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Sea level satellite mission: Jason-2 calibration

Discussion in 'Environmental Discussion' started by bwilson4web, Dec 17, 2012.

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  1. bwilson4web

    bwilson4web 03 and 10 Prius

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    Hi,

    Having worked on the Landsat and supported other satellite missions, one comes to appreciate the marvels of these machines. So when someone calls the 20 year missions to measure sea level "a lie," it makes sense to review what is going on. There have been and continues, three, sea level measuring satellite missions:
    • Topex/Poseidon (1993-2002)
    • Jason-1 (2002-2008)
    • Jason-2 (2008-continues)
    The primary technical challenge is how is sea level measured down to millimeter resolution from a low earth orbiting satellite, ~150 miles at ~17,500 mph. The first challenge is knowing the exact satellite location:
    • Doppler Orbitography and Radio-positioning Integrated by Satellite (DORIS)
      DORIS, supplied by CNES, uses a ground network of 60 orbitography beacons around the globe, which send signals, in two frequencies, to a receiver on the satellite. The relative motion of the satellite generates a shift in the signal's frequency (called the Doppler shift) that is measured to derive the satellite's velocity. These data are then assimilated in orbit determination models to keep permanent track of the satellite's precise position (to within three centimetres) in its orbit.
    • Global Positioning System Payload (GPSP)
      The NASA-supplied GPSP (Global Positioning System Payload), previously referred to as TRSR-2 (Turbo Rogue Space Receiver-2), uses the Global Positioning System (GPS) to determine the satellite's position by triangulation. At least three GPS satellites are needed to establish the satellite's exact position at a given instant. Positional data are then integrated into an orbit determination model to continuously track the satellite's trajectory.
    • Laser Retroreflector Array (LRA)
      The LRA, supplied by NASA, is an array of mirrors that provide a target for laser tracking measurements from the ground. By analysing the round-trip time of the laser beam, we can locate where the satellite is in its orbit and calibrate altimetric measurements.
    Source: EUMETSAT - Satellites - Instruments

    Three systems are used because each has non-overlapping limits and accuracies. Knowing the satellite location, the next challenge is measurement instrumentation:
    • Poseidon-3 Altimeter
      Poseidon-3 (supplied by CNES) is the mission's main instrument, derived from the Poseidon-2 altimeter on Jason-1. It is a compact, low-power, low-mass instrument offering a high degree of reliability. Poseidon-3 is a radar altimeter that emits pulses at two frequencies, 13.6 GHz (Ku-band) and 5.3 GHz (C-band), and analyses the return signal reflected by the surface. The signal round-trip time is estimated very precisely, to calculate the range after applying corrections. The primary goal of the dual-frequency operation is to provide a precise ionospheric correction. Besides a differential ionospheric path delay, Ku- and C-band signals are differentially and significantly affected by geophysical quantities, such as atmospheric precipitation and sea surface roughness.
    • Advanced Microwave Radiometer (AMR)
      The AMR, supplied by NASA, measures radiation from the Earth's surface at three frequencies (18, 21 and 37 GHz). These different measurements are combined to determine atmospheric water vapour and liquid water content. Once the water content is known, it is possible to determine the correction to be applied for radar signal path delays.
    But just as the National Atomic Clock requires a periodic "leap second," calibration for any instrument is always a challenge. For example, the Landsat missions I worked on required "ground truth" surveys to figure out the calibration constants. So Jason-2 calibration is on-going and the raw mission data is being corrected:
    http://www.aviso.oceanobs.com/fileadmin/documents/data/tools/JA2_GDR_D_release_note.pdf

    We'll go over more of these technical, calibration efforts. This is where the meat of the matter resides . . . not in the false claims of non-technical, "cut-and-paste" skeptics.

    Bob Wilson
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  2. JimboPalmer

    JimboPalmer Nadir of Wrongness

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    Whew, I thought you were going to propose a satellite that orbits at sea level. It is hard to conceive of a worse plan.

