gps radio occultation sounding zhen zeng (hao&cosmic)
TRANSCRIPT
GPS Radio Occultation Sounding
Zhen Zeng (HAO&COSMIC)
Occultation is from the Latin word “Occult” meaning “to hide”.
Conditions: both a transmitter and a receiver off the planet of interest;
Value: many “conventional” measurements (comprehensive, continuous, synoptic);
Cost: expansive or cheap?
Establishment of GPS constellation
April, 1995
GPS/MET
Early temperature profile from GPS/MET
CHAMP
SAC-C
GRACE
Ørsted
Sunsat
IOX
T. P. Yunck
Basic measurements: precise measurement of phase delay and amplitude of GPS signals.
Principles of occultation technique
The velocity of GPS relative to LEO must be estimated to ~0.2 mm/sec (velocity of GPS is ~3 km/sec and velocity of LEO is ~7 km/sec) to determine precise temperature profiles
The LEO tracks the GPS phase while the signal is occulted to determine the Doppler
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
LEO
vleo
vGPS
Tangent point
Principles of occultation technique
Atmospheric refractive index where is the light velocityin a vacuum and is the light velocity in the atmosphere
Refractivity
26
25 103.401073.36.77
f
n
T
P
T
PN ew
vcn / cv
)1(106 nN
• Hydrostatic dry (1) and wet (2) terms dominate below 70 km
• Wet term (2) becomes important in the troposphere and can constitute up to 30% of refractivity at the surface in the tropics
• In the presence of water vapor, external information information is needed to obtain temperature and water vapor
• Liquid water and aerosols are generally ignored
• Ionospheric term (3) dominates above 70 km
(1) (2) (3)
6 Satellites was launched:01:40 UTC 15 April 2006
Three instruments:GPS receiver, TIP, Tri-band beacon
Weather + Space Weather data Global observations of:
Pressure, Temperature, HumidityRefractivityIonospheric Electron DensityIonospheric Scintillation
Demonstrate quasi-operational GPS limb sounding with global coverage in near-real time
Climate Monitoring
COSMIC (Constellation Observing System for Meteorology, Ionosphere
and Climate)
COSMIC launch picture provided by Orbital Sciences Corporation
• All six satellites stacked and launched on a Minotaur rocket
• Initial orbit altitude ~500 km; inclination ~72°
• Will be maneuvered into six different orbital planes for optimal global coverage (at ~800 km altitude)
• All satellites are in good health and providing initial data
Launch on April 14, 2006Vandenberg AFB, CA
Chris Rocken
COSMIC Ionospheric Data
GPS Receiver
TEC Profiles Scintillation
TIP
UV Radiation
TBB
ScintillationTEC
From presentation by Stig Syndergaard, UCAR/COSMIC
Absolute TEC Processing
• Pseudorange local multipath
• Phase cycle-slip & outliers (Blewitt, 1990)
• Phase-to-pseudorange leveling
• Differential code bias (DCB) calibrationFrom presentation by Stig Syndergaard, UCAR/COSMIC
Pseudorange Local Multipath Calibration
From presentation by Stig Syndergaard, UCAR/COSMIC
Pseudorange Local Multipath Calibration
From presentation by Stig Syndergaard, UCAR/COSMIC
LEO Differential Code Bias Estimation
From presentation by Stig Syndergaard, UCAR/COSMIC
LEO Differential Code Bias Estimation
From presentation by Stig Syndergaard, UCAR/COSMIC
Electron Density Profile Processing
• Subtracting positive elevation angle data from negative elevation angle (Schreiner et al., 1999)
• Model independent estimate of upper boundary electron density (Syndergaard et al., 2006)
• Profile retrieval based on straight-line and spherical symmetry assumptions
First Collocated Ionospheric Profiles
From presentation by Stig Syndergaard, UCAR/COSMIC
COSMIC Occultation Over South Atlantic: 1815 UTC
Black: CosmicBlue: IRIRed: IDA3D
Horizontal LinesRepresentationError
From presentation by Gary S. Bust, Atmospheric & Space Technology Research Associates
COSMIC NmF2 - 1 week
From presentation by Bill Kuo, UCAR/COSMIC
Quiet
Storm
COSMIC #2 GAIM
COSMIC #2 GAIM
Comparisons During Quiet and Disturbed Conditions
From presentation by Ludger Scherliess, Utah State University
Comparison of NmF2 and HmF2 between COSMIC and GAIM during Apr. 21-28, 2006
Good agreement of NmF2 between COSMIC and GAIM;
Higher peak heights from GAIM than those from COSMIC
Comparison of Ne(h) between COSMIC (red), Ionosondes (green)and TIEGCM (black) on Aug. 17 - 21nd
COSMIC agree well with ionosonde obs, especially the HmF2;
Vertical structures from COSMIC coincide well with TIEGCM in the mid-lat, but not in the tropics.
TIEGCM shows a bit higher HmF2 compared with obs.
COSMIC agree well with ionosonde observations;
Global map of NmF2 revealed from COSMIC is well represented by TIEGCM model, though TIEGCM shows higher peak density in the low latitude.
Maps of NmF2 for: COSMIC (dots), Ionosondes (stars), TIEGCM (contour)
RED = COSMIC satBLUE = GPS sat
Formosat-3/COSMIC Observations of Scintillations
From presentation by Chin S. Lin, AFRL
TIP 135.6-nm passes 14 Sep 2006 FM1 FM3 FM6 0-24 UT (2100 LT)
From presentation by Clayton Coker, NRL
COSMIC
Radiosondes
COSMIC Soundings in 1 Day
Chris Rocken
About 90,000 soundings / month Or ~10 soundings / 2.5 x 2.5 pixel / month
All local times sampled every day!
COSMIC Education Module
A joint effort by COMET and COSMIC.
It covers:
- Basics of GPS radio occultation science
- Applications to weather, climate, and ionosphere
- COSMIC Mission description
http://www.meted.ucar.edu/COSMIC/
http://www.cosmic.ucar.edu
* Select the 'Sign Up ' link under COSMIC
•Accept data use agreement
* Enter information: Name, Address, email, user_id,Password, planned use of data
• An email will be sent within 2-3 business days to indicateaccess has been granted.
COSMIC Data Access
More than 335 users have already registered
Applications
• Atmospheric science and weather forecasting
• Global change and Climate• Ionospheric monitoring and space we
ather
Using GPS temperatures to study the extratropical tropopause
1) Observational characteristics of double tropopauses
Occurs frequently, related to interaction between tropics and extratropics
acd.ucar.edu/~randel
(see also Schmidt et al., GRL, 2006)
2) The tropopause inversion layer
W. Randel
Equatorial QBO-- T. Schmidt, Atmos. Chem. Phys., 2005
Diurnal tides
Some promising forecast scores in the stratosphere. EG, verification
against radiosonde at 100 hPa. Red = COSMIC experiment.
The results in the NH are new. We had not seen improvements in NH with CHAMP. Note that the improvements are generally statistically significant at the 5% level.
NH
TR
SH
Sean Healy
Shu-peng Ho
Validation of Microwave Lower Stratosphere Temperatue using CHAMP RO data
Study ionospheric Irregularities
Pavelyev, GPS solution, 2005
edTECN dS
edTECN dS
edTECN dS
2
4 2
( )I t IS
I
: Amplitude scintillations averaged in 40-80
km interval
Questions for the future
• Retrieval Problem
-- Ionosphere residual
-- Spherical symmetry assumption
-- Orbits
-- Multipath
-- Cycle slips, qualtiy control …• What is optimal assimilation variables?• What is optimal processing strategy for climate records?