Sea Level Rise (SLR) Satellite Altimetry—Fit for Purpose?

Reblogged from Watts Up With That:

by Rud Istvan

ctm and I were having lunch recently near our mutually admired South Florida coral reef system, and over conversation we started speculating about ARGO. I brought up Jason2 SLR as an analog. WUWT readers can see my 2016 WUWT guest post: ‘Sea Level Rise, Acceleration, and Closure’ for details. That ctm lunch has inspired a lot more volunteer WUWT ‘sciency’ research on whether the most modern climate research instrument systems are fit for purpose. This post covers satellite altimetry measured sea level rise (SLR). The short answer is NOPE. The eventual companion post whose results are TBD, because ‘It’s complicated, folks’, and will cover ARGO. Dunno any ARGO answer(s) yet.

There are strong evidentiary reasons to think satellite altimetry does NOT accurately represent SLR change over time. The two most irrefutable observational reasons are:

(1) Satellite altimetry measured trends are about 1.5x higher than differential GPS, (vertical land motion) corrected long record tide gauges (about 3.4 versus about 2.2 mm/yr)

(2) the dGPS tide gauge estimates roughly close, while Jason2 satellite altimetry estimates definitely do NOT. Per my previous above referenced guest post, ‘closure’ is the simple arithmetic that SLR must approximately equal thermosteric rise, as hotter seawater expands in volume, plus ice sheet losses (land based ice when melted adds ocean water), while all other contributions, such as ground water extraction are arguably de minimus.)

So, is the most recent ‘satalt bird’, Jason3, fit for purpose? Satellite altimetry uses radar signal returns reflected off a wavy ocean surface to estimate sea level from the timing of the signal from generation to receipt. This is no different in principle than any other range estimating radar system. And as many military and commercial aviation uses evidence, radar ranging generally IS fit for purpose.

Except, military cruise missile ranging is meters between Syrian aircraft bunkers, not millimeters. Commercial aviation aircraft avoidance is kilometers between planes, not millimeters. But sea level changes are measured in millimeters/yr. That is a different accuracy/precision ball game, and the logical essence of this post.

Rather than a bunch of footnotes and links necessary for ‘whack-a-mole’ guest posts like my recent ‘Antarctic SLR contributions’, this guest post simply extracts irrefutable images and numeric values from the official Jason3, (the newest satalt bird) NASA reference products and mission specs. The official ‘product’ documents are available at for any WUWT reader wanting to double check. The related Jason3 mission/physical instrument specs are available at This guest post uses Jason3 product handbook version 1.5, issued 9/17/18.

Jason3 was launched into polar orbit on 1/17/2016.


It overlaps Jason2, and interestingly shows significantly less SLR in its overlap period. According to NASA, Jason3 instruments and processing algorithms correct for ‘known’, (and noted in my Blowing Smoke ebook SLR essay ‘PseudoPrecision’) Jason2 deficiencies: wave height, sigma naught, tropospheric, and ionospheric humidity. Jason3’s many new instruments now have a spatial aperture ‘pixel’ resolution of 11.2km x 5.1km, allowing closer calibration to land altitude reference pixels in order to better estimate temporal orbital decay.

The following schematic from the official Jason3 NASA information illustrates only some of the data processing problems Jason3 supposedly now ‘overcomes’.


The smaller Jason3 aperture improves its reference orbital accuracy. True. But probably not the processing algorithms for the reference Earth ellipsoid; thanks to geology Earth’s gravity field is anything but a uniform reference ellipsoid. GRACE gravimetric data shows it is a lumpy mess; which means that even though water seeks its own level, ocean seawater is NOT level across a lumpy planetary gravity field.


So, what does the latest NASA information say about Jason3’s algorithmically processed radar return data? Jason3 product manual §1.4.4 says all distance units are reported in units of 1/10 millimeter. WOW! Sure sounds fit for purpose!?!

Not quite. Following is NASA Jason3 ‘product’ manual Table 2.3.1.


For those that are NASA table challenged, the key number is IGDR actual for Total Sea Surface Height hHeightHeighight RMS, (Listed as RSS in the table above is, as noted on the associated EU Jason3 site [it’s a joint mission], just a NASA typo) precision of 3.3 cm. Not mm! This is still an improvement over Jason2, which had an SLR RMS pixel precision of 3.4 cm.

This is defined by repeated pass aperture over the same site ‘pixel’ on the lumpy Earth ellipsoid. I know of no statistics that can reduce a minimum repeatable precision error of >3 cm to an ‘average accuracy’ of 0.1 mm without a ginormous error bar, which NASA ‘conveniently’ DOES NOT provide.

An error term digression is perhaps useful for those who are not long time WUWT readers or Judith Curry “uncertainty monster” cognoscenti. There are two basic error types: precision and accuracy. The simplest layman’s explanation is from target shooting. A tight group is precision. A group on the bullseye is accuracy. An easily understood general illustration is:


NASA Jason3 information say it has BOTH precision and accuracy problems. Its ‘grouping’ precision is 3.3 cm, deceitfully reported to 0.1mm. Its accuracy is 1.5x high off what dGPS corrected long record tide gauges report. For climate purposes it is in quadrants 1/2 rather than 3/4. NOT good.

One further not-so-little satalt factoid. The Jason3 instrument drift spec (column GDR goal, last line) is identical to Jason2, ≤ 1mm/year. So the SLR acceleration that Jason2 ‘sees’ that Jason3 does not (yet) is likely just ‘in spec’ instrument drift between the two.

Ineluctable conclusion: current satellite altimetry measurements of SLR are NOT fit for climate purpose.

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