    Sorry, carry on.
  3. mojo

    mojo Senior Member

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    Let me remind you that you are the one who made a false claim " no one has yet to find it is not rising."
    When the foremost expert on sea levels says the satellites are wrong,maybe you need to go back to the drawing board.
    When positioning is within 3 centimeters and you are making millimeter measurements,isnt that a rather large margin for error?


  4. Zythryn

    Zythryn Senior Member

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  5. bwilson4web

    bwilson4web 03 and 10 Prius

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    Then provide his supporting evidence, the facts and data. Bring something technical to the party . . . if there is anything there. But these false claims stood out:


    Source: Are sea-levels rising? Nils-Axel Morner documents a decided lack of rising seas « JoNova: Science, carbon, climate and tax

    While we wait, I found another fascinating report on Jason-2 calibration:http://www.aviso.oceanobs.com/fileadmin/documents/calval/validation_report/J2/BilanCalval_J2_Cycle_151.pdf

    One of the things that happens when one satellite replaces another is they fly them in formation and compare their data:


    pp 16.

    So we have continuity of data, consistent reports of rising sea level, and on-going calibration efforts that are necessary to deal with instrument aging and better understandings of orbital mechanics (aka., the gravity fields), position information (aka., GPS), and ground processing. These calibration efforts don't care about what the satellite data reveals as long as it is as accurate as we can make it.

    Now if your source has some hard data . . . perhaps the source of the rumors or other credible, engineering comments about Jason-2 and the calibration efforts, bring it on. In the meanwhile, this data stands:
    [​IMG]
    Source: CU Sea Level Research Group | University of Colorado

    The current Jason-2 calibration effort is scheduled to be done by the end of this year. So we should see a slightly different graph in the 2008-2012 period. But the existing calibration reports provide credible, engineering data showing this is a real, global effect.

    Bob Wilson
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  6. fuzzy1

    fuzzy1 Senior Member

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    Maybe you should re-read your own references a little closer.


    Don't confuse the momentary error of a single reference system with the long term average error of three combined reference systems.
  7. bwilson4web

    bwilson4web 03 and 10 Prius

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    Climate deniers use a lot of silly tricks to spread their nonsense. In this particular case, several stand out:
    • 'somebody said' - an unidentified source is claimed to have revealed that climate data is false. Whether done from a 'cut-and-paste' warrior or claims by some anyone else, this is simply a way to inject a red-herring, a lie, and try to get it treated as fact. FOX news does this when they site someone else's false claim as if the story of the false claim has merit by itself. So we find "One of the keepers of the satellite record told Professor Mörner that the record had been interfered with to show sea level rising," which means nothing.
    • 'cherry picked data' - is when someone take a snippet of noisy data and tries to claim the opposite of what the total record shows. In this case, ~114 tide days out of a record spanning ~6,900 days, 1993-2012. We already know there is variability due to many different factors. The real world is not a smooth operation but with fits and bursts, the noise.
    • 'inflammatory rhetoric' - notice the use of thread titles with words like "lie" or "vs." or "proof" that attempt to get attention for their particular nonsense. Unfortunately, responding to these threads is counter productive because the activity gives the illusion 'something is there.' A better response is to start a fresh tread using cold, dispassionate, facts and data to carry the response. Let the cut-and-paste propagandist threads become the lonely, last leaf of fall, thread generator. We can have a lot more fun dealing with the facts and data and need not give a thread title with inflammatory phrases any further attention. After all, it worked so well for President Newt Gingrich . . . or did he win?
    If nothing else, the Internet has made detecting BS so much easier from the near constant practice.

    Bob Wilson
  8. 2k1Toaster

    2k1Toaster HID Guru

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    Bob, I love your posts, and the knowledge from them. I didn't know you actually worked on satellite tech, that is impressive!

    However, you cannot fight idiocy with science. It just doesn't work. It is like teaching Schrodinger's cat how to use his wave equation.
  9. mojo

    mojo Senior Member

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  10. bwilson4web

    bwilson4web 03 and 10 Prius

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    Apparently you don't understand, bring forth your facts and data. Not some worthless link cut-and-pasted from a trash site. Quote from that source and let's 'talk technical.'

