Jason-3 range measurement over inland water?

[tsaiallen]

Dear all,

I am a newbie to altimetry data, now I want to use Jason-2/3 to calculate the time-series inland water level. However, I found out there is almost no range measurement (range_ku) over land regions but only oceans. Did I use the wrong data? Any suggestion is appreciated.

P.S. I am using data
JA3_GPN_2PdP022_154_20160918_151841_20160918_161454

Regards,
Allen

[jhausman]

Hi Allen
The default algorithms for altimetry are designed for the ocean. You can get inland measurements, but you'll need to use the SGDR/GPS data with the waveforms, as that has the 20 hz signal. The GDR/GPN only has 1hz. If you're interested in lakes check out https://ipad.fas.usda.gov/cropexplorer/ ... reservoir/. The time series have already been calculated for multiple lakes and reservoirs.

[tsaiallen]

Dear jhausman,

thanks for the explanation! Yes I know the great G-REALM project, but I would like to calculate it myself by applying the correction formula to derive corrected orthometric height of lake water level.
So you mean I must use the waveform data of SGDR, but not the "range_20hz_ku" field of SGDR?

P.S. I did found more data points of "range_20hz_ku" field stored in SGDR than GDR. However they are still not continous measurements and have many void value. Is it normal?

Thanks
Allen

[jhausman]

Hi Allen
You can use the range_20hz_ku field. That will give you better values than using the 1hz signal. But it will still contain some land contamination. The best way to do it is to use the waveforms. That way you can actually see what parts of the signal are contaminated and remove them. However that technique does require some expertise in knowing what the waveforms should look like over land and water. For now the 20hz should do. If you find that you are getting large errors from land contamination then you may need to research the technique I mentioned earlier.

[tsaiallen]

Hi jhausman,

thank you, I did find more comprehensive records in SGDR! But I wonder why there is a difference between range_20hz_ku field of SGDR and GDR? As I checked the handbook of Jason-3 it said SGDR is just GDR adding waveforms values?

Also, now I am using Jason-2 data to get denser observations; however, when I use the same formula I used for Jason-3 to calculate the rough water level of inland water, the value is not reasonable. Do you know the reasons?

alt_20hz - range_20hz_ku - geoid

Thank you
Allen

[jhausman]

Hi Allen
Since the SGDRs contain the waveforms, the high rate information can also be included. That's why the 20hz signal is included.

Jason-2 is currently in a geodetic orbit, so it depends on when you are getting the data. If it is in the nominal orbit, the current one Jason-3 is in then you should be seeing similar errors. If you are trying to use Jason-2 data October 2016 or later, that will be the geodetic orbit and the errors will be greater.

[tsaiallen]

Thanks jhausman! I have another question, what is the common way to compare/unify different orthometric heights calculated based on different ellipsoids (such as EGM96 of Jasons and EGM2008 of Sentinel-3)?