1. Satellite Altimetry Serge Desjardins & Devon Telford (author) EC/NOAA Bi-Lateral Marine Workshop Ann Arbor, MI May 3-5, 2016

2. Motivation Way back… in August 2014, a forecaster in Gander had asked me where one could find significant wave height measurement obtained by satellite altimetry data. He believe that this data would provide observations to verify and initialize our sea state forecasts where observational data is sparse if not existent, such as the Labrador sea. After a quick Google of the internet and a poll of our colleagues we determined that although many had seen images of measured significant wave height in presentations no one knew where, or if, these images where available in near real-time to be operationally useful. But again after a quick Google of the internet we were able to find where the Jason-2 satellite data resided. So I thought I’d try and make Todd some images.

3. So For… I’ve been looking at 3 Satellites –Jason-2 –Saral (Satellite with ARgos and Altika) –CryoSat-2

4. Jason-2 Orbit Flies on the same ground-track as Jason-1 and the original T/P with a 254 pass, 10-day exact repeat cycle.

5. Jason-2 Data 3 different data ‘families’ are available for Jason-2 –The Operational Geophysical Data Record (OGDR) is produced within 1-2 hours of the satellite overflight. It has the lowest quality data and the most missing data. However, it is useful for time-critical applications. –The Interim GDR, IGDR, is produced within 1-2 days of over flight. The orbital quality is far better. –The science-quality "final" GDR is produced with a 60-day time lag. So for operational uses I’m downloading the ORGR data

6. Jason-2 Data Processing centers perform functions such as science data processing, data verification and orbit determination. There are three levels of processed data: –Telemetry data (raw data or level0) –Sensor Data Records (engineering units or level1) –Geophysical Data Records (geophysical units or level2) Real-time processing and distribution (EUMETSAT and NOAA) –The operational geophysical data record (OGDR) is available with a latency of 3-5 hours. –Note: that this is a non-validated product.

7. Jason-2 Data

8. Ocean Waves (sea state bias) what is measuring Sea-state effects are an intrinsic property of the large footprint radar measurements. The surface scattering elements do not contribute equally to the radar return; troughs of waves tend to reflect altimeter pulses better than do crests. The accuracy of sea state bias algorithms remains limited and continues to be a topic of research. For a typical significant wave height (SWH) of 2 meters, the sea state bias is about 10 cm, and the error (bias) in the sea state bias correction is approximately 1-2 cm. Resolution: ~7 km along track (1 Hz)

9. Ku vs c band Radar “In general the problem with the c band is that there are more stringent restrictions on how much power the instruments can emit at, in order to avoid interference with terrestrial applications in the same band.” “The ku band is dedicated to satellite activity so the afore-mentioned restrictions on the power are unnecessary.” “On the other hand the ku band is higher frequency and therefore more susceptible to rain fade, but.. its practical impact isn't too severe.” “In practice, in some of the documentation which I consulted when I first started working on this, there was mention of some operational problem with the c band (at least on some platforms?) and so I've only ever worked with the ku band. Upon receipt of your email I took a closer look at the swh's determined in the two bands and there can be quite a difference.” -from correspondence with Syd Peel

10. Rain_Flag Liquid water along the pulse's path reduces the energy returned to the altimeter, mainly at Ku band. In heavy rain, there are competing effects from attenuation and surface changes. The small scale nature of rain cells tends to produce rapid changes in the strength of the echo as the altimeter crosses rain cells. Both effects degrade the performance of the altimeter. Data contaminated by rain are rare (most are located in the west equatorial pacific), flagged and should be ignored (see rain_flag parameter).

11. So what is the SARAL Satellite? Built on the heritage of the TP and Jason-1/2 missions Satellite ofr Argos and AltiKa (SARAL) Launched in 2013

12. SARAL Orbit Flies on the same ground-track as the ENIVISAT with a 1002 pass, 35-day exact repeat cycle.

13. SARAL Data Same basic idea as the Jason-2 But a larger data latency for the Operational Geophysical Data Record (OGDR), 7-9 hrs. Same sea state bias effects Jason-2. Even more sensitive to rain and cloud then Jason-2.

