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Radar Altimetry Johnny A. Johannessen Nansen Environmental and Remote Sensing Center, Bergen, Norway. OUTLINE PRINCIPLES OF ALTIMETRY FROM SATELLITE HEIGHT TO SURFACE HEIGHT GEOPHYSICAL PARAMETERS AND APPLICATIONS FUTURE ALITMETRY.

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Presentation Transcript
slide1

Radar Altimetry

Johnny A. Johannessen

Nansen Environmental and Remote Sensing Center,

Bergen, Norway

slide2

OUTLINE

  • PRINCIPLES OF ALTIMETRY
  • FROM SATELLITE HEIGHT TO SURFACE HEIGHT
  • GEOPHYSICAL PARAMETERS AND APPLICATIONS
  • FUTURE ALITMETRY
slide3

OCEAN SURFACE QUANTITIES MEASURED FROM SPACE?

SEA ICE

WAVES

SEA ICE THICKNESS

CHLOROPHYLL

GEOID & MDT

SURFACE SALINITY

SURFACE CURRENT

NEAR SURFACE WIND

SURFACE TEMPERATURE

SEA LEVEL

ICEBERG

satellite altimetry coverage
Satellite altimetry coverage

TOPEX/Poseidon Sampling

Exact repeat orbits (to within 1 km)

  • Spatial coverage :
  • global
  • homogeneous
  • Nadir (not swath)
  • Temporal coverage :
  • repeat period
  • 10 days, T/P-Jason-1
  • 35 days ERS/ENVISAT

1 measure/1 s (every 7 km)

all weather (radar)

error budget for altimetric missions

orbit error

RA error

Ionosphere

Troposphere

EM Bias

Error Budget for altimetric missions

Centimeters

100

90

80

70

60

50

40

EMR

PRARE

30

TMR

GPS/DORIS

20

10

0

Jason

ERS

T/P

Geos 3

SEASAT

GEOSAT

principles of radar altimetry
Principles of radar altimetry.
  • Active radar sends a microwave pulse towards the ocean surface, f = 13.5 Ghz
  • Precise clock onboard mesures the return time of the pulse, t

t = 2d/c

Centimetre Precision (10-8)

from an altitude of 800 – 1350 km

  • Measures the backscatter power (wind speed)
  • Measures ocean wave height
slide7

x :

t :

Time to reach mid-power point :

Back slope :

Distance, R

antenna mispointing

Pu :

Energy of the pulse :

backscatter

Coefficient, s

SWH :

Leading edge slope :

Pb:

Instrument noise

Wave height

t

Physical parameters from the waveform

t = 2d/c

slide8

Pulse Limited Footprint

Full Area illuminated

stays constant

The full area has a radius

R=(2hcp)1/2

Position of pulse

t=T

t=T+p

t=T+2p

t=T+3p

Position of pulse

t=T

t=T+p

t=T+2p

t=T+3p

sea state effects
Sea State Effects

Electromagnetic bias

The concave form of wave troughs tends to concentrate and better reflect the altimetric pulse. Wave crests tend to disperse the pulse. So the mean reflecting surface is shifted away from mean sea level toward the troughs.

Mean Sea Level

Mean Reflecting Surface

sea state bias
Sea State Bias

Skewness bias

For wind waves, wave troughs tend to have a larger surface area than the pointy crests – the difference leads to a skewness bias.

Again, the mean reflecting surface is shifted away from mean sea level toward the troughs

The EM Bias and skewness bias (= Sea State Bias or SSB) vary with increasing wind speed and wave height, but in a non-linear way.

SSB is estimated using empirical formulas derived from altimeter data analysis (crossover, repeat-track differences and parametric/non-parametric methods). The range correction varies from a few to 30 cm. EM bias accuracy is ~2 cm, skewness bias accuracy is ~1.2 cm.

Empirical estimation of the SSB also includes tracker bias (depends on H1/3). .

atmospheric pressure forcing
Atmospheric Pressure Forcing

Evolving atmospheric pressure field with highs and lows leads to spatial and temporal variation of the sea level pressure

lows

low

high

- -

++

- - -

Sea surface

+++

Bottom pressure

Sea level rises (falls) as the low (high) pressure systems pass. The inverse barometer effect implies that 1 mbar of relative pressure change leads to a 1 cm sea level change

from satellite height to surface height
FROM SATELLITE HEIGHT TO SURFACE HEIGHT

Orbit errors in position of satellite

SSH = Orbit – Range – S Corr

  • Precision of the SSH :
  • Orbit error
  • Errors on the range
    • Instrumental noise
    • Various instrument errors
    • Various geophysical errors (e.g., atmospheric attenuation, tides, inverse barometer effects, …)
ssh geoid dynamic topography noise
SSH = Geoid + dynamic topography + «noise»
  • hg : geoid 100 m
  • hd : dynamic topography 2 m
  • hT : tides 1-20 m
  • ha : inverse barometer 1 cm/mbar
slide20

OCEAN DYNAMICS FROM ALTIMETRY

LARGE SCALE SSH ANOMALIES

MESOSCALE VARIABILITY

PLANETARY WAVES

SEA LEVEL CHANGE

slide22

Mesoscale variability

T/P

Jason-1

ENVISAT

Jason-1 + ENVISAT

slide24

Sea surface

--

+ + +

West

East

horizontal plane

Pressure force

North

Vertical plane

Pressure force

Coriolis force

West

East

South

Geostrophic Currents

Geostrophic Balance :

Horizontal gradients in the pressure field create a downgradient force. On a rotating earth this is balanced by the Coriolis force.

N Hemisphere : high P is to the right of the flow.

S Hemisphere : high P is to the left of the flow.

slide25

Geostrophic Currents from altimetry

B

A

With altimetry, we measure the sea surface height along a groundtrack. Geostrophic currents calculated from the alongtrack slope will be perpendicular to the groundtrack.

Groundtrack A

Groundtrack B

h’

h’

v’

v’

Groundtrack A perpendicular to slope : strong currents

Groundtrack B parallel to slope : weak currents

slide27

IMPORTANCE OF MAPPING FREQUENCY AND COVERAGE

EKE

estimated with 4 satellites missions

(Jason-1, T/Pi,ERS-2/ENVISAT,GFO)

Units are in cm2/s2

0

800

EKE differences

between 4 and 2 satellites

missions

Units are in cm2/s2

Courtesy of CLS

0

400

slide28

IMPORTANCE OF MAPPING FREQUENCY AND COVERAGE

Cyclonic eddy of the Gulf Stream. 2 ALTIMETERS LEFT

4 ALTIMETERS RIGHT Courtesy of CLS

slide29

PLANETARY WAVES

Surface Layer

(warmer, lighter)

Deep Layer

(cooler, denser)

slide30

Hovmuller diagrams and propagating Rossby waves

Sea Level Variance

Courtesy of Remko Scharroo, DEOS, TU Delft, NL

slide38

Computation of Mean Dynamic Topography (MSS - Geoid)

Compute the geoid relative to the TP ellipsoid and in the mean tide system

Substract the geoid from the mean sea surface

Apply a Gaussian filter with a 400 km width

MDTS

Geoid

MSS CLS01-EIGENGL04S

cm

m

geoid

From GUTS Study, Courtesy of Rio, 2007

slide42

T

H

A

N

K

Y

O

U

slide43

Principles of radar altimetry

Beam limited footprint < pulse limited footprint

Pulse limited

Beam limited

L

L:antenna size

:wavelength

d

d

L

= B/2d

B=d/L

(P/2)2 + d2 =

(d+ c2

P = 2(2cd)1/2

Pulse length = c

B

P