    For example:
    [​IMG]
    Source: CU Sea Level Research Group | University of Colorado

    I have my facts and data and am very patient. So bring yours, if you have any.

    Bob Wilson
  11. mojo

    mojo Senior Member

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    Heres the actual data before its been "adjusted" as in your graph.
    Why bother to make any measurements at all, if you are just going to alter them to suit your agenda?

    http://ruby.fgcu.edu/courses/twimberley/EnviroPhilo/PastRecords.pdf

    "Fig. 2. Sea level changes in mm as recorded by TOPEX/POSEIDON between October 1992 and April 2000: raw data before any filtering or
    sliding mean average. The variability is high, in the order of F5– 10 mm. From 1993 to 1996, no trend is recorded, just a noisy record around
    zero. In 1997, something happens. High-amplitude oscillations are recorded; a rapid rise in early 1997 at a rate in the order of 2.5 mm/year,
    followed by a rapid fall in late 1997 and early 1998 at a rate in the order of 1.5 mm/year, and finally, in late 1998 and 1999, a noisy record with
    unclear trends. The new factor introduced in 1997 and responsible for the high-amplitude oscillations, no doubt, is the global ENSO event,
    implying rapid redistribution of oceanic water masses (characteristic for mode III in Table 1). This means that this data set does not record any
    general trend (rising or falling) in sea level, just variability around zero plus the temporary ENSO perturbations."

    Attached Files:

  12. JimboPalmer

    JimboPalmer Nadir of Wrongness

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    (I am not a climatologist, nor do I play one on TV)

    Two interesting differences between the graphs.
    One is for 7 years, one is for 20, when one is looking for a trend, the longer the baseline the better. (If I had a 5000 year baseline, I would have an opinion on climate change)

    One seems to be measuring the sea level, the other measuring the rate of change in the sea level. Only measuring the change from the previous year lowers the slope quite a lot compared to measuring the cumulative change.

    It is possible neither graph is 'wrong' until humans try to distort what it means. As I read it the seven year graph is NOT measuring the difference in sea level, but the rate of change in the slope of the sea level rise. Sea level that rose uniformly year to year would be 'flat' on that graph.

    Have I misread how they are labeled?
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  13. bwilson4web

    bwilson4web 03 and 10 Prius

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    Thank you! Now we can talk technical:


    I opened the PDF file and quickly found "Fig. 2":
    [​IMG]
    Well at least the time scale begins to make more sense than this version:
    [​IMG]
    The "Tidal cycle no." is totally bogus. The note about the time interval is OK except the X-axis is not the same units.

    So let's see what we find:
    [​IMG]
    Source: CU Sea Level Research Group | University of Colorado

    We can see some part of the 1998 pulse in both but it at least makes more sense than the earlier, 'Titdal cycle no." chart. Still, let's see what might be going on in the calibration report:http://www.aviso.oceanobs.com/fileadmin/documents/calval/validation_report/J2/BilanCalval_J2_Cycle_151.pdf



    Source: http://www.aviso.oceanobs.com/fileadmin/documents/calval/validation_report/J2/BilanCalval_J2_Cycle_151.pdf , pp 5.

    There are two charts of which one is especially interesting:
    [​IMG]
    NOTE: in the original PDF it has to be significantly expanded to make a useful screen capture.

    So here we see two calibration adjustments, decreasing the distance of the difference of travel of the two signals used to measure the altitude. Unfortunately, we don't know from the text what "cycle" means. However, JPL says:


    Source: OSTM - JASON 2 | PO.DAAC
    This is interesting because with the cycle times and this chart of the calibration changes we can determine the rate of change and see if it matches the 3.1 mm/year projected from the 20 year period of satellite measurements.

