of wave heights change the algorithm?
JF.
“…gauge measurements by a factor of about 1.5x. “ out compared to the best available.
That sounds really good to me for government funded operation!
😉
“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’.”
Gosh, do you really think it’s that corrupt? If so, it’s all very depressing. Just the reverse of the way science and data collection should work. We will never move ahead.
In the above article, Rud Istvan states as the first of his 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.
Well, my understanding it that neither precise triangulation-based surveying nor WAAS GPS can provide orthometric vertical elevation of land to 1 cm vertical accuracy over km-scale distances. In fact, there is a major effort presently underway, the NOAA-National Geodetic Survey’s “Gravity for the Redefinition of the American Vertical Datum” (GRAV-D) project to provide much-improved topographic elevation measurements for most of the US . . . but to only 2 cm accuracy, which itself is about a factor of ten better than existing topographic elevation accuracy (Ref: https://www.ngs.noaa.gov/GRAV-D/ ) This website provides a link to a free pdf download of the GRAV-D project plan, wherein beginning on page 1 there is a good description of the difference between reference ellipsoid vertical height (which GPS determines) and reference geoid orthometric vertical height (“colloquially—although not quite appropriately—referred to by the more common term ‘height above sea level’ “)
So given that past orthometric land elevations had measurement accuracy far less than 2 cm, I have to question Rud’s statements about (a) tide gauge estimates being calibrated to vertical land motion, especially to the extent of providing 0.1 mm/year precision, and (b) why assertions of 1 cm altitude accuracy over oceans for Sentinel-6 are criticized but claims of 2 cm orthometric vertical height of land at the end the GRAV-D project are not treated as being even more problematic (vis-a-vis closure with “calibrated” tide gauges).
Also, on the subject of comparing satellite measurements of SLR to tide gauge measurements, one must consider that the locations of tide gauges represents a very small and special subset of the extent of Earth’s oceans. Tide gauges are mostly located in shallow waters along coastlines. At these locations—depending on many non-instrumental factors such as wind fetch, ocean circulation patterns, local seabed topography, thermal gradients, tidal gradients in surrounding bays or channels or estuaries or rivers—the local “sea level” can vary greatly from the gravity-based vertical datum of “sea level” (the reference geoid). To the best of my knowledge, there has been no attempt to correct tide gauges for such conditions that may persist over months, years or decades.
Bottom line IMHO: considering the best of current science and applied technology, it is a joke to report any SLR numerical value to 0.1 mm/year resolution . . . moreover, reporting SLR to even 1 mm/year resolution needs 20 or more years of data from the exact same instrumentation at the exact same location with precise, periodic corrections for local land/seafloor subsidence and/or uplift. Very challenging to accomplish successfully.
Could you please provide a reference? I’ve been using these and would love to have additional references:
http://sealevel.colorado.edu/content/tide-gauge-sea-level
https://link.springer.com/article/10.1007/s10712-011-9119-1
I just sent this to Charles “The Moderator’ and after another glass of “Adult Beverages” I decided to plug it into a comment here :
I just read most of the thread about SLR and found it amusing that NASA touts 1 cm accuracy, hmmm… 10mm +/- and they want to resolve things to 2.?mm? Where did those jerks go to school?
Oh, I forgot “New Math”…
It’s not just the World Ocean that needs to be looked at; read this entertaining discussion on my favorite boating site. I took the pic in the last comment.
http://continuouswave.com/forum/viewtopic.php?f=4&t=4531
The Great Lakes are very much affected by isostatic rebound which makes basin wide analyses a crap shoot since the rebound can be very local.
Why can’t we know for sure, you ask, eh?
God Said……..
😉
Rud Istvan
” Satalt simply does not correspond with the BEST SLR tide gauge measurements by a factor of about 1.5x. ”
Could you show the data sustaining such a claim?
*
Let us compare available data:
– the most complex and most accurate PMSL tide gauge evaluation by Sönke Dangendorf & alii
– the NOAA statellite altimetry (TOPEX/Poseidon, Jason-1, Jason-2, Jason-3)
Here are the charts made out of the downloaded data (the sources will come out in a subsequent comment). All plots wrt the respective means in the common reference period 1993-2013.
1. Dangendorf 1900-2015 together with NOAA 1993-2020
https://drive.google.com/file/d/1J9cieS73FKJ_Wa8pLo6zVKPkz0VN-5cr/view
2. Dangendorf 1993-2015 together with NOAA 1993-2020
https://drive.google.com/file/d/1XPtw2L5OFkwDsju9GelE9WIKU0YdorgP/view
Trends in mm/year, 2 sigma
– Dangendorf 1900-2015 : 1.4 ± 0.01
– Dangendorf 1993-2013 : 2.7 ± 0.02
– NOAA sat: 1993-2013 2.6 ± 0.03
– NOAA sat: 1993-2020: 2.9 ± 0.03
The Dangendorf paper (behind paywall for me)
Persistent acceleration in global sea-level rise since the 1960s
Sönke Dangendorf, Carling Hay, Francisco M. Calafat, Marta Marcos, Christopher G. Piecuch, Kevin Berk & Jürgen Jensen
Nature Climate Change volume 9, pages705–710 (2019)
Abstract
Previous studies reconstructed twentieth-century global mean sea level (GMSL) from sparse tide-gauge records to understand whether the recent high rates obtained from satellite altimetry are part of a longer-term acceleration.
However, these analyses used techniques that can only accurately capture either the trend or the variability in GMSL, but not both. Here we present an improved hybrid sea-level reconstruction during 1900–2015 that combines previous techniques at time scales where they perform best.
We find a persistent acceleration in GMSL since the 1960s and demonstrate that this is largely (~76%) associated with sea-level changes in the Indo-Pacific and South Atlantic.
We show that the initiation of the acceleration in the 1960s is tightly linked to an intensification and a basin-scale equatorward shift of Southern Hemispheric westerlies, leading to increased ocean heat uptake, and hence greater rates of GMSL rise, through changes in the circulation of the Southern Ocean.
{ Though clearly shown in the spreadsheet’s quadratic fits, I’m not interested in any discussion about SLR acceleration. }
J.-P. D.
Rud Istvan (ctnd)
I preferred to post the data sources in this separate comment (problems with comments containing many links)
1. Data for Dangendorf 1900-2015
https://static-content.springer.com/esm/art%3A10.1038%2Fs41558-019-0531-8/MediaObjects/41558_2019_531_MOESM2_ESM.txt
2. Data for NOAA’s sat altimetry:
http://www.star.nesdis.noaa.gov/sod/lsa/SeaLevelRise/slr/slr_sla_gbl_free_txj1j2_90.csv
J.-P. D.