GEOF334 – Spring 2010 Radar Altimetry Johnny A. Johannessen Nansen Environmental and Remote...

GEOF334 – Spring 2010

Radar Altimetry

Radar Altimetry

Johnny A. Johannessen Nansen Environmental and Remote Sensing Center,

Bergen, Norway


Radar Altimetry

OUTLINE

PRINCIPLES OF ALTIMETRY

FROM SATELLITE HEIGHT TO SURFACE HEIGHT

GEOPHYSICAL PARAMETERS AND APPLICATIONS

FUTURE ALITMETRY


Radar Altimetry

OCEAN SURFACE QUANTITIES MEASURED FROM SPACE?

SEA LEVEL

CHLOROPHYLL

NEAR SURFACE WIND

WAVES

SURFACE TEMPERATURE

ICEBERG

SEA ICE

SURFACE CURRENT

SURFACE SALINITY

SEA ICE THICKNESS

GEOID & MDT


Radar Altimetry

• Spatial coverage :

TOPEX/Poseidon Sampling

- global

- homogeneous

• Temporal coverage :- repeat period10 days, T/P-Jason-135 days ERS/ENVISAT

1 measure/1 s (every 7 km) all weather (radar)

Satellite altimetry coverageSatellite altimetry coverage

- Nadir (not swath)

Exact repeat orbits (to within 1 km)


Radar Altimetry

Error Budget for altimetric missions

Error Budget for altimetric missions

0

10

20

30

40

50

60

70

80

90

orbit error

RA error

Ionosphere

Troposphere

EM Bias

100

Centimeters

Geos 3 SEASAT GEOSAT ERS T/P Jason

EMR

PRARE TMR

GPS/DORIS


Radar Altimetry

h Measures the backscatter power (wind speed)h Measures ocean wave height

h Active radar sends a microwave pulse towards the ocean surface, f = 13.5 Ghz

h 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

Centimetre Precision (10-8)from an altitude of 800 – 1350 km

Principles of radar altimetry.

Principles of radar altimetry.


Radar Altimetry

Energy of the pulse : backscatterCoefficient,

Pu

Back slope : antenna mispointing

Leading edge slope : Wave height

SWH :

Time to reach mid-power point :

Distance, R

Instrument noise Pb:

t

Physical parameters from the waveformPhysical parameters from the waveform

t = 2d/c


Radar Altimetry

Pulse Limited FootprintPulse Limited Footprint

t=T t=T+p

t=T+2p t=T+3p

t=T t=T+p

t=T+2p t=T+3p

Position of pulse

Position of pulse

Full Area illuminated stays constant

The full area has a radiusR=(2hcp)1/2


Radar Altimetry

Pulse Limited FootprintPulse Limited Footprint


Radar Altimetry

Sea State Effects

Sea State Effects

Electromagnetic biasElectromagnetic 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


Radar Altimetry

Sea State BiasSea State BiasSkewness biasSkewness 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). .


Radar Altimetry

Atmospheric Pressure ForcingAtmospheric Pressure Forcing

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

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

+++

++

lows

- - -

- -low

Sea surface

Bottom pressure

high


Radar Altimetry


Radar Altimetry

SINGLE PULSE STRUCTURESINGLE PULSE STRUCTURE


Radar Altimetry

MULTIPLE PULSE AVERAGINGMULTIPLE PULSE AVERAGING


Radar Altimetry

FROM SATELLITE HEIGHT TO SURFACE HEIGHT FROM SATELLITE HEIGHT TO SURFACE HEIGHT

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 = Orbit – Range – Corr Orbit errors in position of satellite


Radar Altimetry

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


Radar Altimetry

OCEAN DYNAMICS FROM ALTIMETRYOCEAN DYNAMICS FROM ALTIMETRY

LARGE SCALE SSH ANOMALIES

MESOSCALE VARIABILITY

PLANETARY WAVES

SEA LEVEL CHANGE


Radar Altimetry

Coverage, interpolation and gridding to SSH anomalies

Coverage, interpolation and gridding to SSH anomalies


Radar Altimetry

ENVISAT

Jason-1

Jason-1 + ENVISAT

T/P

Mesoscale variabilityMesoscale variability


Radar Altimetry

Along track SSHAlong track SSH


Radar Altimetry

Geostrophic CurrentsGeostrophic Currents

Sea surface

---- + + ++ + +

Pressure forcePressure force

Vertical plane

West East

East

horizontal plane

West

Pressure forcePressure force Coriolis forceCoriolis force

North

South

Geostrophic BalanceGeostrophic 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.

fvPxfuPy f=−=⎧⎨⎪⎪⎩⎪⎪ =11 2 20

0ρ∂∂ρ∂∂ θ(1)() sinΩ


Radar Altimetry

Geostrophic Currents from altimetryGeostrophic Currents from altimetry

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

A

Groundtrack A

h’h’

v’v’

B

Groundtrack B

h’h’

v’v’Groundtrack A perpendicular to slope : strong currents

Groundtrack B parallel to slope : weak currents


Radar Altimetry

Global Observations – Geostrophic Current


Radar Altimetry

EKEestimated with 4 satellites missions (Jason-1, T/Pi,ERS-2/ENVISAT,GFO)

Units are in cm2/s2

EKE differencesbetween 4 and 2 satellites missions

Units are in cm2/s2

0 800

0 400Courtesy of CLS

IMPORTANCE OF MAPPING FREQUENCY AND COVERAGE


Radar Altimetry

Cyclonic eddy of the Gulf Stream. 2 ALTIMETERS LEFT4 ALTIMETERS RIGHT Courtesy of CLS

IMPORTANCE OF MAPPING FREQUENCY AND COVERAGE


Radar Altimetry

Surface Layer(warmer, lighter)

Deep Layer(cooler, denser)

PLANETARY WAVES


Radar Altimetry

Hovmuller diagrams and propagating Rossby wavesHovmuller diagrams and propagating Rossby waves

Sea Level Variance

Courtesy of Remko Scharroo, DEOS, TU Delft, NL


Radar Altimetry

Global Sea Level Change


Radar Altimetry

SPATIAL TRENDS


Radar Altimetry

EL NINO 1997


Radar Altimetry

Waveform and SWHWaveform and SWH


Radar Altimetry


Radar Altimetry

Wind speed retrievalsWind speed retrievals


Radar Altimetry

Altimetry TimelineAltimetry Timeline


Radar Altimetry

Apply a Gaussian filter with a 400 km width

MSS CLS01-EIGENGL04S

Computation of Mean Dynamic Topography (MSS - Geoid)

Substract the geoid from the mean sea surface

MDTS

m cm

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

Geoid

From GUTS Study, Courtesy of Rio, 2007geoid


Radar Altimetry

CRYOSAT 2 - Altimeter Thickness Observations

aa

1h2h3h4h5h

Δ = -h hh

Δ 3hafter Laxon


Radar Altimetry


Radar Altimetry

SummarySummary


Radar Altimetry

THANK

YOU


Radar Altimetry

Principles of radar altimetry Principles of radar altimetry

Beam limited Pulse limited

Beam limited footprint < pulse limited footprint

LL:antenna size:wavelength

Pulse length = c

22L = B/2dB=d/L

B P

d d

(P/2)2 + d2 =(d+ c2

P = 2(2cd)1/2

GEOF334 – Spring 2010 Radar Altimetry Johnny A. Johannessen Nansen Environmental and Remote...

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