Slide 12nd GRAS SAF User Workshop, LO-Skolen, Helsingør, June 11 - 13, 2003

Introduction to Radio Introduction to Radio OccultationsOccultations

Georg Bergeton Larsen

GRAS SAF Project ManagerAtmosphere Ionosphere Research Division (AIR)

Danish Meteorological Institute (DMI)Copenhagen

Slide 22nd GRAS SAF User Workshop, LO-Skolen, Helsingør, June 11 - 13, 2003

Outline of presentationOutline of presentation

• The satellite system

• Derivation of atmosphere parameters – Bending angle– Refractivity– Temperature– Humidity

• Distribution of measurements

• Advantages and limitations

• Satellite missions

• Summary

Slide 32nd GRAS SAF User Workshop, LO-Skolen, Helsingør, June 11 - 13, 2003

The satellite systemThe satellite system

The GPS constellationThe GPS constellation

The occultation measurementThe occultation measurement

Slide 42nd GRAS SAF User Workshop, LO-Skolen, Helsingør, June 11 - 13, 2003

GRAS Atmosphere profilingGRAS Atmosphere profiling

t1 t2

t3

GPS

Metop

Slide 52nd GRAS SAF User Workshop, LO-Skolen, Helsingør, June 11 - 13, 2003

Neutral Atmosphere Bending AngleNeutral Atmosphere Bending Angle

• The bending angle is computed from the atmospheric phase delay

• Ionosphere correction on bending angle

22

21

22

212

1 )()()(

ff

afafa

Slide 62nd GRAS SAF User Workshop, LO-Skolen, Helsingør, June 11 - 13, 2003

Refractivity profileRefractivity profile

• The refractivity is determined using the Abel transform

• Uncertainty: 0.3 % measurement range: 4 - 450 N-units

))(1

exp()(22

a ax

dxxa

curveRa

ahN

)(,10)1( 6

Slide 72nd GRAS SAF User Workshop, LO-Skolen, Helsingør, June 11 - 13, 2003

Atmosphere ParametersAtmosphere Parameters

• The density of the dry atmosphere is computed by assuming an ideal gas

• where Rd = 0.287 J/(gK) and k1=77.6 K/hPa, k2 = 37,39104 K2/hPa, k3 = 70,40 K/hPa. The pressure profile is now obtained by using hydrostatic equilibrium

• Uncertainty: 0.3 % measurement range: 10-1100 mb

T

Pk

T

Pk

T

PkN wwd

3221

)(1

)(1

zNRk

zd

z

dry dzzgzzP ')'()'()(

Slide 82nd GRAS SAF User Workshop, LO-Skolen, Helsingør, June 11 - 13, 2003

Temperature profileTemperature profile

• The temperature profile is derived by using the ideal gas law and the estimated profiles of refractivity and pressure.

• Uncertainty: 1K measurement range: 180K - 335 K

)(

)()( 1

zN

zPkzT dry

dry

Slide 92nd GRAS SAF User Workshop, LO-Skolen, Helsingør, June 11 - 13, 2003

Water vapor profileWater vapor profile

• The water vapor pressure is derived by an iterative process using T(z) from NWP model (Offline Products) and using the 1DVAR method (NRT Products).

• 1) Total pressure

• 2) Water vapor pressure

• 3) Total density

• Equations 1) to 3) are solved with the dry pressure as initial input in eq. 1) and Rw=0.461 J/(gK).

• Uncertainty: < 20 % Measurement range: 1 - 45 mb • Assumed uncertainty on T(z) less than 1K

z

dzzgzzP ')'()'()(

2

1 )())()()(()(

k

zTzPkzTzNzPw

)(

)(1

)(

)(1)(

zT

zP

R

R

zT

zP

Rz w

w

d

d

Slide 102nd GRAS SAF User Workshop, LO-Skolen, Helsingør, June 11 - 13, 2003

GRAS SAF Prototype TemperatureGRAS SAF Prototype TemperatureProfile from CHAMP DataProfile from CHAMP Data

Slide 112nd GRAS SAF User Workshop, LO-Skolen, Helsingør, June 11 - 13, 2003

Retrieved Water Vapour ProfileRetrieved Water Vapour Profile

GPS/MET occultation Feb 9, 1997 at UT 16:15 lat: –14 lon: 141

Tdry

TNWP

eocc

esaturated

Slide 122nd GRAS SAF User Workshop, LO-Skolen, Helsingør, June 11 - 13, 2003

Distribution of GRAS measurementsDistribution of GRAS measurements

GRAS occultations during 24 hrs. Approximately 600 atmosphere profiles distributed globally

Distribution of NWP Radio sondes

Slide 132nd GRAS SAF User Workshop, LO-Skolen, Helsingør, June 11 - 13, 2003

Advantages and limitations of GPS Advantages and limitations of GPS Atmosphere ProfilingAtmosphere Profiling

• Absolute measurement♦ The basics of the observations are a measurement of time. Calibration of

clocks can be achieved using assisting ground observations.

• Global coverage♦ The geometry of the observation leads for one satellite to evenly distributed

data on a 24-hour interval. Observations over seas and oceans (covering 70% of the Earth) minimize the major error source concerning accuracy of weather forecast and climate models.

• High vertical resolution♦ The vertical resolution limited by the Fresnel zone of the observation leads to

information of atmosphere phenomena with scale sizes less than 1 km.

• Insensitive to clouds and precipitation♦ The wavelengths applied makes the measurement transparent to clouds and

rain hampering other space techniques.

Slide 142nd GRAS SAF User Workshop, LO-Skolen, Helsingør, June 11 - 13, 2003

Satellite missions Satellite missions

• Research / demonstration• GPS-Met (1995-97)• Ørsted (1999-)• CHAMP (2000-)• SAC-C (2000-)• GRACE• FedSat

• Operational• METOP/GRAS (2005-) • NPOESS/GPSOS (2010-)

• Constellations of micro-satellites• COSMIC (2006-)• ACE+ (2008-)

Slide 152nd GRAS SAF User Workshop, LO-Skolen, Helsingør, June 11 - 13, 2003

SummarySummary

• Key parameters and method introduced – Bending angle

– Refractivity

– Temperature

– Humidity

• Examples and distribution of RO measurements

• Advantages and limitations– Global coverage (not synoptic)

– High vertical resolution (averaged horizontally)

– Insensitive to clouds

• Satellite missions– GRAS on Metop - first operational RO mission