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

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

3. 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

4. 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.

5. 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

6. 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: Hz (altim noise = 0.75 cm): 20 Hz LRM = 3.2 cm (sqrt averaging)

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

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

9. 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

10. 1 Hz ”Noise”
Entire profile
North part (42->)
L2 – Proc
3.5 cm
2.4 cm (no jumps)
L2 – Proc
2.8 cm
3.1 cm
RADS 2012

11. 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)

12. 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

13. 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

14. DTU12? Gravity

15. Coastal regions heavily improved
DTU 2012 Beta
Coastal regions heavily improved
Preliminary testing using
1.6 years of NOAA C2 LRM
Data
All > 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 ( m)
3.22
3.27
3.52
3.23
Green ( m)
3.53
3.49
3.79
3.51
Yellow ( m)
4.33
4.30
4.35
Red/Pink (>1000 m)
4.83
4.82
4.69
4.85
All
3.72
3.81
4.09
3.83

16. Summary ESA L2 LRM are very good for marine gravity
Processor upg ( ) 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).