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