14. And what is the CryoSat Satellite? CryoSat is a 3.5 year radar altimetry mission, launched on 8 April 2010, to monitor variations in the thickness of the Earth’s marine ice cover and continental ice sheets. The CryoSat ‐ 2 satellite replaces the original CryoSat which was lost due to a launch failure in October 2005

15. CryoSat Orbit CryoSat will orbit at an unusually high inclination, of about 92 degrees. The repeat cycle for CryoSat is 369 days and 5344 orbits.

16. SIRAL Altimeter Low Rate Mode (LRM) is pulse-limited and insures that the returning echoes are uncorrelated and then averaged to reduce speckle. Synthetic Aperture Radar (SAR) mode is used over flat areas of sea ice. SAR Interferometry, or SARIN, is used across ice sheet margins where the ice surface may be sloping.

17. CryoSat Data Level 2 available once per day. So, a data latency of ~2 to 26 hrs. Using Ku band (similar as the others) Presently download using the Radar Altimetry Database System (RADS) which is running on whxlab3.dart.ns.ec.gc.ca

18. 2) Work Flow: BRAT (to create a static image) Both Brat operations create new files with coarser resolution. For example… Jason-2 samples at 1 Hz or ~ 7 km resolution. This is pretty fine and hard to see unless you are zoomed right it. By decreasing the resolution and plotting the arithmetic mean of the samples the image become more usable. The data computation could be set to the maximum value instead.

19. 3) Work Flow: GrADs If the new file contained no data with the domain defined by the Brat operation files the process stops there. Otherwise, the Grid Analysis and Display System (GrADs) is called to create a PNG image file from the truncated NetCDF file and the SHP files that contain the marine areas.

20. The images are available at: http://whxlab3.dart.ns.ec.gc.ca/~telfordd/Altimetry/ Hour Minute Second that the Pass finished 3) Work Flow: GrADs Name of the Satellite Name of the variable Year Month Day that the Pass finished Color Bar/Legend in meters

21. Concluding Remarks Images currently available at http://whxlab3.dart.ns.ec.gc.ca/~telfordd/Altimetry/ http://whxlab3.dart.ns.ec.gc.ca/~telfordd/Altimetry/ Most recent data at top-left corner in the Domain folder [NW_Atl, NE_Pac, HSB_MBL and Arctic] This was just a ‘quick and dirty’ means of getting the data to the forecasters and in doing so other impotent can be made. Perhaps the addition of ice flags to the variable the BRATs.

22. References http://www.nodc.noaa.gov/SatelliteData/jason/ https://www.eumetsat.int/cs/idcplg?IdcService=GET_FILE&dDocName=pdf_conf_p_s9_36_rosm ordu_p&allowInterrupt=1&noSaveAs=1&RevisionSelectionMethod=LatestReleased https://www.eumetsat.int/cs/idcplg?IdcService=GET_FILE&dDocName=pdf_conf_p_s9_36_rosm ordu_p&allowInterrupt=1&noSaveAs=1&RevisionSelectionMethod=LatestReleased http://earth.eo.esa.int/brat/html/data/toolbox_en.htm OSTM/Jason-2 Products Handbook SARAL/AltiKa Products handbook CryoSat product handbook Brat 3.1 User Manual RADS version 3.1 User Manual and Format Specification

23. So what is the Jason-2 Satellite? The Jason-2 satellite launched 20 June 2008 and is the latest in a series of Ocean Surface Topography Missions (OSTM) designed to observe ocean circulation, sea level rise, and wave heights. R

24. Poseidon-3 Altimeter The two-frequency solid-state altimeter, measuring range with accurate ionospheric corrections, is derived from the two-frequency Poseidon-2 altimeter embarked on Jason-1 mission and from the SIRAL altimeter. It operates at 13.575 GHz (Ku-band) and 5.3 GHz (C-band). R

25. AltiKa Altimeter Mono-frequency Ka band (35.75 GHz) It’s functions are based on proven concepts and already developed sub-systems, as it inherits from the Siral (CYROSAT mission) and the Poseidon3 (Jason-2 mission). R

26. SIRAL Altimeter SIRAL has been designed with three different operational modes for sea ice, sloping ice sheets and flat ice sheet interiors (also suitable for operation over open ocean) The mode of operation is selected from a mask of geographical zones that is updated every two weeks to allow for changes in sea ice extent.

27. 3) Work Flow: GrADs The images are available at: http://whxlab3.dart.ns.ec.gc.ca/~telfordd/Altimetry/