    So let's do the math:
    2011/09/25 02:26:06.643 - cycle 119 begins​
    2008/07/12 01:20:05.1082 - cycle 1 begins​
    --------------​
    1,170.046 days​

    90.53 @1 cycle​
    90.445 @120 cycle​
    --------​
    0.085 cm / 1,170.046 days​

    0.85 mm / 3.203 years​

    0.265 mm / year << 3.1 mm / year​

    Although we can find a somewhat linear calibration curve for the altimeter, the value of the calibration offset over time does not come close to the reported 3.1 mm /year.

    The calibration report is available describing the metrics and outlining the methods. So this begins to suggest we need to look elsewhere for the error. Perhaps the original 'claim' of the unadjusted data.

    Soon enough, we'll go to the data repository and see if we can replicate the Nils-Axel 'raw data' analysis. Only this time, we'll use the Jason-2 data since that is the one we have calibration reports and constants.

    Bob Wilson




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  14. tochatihu

    tochatihu Senior Member

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    I hope that it seems relevant to again mention Church and White 2011 in Surveys in Geophysics
    DOI: 10.1007/s10712-011-9119-1

    Abstract:
    We estimate the rise in global average sea level from satellite altimeter data for 1993–2009 and from coastal and island sea-level measurements from 1880 to 2009. For 1993–2009 and after correcting for glacial isostatic adjustment, the estimated rate of rise is 3.2 ± 0.4 mm year−1 from the satellite data and 2.8 ± 0.8 mm year−1 from the in situ data. The global average sea-level rise from 1880 to 2009 is about 210 mm. The linear trend from 1900 to 2009 is 1.7 ± 0.2 mm year−1 and since 1961 is 1.9 ± 0.4 mm year−1. There is considerable variability in the rate of rise during the twentieth century but there has been a statistically significant acceleration since 1880 and 1900 of 0.009 ± 0.003 mm year−2 and 0.009 ± 0.004 mm year−2, respectively. Since the start of the altimeter record in 1993, global average sea level rose at a rate near the upper end of the sea level projections of the Intergovernmental Panel on Climate Change’s Third and Fourth Assessment Reports. However, the reconstruction indicates there was little net change in sea level from 1990 to 1993, most likely as a result of the volcanic eruption of Mount Pinatubo in 1991.

    If you have difficulty accessing the full publication online, contact me or your friendly local librarian.

    The (global) tidal gauge data are accumulated here
    Permanent Service for Mean Sea Level (PSMSL)

    and their sea-level explorer is an interesting tool to see how the patterns differ across the globe. Isostatic rebound in Fennoscandia really stands out.

    That is Morner's home turf (Sweden, specifically), and via google scholar you can find his many publications on geological and other earth-system processes that affect local measurements. His descriptions of those processes make good sense to me . Except perhaps connecting earth orbital slowing to sea-level rise and to nothing else, but I claim no expertise on that. I cannot say whether Morner's expertise extends to deconvoluting travel-time estimates for Topex Poseidon and all of those. It seems quite a different field.

    On a related thread, I and at least two others linked to a recent Science article where the (about) 3 mm/year was partitioned into thermal expansion and melt of grounded ice. The thermal expansion (as near as I can tell) matches other published studies on ocean heat content. Willis, Levitus, Nuticelli are some names that pop up in relation to OHC.

    Overall, it would appear that if one needs to discard 3 mm/year, one will have to discard a lot of research from a lot of different areas. Tough sledding. Not a path I'd choose.
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  15. bwilson4web

    bwilson4web 03 and 10 Prius

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    One thing we've learned from TOPAX/Poisidon/Jason missions is the world-wide variability in local sea levels. Not just storms but also larger patterns. The Western Pacific is especially interesting for wind and wave actions suggesting those areas are most likely to suffer the earliest effects of global sea level rise. As Katrina and Sandy showed, variability is deadly.

    I have to admit I'm enjoying learning more about this particular mission . . . actually if you ever have a chance to be involved in a satellite mission . . . go for it! . . . and bring your "A" game. Manned missions are not so interesting but science based, satellite missions are awesome because they advance the art and explore the truly unknown and risky areas.

    Bob Wilson
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