Global altimetric marine gravity field mapping The impact of Cryosat-2 Ole B. Andersen and L. Stenseng

Outline Satellite altimetry – State of the art gravity Cryosat-2 (what it offers). Noise estimation (20 Hz vs 1 Hz) for Gravity DTU12 Beta.

Altimetry – basic principles Satellite orbit Range (R) Altitude (H) Emitting a electromagnetic pulse ... Simply by measuring the return time h ellipsoidal height H the altitude (or orbital distance) of the satellite above the reference ellipsoid R the range from the satellite to the surface Sea surface Geoid Reference ellipsoid

Rule of Thumb: 1 cm Height difference over 10 km equals 1 mGal. sea mount Gravity field The (mean) Sea surface height mimicks the geoid Gravity from MDT corrected MSS (= Geoid) Laplace Slope of the sea surface ... 1 mGal -> Alongtrack heights differences need to be accurate to <1 cm Rule of Thumb: 1 cm Height difference over 10 km equals 1 mGal.

State of the Art: 321.400 obs Mean Max Dev Note KMS02 0.44 49.38 Std Dev. Max Dev Note KMS02 0.44 5.15 49.38 DNSC08 DTU10 0.39 3.91 3.82 36.91 36.89 Double Retrack SS V12.1 0.62 5.79 82.20 GSFC 00.1 0.68 6.14 89.91 NTU01 0.79 6.10 92.10 SS V16.1 SS V18.1 0.59 0.41 4.88 3.96 45.29 36.99 Retracked ERS1+GSA

Cryosat ”promises” Conventional DD Cryosat Factor of 2 on accuracy over conventional altimetry Accuracy independent of Sea state: Jason-1: 1 Hz (altim noise = 1.5 cm): 20 Hz = 6.4 cm C2 DD: 1 Hz (altim noise = 0.75 cm): 20 Hz LRM = 3.2 cm (sqrt averaging)

C2 will improve Shortwavelength scale signal Bathymetry -> oceanography Long wavelength signal still in error.

ESA Level 2 Products. (20 Hz data) Identical profile 2011 / 2012 SAR-In SAR LRM Hawaii

18 Hz Cryosat ”Noise” (Standard Dev) LRM SAR SAR-In >100 mGal 2011 V 2.1 2012 V 2.4 Large ”jumps” at mode shift SOUTH North ”Noise” (Standard Dev) LRM SAR SAR-In 2011 Proc 2.1 (20Hz) 6.9 cm 13.46 cm 27.1 cm 2012 Proc 2.4 (20 Hz) 10.1 cm 9.9 cm 14.3 cm

1 Hz ”Noise” Entire profile North part (42->) L2 – Proc 2.1 2011 3.5 cm 2.4 cm (no jumps) L2 – Proc 2.4 2012 2.8 cm 3.1 cm RADS 2012

ERS-1 GEOSAT ERM CRYO-2 Number 8936 13698 3783 12430 Std All / Alongtrk 8.9 (5.7) 7.2 (4.8) 5.2 (2.4) 7.5 (5.7)

Coastal regions heavily improved DTU 2012 Beta Coastal regions heavily improved Including CRYOSAT LRM data - 30% more data. Currently investigating ESA L2 LRM vs NOAA LRM data. Including Retracked L1 CRYOSAT SAR and SAR-In data Has to solve for ”mode jumps first” Updating all existing GM data. Ocean tide correction updated to GOT 4.7 SSB correction updated (N/A for Cryosat-2) Dynamic Atmosphere updated from IB alone Standards DNSC08/DTU10 DTU12 Beta Dry troposphere ECMWF Wet troposphere Radiometer Ionosphere Altimeter Dynamic Atmosphere IB (1013 mbar) MOG-2D_IB Ocean tides GOT 00.2 GOT4.7 Sea State Bias BM4 Non-PARAM

Retracking of Cryosat-2 Level 1b SAR waveform (black) with fitted five parameter beta retracker with exponential tail (blue) and surface estimate (red). COG – Center of Gravity Level 1b SAR waveform (black) with fitted five parameter beta retracker with exponential tail (blue) and surface estimate (red). Green is cut off for leading edge Center of Gravity

DTU12? Gravity

Coastal regions heavily improved DTU 2012 Beta Coastal regions heavily improved Preliminary testing using 1.6 years of NOAA C2 LRM Data All > 10.000 obs DTU12 DTU10 SAND 18 EGM2008 Purple (0-20 m DEPTH) 3.01 3.30 3.69 3.20 Dark Blue (20-50 m) 2.79 3.42 2.80 Light Blue (50-200 m) 3.22 3.27 3.52 3.23 Green (200-500 m) 3.53 3.49 3.79 3.51 Yellow (500-1000 m) 4.33 4.30 4.35 Red/Pink (>1000 m) 4.83 4.82 4.69 4.85 All 321.000 3.72 3.81 4.09 3.83

Summary ESA L2 LRM are very good for marine gravity Processor upg (2.1-2.4) removes spikes and is better for SAR-In But increases noise for LRM Jumps between modes are clear and needs to be handled Current Cryosat 2 data are comparable to retracked GM altimetry Aims at releasing DTU12 at “20 years progress of altimetry” Jason-1 EOL Geodetic Mission ? . Will still be a fantastic asset for marine gravity up to the 66 parallel (no Arctic here).